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A Reversibly Switching Surface
Abstract
We report the design of surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential. The change in wetting behavior was caused by surface-confined, single-layered molecules undergoing conformational transitions between a hydrophilic and a moderately hydrophobic state. Reversible conformational transitions were confirmed at a molecular level with the use of sum-frequency generation spectroscopy and at a macroscopic level with the use of contact angle measurements. This type of surface design enables amplification of molecular-level conformational transitions to macroscopic changes in surface properties without altering the chemical identity of the surface. Such reversibly switching surfaces may open previously unknown opportunities in interfacial engineering.
... Lahann et al. 33 paved the way in controlling the wettability in situ using this method. Using gold−thiol chemistry, they adsorbed onto gold surfaces a derivative of 16-mercaptohexadecanoic acid (MHA) that has (2-chlorophenyl) diphenylmethyl ester at the opposite end of MHA to prepare a new thiol molecule with a large headgroup (MHAE). ...
... Note that this proposed mechanism is different from others who induced the conformation change with charge− charge interactions. [33][34][35]37 The details of differences are given at the end of the Results and Discussion section because the results to be presented are needed in explaining why a different mechanism is necessary to explain the observations. Second, the change in the contact angle can be increased (or decreased) by mixing the molecules that would undergo conformational changes in response to electric current (hereafter noted as "controlling molecules") with shorter molecules that would not respond to electric current ("spacing molecules"). ...
... Note that the same experiments were performed on clean gold surfaces and those coated with ethanethiol, and they did not produce statistically meaningful changes in the contact angle (see Figure S1 in the Supporting Information). To account for the possible effects on the contact angles from heat-induced electrical resistance, the temperatures of cleaned and dry gold surfaces with and Similar to the report by Lahann et al. 33 and Luo et al. 37 that showed that the densely packed MHA did not produce a meaningful change in the contact angles of water, MHA 1:0 showed only a small change in contact angle. Indeed, the surface density from Luo et al. (0.87 nmol/cm 2 ) was similar to that reported in the right panel of Figure 3, which shows the adsorption density of MHA. ...
The ability to change wettability in situ would realize active surfaces that can change their functionality and adapt to different environments. This article reports a new and easy method that controls surface wettability in situ. In doing so, three hypotheses were to be proven. First, thiol molecules with dipole moments at the end that were adsorbed onto gold could change the contact angles of nonpolar or slightly polar liquids when an electric current was provided at the gold surface without having to ionize the dipole. It was also hypothesized that the molecules would undergo conformation changes as their dipoles would align with the magnetic field induced by the applied current. Second, the ability to change contact angles was modified by mixing ethanethiol, a much shorter thiol with no dipole, with the abovementioned thiol molecules because it would provide space for the thiol molecules to undergo conformation changes. Third, the indirect evidence of the conformation change was verified with attenuated total reflection Fourier transform infrared (FT-IR) spectroscopy. Four thiol molecules that controlled the contact angles of deionized water and hydrocarbon liquids were identified. The abilities of those four molecules in changing the contact angles were modified by adding ethanethiol. A quartz crystal microbalance was used to infer the possible change in the distance between the adsorbed thiol molecules by investigating adsorption kinetics. The changes in FT-IR peaks with respect to applied currents were also presented as indirect evidence for the conformation change. This method was compared with other reported methods that control wettability in situ. The differences between the voltage-driven method to induce conformation changes of thiol molecules and the method presented in this paper were further discussed to emphasize that the mechanism by which the conformation change was induced in this article was most likely because of the dipole-electric current interaction.
... [5][6][7][8][9][10][11][12][13] In most of these experiments ions move in a liquid solution driven by the applied field, covering and effectively modifying the sliding surfaces, thus inducing changes in friction. [14][15][16][17][18][19][20][21][22] An alternative approach is based on the ability of the electric field to reorient macromolecules, thus changing their conformation in aqueous solution [23][24][25] and dry environments, [26][27][28] with potentially dramatic effects on friction. Within this alternative scheme, here we consider neutral chain molecules tethered to flat substrate surfaces. ...
... These chains represent zwitterionic molecules, which have found applications for colloid stabilization, regulation in wetting and adhesion, the creation of protective coatings and many others. 23,[29][30][31] The surface force apparatus (SFA) provides a highly sensitive way to measure the frictional shear stress between atomically flat surfaces with molecules deposited on them, while applying a controlled normal load. [32][33][34][35][36] While such experimental setup does measure the system rheological and dissipative response in terms of crucial, yet averaged, physical quantities, 37 a viable exploitation of the electrotunable approach requires casting light on the elemental mechanisms and molecular rearrangements occurring at the sheared interface. ...
We theoretically explore the effect of a transverse electric field on the frictional response of a bi-layer of packed zwitterionic molecules. The dipole-moment reorientation promoted by the electric field can lead to either stick-slip or smooth sliding dynamics, with average shear stress values varying over a wide range. A structure-property relation is revealed by investigating the array of molecules and their mutual orientation and interlocking. Moreover, the thermal friction enhancement previously observed in these brushes is shown to be suppressed by the electric field, recovering the expected thermolubricity at large-enough fields. The same holds for other basic tribological quantities, such as the external load, which can influence friction in opposite ways depending on the strength of the applied electric field. Our findings open a route for the reversible control of friction forces via electric polarization of the sliding surface.
... In addition to the use of polymer brushes to achieve efficient lubrication, by adjusting the external conditions to apply stimuli to the lubrication layer, such as solvent [50], light [51], temperature [52], pH [53], electric field [54], and shear stress [55], the conformation of some polymers can be changed accordingly to achieve further modulation of the interface lubricating properties. For example, based on the mimicry of the lubrication performance of fish skin, Wu et al. [56] further introduced the pH-sensitive monomers sodium methacrylate (NaMA) and 2-(dimethylamino)ethyl methacrylate into the temperaturesensitive graphene-pNIPAM gel system, obtaining a hydrogel with the dual responsiveness of the pH and temperature (Figure 6a-d). ...
Organisms in nature have evolved a variety of surfaces with different tribological properties to adapt to the environment. By studying, understanding, and summarizing the friction and lubrication regulation phenomena of typical surfaces in nature, researchers have proposed various biomimetic friction regulation theories and methods to guide the development of new lubrication materials and lubrication systems. The design strategies for biomimetic friction/lubrication materials and systems mainly include the chemistry, surface structure, and mechanics. With the deepening understanding of the mechanism of biomimetic lubrication and the increasing application requirements, the design strategy of multi-strategy coupling has gradually become the center of attention for researchers. This paper focuses on the interfacial chemistry, surface structure, and surface mechanics of a single regulatory strategy and multi-strategy coupling approach. Based on the common biological friction regulation mechanism in nature, this paper reviews the research progress on biomimetic friction/lubrication materials in recent years, discusses and analyzes the single and coupled design strategies as well as their advantages and disadvantages, and describes the design concepts, working mechanisms, application prospects, and current problems of such materials. Finally, the development direction of biomimetic friction lubrication materials is prospected.
... The development of reversible adhesives, which are triggered by a change in temperature, [1] hydration, [2] an electrical potential, [3] exposure to radiation, [4] a magnetic field, [5] or a change in pH, [6] are of interest from both scientific and industrial perspectives. [7][8][9] The mechanism of debonding for each case depends on the interfacial chemistry between two surfaces, which very often precludes their integration into large scale industrial processes. ...
Commercial adhesives typically fall into two categories: structural or pressure sensitive. Structural glues rely on covalent bonds formed during curing and provide high tensile strength whilst pressure‐sensitive adhesives use physical bonding to provide weaker adhesion, but with considerable convenience for the user. Here, a new class of adhesive is presented that is also reversible, with a bond strength intermediate between those of pressure‐sensitive and structural adhesives. Complementary water‐based formulations incorporating oppositely charged polyelectrolytes form electrostatic bonds that may be reversed through immersion in a low or high pH aqueous environment. This electrostatic adhesive has the advantageous property that it exhibits good adhesion to low‐energy surfaces such as polypropylene. Furthermore, it is produced by the emulsion copolymerization of commodity materials, styrene and butyl acrylate, which makes it inexpensive and opens the possibility of industrial production. Bio‐based materials have been also integrated into the formulations to further increase sustainability. Moreover, unlike other water‐based glues, adhesion does not significantly degrade in humid environments. Because such electrostatic adhesives do not require mechanical detachment, they are appropriate for the large‐scale recycling of, e.g., bottle labels or food packaging. The adhesive is also suitable for dismantling components in areas as varied as automotive parts and electronics.
... . 在电化学反应过程中通常会伴随着催化剂原 始稳态结构在电压驱动下的结构演变, 通常表现为原 子结构、配位结构、氧化态的变化并伴随反应中间态 的不断演化形成高活性的亚稳态, 此亚稳态才是反应 的真正活性相 [14,15] . 因此, 电催化过程中活性位点的 微观结构演变和反应中间体的同步演变动力学的实时 探测仍然存在巨大的挑战, 主要由于有效识别电极表 面的微弱信号和识别界面的稀疏中间产物存在很大困 难 [16,17] . 近年来, 通过准原位的静态形貌表征、原位光 谱学以及理论计算模拟的手段来对电化学的反应中结 构演变以及催化反应路径的选择性的研究取得了一些 进展 [18,19] . ...
... 27,30−41 Despite their disordered surface structures, their applications as molecular rectifiers make them highly attractive systems. 39,42,43 SAMs formed from aromatic thiols have also found applications in nanotechnology due to their interesting electronic properties. 5,39 SAMs formed from aromatic organothiol compounds that have been characterized with STM are marked by large domains of bright protrusions and an absence of gold vacancy islands. ...
A two-dimensional (2D) mixture in the form of a self-assembled monolayer composed of two distinct organothiol compounds was created by sequentially depositing 1-naphthalenethiol (1NT) and octanethiol (OT) on a gold surface. By varying the sequence of deposition, two mixed surface systems were created. The surface structure of the resulting mixed monolayer was characterized with Scanning Tunneling Microscopy (STM) and showed surface disorder across all investigated domains. Elemental analysis was carried out with X-ray Photoelectron Spectroscopy (XPS) and indicated that the 1NT monolayer was prone to significant oxidation. Reductive desorption (RD) was used to characterize the binding strength and electrochemical environments of the molecular components in the mixture, and confirmed disordered molecular layers. Due to the presence of oxidized species in the 1NT monolayer, 1NT was displaced by OT resulting in a novel surface structure composed of either OT or 1NT. Monolayers of OT that were exposed to a solution of 1NT resulted in disordered surface structures with a significant amount of gold vacancy islands. To date, there is no experimental phase diagram explaining the chemical behavior of two-dimensional mixtures. This study addresses the need for an experimental understanding of the phase behavior of mixed organothiol self-assembled monolayers (SAMs).
... Carboxylic acid-terminated self-assembled monolayers have attracted considerable interest due to their wide applications in nanoscience and nanotechnology [117][118][119][120][121][122][123][124][125]. However, there were inconsistent values of water contact angles on COOH-SAMs, even after 25 years of study, which still puzzles researches regarding the surface water adsorption behavior on COOH-SAMs. ...
Experiments and theory have revealed versatile possible phases for adsorbed and confined water on two-dimensional solid surfaces, which are closely related to the aspects of various phenomena in physics, chemistry, biology, and tribology. In this review, we summarize our recent works showing that the different water phases with disordered and ordered structures can greatly affect surface wetting behavior, dielectric properties, and frictions. This includes the ordered phase of water structure that induces an unexpected phenomenon, an “ordered water monolayer that does not completely wet water”, at T = 300 K on the model’s surface and some real, solid material, together with the anomalous low dielectric properties due to ordered water.
... To reuse defective products or recycle bonded materials, dismantlable or reversible adhesives are being developed. Dismantlable adhesives are those that can be separated on demand through the application of an external stimuli: 23 a change in temperature, 92,93 exposure to radiation, 94,95 an electrical current, 96,97 or a change in pH, 98,99 among others. Being able to detach bonded materials can be useful for reducing waste associated with defective products that cannot be repaired due to failed parts bonded adhesively or to bonded materials that cannot be reintegrated into the production flow without being first separated. ...
This Feature Article evaluates ongoing efforts to adapt adhesives toward the goal of zero-waste living and suggests the most promising future directions. Adhesives are not always considered in zero-waste manufacturing because they represent only a small fraction of a product and offer no additional functionality. However, their presence restricts the reintegration of constituent parts into a circular economy, so a new generation of adhesives is required. Furthermore, their production often leads to harmful pollutants. Here, two main approaches toward addressing these problems are considered: first, the use of natural materials that replace petroleum-based polymers from which conventional adhesives are made and second, the production of dismantlable adhesives capable of debonding on demand with the application of an external stimulus. These approaches, either individually or combined, offer a new paradigm in zero-waste industrial production and consumer applications.
... [16][17][18][19] Despite extensive progress in tailoring surface morphologies, nature also provides a higher level of biomimetic opportunities, with the intent to in situ tune wettability in response to external environments through a morphological transformation of surface structures, for example, the dry-wet reversible transformation of geckos, 20 tree frogs, 21 octopuses, 22 and springtails. 23 Although smart responsive materials with tunable physical properties in response to external stimuli, for example, chemical, 24 thermal, 25 magnetic, 26 and electric, 27 have emerged and come into use, they are mainly catalyzed (also limited) by progress in material science. In situ tailoring structural morphologies for wettability tuning independent of smart responsive materials remains elusive. ...
Morphological transformation of surface structures is widely manifested in nature and highly preferred for many applications such as wetting interaction; however, in situ tuning of artificial morphologies independent of smart responsive materials remains elusive. Here, with the aid of microfluidics, we develop a pneumatic programmable superrepellent surface by tailoring conventional wetting materials (e.g., polydimethylsiloxane) with embedded flexible chambers connecting a microfluidic system, thus realizing a morphological transformation for enhanced liquid repellency based on a nature‐inspired rigid‐flexible hybrid principle (i.e., triggering symmetry breaking and oscillator coupling mechanisms). The enhancement degree can be in situ tuned within around 300 ms owing to pneumatically controllable chamber morphologies. We also demonstrate that the surface can be freely programmed to achieve elaborated morphological pathways and gradients for preferred droplet manipulation such as directional rolling and bouncing. Our study highlights the potential of an in situ morphological transformation to realize tunable wettability and provides a programmable level of droplet control by intellectualizing conventional wetting materials. Morphological transformation of surface structures is an efficient way to in situ tune the wettability, which mainly relies on smart responsive materials. We develop a pneumatic programmable superrepellent surface by tailoring conventional wetting materials with the aid of microfluidics, showing an in situ morphological transformation for wettability tuning and droplet manipulation (directional rolling and bouncing).
... The use of oxides, for example, has been investigated with nucleate boiling heat transfer to increase wetting speeds (and, therefore, increase the critical heat flux) in thermal management applications [22][23][24]. Moreover, external stimuli such as temperature [25][26][27], electricity [28][29], pH [30], and solvent [31] were used to modulate the wetting dynamics on various surfaces. For example, various methods have been reported to improve the controllable wettability of TiO 2 -derived materials and their applications [32]. ...
This work demonstrates how photoactive compounds can be used to tune the surface chemistry, surface free energy, and the wetting velocity of fluids on Spiropyran functionalized surfaces with different surface microstructures. Evidence of this photowetting effect is based on data showing: (1) the cyclic changes in the static, advancing, and receding contact angles θCA for multiple UV ⇄ vis photoswitching cycles with both smooth and microstructured surfaces and (2) the changes in the fluid wicking velocity (via UV ⇄ vis photoswitching) on different Spiropyran functionalized hemiwicking surfaces. X-ray photoelectron spectroscopy is used to determine the efficiency of photoswitching caused by the quality of the Spiropyran functionalization. Reversible photoisomerization of Spiropyran generates a gradient in surface free energy, resulting in a similar change in contact angle (θCA) in the photoswitchable surfaces. By incorporating these changes into the Owens-Wendt and Van Oss surface energy models, the energy of smooth Au surfaces can also be predicted for both UV and visible light irradiations, where reversible contact angle variations of ΔθCA≈ 5-10 ° are achieved with both water and water-ethanol mixtures due to the conversion of SP to merocyanine, which corresponds to a total free energy change of ∼13% on the photoswitchable surface.
... When the surface experiences a stimulus, wetting changes can be observed, displaying responsive superwetting ( Figure 3c). 113,114 Currently, the external stimuli have been divided into multiple stimuli, such as light, 115−118 pH, 119−121 temperature, 122,123 gas, 124,125 electric field, 126,127 magnetic field, 128,129 etc. As wellknown light-responsive materials, Ogawa's group reported a series of titanium biomaterials based on photofunctionalizations. 133−137 The fluorinated TiO 2 nanoparticles show superhydrophilicity after extending UV illumination due to hydroxyl groups and molecular water adsorption. ...
Molecular‐level insight into the interfacial composition of electrodes at the solid‐electrolyte and the solid‐electrode interface is essential to understanding the charge transfer processes, which are vital for electrochemical (EC) and photoelectrochemical (PEC) applications. However, spectroscopic access to both interfaces, particularly upon application of an external bias, remains a challenge. Here, in situ surface sensitive vibrational sum‐frequency generation (VSFG) spectroscopy is used for the first time to directly access the interfacial structure of a cobalt‐containing Prussian blue analog (Co‐PBA) in contact with the electrolyte and TiO 2 /Au surface. Structural and compositional changes of the Prussian blue layer during electrochemical oxidation are studied by monitoring the stretching vibration of the CN group. At open circuit potential, VSFG reveals a non‐homogeneous distribution of oxidation states of metal sites: Fe III –CN–Co II and Fe II –CN–Co III coordination motifs are dominantly observed at the Co‐PBA|TiO 2 interface, while it is only the Fe II –CN–Co II unit at the electrolyte interface. Upon increasing the potential applied to the electrode, the partial oxidation of Fe II –CN–Co II to Fe III –CN–Co II is observed followed by its transformation to Fe II –CN–Co III via charge transfer and, finally, the formation of Fe III –CN–Co III species at the interface with TiO 2 and the electrolyte.
Surface-tension-confined microfluidic devices are platforms for manipulating 2D droplets based on patterned surfaces with special wettability. They have great potential for various applications, but are still in the early stages of development and face some challenges that need to be addressed. This study, inspired by the Wenzel and slippery transition of rose petal, develops a Patterned Oil-triggered Wenzel-slippery Surface (POWS) to examine the microfluidic devices. A laser-chemical composite method is established to fabricate POWSs, which take rose-petal-like microstructures as wettability pattern and a superamphiphobic surface as the background. The prepared POWSs switched between high adhesion superhydrophobic state and the slippery liquid-infused surface state through adding or removing the lubricant oil. In the high adhesion superhydrophobic state, the droplets can be sticked on the surface. In the slippery liquid-infused state, the droplet can slide along the wettability pattern as the designed route. A POWS-based droplet reactor is further constructed, on which, the droplets can be remotely controlled to move, mix and react, as required. Such a POWS, which manipulates droplets with surface tension controlled by the switchable wettability patterns, would be a promising candidate to construct multiple surface-tension-confined microfluidic devices. In addition, the fabrication technique and design principle proposed here may aid the development of various field related to the bio-inspired surfaces, such as water collection, desalination and high throughput analysis, etc.
Under the influence of electric fields, the chains in polyelectrolyte brushes can stretch and collapse, which changes the structure of the brush. Copolymer brushes with charged and uncharged monomers display a similar behavior. For pure polyelectrolyte and random copolymer brushes, the field-induced structure changes only the density of the brush and not its local composition, while the latter could be affected if charges are distributed inhomogeneously along the polymer backbone. Therefore, we systematically study the switching behavior of gradient polyelectrolyte brushes in electric fields for different solvent qualities, grafting densities, and charges per chain via coarse-grained molecular dynamics simulations. Similar to random copolymers and pure polyelectrolytes, these brushes show a mixed-phase transition: intermediate states between fully stretched and collapsed are characterized by a bimodal chain-end distribution. Additionally, we find that the total charge of the brush plays a key role in the critical field required for a complete transition. Finally, we find that gradient polyelectrolyte brushes are charge-enriched at the brush–solvent interface under stretched conditions and charge-depleted under collapsed conditions, allowing for control over the local composition and thus the surface charge of the brush due to the inhomogeneous charge along the grafted chains.
Throughout the last few decades, a significant amount of research has been conducted on the wetting and deformation of soft materials. Employing high-speed videography, we report the spatiotemporal wetting and hemiwicking velocities for different fluids with micropillar hemiwicking arrays of various pillar geometries, spacing configurations, and Young's moduli ranging from E = 61 kPa to 12 GPa. The wetting and wicking dynamics are analyzed with comparisons to that predicted by contemporary scaling laws or derived analytic solutions. The results show that our analytical models can predict the inertial wetting, deformation, and hemiwicking dynamics within typically <50% of that experimentally observed. For the wide range of sample-fluid couples studied, the upper-limit (maximum) hemiwicking and inertial wetting velocities are typically observed within the narrow ranges of 10~30 mm/s and 0.8~1.6 m/s, respectively. Aside from the ultrasoft (~61 kPa) Ecoflex sample, sample stiffness had no notable effect on the maximum speed that a fluid can wet the non-textured surface region. However, for soft materials with elastic moduli below ~150 kPa, the rapid coupling between transient swelling and elastocapillary deformations leads to recoil wetting and wicking dynamics and notable reductions in the hemiwicking velocity.
Commercial adhesives typically fall into two categories: structural or pressure sensitive. Structural glues rely on covalent bonds formed during curing and provide high tensile strength whilst pressure‐sensitive adhesives use physical bonding to provide weaker adhesion, but with considerable convenience for the user. Here, a new class of adhesive is presented that is also reversible, with a bond strength intermediate between those of pressure‐sensitive and structural adhesives. Complementary water‐based formulations incorporating oppositely charged polyelectrolytes form electrostatic bonds that may be reversed through immersion in a low or high pH aqueous environment. This electrostatic adhesive has the advantageous property that it exhibits good adhesion to low‐energy surfaces such as polypropylene. Furthermore, it is produced by the emulsion copolymerization of commodity materials, styrene and butyl acrylate, which makes it inexpensive and opens the possibility of industrial production. Bio‐based materials have been also integrated into the formulations to further increase sustainability. Moreover, unlike other water‐based glues, adhesion does not significantly degrade in humid environments. Because such electrostatic adhesives do not require mechanical detachment, they are appropriate for the large‐scale recycling of, e.g., bottle labels or food packaging. The adhesive is also suitable for dismantling components in areas as varied as automotive parts and electronics.
Area-selective atomic layer deposition (AS-ALD) of insulating metallic oxide layers could be a useful nanopatterning technique for making increasingly complex semiconductor circuits. Although the alkanethiol self-assembled monolayer (SAM) has been considered promising as an ALD inhibitor, the low inhibition efficiency of the SAM during ALD processes makes its wide application difficult. We investigated the deposition mechanism of Al2O3 on alkanethiol-SAMs using temperature-dependent vibrational sum-frequency-generation spectroscopy. We found that the thermally induced formation of gauche defects in the SAMs is the main causative factor deteriorating the inhibition efficiency. Here, we demonstrate that a discontinuously temperature-controlled ALD technique involving self-healing and dissipation of thermally induced stress on the structure of SAM substantially enhances the SAM's inhibition efficiency and enables us to achieve 60 ALD cycles (6.6 nm). We anticipate that the present experimental results on the ALD mechanism on the SAM surface and the proposed ALD method will provide clues to improve the efficiency of AS-ALD, a promising nanoscale patterning and manufacturing technique.
We theoretically explore the effect of a transverse electric field on the frictional response of a bi-layer of packed zwitterionic molecules. The dipole-moment reorientation promoted by the electric field can lead to either stick-slip or smooth sliding dynamics, with average shear stress values varying over a wide range. A structure-property relation is revealed by investigating the array of molecules and their mutual orientation and interlocking. Moreover, the thermal friction enhancement previously observed in these molecules is shown to be suppressed by the electric field, recovering the expected thermolubricity at large-enough fields. The same holds for other basic tribological quantities, such as the external load, which can influence friction in opposite ways depending on the strength of the applied electric field. Our findings open a route for the reversible control of friction forces via electric polarization of the sliding surface.
A hierarchically porous monolith with macro-and meso-pores was synthesized via a sol-gel and phase separation process. Due to the surface modification by organic silanes, the wettability of the silica material was effectively controlled. A series of hydrophobic porous silica monoliths (HPSM) were obtained. Using a "reverse membrane emulsification" process, the HPSM not only cleared oil away from water, but also broke the micro-emulsion efficiently, even when emulsion stabilizer was in the system. As the filtration layer, 1.0 g of HPSM could treat 944 mL of oil containing water or 667 mL of surfactant-stabilized micro-emulsion. HPSM materials could remove at maximum 96.5% of the oil in water and 100% surfactant in the micro-emulsion. In addition, the material could be reused through a simple treatment. The excellent separating effect was kept even after 8 times of regeneration. Special selectivity, easy operation and excellent recyclability make the material have great potential for practical application.
In this study, thermosensitive amino‐silica@PDVB/PNIPAM Janus particles (JPs) are synthesized by seed emulsion polymerization‐induced phase separation and selective modification methods. Amino‐modified silica moieties are covalently bonded to a diverse choice of substrates to achieve robust composite coatings, and a PDVB/PNIPAM abdomen forms a micro‐nano‐scale hierarchical surface. PNIPAM has a lower critical solution temperature (LCST), which allows the hydrophilic and hydrophobic properties of the coating to reverse with a change in temperature. When the fabrics are coated with the thermosensitive Janus particles, water repellency is observed above 32 °C, while hydrophilicity is revealed below 32 °C. Then, after the composite fabric is worn, the side next to the skin becomes hydrophobic due to the high temperature, and the side facing the environment is hydrophilic. Therefore, sweat can be pumped from the hydrophobic side to the hydrophilic side through the dynamic Janus fabric. The dynamic hydrophobic–hydrophilic Janus structure enables the efficient and fast evaporation of sweat. The perspiration rate of Janus fabrics is five times higher than that of commercial cotton fabrics. While the wettability of the composite coating remains reversible after 20 temperature cycles and 20 tape adhesion cycles, showing good mechanical durability. The reversible thermal sensitivity remains after repeated rubbing and ultrasonic immersion.
Wetting functionalities of rough surfaces are largely determined by the Laplace pressure generated across liquid–gas interfaces formed within surface structures. Typically, rough wetting surfaces create negative Laplace pressures, enabling capillary wicking, while rough non‐wetting surfaces create positive Laplace pressures, exhibiting fluid repellency. Here, with microfabricated reentrant structures, it is shown that the same surface can exhibit either a negative or positive Laplace pressure, regardless of its intrinsic wettability. This material‐independent Laplace pressure duality enables or enhances a range of wetting functionalities including wicking, switchability, and selectivity. On the same surface, capillary rise, capillary dip, and the combination of the two which leads to further enhancement of the total sustainable capillary height and Laplace pressure, the driving force for wicking is demonstrated. Further, active switching of wetting states between the hemiwicking and the repellent Cassie state on reentrant structures is shown. Moreover, with a water‐hexane mixture system, selective wetting of reentrant structures are demonstrated, that is, water can be selectively wicked or repelled in the presence of hexane, and vice versa. These functionalities are achieved, which would typically require complex chemical coatings, solely using surface structures, thus largely expanding the design space for a wide range of thermofluidic applications. The dual Laplace pressure of reentrant structures, which enables tailoring the surface wetting state to either repellent or hemiwicking state regardless of intrinsic wettability, is demonstrated by capillary rise/dip tests. Wetting functionalities such as switchability and selectivity are achieved by harnessing the dual Laplace pressure. This work has important implications in enabling extreme wettabilities by surface structures alone.
Organic luminescent materials are one of the research hotspots worldwide. The luminescent efficiency of these materials can be tuned by introducing noncovalent interactions. Ion−π interactions, mainly including anion−π interactions and cation−π interactions, are relatively new but powerful noncovalent interactions with extensive applications in supramolecular chemistry. The use of ion−π interactions to fabricate highly luminescent materials is initiated since they were introduced to develop aggregation‐induced emission luminogens in 2017. Then, this research field is undergoing through challenging but very prospective developments. In this minireview, we start with a brief introduction of ion−π interactions and then make a summary of the reports on organic fluorescence based on ion−π interactions and room temperature phosphorescence induced by ion−π interactions. The applications of organic luminescent materials based on ion−π interactions in bioimaging, imaging‐guided anticancer and antibacterial treatment are also discussed. The challenges and prospects of using anion−π and cation−π interactions for constructing organic luminogens are presented in the final part. It is expected that this minireview will provide some guidelines for fabricating novel organic luminescent materials and further extend the application potential of ion−π interactions. The construction of organic fluorescence and room temperature phosphorescence materials by using ion−π interactions including anion−π interactions and cation−π interactions is presented in this minireview. The applications of these ion−π type organic luminescent materials in bioimaging, imaging‐guided anticancer, and antibacterial treatment are also summarized. The perspective and challenges for constructing ion−π type organic luminescent materials are discussed.
Surfactant-based viscoelastic (SBVE) fluids are innovative nonpolymeric non-newtonian fluid compositions that have recently gained much attention from the oil industry. SBVE can replace traditional polymeric fracturing fluid composition by mitigating problems arising during and after hydraulic fracturing operations are performed. In this study, SBVE fluid systems which are entangled with worm-like micellar solutions of cationic surfactant: cetrimonium bromide or CTAB and counterion inorganic sodium nitrate salt are synthesized. The salt reagent concentration is optimized by comparing the rheological characteristics of different concentration fluids at 25 °C. The study aims to mitigate the primary issue concerning these SBVE fluids: significant drop in viscosity at high temperature and high shear rate (HTHS) conditions. Hence, the authors synthesized a modified viscoelastic fluid system using ZnO nanoparticle (NPs) additives with a hypothesis of getting fluids with improved rheology. The rheology of optimum fluids of both categories: with (0.6 M NaNO3 concentration fluid) and without (0.8 M NaNO3 concentration fluid) ZnO NPs additives were compared for a range of shear rates from 1 to 500 Sec−1 at different temperatures from 25 °C to 75 °C to visualize modifications in viscosity values after the addition of NPs additives. The rheology in terms of viscosity was higher for the fluid with 1% dispersed ZnO NPs additives at all temperatures for the entire range of shear rate values. Additionally, rheological correlation function models were derived for the synthesized fluids using statistical analysis methods. Subsequently, Herschel–Bulkley models were developed for optimum fluids depending on rheological correlation models. In the last section of the study, the pressure-drop estimation method is described using given group equations for laminar flow in a pipe depending on Herschel–Bulkley-model parameters have been identified for optimum fluids are consistency, flow index and yield stress values.
Membrane separation has been widely used in water and wastewater treatment due to its relatively low cost, easy operation and high efficiency. However, current membranes suffer from a trade-off between selectivity and permeability as well as membrane fouling. Recently, numerous studies have focused on the development of carbon nanomaterial (typically, carbon nanotube and graphene)-based membranes with high performance for water and wastewater treatment. Many studies have demonstrated that benefiting from the good electrical conductivity of carbon nanomaterial-based membranes, their permselectivity and fouling resistance can be further improved under electrochemical assistance. Herein, we review the recent progress in the preparation, mechanisms and applications of electroconductive nanocarbonaceous membranes for water purification and wastewater treatment, aiming at improving the fundamental understanding of the combination of membrane separation and electrochemistry. Firstly, we summarize the recent methods for the preparation of some typical electroconductive membranes, for example, carbon nanotube membranes and graphene membranes. Subsequently, we discuss the underlying mechanisms (e.g., electro-oxidation, electro-adsorption, and electrostatic interactions) for the electrochemically enhanced permselectivity, antifouling and regeneration performance, and finally highlight the practical limitations of membrane/electrochemistry systems and outline possible solutions.
The local electric field affects the morphology of ionomer films on electrodes and thus the transport phenomena in ionomer films. Therefore, tuning the local electric field is a potential way...
Smart oil-water separation materials with stimuli-responsive wettability surfaces have attracted wide attention. In this paper, a novel, simple and low-cost method is developed to prepare a pH-responsive melamine sponge (MS) with switchable wettability. This approach involves the hydrophobic modification of hexadecyltrimethoxysilane (HDTMS) on the MS surface modified by polydopamine (PDA) and the Michael addition reaction and Schiff base reaction between 12-aminododecanoic acid (NH2(CH2)11COOH) and PDA. Hydrophobic/oleophilic HDTMS and pH-triggered NH2(CH2)11COOH enable the functionalized sponge to repeatedly switch between the two extreme wettability under different pH environments. Benefiting from the excellent 3D porous surface structure and smart wettability, the as-prepared sponge can be applied in the on-demand separation of various oil-water mixtures by adsorption or filtration with a separation efficiency over 98.5% and high absorption capacity of 72.7-161.3 g·g⁻¹. With the assistance of pump-driven, continuous adsorption separation can be achieved in two cases by utilizing the wettability transformation. Moreover, in an alkaline solution, the sponge can quickly release most of the absorbed oil, showing a controllable desorption process. This smart wettability material with simple preparation and green has application potential in controllable oil-water separation and on-demand oil-spills treatment.
Electrochemical interfaces are key structures in energy storage and catalysis. Hence, a molecular understanding of the active sites at these interfaces, their solvation, the structure of adsorbates, and the formation of solid‐electrolyte interfaces are crucial for an in‐depth mechanistic understanding of their function. Vibrational sum‐frequency generation (VSFG) spectroscopy has emerged as an operando spectroscopic technique to monitor complex electrochemical interfaces due to its intrinsic interface sensitivity and chemical specificity. Thus, this review discusses the happy get‐together between VSFG spectroscopy and electrochemical interfaces. Methodological approaches for answering core issues associated with the behavior of adsorbates on electrodes, the structure of solvent adlayers, the transient formation of reaction intermediates, and the emergence of solid electrolyte interphase in battery research are assessed to provide a critical inventory of highly promising avenues to bring optical spectroscopy to use in modern material research in energy conversion and storage.
Adhesives have a long and illustrious history throughout human history. The development of synthetic polymers has highly improved adhesions in terms of their strength and environmental tolerance. As soft robotics, flexible electronics, and intelligent gadgets become more prevalent, adhesives with changeable adhesion capabilities will become more necessary. These adhesives should be programmable and switchable, with the ability to respond to light, electromagnetic fields, thermal, and other stimuli. These requirements necessitate novel concepts in adhesion engineering and material science. Considerable studies have been carried out to develop a wide range of adhesives. This review focuses on stimuli‐responsive material‐based adhesives, outlining current research on switchable and controlled adhesives, including design and manufacturing techniques. Finally, the potential for smart adhesives in applications, and the development of future adhesive forms are critically suggested. In this review, the authors focused on stimuli‐responsive materials‐based adhesives, summarizing the current works of switchable and controllable adhesives, including the design and fabrication strategies. Finally, the challenges and opportunities for smart adhesives in applications and future forms of adhesives are discussed.
The in situ control of reversible protein adsorption to a surface is a critical step towards biofouling prevention and finds utilisation in bioanalytical applications. In this work, adsorption of peptides is controlled by employing the electrode potential induced, reversible change of germanium (100) surface termination between a hydrophobic, hydrogen terminated and a hydrophilic, hydroxyl terminated surface. This simple but effective 'smart' interface is used to direct adsorption of two peptides models, representing the naturally highly abundant structural motifs of amphipathic helices and coiled-coils. Their structural similarity coincides with their opposite overall charge and hence allows the examination of the influence of charge and hydrophobicity on adsorption. Polarized attenuated total reflection infrared (ATR-IR) spectroscopy at controlled electrode potential has been used to follow the adsorption process at physiological pH in deuterated buffer. The delicate balance of hydrophobic and electrostatic peptide/surface interactions leads to two different processes upon switching that are both observed in situ: reversible adsorption and reversible reorientation. Negatively charged peptide adsorption can be fully controlled by switching to the hydrophobic interface, while the same switch causes the positively charged, helical peptide to tilt down. This principle can be used for 'smart' adsorption control of a wider variety of proteins and peptides and hence find application, as e.g. a bioanalytical tool or functional biosensor.
The ability to control interfacial tension electrochemically is uniquely available for liquid metals (LMs), in particular gallium‐based LM alloys. This imparts them with excellent locomotion and deformation capabilities and enables diverse applications. However, electrochemical oxidation of LM is a highly dynamic process, which often induces Marangoni instabilities that make it almost impossible to elongate LM and manipulate its morphology directly and precisely on a 2D plane without the assistance of other patterning methods. To overcome these limitations, this study investigates the use of an LM–iron (Fe) particle mixture that is capable of suppressing instabilities during the electrochemical oxidation process, thereby allowing for superelongation of the LM core of the mixture to form a thin wire that is tens of times of its original length. More importantly, the elongated LM core can be manipulated freely on a 2D plane to form complex patterns. Eliminating Marangoni instabilities also allows for the effective spreading and filling of the LM–Fe mixture into molds with complex structures and small features. Harnessing these excellent abilities, a channel‐less patterning method for fabricating elastomeric wearable sensors is demonstrated to detect motions. This study shows the potential for developing functional and flexible structures of LM with superior performance.
We investigate the concept of molecular-sized outward-swinging gate, which allows for entropy decrease in an isolated system. The theoretical analysis, the Monte Carlo simulation, and the direct solution of governing equations all suggest that under the condition of local nonchaoticity, the probability of particle crossing is asymmetric. It is demonstrated by an experiment on a nanoporous membrane one-sidedly surface-grafted with bendable organic chains. Remarkably, through the membrane, gas spontaneously and repeatedly flows from the low-pressure side to the high-pressure side. While this phenomenon seems counterintuitive, it is compatible with the principle of maximum entropy. The locally nonchaotic gate interrupts the probability distribution of the local microstates, and imposes additional constraints on the global microstates, so that entropy reaches a nonequilibrium maximum. Such a mechanism is fundamentally different from Maxwell's demon and Feynman's ratchet, and is consistent with microscopic reversibility. It implies that useful work may be produced in a cycle from a single thermal reservoir. A generalized form of the second law of thermodynamics is proposed.
The opportunity to manipulate cell functions by regulating bioactive surfaces is a potentially promising approach for organic bioelectronics. Here, the tuning of the orientation of charged peptides by means of an electrical input observed via optical tensiometry is reported. A stimuli-responsive self-assembled monolayer (SAM) with specially designed charged peptides is used as a model system to switch between two separate hydrophilic states. The underwater contact angle (UCA) technique is used to measure changes in the wetting property of a dichloromethane droplet under electrical stimuli. The observed changes in the UCA of the bio-interface can be understood in terms of a change in the surface energy between the ON and OFF states. Molecular dynamics simulations in an electric field have been performed to verify the hypothesis of the orientational change of the charged peptides upon electrical stimulation. In addition, X-ray photoelectron spectroscopy (XPS) is performed to clarify the stability of the functionalized electrodes. Finally, the possibility of using such a novel switching system as a tool to characterize bioactive surfaces is discussed.
Hybrid organic‐inorganic heterogeneous catalytic interfaces, where traditional catalytic materials are modified with self‐assembled monolayers (SAMs), create promising features to control a wide range of catalytic processes through the design of dual organic‐inorganic active sites and the induced confinement effect. To provide a fundamental insight, we investigated CO2 electroreduction into valuable C2 chemicals (CO2RR‐to‐C2) over SAM‐modulated Cu. Our theoretical results show that 1/4 monolayer aminothiolates improve the stability, activity and selectivity of CO2RR‐to‐C2 by: (1) decreasing surface energy to suppress surface reconstruction; (2) facilitating CO2 activation and C−C coupling through dual organic‐inorganic (i. e., −NH, Cu) active sites; (3) promoting C−C coupling via confinement effects that enlarge the adsorption energy difference between CO* and COH*; (4) inducing local electric fields to Cu surface and changing its dipole moment and polarizability to be in favor of C−C coupling under electrode/electrolyte interfacial electric field.
Electronic structure methods based on quantum mechanics (QM) are widely employed in the computational predictions of the molecular properties and optoelectronic properties of molecular materials. The computational costs of these QM methods, ranging from density functional theory (DFT) or time-dependent DFT (TDDFT) to wave-function theory (WFT), usually increase sharply with the system size, causing the curse of dimensionality and hindering the QM calculations for large sized systems such as long polymer oligomers and complex molecular aggregates. In such cases, in recent years low scaling QM methods and machine learning (ML) techniques have been adopted to reduce the computational costs and thus assist computational and data driven molecular material design. In this review, we illustrated low scaling ground-state and excited-state QM approaches and their applications to long oligomers, self-assembled supramolecular complexes, stimuli-responsive materials, mechanically interlocked molecules, and excited state processes in molecular aggregates. Variable electrostatic parameters were also introduced in the modified force fields with the polarization model. On the basis of QM computational or experimental datasets, several ML algorithms, including explainable models, deep learning, and on-line learning methods, have been employed to predict the molecular energies, forces, electronic structure properties, and optical or electrical properties of materials. It can be conceived that low scaling algorithms with periodic boundary conditions are expected to be further applicable to functional materials, perhaps in combination with machine learning to fast predict the lattice energy, crystal structures, and spectroscopic properties of periodic functional materials.
Current major approaches to access surface hydrophobicity include directly introducing hydrophobic nonpolar groups/molecules onto the surface or elaborately fabricating surface roughness. Here, for the first time, molecular dynamics simulations show an unexpected hydrophobicity with a contact angle of 82° on a flexible self-assembled monolayer terminated only with two hydrophilic OH groups ((OH)2-SAM). This hydrophobicity, verified by a water slip phenomenon characterizing the friction on the (OH)2-SAM surface, is attributed to the formation of a hexagonal-ice-like H-bonding structure in the OH matrix of (OH)2-SAM, which sharply reduces the hydrogen bonds between the surface and the water molecules above. The unique simple interface presented here offers a significant molecular-level platform for examining the bio-interfacial interactions ranging from biomolecule binding to cell adhesion.
The effect of swift heavy silicon ion irradiation on hydroxyapatite prepared by hydrothermal technique was analyzed. The physical and biological properties were studied using G1XRD, Micro-Raman, photoluminescence, AFM, SEM, EDX, in vitro bioactivity, antimicrobial activity and drug release. When compared with pristine the crystallinity increases with an increase in fluence. Slight reduction in crystallinity was noted for irradiated samples at higher fluence (1×10¹³ and 1×10¹⁴ ions/cm²) than lower fluence (1×10¹² ions/cm²). After irradiation the υ4 O-P-O asymmetric bending mode peak at 583 cm⁻¹ disappeared. The PL intensity increases with an increase in fluence and it had become narrow and showed red shift. This suggests the material could find application as phosphors. The enhancement in band gap was attained for irradiated samples than Hpris (5.38–5.5 eV). AFM studies revealed the enhancement of roughness, particle size and pores on irradiation. SEM exposed the creation of pores along with an increase in porosity (2 µm to 30 µm). Formation of platy crystals was noted for 1×10¹³ ions/cm² sample. In vitro bioactivity results with the formation of spherical (2 µm) apatite deposition along with the presence of interconnected pores (2–10 µm) which could enhance osteointegration and osteoconduction. In addition, Ca/P ratio was found to increase after soaking in SBF. Initial burst release followed by sustained release was obtained upto 45 h. Therefore it could assist in the treatment of bone and joint infection. Hence, SHI had improved the material to be used in biomedical field as an implant due to its enhanced properties such as crystallinity, pore size, surface roughness, PL intensity, antimicrobial activity, drug release and bioactivity.
In recent decades novel solid substrates have been designed which change their wettability in response to light or an electrostatic field. Here, we investigate a droplet on substrates with oscillating uniform wettability by varying minimum and maximum contact angles and frequency. To simulate this situation, we use our previous work [Grawitter and Stark, Soft Matter, 2021, 17, 2454], where we implemented the boundary element method in combination with the Cox-Voinov law for the contact-line velocity, to determine the fluid flow inside a droplet. After a transient regime the droplet performs steady oscillations, the amplitude of which decreases with increasing frequency. For slow oscillations our numerical results agree well with the linearized spherical-cap model. They collapse on a master curve when we rescale frequency by a characteristic relaxation time. In contrast, for fast oscillations we observe significant deviations from the master curve. The decay of the susceptibility is weaker and the phase shift between oscillations in wettability and contact angle stays below the predicted π/2. The reason becomes obvious when studying the combined dynamics of droplet height and contact angle. It reveals non-reciprocal shape changes during one oscillation period even at low frequencies due to the induced fluid flow inside the droplet, which are not captured by the spherical-cap model. Similar periodic non-reciprocal shape changes occur at low frequencies when the droplet is placed on an oscillating nonuniform wettability profile with six-fold symmetry. Such profiles are inspired by the light intensity pattern of Laguerre-Gauss laser modes. Since the non-reciprocal shape changes induce fluid circulation, which is controllable from the outside, our findings envisage the design of targeted microfluidic transport of solutes inside the droplet.
Thin coatings of photoresponsive, pyrimidine-terminated molecules, attached to gold or quartz substrates in contact with water, undergo dimerization and wettability changes when irradiated with UV light at 280 and 240 nm. Self-assembled monolayers of long chain thymine-terminated thiols give the largest, reversible photoinduced contact angle changes. The latter are caused by a decrease in surface charge as the thymine monomer dimerizes upon irradiation, a process which is accompanied by an increase in the acidity constant of the dimer. Uracil self-assembled monolayers photodimerize but do not photocleave; there is an irreversible change in contact angle. Spin-cast films of thymines give smaller contact angle changes, the maximum values corresponding to films which are composed of a mixture of crystalline and amorphous states.
Experiments were carried out on a variety of surfactant-coated mica surfaces using the surface forces apparatus technique and contact angle measurements. The experiments were designed to clarify the molecular mechanisms underlying adhesion hysteresis (during loading-unloading cycles) and contact angle hysteresis (of advancing/receding liquids), and to explore any possible relationship between these two energy-dissipating phenomena. We found that hysteresis effects are not simply due to surface imperfections, such as roughness or chemical heterogeneity. Even surfaces that are initially smooth and chemically homogeneous can exhibit large adhesion and contact angle hysteresis effects. Our results indicate that, for such surfaces, hysteresis arises because of molecular rearrangements occurring at solid-solid or solid-liquid interfaces after they have come into contact. This results in a lower surface free energy during the approach of two surfaces (or during spreading) than during separation (or retraction). We have studied a number of factors that enhance hysteresis: (i) increasing the freedom of the surface molecules to reorder, (ii) increasing the load and time surfaces are allowed to remain in contact, and (iii) increasing the rate of separation (or retraction). These findings highlight the inherent nonequilibrium nature of most loading-unloading and wetting-dewetting cycles and suggest ways for reducing the energy-dissipating hysteresis associated with such processes. Our results further indicate that the adhesion or pull-off force F between two curved surfaces of radius R is related to the surface energy-gamma by the Johnson-Kendall-Roberts theory, for example, F = 3-pi-R-gamma for a sphere on a flat surface, but only when the separation occurs under equilibrium conditions. Preliminary results also indicate a correlation between adhesion hysteresis and friction/stiction.
The macroscopic motion of liquids on a flat solid surface was manipulated reversibly by photoirradiation of a photoisomerizable monolayer covering the surface. When a liquid droplet several millimeters in diameter was placed on a substrate surface modified with a calix[4]resorcinarene derivative having photochromic azobenzene units, asymmetrical photoirradiation caused a gradient in surface free energy due to the photoisomerization of surface azobenzenes, leading to the directional motion of the droplet. The direction and velocity of the motion were tunable by varying the direction and steepness of the gradient in light intensity. The light-driven motion of a fluid substance in a surface-modified glass tube suggests potential applicability to microscale chemical process systems.
A surface having a spatial gradient in its surface free energy was capable of causing drops of water placed on it to move uphill. This motion was the result of an imbalance in the forces due to surface tension acting on the liquid-solid contact line on the two opposite sides ("uphill" or "downhill") of the drop. The required gradient in surface free energy was generated on the surface of a polished silicon wafer by exposing it to the diffusing front of a vapor of decyltrichlorosilane, Cl(3)Si(CH(2))(9)CH(3). The resulting surface displayed a gradient of hydrophobicity (with the contact angle of water changing from 97 degrees to 25 degrees ) over a distance of 1 centimeter. When the wafer was tilted from the horizontal plane by 15 degrees , with the hydrophobic end lower than the hydrophilic, and a drop of water (1 to 2 microliters) was placed at the hydrophobic end, the drop moved toward the hydrophilic end with an average velocity of approximately 1 to 2 millimeters per second. In order for the drop to move, the hysteresis in contact angle on the surface had to be low (</=10 degrees ).
Elastomeric stamps and molds provide a great opportunity to eliminate some of the disadvantages of photolithograpy, which is currently the leading technology for fabricating small structures. In the case of "soft lithography" there is no need for complex laboratory facilities and high-energy radiation. Therefore, this process is simple, inexpensive, and accessible even to molecular chemists. The current state of development in this promising area of research is presented here.
Electrochemical methods were combined with redox-active surfactants to actively control the motions and positions of aqueous
and organic liquids on millimeter and smaller scales. Surfactant species generated at one electrode and consumed at another
were used to manipulate the magnitude and direction of spatial gradients in surface tension and guide droplets of organic
liquids through simple fluidic networks. Solid microparticles could be transported across unconfined surfaces. Electrochemical
control of the position of surface-active species within aqueous films of liquid supported on homogeneous surfaces was used
to direct these films into periodic arrays of droplets with deterministic shapes and sizes.
The microscopic interactions between organic monolayers and various liquids have been studied by infrared-visible sum-frequency spectroscopy. The spectra of a series of alkoxy-terminated hexadecanethiols (CH3(CH2)nO-(CH2)16SH, n = 0-3) on gold and silver were recorded under air, hexane, acetonitrile, and water in the infrared C-H stretching region. The spectra of monolayers of octadecanethiol (ODT), which contains pure linear hydrocarbon chains, were also recorded for purposes of comparison. Intermolecular forces between the liquids and the polar ether group change the positions, line widths, and intensities of the terminal methyl group resonances in monolayers of the methyl ether (CH3O-(CH2)16SH). These changes were most pronounced under water and are consistent with the formation of hydrogen bonds between water and the oxygen atom of the ether. The spectra of the longer ether analogues reveal the dependence of these hydrogen bonding interactions on the depth of the ether oxygen atom beneath the monolayer surface. As the oxygen atom was progressively ''buried'', its influence became less pronounced and it no longer had an observable effect on the spectra of the butyl ether (CH3(CH2)3O-(CH2)16SH). The microscopic interactions inferred from the sum-frequency spectra are broadly consistent with macroscopic contact angle measurements.
Values of ΔGs(g → aq) and ΔGs(liq → aq) at 298 K are documented for 14 homologous series of gaseous and liquid solutes, and corresponding enthalpies of solution listed for 7 homologous series. It is shown by a thermodynamic argument that only parameters for the process g → aq can be used to assess solute–water interactions and that the standard state of pure liquid solute includes a different solute–solute interaction term for each solute standard state. For most of the homologous series, parameters for the process g → aq are linear in the number of carbon atoms in the solute; from such linear equations, methylene and group contributions are obtained. It is shown that the methylene increments to ΔGs(g → aq) and to ΔHs(g → aq) are not constant but vary from one homologous series to another. In a few homologous series the methylene increment is not constant, the most outstanding examples being the alkan-1-ols and n-alkanes. Above dodecan-1-ol, ΔGs(g → aq) becomes gradually more negative than expected, so that octadecan-1-ol is 16 times as soluble as calculated from results on the low alkan-1-ols. A similar, but much larger, effect is observed for the n-alkanes: n-octadecane is more soluble than expected by a factor of 5 × 103(the factor for n-hexatriacontane is 2 × 1018) and it is deduced that the n-alkane C51H104 will be as soluble in water as in the non-aqueous solvents ethanol and phenol.
A theoretical treatment of the effect of surface heterogeneity on contact angles is given, by means of a model employing the capillary rise of a liquid in contact with a stripwise heterogeneous surface. Local contortions of the liquid-vapor surface are postulated to conform to an assumed periodic shape of the three-phase line. A minimum of free energy is found in a configuration in which Young's equation is obeyed locally. When the width of the strips is below some value of the order of 0.1 μ, the amplitude of the periodic contortion of the three-phase line is less than about 10 A, which is operationally indistinguishable from a straight line. Extension of this model is made to a patchwise heterogeneous surface, and a mechanism for hysteresis is developed. For patches smaller than about 0.1 μ, it is shown that heterogeneity should make a negligible contribution to hysteresis.
The spreading velocity of liquids on the surface of a liquid crystalline polymer can be tremendously affected by a slight
temperature change. Indeed, a bulk transition between a highly ordered smectic and an isotropic phase induces a sharp change
from a rigid to a soft behavior, with consequent effects on the tack properties of the liquid crystalline polymer and on the
dewetting dynamics of a liquid on its surface.
Optical second harmonic generation (SHG) is especially useful in surface
studies because it can be applied to all interfaces accessible to light.
The advantages of SHG in the study of metal surfaces, semiconductor
surfaces, liquid/solid interfaces, electrochemistry, gas/liquid
interfaces, biological systems, and surface monolayer microscopy are
examined in this paper. The use of SHG in sum-frequency generation is
addressed.
We have investigated temperature modulation of surface properties for hydrophilic/hydrophobic changes using poly(N-isopropylacrylamide) (PIPAAm). Two types of PIPAAm were used as surface modifiers: an end-functionalized PIPAAm with a carboxyl end group and a poly(IPAAm-co-acrylic acid) copolymer. By means of dynamic contact angle measurements in water, the wettability of terminally polymer grafted surfaces using end-functionalized PIPAAm with a carboxyl end group were compared with that of multipoint polymer grafted surfaces using PIPAAm copolymers containing carboxyl groups along the polymer chain. Each PIPAAm grafted surface showed completely hydrophilic properties under 20 degrees C. Although multipoint grafted surfaces demonstrated surface property changes near 24 degrees C, the extent of decrease in the hydrophilic property was small compared to that of the terminal grafted surfaces. Terminal grafted surfaces demonstrated hydrophilic/hydrophobic surface property changes at 24 degrees C with small temperature increases. The value of cos theta changes from 0.63 at 20 degrees C to 0.05 at 26 degrees C. Temperature-responsive surface property changes which terminal grafted surfaces demonstrated were more rapid and significant than that of multipoint grafted surfaces;demonstrated. These features were suggested to be due to more effective restrited conformational freedom for PIPAAm graft chains which influence polymer dehydration and hydrogen bonding with water molecules.
Surface Raman spectroscopy using an emersion approach has been utilized to study the effects of electrolyte and applied potential on the in situ structure and conformational order of self-assembled monolayers (SAMs) formed from n-alkanethiols on smooth Ag surfaces. Vibrational information in the ν(C−S), ν(C−C), δ(C−H), and ν(C−H) regions is particularly useful in elucidating the degree of order and presence of defects in the monolayer before and after exposure of the SAM to electrolyte and potential. Spectral results from these studies suggest that aqueous 0.1 M electrolytes (NaF, NaCl, NaSCN, NaOH, H2SO4) and potential have different effects on the order of the SAMs studied. The order of short chain alkanethiol SAMs is more easily affected by electrolyte and potential than that of the longer chain SAMs.
We report the development of a molecular-thermodynamic model for Gibbs monolayers formed from the redox-active surfactant (11-ferrocenylundecyl)trimethylammonium bromide (II+), or oxidized II+ (II2+), at the surfaces of aqueous solutions. This model provides an account of past experimental measurements (Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116−4124) which demonstrated electrochemical oxidation of II+ to II2+ to lead to large and reversible changes in the excess surface concentrations and surface tensions of aqueous solutions of this redox-active surfactant. The results of the model lead us to conclude that II+ assumes a looped conformation at the surfaces of aqueous solutions. This looped conformation lowers the surface tensions of aqueous solutions of II+ to 49 mN/m at a limiting surface area of 85 Å2/molecule (in 0.1 M Li2SO4). The underlying cause of the reduction in surface tension is not an electrostatic contribution to the surface pressure (as is the case with classical ionic surfactants) but rather an entropic contribution due to the constrained (looped) configuration of the surfactant at the surface of the solution (chain packing). At concentrations around the critical micelle concentration (CMC) of II+ (0.1 mM), oxidation of II+ to II2+ results in the desorption of surfactant from the surface of the solution and an increase in surface tension from 49 to 72 mN/m. The process of desorption is driven by an oxidation-induced decrease in the hydrophobic driving force for self-association of the surfactants as well as an electrostatic repulsion between adsorbed surfactants. In contrast, at concentrations of II+ that substantially exceed its CMC, oxidation of II+ to II2+ drives the disruption of micelles to monomers in the bulk solution, thus increasing the chemical potential and excess surface concentration of surfactant: the oxidation-induced increase in excess surface concentration of surfactant leads to a decrease in surface tension. These results, when combined, provide principles for the design of redox-active surfactants.
Electrochemistry, FTIR-external reflection spectroscopy (FTIR-ERS), and scanning tunneling microscopy (STM) were used to study the ability of n-alkanethiol self-assembled monolayers (SAMs) to protect Au from corrosion in aqueous Br- solutions. The thickness and terminal functional group of the SAMs were varied to determine which factors lead to the greatest corrosion passivation. SAMs prepared on Au from the following molecules were studied: HS(CH2)15CH3, HS(CH2)11CH3, HS(CH2)10COOH, and HS(CH2)11OH. The data reveal that, prior to corrosion, the SAMs are crystalline and highly ordered but that afterward they are disordered and oriented parallel to the surface plane. The results show that, for SAMs containing the same terminal functional group, corrosion resistance increases as the SAM thickness increases. For SAMs that are equal in thickness but contain different terminal functional groups, the end groups resulting in the most corrosion resistance follow the order OH > COOH > CH3. The hydrophilic SAM-modified Au surfaces corrode smoothly in a layer-by-layer fashion while the methyl-terminated, hydrophobic SAM-modified surfaces undergo localized corrosion (pitting). Since hydrophilic SAMs are generally more defective than methyl-terminated SAMs but provide the Au more protection, we conclude that initial SAM defectiveness is not a reliable predictor of its barrier properties.
The in situ infrared external reflection spectra of a monolayer film of n-octadecanethiol at gold in aqueous and various nonaqueous solvents are reported. Differences in the peak positions and widths of bands in the C-H stretching region relative to an ex situ (dry Nâ atmosphere) spectrum provide preliminary insights into the interactions between solvent and monolayer. Changes in the width and positions of the CHâ stretches, which qualitatively correlate with the polarizability of the solvent, suggest a slight increase in conformational disorder near the chain terminus.
Physical-organic methods are useful in studying the surface chemistry of organic solids. These methods complement the usual spectroscopic approaches in characterizing the solid-liquid interface. This paper focuses on two topics drawn from physical-organic surface chemistry: preparations of ordered organic surfaces by self-assembly of organic molecules on inorganic supports and uses of wetting in characterizing these and other surfaces. Monolayer films prepared by chemisorption of alkanethiols and dialkyl disulfides on gold are the best characterized and most widely studied of the self-assembled systems. Wetting is uniquely valuable in characterizing surfaces for its combination of high surface sensitivity and applicability to disordered surfaces.
Alkyl mercaptans with long hydrocarbon chains (C12, C14, C16, and C18) spontaneously form organized monolayers on gold during adsorption from solution. The oriented monolayers are stable over a wide potential range in aqueous electrolytes. They strongly block electrochemical oxidation of gold and also electron transfer with redox couples in solution. Tafel plots exhibit anomalously low but nonzero slopes for overpotentials up to 0.6 V and are often nonlinear. Electron tunneling across the full width of the oriented monolayer is contraindicated. Faradaic current appears to be composed of electron transfer at defect sites and electron tunneling at "collapsed" sites in the monolayer.
Electrochemical effects on self-assembled monolayers of alkanethiols, CnH2n divided by 1SH (n = 9, 10, 18), on Au, Ag(111), and Pt(111) are investigated using sum-frequency generation. Layers on Au and Ag(111) show no noticeable effects. Layers on Pt(111) show gauche transformations that are reversibly eliminated at negative or positive potentials.
We have used applied electrical potentials, in combination with self-assembled monolayers (SAMs) of alkanethiolates supported on gold films, to control the wettability of a surface over wide ranges. A surface can be transformed from nonwetting to wetting, or the reverse, with time constants of seconds. The method is based on a competition between reductive electrochemical desorption of a hydrophobic SAM and its re-formation from alkanethiol in solution. Self-assembled monolayers formed from either CH3(CH2)(15)-SH or CH3(CH2)(2)SH were hydrophobic (80 degrees < theta < 110 degrees, theta = contact angle) toward aqueous solutions of electrolyte at neutral potentials but became hydrophilic (theta similar to 10 degrees) at reducing potentials (E < -1.3 V vs Ag wire): Contact angles of aqueous solutions containing CH3(CH2)(2)SH returned to their initial values (theta similar to 80 degrees) after the reducing potentials were removed. Because the change in wettability was dependent on the structure of the organic molecule in the monolayer, it was possible to prepare patterned SAMs in which certain regions were transformed from hydrophobic to hydrophilic by changing potential, while other regions were inert.
The vibrational spectroscopy sum frequency generation (SFG) is used to investigate the adsorption of carbon monoxide on the single crystal (111) and polycrystalline platinum surfaces. By varying the frequency and polarization of the light beams, different surface species of CO species are probed. SFG signal intensities for different polarization indicate that adsorbed CO polarizability is significantly perturbed from the gas-phase molecule. The SFG signal of CO disappears well below the main oxidation potential of CO to CO2. The disappearance of the CO signal is interpreted as a transformation in the CO layer to a state which is invisible to SFG. The invisible state is suggested to be CO with the bond axis nearly parallel to the platinum surface.
Computational studies including geometry optimizations and molecular dynamics (MD) simulations are carried out for self-assembled monolayers of n-alkanethiols (RSH, R = C16H33, C17H35) and 4‘-alkoxybiphenyl-4-thiols (ROC12H8SH, R = C16H33, C17H35) on the (111) surface of gold with a full atomic representation force field. In this work, we combine the information derived from scanning tunneling microscopy (STM), surface reflection infrared spectra (IR), and computational studies to uncover the origins of different odd−even effects observed by IR for long chain n-alkanethiols and 4‘-alkoxybiphenyl-4-thiols and then to establish a relationship between chemical structures of the headgroups and packing structures of thiols on Au(111). Computationally, the odd−even effect is monitored by the relative magnitude of z-components (the direction normal to the Au surface) of the methyl group. Although both n-alkanethiols and 4‘-alkoxybiphenyl-4-thiols occupy the same spacing on Au(111), according to our simulation results, their favored packing structures are different. Because the headgroup of 4‘-alkoxybiphenyl-4-thiol (−SC12H8O−) is more rigid than that of n-alkanethiol (−S−), 4‘-alkoxybiphenyl-4-thiol prefers more structured packing arrangements and thus its odd−even effect is stronger than that of n-alkanethiol. In other words, the flexibility of the headgroup greatly influences the variety of possible packing structures. Finally, with this new relationship, we are able to rationalize the strong odd−even effect of a new SAM molecule, n-alkyldithioic acid, on Au(111).
We report an in situ vibrational study of the reductive desorption and the oxidative adsorption of an alkanethiol monolayer on Au(111) in an alkaline solution between 5 and 37 °C. Cyclic voltammograms show that the reduction of a hexadecanethiol monolayer gives rise to two current peaks at low temperature. These two peaks slowly merge into a single peak as the temperature increases. The oxidative chemisorption displays a similar behavior. Three oxidative current peaks are observed at low temperature and only one at higher temperature. In situ vibrational spectroscopy shows that these electrochemical changes are related to a sharp phase transition. A sudden increase of the wavenumber of the methylene asymmetric CH stretching d- band from 2918 cm-1 below 12 °C to 2927 cm-1 above 12 °C indicates an abrupt disordering of the monolayer. A sharp transition at the same temperature of 12 °C is observed for the intensity of the differential reflectance CH stretching bands. This shows that changes in orientation of the alkane chains occur simultaneously with the disordering of the monolayer. We suggest that the phase transition from an ordered phase to a disordered phase is related to a sudden increase of ion permeation into the hexadecanethiol monolayer above 12 °C.
: The amide formed from polyethylene carboxylic acid (PE-CO2H) and anthranilic acid shows an exceptionally large change in wettability by water with pH. The values of advancing contact angles theta(a) are theta(a)(pH 1) = 110 degs and theta(a)(pH 12) = 33 degs. Comparison of these values with those for corresponding amides of meta- and para-aminobenzoic acid and aniline suggest that both conformational mobility of the polar functional group at the solid- water interface and surface roughness contribute to this large value.
omega-Alkoxy-n-alkanethiols (HS(CH2)(n)OR) adsorb from solution onto the surfaces of evaporated silver and gold films and form oriented self-assembled monolayers (SAMs). For many of these SAMs (R greater than or equal to propyl), the wetting properties of the SAMs by various polar and nonpolar liquids are indistinguishable from those of SAMs derived from n-alkanethiols and suggest that the presence of the ether oxygen atom is not sensed by contacting liquids. The structures of the SAMs that form from these adsorbates on silver and gold are different from, but reminiscent of, the canted structures that form upon adsorption of n-alkanethiols (CH3(CH2)(x)SH) onto these metal surfaces. The structural differences that exist between the SAMs on the two metals do not affect the wetting properties of the SAMs. The structure of the SAMs on the two metals has been determined using X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and sum-frequency generation spectroscopy (SFS). Application of these techniques indicates that the ether oxygen atom causes a local disordering and increases the population of gauche conformations. The magnitude of this disordering depends sensitively on the position of the oxygen atom along the chain. When the oxygen atom was located similar to 2 or more methylene units away from the chain end, the terminal methyl group in the SAMs exhibited the same molecular orientations as is found in SAMs that do not contain the heteroatom This observation suggests that this type of substitution constitutes a weak perturbation of chain ordering, and one which need not affect the structure of an extended chain.
Cyclic voltammetry, differential capacity and chronocoulometry have been employed to study pyrazine adsorption quantitatively at the Au(111) | aqueous solution interface. The adsorption isotherms, Gibbs energies of adsorption and the electrosorption valency for pyrazine adsorption at the Au(111) surface have been determined. The results indicate that pyrazine adsorption at gold has the character of weak chemisorption which involves either the interaction of π orbitals of the aromatic ring with free electrons at the metal surface or a mixing of the n orbital at the nitrogen atom with d electronic states in the metal. Consequently the pyrazine molecule may assume either the π-bonded flat or the N-bonded vertical surface coordinations. The flat orientation is stabilized at the negatively charged surface, the vertical orientation is stabilized at the positively charged surface. A reorientational phase transition in the monolayer of adsorbed pyrazine was observed at the potential of zero charge.
Several factors contribute to the loss and retention of self-assembled monolayers, SAMs, of organothiols at gold electrodes under potential control in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile and methylene chloride. Near 0 V vs. Ag/AgCl (saturated KCl), SAMs of dodecanethiol have maximum stability. Under driest and most oxygen-free conditions at negative potentials, such as −1.8 V, SAMs exhibit a 90% loss in surface coverage in 1 h in methylene chloride and in less than 1 min in acetonitrile. At more positive potentials such as + 0.8 V, the rate of SAM loss is slower, and only ca. 33% of surface-confined molecules were removed after l h in methylene chloride, but 100% after 10 min in acetonitrile. Although the rates of removal are different for the two solvents, the potential window of greatest stability is similar, centered near 0 V and extending to −1.0 V and +0.5 V. This window is independent of the type of gold substrate: Au/Cr/glass, Au/(3-mercaptopropyl)trimethoxysilane/glass, and Au foil. In both solvent systems, water greatly diminishes the stability of the surface-bound species at negative and positive potentials. The addition of oxygen, however, greatly enhances the stability of the surface-bound species. Details of the mechanisms for the effect of oxygen and water are not known at this time. However, positive potentials (+ 0.7 V) cause formation of gold oxide, which increases with water concentration. At negative potentials (− 1.5 V) reduction of residual oxygen is facilitated by the presence of water and may form OH− that interacts with SAMs. Stirring under both argon-purged and oxygenated conditions at negative potentials diminishes SAM stability; this is thought to be explained by enhanced flux of either water to the surface or desorbed species away from the surface. Uncompensated resistance and shifts in liquid junction potentials of the reference electrode do not explain differences in potential-dependent SAM stability between the two solvents.
We present the derivation of a new molecular mechanical force field for simulating the structures, conformational energies, and interaction energies of proteins, nucleic acids, and many related organic molecules in condensed phases. This effective two-body force field is the successor to the Weiner et al, force field and was developed with some of the same philosophies, such as the use of a simple diagonal potential function and electrostatic potential fit atom centered charges. The need for a 10-12 function for representing hydrogen bonds is no longer necessary due to the improved performance of the new charge model and new van der Waals parameters. These new charges are determined using a 6-31G basis set and restrained electrostatic potential (RESP) fitting and have been shown to reproduce interaction energies, free energies of solvation, and conformational energies of simple small molecules to a good degree of accuracy. Furthermore, the new RESP charges exhibit less variability as a function of the molecular conformation used in the charge determination. The new van der Waals parameters have been derived from liquid simulations and include hydrogen parameters which take into account the effects of any geminal electronegative atoms. The bonded parameters developed by Weiner et al. were modified as necessary to reproduce experimental vibrational frequencies and structures. Most of the simple dihedral parameters have been retained from Weiner et. al., but a complex set of phi and psi parameters which do a good job of reproducing the energies of the low-energy conformations of glycyl and alanyl dipeptides has been developed for the peptide backbone.
Polarization modulation Fourier transform infrared reflection absorption spectroscopy (PM-FTIRRAS) has been utilized to study the in situ structure of octadecanethiol when adsorbed to gold electrode surfaces and with the application of potential to the substrate. It is found that applied potential has little effect upon the spectrum of the monolayer in the presence of D2O solutions containing 0.10 M NaClO4. The presence of an acetonitrile solution contacting the monolayer leads to small changes in the infrared spectrum relative to that seen in D2O solutions, suggestive of some small disordering of the monolayer caused by the presence of the acetonitrile. With the application of potential, however, the infrared peak positions for the methyl and methylene modes return to the positions observed in the pseudo-crystalline ex situ spectrum, suggesting that the applied potential has an organizing effect upon the monolayer in the acetonitrile solutions.
We present the development of a force field for simulation of nucleic acids and proteins. Our approach began by obtaining equilibrium bond lengths and angles from microwave, neutron diffraction, and prior molecular mechanical calculations, torsional constants from microwave, NMR, and molecular mechanical studies, nonbonded parameters from crystal packing calculations, and atomic charges from the fit of a partial charge model to electrostatic potentials calculated by ab initio quantum mechanical theory. The parameters were then refined with molecular mechanical studies on the structures and energies of model compounds. For nucleic acids, we focused on methyl ethyl ether, tetrahydrofuran, deoxyadenosine, dimethyl phosphate, 9-methylguanine-1-methylcytosine hydrogen-bonded complex, 9-methyladenine-1-methylthymine hydrogen-bonded complex, and 1,3-dimethyluracil base-stacked dimer. Bond, angle, torsional, nonbonded, and hydrogen-bond parameters were varied to optimize the agreement between calculated and experimental values for sugar pucker energies and structures, vibrational frequencies of dimethyl phosphate and tetrahydrofuran, and energies for base pairing and base stacking. For proteins, we focused on Φ,Ψ maps of glycyl and alanyl dipeptides, hydrogen-bonding interactions involving the various protein polar groups, and energy refinement calculations on insulin. Unlike the models for hydrogen bonding involving nitrogen and oxygen electron donors, an adequate description of sulfur hydrogen bonding required explicit inclusion of lone pairs.
The wetting of solids by liquids is connected to physical chemistry (wettability), to statistical physics (pinning of the contact line, wetting transitions, etc.), to long-range forces (van der Waals, double layers), and to fluid dynamics. The present review represents an attempt towards a unified picture with special emphasis on certain features of "dry spreading": (a) the final state of a spreading droplet need not be a monomolecular film; (b) the spreading drop is surrounded by a precursor film, where most of the available free energy is spent; and (c) polymer melts may slip on the solid and belong to a separate dynamical class, conceptually related to the spreading of superfluids.
Mixed monolayers of 3-mercaptopropionic acid (MPA) and alkanethiols of various chain lengths have been constructed on Au based on a novel concept, namely, control of the composition of the component thiols in mixed monolayers by controlling the surface structure of the substrate. The Au substrate surface was first modified with underpotentially deposited Pb (UPD Pb) atoms, followed by the formation of a self-assembled monolayer (SAM) of alkanethiol. The UPD Pb atoms were then oxidatively stripped from the surface to create vacant site, on which MPA was adsorbed to finally form the mixed monolayers. The surface coverages of Pb, alkanethiol and MPA, and the total numbers of thiols were determined using an electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and reductive desorption voltammetry. These results demonstrate that the surface coverage of MPA in the mixed monolayers is determined by the initial coverage of UPD Pb. Fourier transform infrared spectra also support this conclusion. The observed single peak in the cyclic voltammogram for the reductive desorption shows that MPA and alkanethiol do not form their single-component domains. Scanning tunneling microscopy revealed the single-row pinstripe structure for all the thiol adlayers formed during each step of the preparation. This shows that the surface structure of the mixed monolayers is determined by the structure of the initially formed SAM on Au partially covered with UPD Pb.
Synthetic polymers offer a wealth of opportunities to design responsive materials triggered by external stimuli. Changing
the length, chemical composition, architecture, and topology of the chains allows response mechanisms and rates to be easily
manipulated; and devices based on the entropy of the chains, surface energies, and specific segmental interactions can readily
be made. Although numerous applications exist, intriguing possibilities are emerging that have tremendous potential to further
developments in surface-responsive materials.