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A diagrammatic representation of the alkylbenzene sulphonate ion, showing the 3 and 6 positional isomers.
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A commercial linear alkylbenzene sulphonate (LAS)/H2O surfactant system was studied to develop understanding of the phase diagram and the important mechanisms determining the behaviour. This is an important area of understanding for formulations. Optical microscopy, differential scanning calorimetry, X-ray scattering and nuclear magnetic resonance...
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Context 1
... detergent formulations are based around surfactant systems that are largely responsible for the micro-structure (col- loidal and phase structure) of the detergent slurry. This structure controls the slurry and subsequent product properties. These material properties interact with the manufacturing process and there- fore determine the process conditions. The slurries normally consist of a lamellar liquid crystal phase [1], along with some inorganic material, dispersed in a concentrated electrolyte solution. There are complex interactions between the ingredients that limit their inclu- sion in the formulation. The slurry must be processable. The product must have good storage properties, good dissolution characteristics and be an effective cleaning agent. Very often these characteristics are seen to be antagonistic, and being able to manipulate these behaviours is a very useful tool. Thus great importance is placed on developing a good understanding of the phase diagram as is seen by the many data available, both published and unpublished [1]. The alkylbenzene sulphonate (AS) group of surfactants are amongst the most commonly used class of anionic detergents [2,3] yet in spite of their ubiquity there is relatively little published information available. Typically micelles and lamellar liquid crystals are observed within the system, however, changing the composition can have dramatic effects on the phase behaviour and on the architecture of the micro-structures that are formed [4,5]. Some results have even indicated the presence of multiple lamellar phases [6,7]. Furthermore, in the presence of electrolytes micelles are known to transform into uni-lamellar and multi-lamellar structures [8,9] so drastically altering the system’s equilibrium position. This paper presents a study into the phase behaviour of a commercial linear alkylbenzene sulphonate (LAS)/water system. Light polarising optical microscopy, differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and small angle X-ray scattering (SAXS) have been employed to study the fundamen- tal characteristics of the system, and the important mechanisms that determine this phase behaviour. Particular emphasis has been placed on the influence of temperature in order to aid understanding of these mechanisms in both the processing and dissolution stages of the detergent life-cycle. Commercial alkylbenzene sulphonic acid (HLAS) was provided by Procter and Gamble. The HLAS is composed of a number of different alkyl chain lengths and positional isomers (the benzene ring is substituted on any carbon other than those at either end of the alkyl chain, see Fig. 1). A typical composition is shown in Table 1. First the HLAS was neutralised by titration against known NaOH solutions. This allowed a pH curve to be plotted and provided an implied molecular weight of 318 g/mol, which gives an average chain length of 11.3 carbon atoms. The true average molecular weight is slightly higher (11.8 carbons) due to the presence of H 2 SO 4 (ca. 0.8 wt%) which is also neutralised, and means that the presence of some Na 2 SO 4 is intrinsic to the system. The addition of salts is known to influence the behaviour of surfactant systems [1,4,9]. There will also be <1.5% by mass of organic matter, largely composed of unsulphonated LAB brought in from the HLAS. The samples were prepared by neutralisation of the HLAS, the end point of which was determined via a titration. The HLAS was combined with an appropriate weight and concentration of NaOH (aq) (the NaOH was purchased from Sigma Aldrich and dissolved in deionised water) to achieve the desired composition. This is an exothermic reaction, and the sample is naturally heated to between 40 and 70 ◦ C. Samples were homogenised by vigorous shaking. After preparation samples were stored in a sealed airtight container at ambient conditions. Typically samples were studied between 1 day and 1 week after preparation. The main experimental techniques used were optical microscopy, DSC, NMR and SAXS. In optical microscopy the phase penetration scans [10] were carried out by contacting a surfactant rich sample with water in a square capillary tube, allowing a surfactant/water interface to develop. This tube was mounted on a Linkam hot-stage and subjected to heat–cool temperature cycles driven by a Linkam temperature controller. In other microscopy experiment samples were sealed (using epoxy resin) between two cover slips and again mounted on the linkam hot-stage. The hot-stage is able to control temperature to within ± 0.5 ◦ C. The DSC experiments were carried out on a Setaram Micro-DSC III. Sealed samples of ca. 500 mg were subjected to heat–cool temperature cycles at 1 ◦ C/min between 5 and 85 ◦ C. The control cell was loaded with deionised water. This technique allows the obser- vation of physio-chemical transitions as characterised by changes in the thermal properties of the sample. Nuclear magnetic resonance was carried out on a Varian 300 MHz spectrometer with a 2 H probe. The spectrometer allowed the temperature ( ± 2 ◦ C) to be controlled from room temperature to 80 ◦ C. The quadrupolar splitting ( ) value is taken as the separation between the pairs of maxima exhibited in a typical powder pattern. However spectra are sometimes indistinct and there may be a significant error within this value, this can be as large as ± 6%. The SAXS experiments were carried out at the CCLRC Syn- chrotron radiation source at Daresbury, UK. The experiments were carried out on beamlines 2.1 [11] and 16.1 and calibrated using wet rat’s tail collagen, which has well known d -spacings [12]. Some wide-angle experiments were also performed on station 14.1, and these were calibrated using beeswax [13]. Samples were flame sealed in Lindemann capillary tubes and mounted on a specially adapted Linkam hot-stage to allow temperature control. These experiments enable the determination of the bilayer repeat distance ( d -spacing) from the scattering vector ( q ) using Braggs law ( = 2 d sin (2 Â )) [14]. The analysis of sample compositions was carried out by Procter and Gamble at the Newcastle Technical Centre. The samples were first separated by centrifugation at 25 ◦ C, the phases indi- vidually removed and dissolved in methanol (0.1 g in 10 mL). A 100 L aliquot of this solution was then dissolved in 900 L of deionised water. A widely used liquid chromatography technique was used to determine the positional isomer and chain length distributions [15,16] of the resultant solution with detection at 210 nm. The results are found to be accurate to within ± 2%. The chain length distribution results were later confirmed using mass spectrome- try. When LAS/water samples were examined, samples of over 27 wt% LAS formed a cloudy white precipitate at room temperature. This precipitate co-exists with an isotropic solution up to ca. 65 wt%. The cloudy white precipitate is considered to be indicative of a lamellar phase being present. In fact this has been shown to be the region of two-phase co-existence between a lamellar and micellar phase [4,6,15,17]. The initial observations were made using a phase penetration scan (Fig. 2), whereby a high active LAS paste (65 wt% in this case) was contacted with deionised water in a glass capillary. As expected (in accordance with the literature [5]) an isotropic water/micellar region was observed, this was followed by a band in which some spherulites (maltese crosses when viewed through crossed polars) were observed, and there was a two-phase region where the lamellar phase formed myelin structures reaching into the aqueous ...
Context 2
... alkylbenzene sulphonic acid (HLAS) was provided by Procter and Gamble. The HLAS is composed of a number of different alkyl chain lengths and positional isomers (the benzene ring is substituted on any carbon other than those at either end of the alkyl chain, see Fig. 1). A typical composition is shown in Table 1. First the HLAS was neutralised by titration against known NaOH solutions. This allowed a pH curve to be plotted and pro- vided an implied molecular weight of 318 g/mol, which gives an average chain length of 11.3 carbon atoms. The true average molec- ular weight is slightly higher (11.8 ...
Citations
... Consequently, direct examination of the formation of isolated myelins under defined thermodynamic conditions has remained challenging. 8,19,20 Here, we investigate the growth of isolated myelin figures from multilamellar vesicles (MLVs) of an anionic surfactant, Sodium Linear Alkylbenzene Sulfonate (NaLAS), formed spontaneously through a thermally-induced phase transformation, 21,22 see SI for more detail. In a contact-dilution experiment (Fig. 1a), a concentrated NaLAS lamellar phase A is diluted in water, and yields a region of populated tubular lamellar structures B at its interface, followed by an area covered with an isotropic micellar phase I C . ...
... [13][14][15] The shape, size and structure of the micelles at nanoscale determine the physical properties and functionality of the surfac-d-spacing, and this effect was more pronounced at lower NaLAS concentrations. 21 Below 30 wt%, the micellar phase of NaLAS has been reported to spontaneously transform into multilameller vesicles (MLVs) upon cooling from room temperature (Fig. 1b). 23,24 Fig. 1 (a) Distribution of phenyl positional isomers with different alkyl chain lengths in an industrial mixture of NaLAS. ...
We investigate the shape, dimensions, and transformation pathways of micelles of linear sodium alkylbenzenesulfonate (NaLAS), a common anionic surfactant, in aqueous solution. Employing Small Angle Neutron Scattering (SANS) and surface tensiometry, we quantify the effects of surfactant concentration (0.6-15 w/w%), temperature (5-40 C) and added salt (Na2SO4, ≤ 0.35 M). Spherical micelles form at low NaLAS (≤ 2.6 wt%) concentration in water, and become elongated with increasing concentration and decreasing temperature. Addition of salt reduces the critical micelle concentration (CMC) and thus promotes the formation of micelles. At fixed NaLAS concentration, salt addition causes spherical micelles to grow into cylindrical micelles, and then multilamellar vesicles (MLVs), which we examine by SANS and cryo-TEM. Above a threshold salt concentration, the MLVs reach diameters of 100s of nm to few µm, eventually causing precipitation. While the salt concentrations associated with the micelle-to-cylinder transformation increase only slightly with temperature, those required for the cylinder-to-MLV transformation exhibit a pronounced, linear temperature dependence, which we map in detail. Our study establishes a solution structure map for this model anionic surfactant in water, quantifying the combined roles of concentration, temperature and salt, at practically relevant conditions.
... For example, for the position of the benzenesulfonate group, the C12 product is a mixture of five secondary phenyldodecanes (Swisher, 1963). Knowing the exact composition of alkylbenzene sulfonates is important to understand the behavior and stability of commercial formulations for their appropriate treatment before final disposal (Stewart et al., 2009). However, the composition of the surfactant or the mixtures used in formulations is not specified in manufactured soaps and detergents. ...
... This information allows us to predict if reactions could continue with subsequent enzymatic steps towards total biodegradation. Theoretical results from this work agree with those obtained by different authors (Stewart et al., 2009;Pakou et al., 2007;Alexander, 1973;Swisher, 1963), so we did not consider it necessary to apply other modeling techniques involving more complex calculations. For instance, other techniques would allow modeling chemical bond breaking and formation if we wanted to evaluate the enzymatic mechanism until complete mineralization. ...
Gray water constitutes an important fraction of total wastewater. Some of the most problematic compounds in gray water are the anionic surfactants used as an ingredient for domestic and industrial soaps and detergents. The alkylbenzene sulfonates used in commercially available formula are highly complex mixtures of linear (LAS) and branched (BAS) molecules. LAS are classified generally as biodegradable, although their widespread use generates accumulation in the environment. Docking tools, widely used in recent years in the bioremediation field, allow molecular modeling of the ligand-enzyme interaction, which is key to understanding and evaluating the possibility of biodegradation. In this work, molecular details that allow us to establish a biodegradation pattern for some alkylbenzene sulfonates were elucidated. Two hydrogen bonds, key for the anchorage of surfactants to the monooxygenase active site involved in the initial biodegradation, were found. These bonds determine the way surfactants locate in the hydrophobic pocket of the enzyme affecting the biodegradation rate in a structurally dependent manner. For C10 to C12 linear isomers, the degradation rate increased together with the length of the hydrocarbon chain. For C13 and C14 isomers, steric difficulties to accommodate the surfactant molecule in the catalytic site were observed. For branched chain isomers, little or no biodegradation was found. In addition, biodegradation was lower in mixtures than for the pure isomers. These results will allow an intelligent design of this family of anionic surfactants to attenuate their contaminating effects in waters and soils. This study constitutes, to the best of our knowledge, a novel contribution towards the design of environmentally friendly surfactants with higher probabilities of being biodegraded to complete mineralization.
... For the model system employed here, NaLAS/water micellar solutions at <30% mass fraction surfactant concentrations, the phase boundaries are within 0-40 ○ C according to previous studies. 32,34 Therefore, we set the dual Peltier stages at 40 ○ C on the inlet side (corresponding to homogeneous L 1 micellar) and 0 ○ C on the outlet side [mixed L 1 + Lα(MLV) phase], thus creating a thermal gradient along the microchannel spanning these values. The temperature of the microfluidic system is allowed to reach steady state before phase mapping. ...
Measurement of the phase behavior and (meta)stability of liquid formulations, including surfactant solutions, is required for the understanding of mixture thermodynamics, as well as their practical utilization. We report a microfluidic platform with a stepped temperature profile, imposed by a dual Peltier module, connected to an automated multiwell plate injector and optical setup, for rapid solution phase mapping. The measurement protocol is defined by the temperature step ΔT ≡ T1 − T2 (≲100 °C), volumetric flow rate Q ≡ ΔV/Δt (≲50 μl/min), which implicitly set the thermal gradient ΔT/Δt (≃0.1–50 °C/min), and measurement time (which must exceed the intrinsic timescale of the relevant phase transformation). Furthermore, U-shaped microchannels can assess the reversibility of such transformations, yielding a facile measurement of the metastable zone width of the phase diagram. By contrast with traditional approaches, the platform precisely controls the cooling and heating rates by tuning the flow rate, and the absolute temperature excursion by the hot and cold thermal profile, which remain stationary during operation, thus allowing the sequential and reproducible screening of large sample arrays. As a model system, we examined the transition from the micellar (L1) to the liquid crystalline lamellar phase (Lα), upon cooling, of aqueous solutions of sodium linear alkylbenzene sulfonate, a biodegradable anionic surfactant extensively employed in industry. Our findings are validated with quiescent optical microscopy and small angle neutron scattering data.
... Aqueous NaLAS systems show this typical phase behaviour, though typically no hexagonal phase is observe in the phase diagram e.g. Liaw et al. [50]; Richards et al. [46] and Stewart et al. [47]. The addition of electrolytes to low concentration aqueous surfactant solutions is known to 'salt out' the surfactant. ...
... The occurrence of coexisting lamellar phases of NaLAS in liquid detergent systems has been previously reported by Richards et al. [46] and Stewart et al. [47], which, according to the authors, was attributed to the natural heterogeneity of the chemical structure of NaLAS. It was suggested that NaLAS isomers possessing a benzene ring substituted in the middle of hydrocarbon chain, i.e. v-shaped isomers, trigger the formation of lamellar phases, while NaLAS isomers with a benzene ring at the extremities of the hydrocarbon chain, but not terminal -CH 3 , promote the formation of micellar phases. ...
The complex multi-scale structure of spray-dried detergent granules has been characterized using a range of techniques including microscopy, wide-angle and small-angle X-ray scattering and X-ray microtomography. Four simple model formulations based on linear alkyl benzene sulphonate (NaLAS) and sodium sulphate were used to probe the influence of initial slurry water content and sodium silicate on the structure. The structure can be viewed as a porous matrix consisting of liquid crystalline NaLAS, sodium sulphate and binder in which large crystals of sodium sulphate are embedded. These large crystals were initially undissolved in the slurry and are consequently reduced in number in the product made from higher water content slurry. The both slurry water content and sodium silicate changed the polymorphs, and the d-spacing of the lamellae. The surface micro-structure and particle morphology can also be significantly affected with the high initial water content; particles having a distinct agglomerated and blistered structure.
... Previous investigations of the phase behaviour of LAS were mainly dedicated to higher concentrations (>30 wt%), for which a densely packed planar lamellar phase L a coexists with a micellar phase L 1 at room temperature [13][14][15]. For intermediate LAS concentrations (40-60 wt%), a nearly composition-independent line in the phase diagram, around 50 C, defines the boundary between an opaque planar lamellar phase (L a in Fig. 1) with a uniform dspacing (32-34 Å), and a semi-transparent lamellar phase (L 0 a in Fig. 1). ...
... For intermediate LAS concentrations (40-60 wt%), a nearly composition-independent line in the phase diagram, around 50 C, defines the boundary between an opaque planar lamellar phase (L a in Fig. 1) with a uniform dspacing (32-34 Å), and a semi-transparent lamellar phase (L 0 a in Fig. 1). The d-spacing of the L 0 a phase was found to significantly increase upon heating the solution, especially at lower LAS concentrations (up to around 44 Å at 35 wt% of LAS) [14]. ...
... The phase behaviour of aqueous NaLAS solutions has been previously studied at relatively higher concentrations (>30 wt%) and temperatures (20-90 C) [13,14]. A broad two-phase region with coexisting micellar L 1 and lamellar L a phases was reported for the range of 30-60 wt% NaLAS concentration, see Fig. 1. ...
We report the spontaneous formation of multilamellar vesicles (MLVs) from low concentration (<30 wt%) aqueous micellar solutions of sodium linear alkylbenezene sulfonate (NaLAS) upon cooling, employing a combination of optical microscopy (OM), Small Angle Neutron Scattering (SANS), and Cryo-TEM. Upon cooling, MLVs grow from, and coexist with, the surfactant micelles, attaining diameters ranging from hundreds of nanometers to a few micrometers depending on the cooling rate, whilst the d-spacing of internal lamellae remains unchanged, at 3 nm. While microscale fluid and flow properties of the mixed MLVs and micellar phase depend on rate of cooling, the corresponding nanoscale structure of the surfactant aggregates, resolved by time-resolved SANS, remains unchanged. Our data indicate that the mixed MLV and micellar phases are in thermodynamic equilibrium with a fixed relative volume fraction determined by temperature and total surfactant concentration. Under flow, MLVs aggregate and consequently migrate away from the channel walls, thus reduce the overall hydrodynamic resistance. Our findings demonstrate that the molecular and mesoscopic structure of ubiquitous, low concentration NaLAS solutions, and in turn their flow properties, are dramatically influenced by temperature variation about ambient conditions.
... The dissolution process is complicated for surfactant-water systems due to the formation of highly viscous liquid crystalline phases, such as hexagonal and lamellar phases [8] that could potentially inhibit the dissolution process [1]. Of late, several studies on the phase behavior of NaLAS have been reported [9][10][11][12][13]. In order to compare dissolution performance, dissolution profiles should be obtained however few studies have investigated the dissolution profiles and release kinetics of NaLAS. ...
In this work, the dissolution behaviors of a series of sodium alkylbenzenesulfonates (NaLAS) tablets with different moisture contents and neutralization degrees were investigated in aqueous solution. The ANOVA-based, model-independent and model-dependent methods were employed to perform comparison analyses on dissolution profiles. The measurements of powder X-ray diffraction patterns and mechanical properties elucidate distinct differences in each formula. The results show that ANOVA provides a possibility for finding the source of differences among different variables, and the model-independent methods including the k values and mean dissolution time are easy to interpret and perform comparison analyses. The Hixson–Crowell model gives satisfactory correlation results for the dissolution data and the dissolution kinetics parameters are obtained. The inhibition effects of neutralization degree and moisture content on NaLAS dissolution were examined, which reveals that the increase in lamellar phase proportion leads to the reduction of dissolution rate. The comparison analyses performed in this work form part of a methodology for dissolution profile prediction and comparison.
... 47 The fluid is a commercial sample of industrial grade and is used without any further purification. The phase diagram of the HLAS-water system 48 can be divided in three different regions depending on HLAS concentration and temperature. At the temperature of 25° C, the phase diagram shows an isotropic micellar (L1) phase at HLAS concentrations lower than 28% wt. ...
Surfactant solutions exhibit multilamellar surfactant vesicles (MLVs) under flow conditions and in concentration ranges which are found in a large number of industrial applications. MLVs are typically formed from a lamellar phase and play an important role in determining the rheological properties of surfactant solutions. Despite the wide literature on the collective dynamics of flowing MLVs, investigations on the flow behavior of single MLVs are scarce. In this work, we investigate a concentrated aqueous solution of linear alkylbenzene sulfonic acid (HLAS), characterized by MLVs dispersed in an isotropic micellar phase. Rheological tests show that the HLAS solution is a shear-thinning fluid with a power law index dependent on the shear rate. Pressure-driven shear flow of the HLAS solution in glass capillaries is investigated by high-speed video microscopy and image analysis. The so obtained velocity profiles provide evidence of a power-law fluid behaviour of the HLAS solution and images show a flow-focusing effect of the lamellar phase in the central core of the capillary. The flow behavior of individual MLVs shows analogies with that of unilamellar vesicles and emulsion droplets. Deformed MLVs exhibit typical shapes of unilamellar vesicles, such as parachute and bullet-like. Furthermore, MLV velocity follows the classical Hetsroni theory for droplets provided that the power law shear dependent viscosity of the HLAS solution is taken into account. The results of this work are relevant for the processing of surfactant-based systems in which the final properties depend on flow-induced morphology, such as cosmetic formulations and food products.
... The remaining 20% are mainly isomers of the 'ideal' SDBS molecule, where the phenyl ring is attached at various intermediate carbons in the dodecyl chain. 31,32 The mixture of isomers present in the SDBS used in this study (and all other studies of SDBS-dispersed CNTs that we are aware of) should be kept in mind when interpreting the observations, an issue we will come back to below. ...
In this chapter we discuss the benefits, peculiarities and main challenges related to nanoparticle templating in lyotropic liquid crystals. We first give a brief bird’s-eye view of the field, discussing different nanoparticles as well as different lyotropic hosts that have been explored, but then quickly focus on the dispersion of carbon nanotubes in surfactant-based lyotropic nematic phases. We discuss in some detail how the transfer of orientational order from liquid crystal host to nanoparticle guest can be verified and which degree of ordering can be expected, as well as the importance of choosing the right surfactant and its concentration for the stability of the nanoparticle suspension. We introduce a method for dispersing nanoparticles with an absolute minimum of stabilizing surfactant, based on dispersion below the Krafft temperature, and we discuss the peculiar phenomenon of filament formation in lyotropic nematic phases with a sufficient concentration of well-dispersed carbon nanotubes. Finally, we describe how the total surfactant concentration in micellar nematics can be greatly reduced by combining cat- and anionic surfactants, and we discuss how nanotubes can help in inducing the liquid crystal phase close to the isotropic–nematic boundary.
... If we exclude other ingredients and simply estimate the concentration of surfactant in water, it comes out to be about 33 wt%. The phase diagram of LAS in water suggests that at this concentration, LAS is observed to be in co-existing isotropic and lamellar biphasic region [31], while the phase diagram of SLES suggests that, for the mentioned concentration, it is in co-existing isotropic and hexagonal biphasic region (unpublished results). On the other hand AOS is observed to be in isotropic phase at 33 wt% concentration (unpublished results). ...
In this work, we study the rheological properties and the yielding behavior of cleaning pastes containing surfactant and abrasive particles, for three different types of surfactants, namely linear alkyl benzene sulfonate (LAS), alpha olefin sulfonate (AOS) and sodium lauryl ether sulfonate (SLES). All the pastes were observed to have soft solid-like consistency with their elastic modulus significantly greater than the viscous modulus. With around 36 volume percent of particulate matter, the high stiffness of the pastes suggests that particles form a space spanning network. Interestingly, when subjected to oscillatory shear deformation with increasing strain amplitude, the elastic modulus undergoes a decrease in two steps thereby showing a two-step yielding behavior. It is observed that the first yield stress does not show frequency dependence, and for LAS-containing paste was the largest followed by AOS- and SLES-containing pastes, respectively. The second yield stress, on the other hand, for all the three pastes is observed to increase with frequency. Careful assessment of the experimental data suggests that the first yielding event is due to rupture of the network which leads to formation of particulate aggregates. The second yielding event is attributed to breakage of aggregates. In both yielding phenomena, surfactants play an important role. Since, the phase behavior of surfactant in water determines the inter-particle interaction and network density, the nature of surfactant has a pivotal influence on both the yielding phenomena in surfactant suspension pastes used for cleaning purposes.
