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We have studied the effect of shape of an amphiphilic molecule on micellization properties by carrying out stochastic molecular dynamics simulation on a bead-spring model of amphiphiles for several sizes of hydrophilic head group with a fixed hydrophobic tail length. Our studies show that the effect of geometry of an amphiphile on shape and cluster...
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... axes. We also notice that the ratio ( L 1 L 2 )/2 L 3 , is slightly larger for hh ϭ 2 tt . These values suggest that the most probable micelles in both systems are ellipsoidal but the micelles made off amphiphiles with smaller head size ( hh ϭ 1.5 tt are slightly more spherical. A snapshot of 3D simulation box is given in Fig. 7. Two typical aggregates from our simulations are isolated and shown in Fig. 8. The cluster distributions and characteristic ratios for h 1 t 4 with hh ϭ 2.0 tt and h 2 t 4 with hh ϭ 1.58 tt are shown in Fig. 9 for comparison. These two types of amphiphilic molecules are chosen since they have the same volume of the hydrophilic segments. Earlier we found in Fig. 1 that their CMC values are close. Here we notice that the cluster distributions are almost indistinguishable. Additional information obtained from shape parameters reveal very similar properties as shown in Fig. 6. The ratio ( L 1 ϩ L 2 )/2 L 3 , is slightly larger for hh ϭ 2.0 tt around the peak of the cluster distribution. It will be interesting to find the optimal size of the hydrophilic head for which both the characteristic ratios are close to unity leading to formation of near spherical micelles. In summary, we have studied the role of the head group geometry in amphiphilic self-assembly for a bead-spring model of flexible amphiphiles using Brownian dynamics simulation. The principal idea of this work is to demonstrate that the shape and size of the micelles can be controlled by varying the size of the hydrophilic head only. We show that the CMC increases for larger size of the hydrophilic head segment. This is consistent with the experimental findings. For a given length of the hydrophobic tail, micelles of desired shape can be obtained by a proper choice of the hydrophilic head. Amphiphiles with larger heads form micelles with very sharp cluster distribution. In addition, we find that a peak in the specific heat and in the characteristic autocorrelation time at the onset of micellization is a generic feature of the self-assembly. Previous simulation studies by other groups based on lattice and off-lattice models discussed dependence of amphiphilic self-assembly of concentration, temperature, and chain length. In this paper we have focused on a systematic investigation of geometric effects in amphiphilic self-assembly. We find that the geometric effects are rather nontrivial and a simulation based knowledge can be very useful for nano mask fabrication and other surfactant mediated templating methods. We are extending these investigations for other types of amphiphiles, e.g., double tailed surfactant which are the building blocks of lipid bilayers. We have seen that for the same temperature and concentration studied here double tailed surfactants after initial formation of large micelles eventually form bilayers. 36 Since vesicles and bilayers are the key ingredients of cell membranes, synthesis of these structures will have enormous applications. Our simulations are aimed to provide useful information for controlled synthesis of these structures by suitable choice of amphiphilic geometry. The research reported here was supported in part by a grant from the National Science Foundation NIRT ͑ ENG/ ECS and CISE/EIA ͒ under Grant No. 0103587. The authors thank Weili Luo and Kevin Belfield for various ...
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Citations
... In the future work, the investigated scale can be extended to the molecular scale. And the molecular dynamics simulation can be used to investigate the effect of amphiphilic boundary condition of the catalyst pellets on the catalyst layer efficiency factor [47][48][49][50]. ...
Reactive distillation is an important intensification technology to overcome the unfavorable equilibrium conversion for the direct hydration of cyclohexene. In this work, a multiphase flow and multicomponent reactive transport model in the catalyst layer was established. By virtue of the model, the influences of structural parameters of the catalyst layer, physical parameters of two-phase fluid and reaction conditions on the catalyst layer efficiency factor were investigated. It was found that physical parameters of two-phase fluid can influence the two-phase distribution, thus affecting the catalyst layer efficiency factor. The catalyst layer efficiency factor decreases slightly with the rise of solubilities of cyclohexene and cyclohexanol, although concentration of cyclohexanol and reaction conversion can be improved by the addition of co-solvents. Furthermore, it was found that the catalyst layer efficiency factor is independent on structural parameters approximately. Therefore, the design of process-specific catalytic packings has remarkable difference between miscible and immiscible reaction systems.
... CG models are nowadays routinely used in polymer and biophysics, mostly for equilibrium [127][128][129][130][131][132][133], but also for non-equilibrium studies [75,[134][135][136] of lipid bilayer membranes and provide a description of reduced complexity with respect to the molecular degrees of freedom [72,137,138]. Practically all reported simulation studies of the static and dynamic properties of biomembranes have been performed very close to equilibrium. ...
My intention in this review article is to briefly discuss several major topics of present-day computational materials science in order to show their importance for state-of-the-art materials modeling and computer simulation. The topics I discuss are multiscale modeling approaches for hierarchical systems such as biological macromolecules and related coarse-graining techniques, which provide an efficient means to investigate systems on the mesoscale, and shock wave physics which has many important and interesting multi- and interdisciplinary applications in research areas where physics, biology, chemistry, computer science, medicine and even engineering meet. In fact, recently, as a new emerging field, the use of coarse-grained approaches for the simulation of biological macromolecules such as lipids and bilayer membranes and the investigation of their interaction with shock waves has become very popular. This emerging area of research may contribute not only to an improved understanding of the microscopic details of molecular self-assembly but may also lead to enhanced medical tumor treatments which are based on the destructive effects of High Intensity Focused Ultrasound (HIFU) or shock waves when interacting with biological cells and tissue; these are treatments which have been used in medicine for many years, but which are not well understood from a fundamental physical point of view.
... The standard numerical technique for the simulation of phospholipid bilayers is molecular dynamics (MD) [18,[27][28][29][30] and most of the existing body of MD simulation studies of biomembrane properties has almost exclusively been performed at or very near at equilibrium [31][32][33][34][35][36][37]. All-atom MD simulations of lipid bilayers which resolve the dynamics of individual atoms-treated with classical force fieldsare generally limited to very small membrane samples (tens of nanometers in extension), even on the largest computer systems [38,39]. ...
We report on the results of particle-based, coarse-grained molecular dynamics simulations of amphiphilic lipid molecules in aqueous environment where the membrane structures at equilibrium are subsequently exposed to strong shock waves, and their damage is analyzed. The lipid molecules self-assemble from unbiased random initial configurations to form stable bilayer membranes, including closed vesicles. During self-assembly of lipid molecules, we observe several stages of clustering, starting with many small clusters of lipids, gradually merging together to finally form one single bilayer membrane. We find that the clustering of lipids sensitively depends on the hydrophobic interaction \(h_\mathrm{c}\) of the lipid tails in our model and on temperature T of the system. The self-assembled bilayer membranes are quantitatively analyzed at equilibrium with respect to their degree of order and their local structure. We also show that—by analyzing the membrane fluctuations and using a linearized theory— we obtain area compression moduli \(K_\mathrm{A}\) and bending stiffnesses \(\kappa _\mathrm{B}\) for our bilayer membranes which are within the experimental range of in vivo and in vitro measurements of biological membranes. We also discuss the density profile and the pair correlation function of our model membranes at equilibrium which has not been done in previous studies of particle-based membrane models. Furthermore, we present a detailed phase diagram of our lipid model that exhibits a sol–gel transition between quasi-solid and fluid domains, and domains where no self-assembly of lipids occurs. In addition, we present in the phase diagram the conditions for temperature T and hydrophobicity \(h_\mathrm{c}\) of the lipid tails of our model to form closed vesicles. The stable bilayer membranes obtained at equilibrium are then subjected to strong shock waves in a shock tube setup, and we investigate the damage in the membranes due to their interaction with shock waves. Here, we find a transition from self-repairing membranes (reducing their damage after impact) and permanent (irreversible) damage, depending on the shock front speed. The here presented idea of using coarse-grained (CG) particle models for soft matter systems in combination with the investigation of shock-wave effects in these systems is a quite new approach.
... Since then, CG models have found their way into polymer physics as so-called bead-spring models, taking advantage of universal scaling laws of long polymer chains due to their fractal nature [33][34][35][36], [37], as well as into geophysics, engineering and other areas of computational research [39], [40]. As reviewed by Saunders and Voth [41], CG models are nowadays routinely used in polymer-and biophysics for equilibrium [41][42][43][44][45][46] and non-equilibrium studies [47][48][49] of lipid bilayer membranes and provide a description of reduced complexity with respect to the molecular degrees of freedom [18], [50], [51]. Several past attempts have been made to obtain a stable, solvent-free, fluid phase bilayer in MD simulations. ...
... 1,4,[10][11][12][13][14][15][16] Idealized chain fluids, such as long or short chains with hard-body, square-well or Lennard-Jones interactions, have been of interest over the years to model elongated and flexible molecules like polymers or amphiphiles. 4,[17][18][19][20][21][22][23][24][25][26][27][28][29] These simplified molecular models offer the possibility of understanding the effects of molecular size, shape, and potential on thermodynamic properties, and comparison between theory and simulation results is straightforward. In a different perspective, simplified potential systems have been used to model colloidal self-assembly, following an approach that scales molecular models into real colloidal systems. ...
... In particular, one of the cases analyzed here is similar to amphiphilic models studied by other authors. 12,[22][23][24][25][26][27][28][29]42 In such a case, micellar formation can be confused with macroscopic phase separation, [42][43][44][45] and a systematic study of specific properties is necessary to correctly identify phase diagram regions. The aim of this work is to quantify the effect of the variableranged potential on the phase diagram and other properties like the surface tension, as well as the gradual induction of self-assembling behavior as the molecular asymmetry in the interactions is increased. ...
... Supramolecular organization was expected, since the K = 0 trimer and other similar chains with asymmetric interactions have been used in previous works by other authors to model amphiphilic molecules. 5,[21][22][23][24][25][26][27][28][29]42 These effective interactions for implicit solvent models are based on the generally accepted assumption that hydrophobic interaction dominates over other forces between amphiphile beads. 22,28 The K = 0 trimer corresponds to one of the models studied in Ref. 28, in which different types of micellar formations were observed varying λ and . ...
In this work, we present Monte Carlo computer simulation results of a primitive model of self-assembling system based on a flexible 3-mer chain interacting via square-well interactions. The effect of switching off the attractive interaction in an extreme sphere is analyzed, since the anisotropy in the molecular potential promotes self-organization. Before addressing studies on self-organization it is necessary to know the vapor liquid equilibrium of the system to avoid to confuse self-organization with phase separation. The range of the attractive potential of the model, λ, is kept constant and equal to 1.5σ, where σ is the diameter of a monomer sphere, while the attractive interaction in one of the monomers was gradually turned off until a pure hard body interaction was obtained. We present the vapor-liquid coexistence curves for the different models studied, their critical properties, and the comparison with the SAFT-VR theory prediction [A. Gil-Villegas, A. Galindo, P. J. Whitehead, S. J. Mills, G. Jackson, and A. N. Burgess, J. Chem. Phys. 106, 4168 (1997)]. Evidence of self-assembly for this system is discussed.
... With an increase in polymer concentration above CMC, the residual solvent is excluded from the core; the micellar structure becomes more compact with reduction in the micelle size, and develops into stable structures. The CMC of the polymeric solution and the kind of aggregates the copolymers can form is dependent on the competition between the enthalpic interaction and the entropic effect in the solution [137,138]. Below CMC, the entropic effects dominate over the enthalpic interaction, whereas the enthalpic contributions are dominant over the entropic effects above CMC, which makes the rapid aggregation of copolymers to form aggregates with particular properties. ...
The generation of supramolecular architectures with well-defined structures and functionalities is recently garnering attraction. Self-assemblage of amphiphilic polymers leads to the formation of polymeric micelles that demonstrate unique set of characteristics such as excellent biocompatibility, low toxicity, enhanced blood circulation time, and solubilization of poorly water-soluble drugs. In this article, we provide an up-to-date review on important aspects of polymeric micelles. Critical factors for solubilization of hydrophobic drugs in the micellar core are discussed. Polymeric micelles can be used as ‘smart’ drug carriers for targeting certain areas of the body. Here, we have especially emphasized on the recent developments in the targetability of certain tissues such as cancerous tissues using polymeric micelles. Different stimuli exploited for creating stimuli-sensitive micelles are discussed comprehensively. Application of polymeric micelles in the photodynamic therapy is also meticulously described.
Introducing a second component is an effective way to manipulate polymerization behaviors. However, this phenomenon has rarely been observed in colloidal systems, such as polymeric nanoparticles. Here, we report the supramolecular polymerization of polymeric nanorods mediated by block copolymers. Experimental observations and simulation results illustrate that block copolymers surround the polymeric nanorods and mainly concentrate around the two ends, leaving the hydrophobic side regions exposed. These polymeric nanorods connect in a side‐by‐side manner through hydrophobic interactions to form bundles. As polymerization progresses, the block copolymers gradually deposit onto the bundles and finally assemble into helical nanopatterns on the outermost surface, which terminates the polymerization. It is anticipated that this work could offer inspiration for a general strategy of controllable supramolecular polymerization.
Introducing a second component is an effective way to manipulate polymerization behavior. However, this phenomenon has rarely been observed in colloidal systems, such as polymeric nanoparticles. Here, we report the supramolecular polymerization of polymeric nanorods mediated by block copolymers. Experimental observations and simulation results illustrate that block copolymers surround the polymeric nanorods and mainly concentrate around the two ends, leaving the hydrophobic side regions exposed. These polymeric nanorods connect in a side‐by‐side manner through hydrophobic interactions to form bundles. As polymerization progresses, the block copolymers gradually deposit onto the bundles and finally assemble into helical nanopatterns on the outermost surface, which terminates the polymerization. It is anticipated that this work could offer inspiration for a general strategy of controllable supramolecular polymerization.
Nanotechnology has played a major contribution in developing different drug-loaded polymeric nanocarriers for therapeutic and diagnostic applications. Among the polymeric nanocarriers, the polymeric micelles have received much attention for the diagnosis and treatment of several complicated diseases. The polymers, which are amphiphilic in nature, are used to develop polymeric nanocarriers to overcome the solubility problems of lipophilic drugs. The size range of these polymeric micelles is 10 to 200 nm. This chapter provides a comprehensive overview of the structure and distinguishing features of polymeric micelles. It also gives insight about various polymers used in preparing polymeric micelles, different methods of preparation, and various types of polymeric nanocarriers. The applications and different factors effecting micelles have also been discussed here. Moreover, their limitations and prospects are been discussed.
Toroids and helices are fundamental geometrical structures in nature. Polymers can self‐assemble into various nanostructures, including both toroids and helices; however, nanostructures combining toroidal and helical morphologies, i.e., helical toroids, are rarely observed. Herein, we report that a binary system containing polypeptide homopolymer and its block copolymer can hierarchically self‐assemble into uniform helical nanotoroids in solution. The formation of the helical toroids is a successive two‐step process. First, the homopolymers aggregate into fibrils and convolve into toroids, resembling the toroidal condensation of DNA chains. Second, the block copolymers self‐assemble on the homopolymer toroids resulting in helical surface patterns. Additionally, the chirality of the surface helical patterns can be varied by the chirality of the polypeptide block copolymers.
