Self-assembled Gemini surfactant film-mediated dispersion stability

The force-distance curves of 12-2-12 and 12-4-12 Gemini quaternary ammonium bromide surfactants on mica and silica surfaces obtained by atomic force microscopy (AFM) were correlated with the structure of the adsorption layer. The critical micelle concentration was measured in the presence or absence of electrolyte. The electrolyte effect (the decrease of CMC) is significantly more pronounced for Gemini than for single-chain surfactants. The maximum compressive force, F(max), of the adsorbed surfactant aggregates was determined. On the mica surface in the presence of 0.1 M NaCl, the Gemini micelles and strong repulsive barrier appear at surfactant concentrations 0.02-0.05 mM, which is significantly lower than that for the single C(12)TAB (5-10 mM). This difference between single and Gemini surfactants can be explained by a stronger adsorption energy of Gemini surfactants. The low concentration of Gemini at which this surfactant forms the strong micellar layer on the solid/solution interface proves that Gemini aggregates (micelles) potentially act as dispersing agent in processes such as chemical mechanical polishing or collector in flotation. The AFM force-distance results obtained for the Gemini surfactants were used along with turbidity measurements to determine how adsorption of Gemini surfactants affects dispersion stability. It has been shown that Gemini (or two-chain) surfactants are more effective dispersing agents, and that in the presence of electrolyte, the silica dispersion stability at pH 4.0 can also be achieved at very low surfactant concentrations ( approximately 0.02 mM).     

Synergistic effects of polymers and surfactants on depletion forces

This work investigates the synergistic effects of a neutral polymer and an anionic surfactant on depletion forces as a function of bulk polymer and bulk surfactant concentration. In this work, we measure the force between a silica particle and a silica plate in aqueous solutions of the polymer and the surfactant using atomic force microscopy. The polymer is the triblock copolymer poly(ethylene oxide-block-propylene oxide-block-ethylene oxide) (Pluronic F108), and the surfactant is sodium dodecyl sulfate (SDS). In F108-only solutions, the force between the silica particle and the silica plate is primarily repulsive for polymer concentrations ranging from 200 to 10 000 ppm. In SDS-only solutions, the net force between the silica surfaces is repulsive at all separations for concentrations below 16 mM SDS and is attractive with a structural force character above 16 mM SDS. When both F108 and SDS are present in the solution, a net attractive force is observed at SDS concentrations as low as 4 mM, a factor of 2 below the critical micelle concentration (cmc). We attribute this synergistic effect to the complexation of F108 with SDS in bulk solution at a critical aggregation concentration (cac) that is less than the cmc, producing a relatively large, charged complex that creates a significant depletion force between the particle and plate.     

Dependence of Colloidal Forces Between Bitumen and Silica on the Chain Lengths of Ammonium Surfactants

Effects of ammonium surfactants with different hydrocarbon chain lengths (C8, C12, C16, and C18) on the colloidal forces between bitumen and silica were studied by atomic force microscopy. The results showed that the chain length of the ammonium surfactants had a significant impact on both the long-range interaction and adhesion forces. With the addition of surfactants with relative short chains of C8 and C12 in the solutions, the long-range repulsive force decreased or even became strong attractive force, while it became repulsive again in solutions of surfactants with long chains of C16 and C18. It was further observed that addition of Ca²⁺ in various surfactants solutions would either depress or enhance the colloidal interactions based on the surfactant chain lengths. It was believed that variation of the interaction behaviors resulted from the mono-layer or bilayer adsorption of various surfactant molecules on the negatively charged surfaces of bitumen and silica, which affected the surface wettability and the surface charge characteristics and then greatly changed the colloidal interactions. The findings indicated that, to have a high bitumen recovery and good froth quality, the surfactant type and concentration of the di-valent metal ions in the oil sand processing slurry must be well considered.     

Transport Properties of Aqueous Solutions of Alkyltrimethylammonium Bromide Surfactants at 25 degrees C

Intradiffusion coefficients, D, of n-alkyltrimethylammonium bromides [CH(3)-(CH(2))(n-1)-N(CH(3))(3)Br, C(n)TAB] (n=6, 8, 10, 12) in mixtures with heavy water were measured by the PGSE-NMR technique at 25 degrees C. The experimental data permitted evaluation of the influence of the alkyl chain length on the surfactant self-aggregation process. For all the surfactants considered, the D trend showed a slope change corresponding to the critical micellar composition (cmc). In the premicellar composition range, D decreased linearly with the square root of the surfactant molality. The D values extrapolated at infinite dilution were related to the limiting mutual diffusion coefficients, determined through the Taylor dispersion technique. In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, D(M), from which the aggregate radii and the aggregation numbers were obtained. The decreasing trend of D(M) with increasing surfactant molality was interpreted in terms of interparticle electrostatic repulsion. D(M) values allowed evaluation of the Gouy-Chapman layer thickness. The solvent intradiffusion coefficient in the heavy water-C(n)TAB mixtures, D(w), was also measured. It decreased with increasing surfactant molality. For n=8, 10, 12 the D(w) trend presented a slope change at the cmc, which could be ascribed to the strong decrease in hydration of surfactant molecules upon micellization. Because of its short hydrophobic tail, C(6)TAB exhibited peculiar aggregation behavior. Its cmc, which is poorly marked, is lower than the value predicted by extrapolating the cmc values obtained for the other terms of the series. The C(6)TAB aggregates do not solubilize TMS molecules; the estimated aggregation number is extremely low ( approximately 3). Finally, no abrupt slope change in the solvent intradiffusion coefficient trend was detected. This evidence suggests that C(6)TAB molecules do not micellize in aqueous solution, but form trimers in which the surfactant hydrophobic tails are not hidden from contact with water molecules. Copyright 2001 Academic Press.     

Effect of the chain-length compatibility of surfactants and mechanical properties of mixed micelles on surfaces

Force/distance curves for silicon nitride tip/flat silica or alumina coated by a layer of mixed micelles of cationic/anionic surfactant are measured by using AFM. Mixtures of SDS/C(n)TAB (with molecular ratios of 3:1 and 20:1) and C(n)TAB/SDS (with molecular ratio of 85:15) were used for alumina and silica substrates, respectively. The number of carbon atoms per C(n)TAB molecule, n, was in the range of 8 to 16. On the basis of the force/distance curves, the elastic modulus, E, and yield strength, Y, of surface micelles are calculated. It is shown that in surfactant mixtures containing SDS the maximal repulsive force (the barrier F(bar)) at which the tip punctured the micelles, as well as the magnitudes of E and Y, attained the maximal values for C(12)TAB ( i.e., when the hydrocarbon chain lengths of two oppositely charged surfactants are the same). Obviously, it can be related to the highest density structure of these micelles. Note that the literature data for the surface micelles from pure C(n)TAB solutions demonstrate a monotonic dependence of F(bar), E, and Y on n in the range of n = 8-16, whereas the oppositely charged mixed surfactant systems yield much higher values of F(bar), E, and Y than does an equivalent chain length from the homologue series plots. The results obtained for mechanical characteristics of mixed micelles at the surface are compared with the results for the relaxation time, tau(2), that characterizes the lifetime (and therefore structure) of the bulk micelles. Both the dependence of F(bar), E, and Y on n for the surface mixed micelles and tau(2) on n for the bulk mixed micelles demonstrate a maximum at n = 12 for the C(n)TAB + SDS system. This correlation between properties of the surface and bulk micelles suggests that the mechanical properties of the surface micelles are largely determined by the interactions between surfactant molecules with surfactant-substrate interactions playing a secondary role.     

Wormlike micelles in poly(oxyethylene) surfactant solution: Growth control through hydrophilic-group size variation

Rheological responses of colloidal gels formed from fumed silica suspensions in aqueous KOH solution at pH 11 by the addition of cationic surfactants, such as dodecyltrimethylammonium chloride (C12 TAC) and hexadodecyltrimethylammonium chloride (C16 TAC) have been investigated as functions of silica and surfactant concentrations. Stable and aggregated fumed silica suspensions with negative charges cause gelling by adding the cationic surfactants through electrical neutralization of their micelles. The resulting critical strain and storage modulus of the gelled silica suspension increase with an increase in the surfactant concentration, irrespective of the cationic surfactant. This means that the higher the surfactant concentration is, the more effective the electrical neutralization interaction through the micelle of the cationic surfactant is. Moreover, the resulting gels can be classified into the strong-link gel and the weak-link one in the presence of C12 TAC and C16 TAC, respectively, from a comparison of the silica volume fraction dependences of critical strain and storage modulus with the fractal gel model.     

Mixed micellization and interfacial properties of dodecyltrimethylammonium bromide and tetraethyleneglycol mono-n-dodecyl ether in absence and presence of sodium propionate

Mixed micelle formation and interfacial properties of aqueous binary surfactant combinations of dodecyltrimethylammonium bromide (C12TAB) and tetraethyleneglycol mono-n-dodecyl ether (C12E4) at 30 degrees C in absence and presence of sodium propionate (NaPr) have been investigated. The critical micelle concentration, aggregation number, micropolarity and interfacial adsorption have been quantitatively estimated by surface tension and steady-state fluorescence measurements. The micellar and adsorption characteristics like composition, activity coefficients and mutual interaction parameters have been estimated following different theoretical treatments like that of Clint, Rubingh, Rodenas, Maeda, Blankschtein and Rosen. The analysis reveals very small mole fraction of cationic surfactant in both the mixed micelles and mixed monolayer inspite of synergism. Blankschtein's model predicts a continuous decrease in synergism due to the salt effect of NaPr; Rubingh's approach, on the contrary, indicates an increase in it above 30 mM of NaPr concentration. Aggregation number variation with NaPr indicates the same. Mixed monolayer shows better synergism compared to that in mixed micelles which increases with the addition of sodium propionate above 30 mM concentration.     

Supercritical ethanol--a fascinating dispersion medium for silica nanoparticles

For the first time, the dispersion stability of silica nanoparticles has been investigated in high-temperature and high-pressure ethanol by measuring the hydrodynamic diffusion coefficient of the particles by means of dynamic light scattering. The silica nanoparticles remain stable in ethanol within a wide temperature range of 24-304 degrees C at 12.3 MPa, and they start to aggregate at T >or= 305 degrees C. Numerical analysis reveals that the net interparticle repulsive potential barrier decreases dramatically with increasing temperature due to the changes in the properties of the medium. We observed that particles remain highly stable in the nonpolar supercritical ethanol in the temperature regime 241-304 degrees C, where the DLVO potential barrier is only 5-2 k(B)T. The dispersion stability of silica nanoparticles at this low potential barrier in high-temperature and high-pressure ethanol, especially in the supercritical ethanol, is fascinating. The silica-ethanol system might be a unique and special example in the colloidal dispersions. Results suggest that silica nanoparticles may be used as a model colloid to investigate the colloidal transport phenomena in the supercritical ethanol.     

Vesicle-micelle transition in aqueous mixtures of the cationic dioctadecyldimethylammonium and octadecyltrimethylammonium bromide surfactants

The vesicle-micelle transition in aqueous mixtures of dioctadecyldimethylammonium and octadecyltrimethylammonium bromide (DODAB and C(18)TAB) cationic surfactants, having respectively double and single chain, was investigated by differential scanning calorimetry (DSC), steady-state fluorescence, dynamic light scattering (DLS) and surface tension. The experiments performed at constant total surfactant concentration, up to 1.0 mM, reveal that these homologous surfactants mix together to form mixed vesicles and/or micelles, depending on the relative amount of the surfactants. The melting temperature T(m) of the mixed DODAB-C(18)TAB vesicles is larger than that for the neat DODAB in water owing to the incorporation of C(18)TAB in the vesicle bilayer. The surface tension decreases sigmoidally with C(18)TAB concentration and the inflection point lies around x(DODAB) approximately 0.4, indicating the onset of micelle formation owing to saturation of DODAB vesicles by C(18)TAB molecules. When x(DODAB)>0.5 C(18)TAB molecules are mainly solubilised by the vesicles, but when x(DODAB)<0.25 micelles are dominant. Fluorescence data of the Nile Red probe incorporated in the system at different surfactant molar fractions indicate the formation of micelle and vesicle structures. These structures have apparent hydrodynamic radius R(H) of about 180 and 500-800 nm, respectively, as obtained by DLS measurements.     

Adsorption of hexyl-alpha,omega-bis(dodecyldimethylammonium bromide) gemini surfactant on silica and its effect on wettability

The adsorption of hexyl-alpha,omega-bis(dodecyldimethylammonium bromide) (C(12)C(6)C(12)Br(2)) gemini surfactant on silica and its effect on wettability have been studied. The structure of the adsorbed aggregates experiences an evolving course with the increase of C(12)C(6)C(12)Br(2) concentration. It is from no aggregates to circular islands, then to semicontinuous islands, and at last to the two-bilayer structure. No matter what kind of aggregates are in existence, their thickness values are to be the same 3.3+/-0.3 nm. The fraction of silica surface covered by the surfactant aggregates also varies from nearly zero at 0.05 mM to approximately 0.92 at 5.0 mM. The variation of contact angle against C(12)C(6)C(12)Br(2) concentration shows two distinct regions. The upward shift indicates that the surfactant molecules are adsorbed with their hydrophobic tails facing air upon increasing concentration, while the downward shift reveals that the surfactant aggregates are in existence with the hydrophilic headgroups facing air. IR spectra suggest that two different courses are involved with the increase of the surfactant concentration. One possible course is that the surfactant tails pack more closely and orderly, and the other may be that the spacer changes from stretched profile to bended conformation upon increasing the surfactant concentration.     

Adsorption of DNA and dodecyl trimethylammonium bromide mixtures at the air/water interface: a neutron reflectometry study

The interactions between dodecyl trimethylammonium bromide (C12TAB) and two samples of DNA with widely differing molecular weights have been studied using surface tension and neutron reflectometry. Neutron reflection data show that the surfactant and polymer are adsorbed together in a highly cooperative fashion over a 1000-fold change in surfactant concentration. Furthermore, the shorter DNA fragments adsorb with C12TAB as trilayers at higher surfactant concentrations, with overall layer thicknesses of 65-70 A. The high molecular weight DNA, however, shows only approximate monolayer adsorption with thicknesses varying from 19 to 26 A over the entire range of C12TAB concentrations. The difference in behavior between the different samples is believed to be a result of the rigid double helical structure of DNA which makes the formation of bulk phase polymer/micelle aggregates much less favorable for the short fragments. The resulting increase in the critical aggregation concentration (CAC) then leads to the adsorption of additional surfactant/polymer complex to the underside of the initial stable surface active DNA/C12TAB complex. Comparison with previous results obtained for synthetic polyelectrolytes shows that DNA/C12TAB complexes are not capable of reducing the surface tensions to the extent that other mixtures such as the poly(styrene sulfonate)/C12TAB mixtures do. A possible reason for this is the high rigidity of DNA combined with the fact that its hydrophobic moieties are positioned within the double helix so that the external molecule is largely hydrophilic.     

Specific interactions of poly(4-vinyl phenol) gel with cationic and anionic surfactants

Binding behaviors of ionic surfactants (decyl- and dodecyltrimethylammonium bromide (C(10)TAB, C(12)TAB), sodium decane sulfonate (SDeSo), and sodium dodecyl sulfate (SDS)) to poly(4-vinyl phenol) (P4VPh) gel were investigated to elucidate a specific swelling behavior that has been found for P4VPh gel in aqueous solutions of tetraalkylammonium salts. With increasing cationic surfactant concentration, P4VPh gel significantly deswelled and then remarkably reswelled at a concentration somewhat below the respective cmc values. On the other hand, in the case of the anionic surfactants, the gel only showed a marked swelling at a concentration just below the respective cmc values. A similar charge-specific behavior of the surfactants was also found for the P4VPh dispersion system studied with a UV-vis spectroscopy; namely, in the cationic surfactant-P4VPh systems, the turbidity of the dispersion first increased with increasing the surfactant concentration and then decreased. This result suggests that aggregation of P4VPh particles first occurred and finally the particles were solubilized. A red shift followed by a blue shift observed for a pi-pi absorption of phenol at around 278 nm was also consistent with the aggregation-solubilization behavior. In the anionic surfactant-P4VPh system, however, only solubilization of the polymer particle was observed, and the UV peak only showed a blue shift. All these results in the gel and the dispersion systems strongly suggest that the cation-pi interaction is involved in the binding of the cationic surfactants to P4VPh.     

Manipulation of the adsorption of ionic surfactants onto hydrophilic silica using polyelectrolytes

This paper demonstrates the use of polyelectrolytes to modify and manipulate the adsorption of ionic surfactants onto the hydrophilic surface of silica. We have demonstrated that the cationic polyelectrolyte poly(dimethyl diallylammonium chloride), poly-dmdaac, modifies the adsorption of cationic and anionic surfactants to the hydrophilic surface of silica. A thin robust polymer layer is adsorbed from a dilute polymer/surfactant solution. The resulting surface layer is cationic and changes the relative affinity of the cationic surfactant hexadecyl trimethylammonium bromide, C16TAB, and the anionic surfactant sodium dodecyl sulfate, SDS, to adsorb. The adsorption of C16TAB is dramatically reduced. In contrast, strong adsorption of SDS was observed, in situations where SDS would normally have a low affinity for the surface of silica. We have further shown that subsequent adsorption of the anionic polyelectrolyte sodium poly(styrene sulfonate), Na-PSS, onto the poly-dmdaac coated surface results in a change back to an anionic surface and a further change in the relative affinities of the cationic and anionic surfactants for the surface. The relative amounts of C16TAB and SDS adsorption depend on the coverage of the polyelectrolyte, and these preliminary measurements show that this can be manipulated.     

Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2'-methoxyphenyl)phthalimide (1): kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of N-(2'-methoxyphenyl)phthalimide (1) decrease nonlinearly with increasing total concentration of nonionic surfactant C(m)E(n) (i.e. [C(m)E(n)](T) where m and n represent the respective number of methyl/methylene units in the tail and polyoxyethylene units in the headgroup of a surfactant molecule and m/n=16/20, 12/23 and 18/20) at constant 2% v/v CH(3)CN and 1.0 mM NaOH. The k(obs)vs. [C(m)E(n)](T) data follow the pseudophase micellar (PM) model at ≤ 50 mM C(16)E(20), ≤ 1.4 mM C(12)E(23) and ≤ 2.0 mM C(18)E(20) where rate of hydrolysis of 1 in micellar pseudophase could not be detected. The values of k(obs) fail to follow the PM model at > ～50 mM C(16)E(20), > ～1.4 mM C(12)E(23) and > ～2.0 mM C(18)E(20) which has been attributed to a micellar structural transition from spherical to rodlike which in turn increases C(m)E(n) micellar binding constant (K(S)) of 1 with increasing values of [C(m)E(n)](T). Rheological measurements show the presence of spherical micelles at ≤ 50 mM C(16)E(20), ≤ 1.4 mM C(12)E(23) and ≤ 3.0 mM C(18)E(20). The presence of rodlike micelles is evident from rheological measurements at > ～50 mM C(16)E(20), > ～1.4 mM C(12)E(23) and > ～3.0 mM C(18)E(20).     

Forces between silica surfaces with adsorbed cationic surfactants: influence of salt and added nonionic surfactants

Forces have been measured between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus. The surfactants used are the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the nonionic surfactant hexakis(ethylene glycol) mono-n-tetradecyl ether (C(14)E(6)) as well as mixtures of these two surfactants. The measurements were made at elevated pH, and the effect of salt was studied. At high pH the glass surface is highly charged, which increases the adsorption of TTAB. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition of C(14)E(6) has a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force is hardly affected, but the adhesion is reduced remarkably. Also, addition of salt decreases the adhesion, but increases the barrier force. In the presence of salt, addition of C(14)E(6) also increases the thickness of the adsorbed layer. The force barrier height is also shown to be related to literature values for surface pressure data in these systems.     

Surfactant adsorption onto cellulose surfaces

The adsorption of the cationic surfactant, hexadecyl trimethyl ammonium bromide, C16TAB, onto model cellulose surfaces, prepared by Langmuir-Blodgett deposition as thin films, has been investigated by neutron reflectivity. Comparison between the adsorption of C16TAB onto hydrophilic silica, a hydrophobic cellulose surface, and a regenerated (hydrophilic) cellulose surface is made. Adsorption onto the hydrophilic silica and onto the hydrophilic cellulose surfaces is similar, and is in the form of surface aggregates. In contrast, the adsorption onto the hydrophobic cellulose surface is lower and in the form of a monolayer. The impact of the surfactant adsorption and the in situ surface regeneration on the structure of the cellulose thin films and the nature of solvent penetration into the cellulose films are also investigated. For the hydrophobic cellulose surface, intermixing between the cellulose and surfactant occurs, whereas there is little penetration of surfactant into the hydrophilic cellulose surface. Measurements show that solvent exchange between the partially hydrated cellulose film and the solution is slow on the time scale of the measurements.     

Control of particle cluster dispersion using responsive polymeric additives

The dispersion behavior of clusters of hydrophilic and hydrophobic silica treated with a thermoresponsive polymer has been investigated. The influence of the thermoresponsive interfacial chemistry was assessed by performing dispersion studies in poly(dimethylsiloxane) (PDMS) at 25 and 68 degrees C for agglomerates of powders incorporating poly(N-isopropyl acrylamide) (PNIPAM), a thermoresponsive polymer, and water. While the dispersion of clusters of hydrophilic silica was found to be temperature independent, dispersion of hydrophobic silica exhibited a temperature dependency. The response of the polymer upon temperature variation brings about modifications in the interparticle interactions of the PNIPAM-treated powders thus leading to changes in the cluster cohesivity. The dependence of the cluster cohesivity on temperature was evidenced through changes in the dispersion mechanism and kinetics of the treated silica clusters at both tested temperatures. In addition, optical microscopy of silica suspensions with and without PNIPAM at 25 and 60 degrees C, illustrated the effect of the polymer presence and configuration on the particle-particle interaction forces. While micrographs of the particles at 25 degrees C show less evidence of particle aggregation, the micrographs at 60 degrees C illustrate an increase in aggregation suggesting an enhancement in the attractive forces between particles.     

Forces between polyethylene surfaces in oxyethylene dodecyl ether solutions as influenced by the number of oxyethylene groups

The atomic force microscopy (AFM) colloidal probe technique was used to study the effect of oxyethylene dodecyl ethers, C12En (n = 1-7), on interactions between hydrophobic polyethylene (PE) surfaces in aqueous solutions. Long-range (colloidal) and contact (pull-off) forces were measured between 10 to 20 microm PE spheres and a flat PE surface at concentrations of surfactant of 1 x 10(-6) and 1 x 10(-4) M. The surface tension of the surfactant solutions and contact angles at PE surfaces were also studied. The influence of the number of oxyethylene groups in the surfactant molecule was examined. Initially, long-range attractive (hydrophobic) forces between the PE surfaces were observed that decreased in range and magnitude with an increase in the number of oxyethylene groups in 1 x 10(-4) M solutions. Above four oxyethylene groups per molecule, repulsive forces were observed. The measured pull-off force between PE surfaces decreased monotonically from approximately 500 mJ/m2 for C12E1 to 150 mJ/m2 for C12E7. The interfacial energy was calculated on the basis of the JKR model, taking into account long-range forces operating outside the contact area. The interfacial energies decreased from 43-47 mJ/m2 for PE-water and PE-C12E1 (1 x 10(-4) M) interfaces to approximately 18 mJ/m2 for PE-C12E7 (1 x 10(-4) M). The interfacial energy was also calculated from measured contact angles and surface tensions using Neumann's equation of state and Young's equation. A similar relationship between interfacial energy and the number of oxyethylene groups was observed on the basis of contact and surface tension measurements. However, interfacial energy values were smaller, within 15-20 mJ/m2, than those calculated from AFM pull-off force measurements.     

A disjoining pressure study of foam films stabilized by mixtures of nonionic and ionic surfactants

Studying the disjoining pressure Pi as a function of the film thickness h (Pi-h curves) of foam films stabilized by ionic and nonionic surfactants, one finds that the surface charge density q0 of films stabilized by ionic surfactants increases with increasing surfactant concentration, while the opposite holds true for nonionic surfactants. Thus, it should be possible to tune the surface charge density with mixtures of nonionic and ionic surfactants. To address this question, we studied foam films stabilized by aqueous solutions of surfactant mixtures. The mixtures consisted of the nonionic beta-dodecylmaltoside (beta-C12G2) and the cationic dodecyl trimethylammonium bromide (C12TAB) with mixing ratios of beta-C12G2/C12TAB = 1:0, 50:1, 1:1, 1:50, 0:1. The addition of small amounts of C12TAB to beta-C12G2 first neutralizes the negative surface charge of the beta-C12G2 films and finally leads to a charge reversal from negatively to positively charged surfaces. On the other hand, by adding small amounts of beta-C12G2 to C12TAB, one observes the formation of stable CBFs which was also observed for the pure C12TAB. However, in contrast to the pure C12TAB, the resulting Pi-h curves for the mixtures cannot be described with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory; the slope of the curves is too steep, and it barely changes with changing electrolyte concentration. A possible explanation for this observation will be given and discussed.     

Morphology and mechanical properties of surfactant aggregates at water-silica interfaces: molecular dynamics simulations

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammonium bromide) surfactant in an aqueous medium that is modeled with empirical potentials. The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force of about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements.