Adsorption of the Lamellar Phase of Aerosol-OT at the Solid/Liquid and Air/Liquid Interfaces

Influence of Adsorption Characteristics of Surfactants Sodium Dodecyl Sulfate and Aerosol–OT on Dynamic Process of Water-Based Lubrication

Surfactant solutions are widely used in industry, and their steady-state lubrication properties have been comprehensively explored, while the “dynamic process” between steady states attracts much less attention. In this study, the lubrication behaviors of sodium dodecyl sulfate (SDS) and sodium bis (2–ethylhexyl) sulfosuccinate (Aerosol–OT, AOT) solutions were comparatively and extensively discussed. Experimental results showed that the duration of the dynamic process of AOT solution lubrication was significantly shorter than that of SDS. The essence of the dynamic process was revealed from the aspects of the running-in of solid surfaces and the adsorption process of surfactant molecules. Unlike the general recognition that the friction force evolution mainly corresponds to the running-in of surfaces, this study indicated that the dynamic adsorption behavior of surfactant molecules mainly contributes to this process. Various experiments and analyses showed that the smaller steric hindrance and lower orientation speed of SDS molecules led to longer diffusion into the confined contact zone and a longer duration of friction force decrease. This work enhances our understanding of the dynamic friction process in water-based lubrication, which could also have important implications for oil-based lubrication and its industrial applications.

Multivalent counterion induced multilayer adsorption at the air-water interface in dilute Aerosol-OT solutions

The formation of surface multilayer structures, induced by the addition of multivalent counterions in dilute surfactant solutions, has been widely observed in a range of anionic surfactants. The phenomenon is associated with the ability to manipulate surface properties, especially in the promotion of enhanced surface wetting, and in the presence of an extensive near surface reservoir for rapid surface delivery of surfactant and other active components.

HYPOTHESIS

In the single alkyl chain anionic surfactants, such as sodium dodecysulfate, SDS, sodium alkylethoxylsulfate, SAES, and alkylestersulfonate, AES, surface multilayer formation is promoted by trivalent counterions such as Al³⁺, and is generally not observed with divalent counterions, such as Ca²⁺ or with monovalent counterions. In the di-alkyl chain anionic surfactant, dodecylbenzenesulfonate, LAS, surface multilayer formation now occurs in the presence of divalent counterions. It is attributed to the closer proximity of a bulk lamellar phase, resulting in a greater tendency for surface multilayer formation, and hence should occur in other di-alkyl chain anionic surfactants.

EXPERIMENTS

Aerosol-OT, AOT, is one of the most commonly used di-alkyl chain anionic surfactants, and is extensively used as an emulsifying, wetting and dispersing agent. This paper reports on predominantly neutron reflectivity, NR, measurements which explore the nature of surface multilayer formation of the sodium salt of AOT at the air-solution interface with the separate addition of Ca²⁺ and Al³⁺ counterions.

FINDINGS

In the AOT concentration range 0.5 to 2.0 mM surface multilayer formation occurs at the air-solution interface with the addition of Ca²⁺ or Al³⁺ counterions. Although the evolution in the surface structure with surfactant and counterion concentration is broadly similar to those reported for SDS, SAES and AES, some notable differences occur. In particular the surfactant and counterion concentration thresholds for surface multilayer formation are higher for Ca²⁺ than for Al³⁺. The differences encountered reflect the greater affinity of the di-alkyl chain structure for lamellar formation, and how the surface packing is controlled in part by the headgroup structure and the associated counterion binding affinity.

Complexation of Pluronic L62 (EO6)-(PO34)-(EO6)/aerosol-OT (sodium bis(2-ethylhexyl)sulfosuccinate) in aqueous solutions investigated by small angle neutron scattering

We investigate the phase behaviours of Pluronic L62 in aqueous solution in the presence of aerosol-OT (AOT) molecules by small angle neutron scattering (SANS). The presence of AOT significantly changes the micellization phenomenon of L62 micelles in aqueous solution, including their critical micelle temperature (CMT), global size, and asphericity. The origin of these observations is attributed to the complexation between the neutral L62 surfactants and the ionic AOT molecules, which additionally provides charge to the mixed micelles: we analyse the data and extract meaningful information using the Ornstein-Zernike integral formalism. As a result, we observe that the co-micellization of L62 and AOT is very stable across a wide temperature range.

Insight into Kinetics and Mechanisms of AOT Vesicle Adsorption on Silica in Unfavorable Conditions

The structure of adsorbed surfactant layers at the equilibrium state has already been investigated using various experimental techniques. However, the comprehension of the formation of structural intermediates in nonequilibrium states and the resulting adsorption kinetics still remain a challenging task. The temporal characterization of these intermediate structures provides further understanding of the layer structure at equilibrium and of the main interactions involved in the adsorption process. In this article, we studied the adsorption kinetics of AOT vesicles on silica at different pHs at ambient temperature. The AOT vesicles were formed in a brine solution. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to obtain information on the kinetics of surfactant adsorption and on the structure of the adsorbed layer at the equilibrium state. Additionally, neutron reflectivity experiments were performed to provide a detailed description of the mean surfactant concentration profile normal to the surface at equilibrium. Results suggest that vesicles in the bulk influence the adsorption mechanisms. In acidic conditions, after a time-dependent structural rearrangement step, followed by the rupture of initially adsorbed vesicles, the formation of a bilayer was observed. At an intermediate and basic pH, in spite of the electrostatic repulsion between the negatively charged surfactants and silica, results demonstrated the existence of an adsorbed layer composed of AOT vesicles. Vesicles are more or less closely packed depending on the pH of the solution. Results show a non-negligible influence of NaCl addition at pH values where adsorption is initially inhibited. Vesicle adsorption at the intermediate and basic pH is probably due to the combination of attractive van der Waals interactions promoted in high ionic strength systems and the formation of hydrogen bonds. Interpretation of adsorption kinetics gave insight into adsorption mechanisms in an electrostatic repulsion environment.

Multilayers formed by polyelectrolyte-surfactant and related mixtures at the air-water interface

The structure and occurrence of multilayered adsorption at the air-water interface of surfactants in combination with other oppositely charged species is reviewed. The main species that trigger multilayer formation are multiply charged metal, oligo- and polyions. The structures vary from the attachment of one or two more or less complete surfactant bilayers to the initial surfactant monolayer at the air-water interface to the attachment of a greater number of bilayers with a more defective structure. The majority of the wide range of observations of such structures have been made using neutron reflectometry. The possible mechanisms for the attraction of surfactant bilayers to an air-water interface are discussed and particular attention is given to the question of whether these structures are true equilibrium structures.

Molecular characterization of water and surfactant AOT at nanoemulsion surfaces

Nanoemulsions and microemulsions are environments where oil and water can be solubilized in one another to provide a unique platform for many different biological and industrial applications. Nanoemulsions, unlike microemulsions, have seen little work done to characterize molecular interactions at their surfaces. This study provides a detailed investigation of the near-surface molecular structure of regular (oil in water) and reverse (water in oil) nanoemulsions stabilized with the surfactant dioctyl sodium sulfosuccinate (AOT). Vibrational sum-frequency scattering spectroscopy (VSFSS) is used to measure the vibrational spectroscopy of these AOT stabilized regular and reverse nanoemulsions. Complementary studies of AOT adsorbed at the planar oil-water interface are conducted with vibrational sum-frequency spectroscopy (VSFS). Jointly, these give comparative insights into the orientation of interfacial water and the molecular characterization of the hydrophobic and hydrophilic regions of AOT at the different oil-water interfaces. Whereas the polar region of AOT and surrounding interfacial water molecules display nearly identical behavior at both the planar and droplet interface, there is a clear difference in hydrophobic chain ordering even when possible surface concentration differences are taken into account. This chain ordering is found to be invariant as the nanodroplets grow by Ostwald ripening and also with substitution of different counterions (Na:AOT, K:AOT, and Mg:AOT) that consequently also result in different sized nanoparticles. The results paint a compelling picture of surfactant assembly at these relatively large nanoemulsion surfaces and allow for an important comparison of AOT at smaller micellar (curved) and planar oil-water interfaces.

An Anionic Surfactant on an Anionic Substrate: Monovalent Cation Binding

Neutron reflectometry has been used to study the adsorption of the anionic surfactant bis(2-ethylhexyl) sulfosuccinate cesium salt on the anionic surface of mica. Evidence of significant adsorption is reported. The adsorption is reversible and changes little with pH. This unexpected adsorption behavior of an anionic molecule on an anionic surface is discussed in terms of recent models for surfactant adsorption such as cation bridging, where adsorption has been reported with the divalent ion calcium but not previously observed with monovalent ions.

Neutron Reflectometry of an Anionic Surfactant at the Solid-Liquid Interface under Shear

Neutron reflectometry with in situ rheology is used to measure the shear response of an adsorbed anionic surfactant (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) at the alumina-water interface. A low surfactant concentration is measured where a single bilayer adsorbs at the interface as well as a higher concentration where a multilamellar structure forms. The low concentration structure does not change with the imposed shear (oscillatory or steady). However, the lamellar phase shows a loss of structure under both steady and oscillatory shear. There are differences between the steady and oscillatory cases, which are discussed, with both showing a strong dependence on the strain amplitude.

Multilayering of Calcium Aerosol-OT at the Mica/Water Interface Studied with Neutron Reflection: Formation of a Condensed Lamellar Phase at the CMC

Using specular neutron reflection, the adsorption of sodium and calcium salts of the surfactant bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT or AOT) has been studied at the mica/water interface at concentrations between 0.1 and 2 CMC. The pH dependence of the adsorption was also probed. No evidence of the adsorption of Na(AOT) was found even at the critical micelle concentration (CMC) while the calcium salt was found to adsorb significantly at concentrations of 0.5 CMC and above. This interesting and somewhat unexpected finding demonstrates that counterion identity may be used to tune the adsorption of anionic surfactants on anionic surfaces. At the CMC, three condensed bilayers of Ca(AOT)₂ were adsorbed at pH 7 and 9 and four bilayers adsorbed at pH 4. Multilayering at the CMC of Ca(AOT)₂ on the mica surface is an unusual feature of this surfactant/surface combination. Only single bilayer adsorption has been observed at other surfaces at the CMC. We suggest this arises from the high charge density of mica which must provide an excellent template for the surfactant.

Multilayering of Surfactant Systems at the Air-Dilute Aqueous Solution Interface

In the last 15 years there have been a number of observations of surfactants adsorbed at the air-water interface with structures more complicated than the expected single monolayer. These observations, mostly made by neutron or X-ray reflectivity, show structures varying from the usual monolayer to monolayer plus one or two additional bilayers to multilayer adsorption at the surface. These observations have been assembled in this article with a view to finding some common features between the very different systems and to relating them to aspects of the bulk solution phase behavior. It is argued that multilayering is primarily associated with wetting or prewetting of the air-water interface by phases in the bulk system, whose structures depend on an overall attractive force between the constituent units. Two such phases, whose formation is assumed to be partially driven by strong specific ion binding, are a concentrated lamellar phase that forms at low concentrations and a swollen lamellar phase that is not space-filling. Multilayering phenomena at the air-water interface then offer a delicate and easy means of studying the finer details of the incompletely understood attraction that leads to these two phases, as well as an interesting new means of self-assembling surface structures. In addition, multilayering is often associated with unusual wetting characteristics. Examples of systems discussed, and in some cases their bulk phase behavior, include surfactants with multivalent metal counterions, surfactants with oligomers and polymers, surfactant with hydrophobin, dichain surfactants, lung surfactant, and the unusual system of ethanolamine and stearic acid. Two situations where the air-water surface is deliberately held out of equilibrium are also assessed for features in common with the steady-state/equilibrium observations.

Neutron reflection study of the adsorption of the phosphate surfactant NaDEHP onto alumina from water

The adsorption of a phosphorus analogue of the surfactant AOT, sodium bis(2-ethylhexyl) phosphate (NaDEHP), at the water/alumina interface is described. The material is found to adsorb as an essentially water-free bilayer from neutron reflection measurements. This is similar to the behavior of AOT under comparable conditions, although AOT forms a thicker, more hydrated layer. The NaDEHP shows rather little variation with added salt, but a small thickening of the layer on increasing the pH, in contrast to the behavior of AOT.

Controlling interfacial film formation in mixed polymer-surfactant systems by changing the vapor phase

Here we show that transport-generated phase separation at the air-liquid interface in systems containing self-assembling amphiphilic molecules and polymers can be controlled by the relative humidity (RH) of the air. We also show that our observations can be described quantitatively with a theoretical model describing interfacial phase separation in a water gradient that we published previously. These phenomena arises from the fact that the water chemical potential corresponding to the ambient RH will, in general, not match the water chemical potential in the open aqueous solution. This implies nonequilibrium conditions at the air-water interface, which in turn can have consequences on the molecular organization in this layer. The experimental setup is such that we can control the boundary conditions in RH and thereby verify the predictions from the theoretical model. The polymer-surfactant systems studied here are composed of polyethylenimine (PEI) and hexadecyltrimethylammonium bromide (CTAB) or didecyldimethylammonium bromide (DDAB). Grazing-incidence small-angle X-ray scattering results show that interfacial phases with hexagonal or lamellar structure form at the interface of dilute polymer-surfactant micellar solutions. From spectroscopic ellipsometry data we conclude that variations in RH can be used to control the growth of micrometer-thick interfacial films and that reducing RH leads to thicker films. For the CTAB-PEI system, we compare the phase behavior of the interfacial phase to the equilibrium bulk phase behavior. The interfacial film resembles the bulk phases formed at high surfactant to polymer ratio and reduced water contents, and this can be used to predict the composition of interfacial phase. We also show that convection in the vapor phase strongly reduces film formation, likely due to reduction of the unstirred layer, where diffusive transport is dominating.

Lipid phase behaviour under steady state conditions

At the interface between two regions, for example the air-liquid interface of a lipid solution, there can arise non-equilibrium situations. The water chemical potential corresponding to the ambient RH will, in general, not match the water chemical potential of the solution, and the gradients in chemical potential cause diffusional flows. If the bulk water chemical potential is close to a phase transition, there is the possibility of forming an interfacial phase with structures qualitatively different from those found in the bulk. Based on a previous analysis of this phenomenon in two component systems (C. Aberg, E. Sparr, K. J. Edler and H. Wennerström, Langmuir, 2009, 25, 12177), we here analyse the henomenon for three-component systems. The relevant transport equations are erived, and explicit results are given for some limiting cases. Then the formalism s applied conceptually to four different aqueous lipid systems, which in addition to water and a phospholipid contain (i) octyl glucoside, (ii) urea, (iii) heavy water, and (iv) sodium cholate as the third component. These four cases are chosen to illustrate (i) a method to use a micelle former to transport lipid to the interface where a multi-lamellar structure can form; (ii) to use a co-solvent to inhibit the formation of a gel phase at the interface; (iii) a method to form pure phospholipid multi-lamellar structures at the interface; (iv) a method to form a sequence of phases in the interfacial region. These four cases all have the character of theoretically based conjectures and it remains to investigate experimentally whether or not the conditions can be realized in practice.

Limitations in the application of the Gibbs equation to anionic surfactants at the air/water surface: sodium dodecylsulfate and sodium dodecylmonooxyethylenesulfate above and below the CMC

This is a second paper responding to recent papers by Menger et al. and the ensuing discussion about the application of the Gibbs equation to surface tension (ST) data. Using new neutron reflection (NR) measurements on sodium dodecylsulfate (SDS) and sodium dodecylmonooxyethylene sulfate (SLES) above and below their CMCs and with and without added NaCl, in conjunction with the previous ST measurements on SDS by Elworthy and Mysels (EM), we conclude that (i) ST measurements are often seriously compromised by traces of divalent ions, (ii) adsorption does not generally reach saturation at the CMC, making it difficult to obtain the limiting Gibbs slope, and (iii) the significant width of micellization may make it impossible to apply the Gibbs equation in a significant range of concentration below the CMC. Menger et al. proposed ii as a reason for the difficulty of applying the Gibbs equation to ST data. Conclusions i and iii now further emphasize the failings of the ST-Gibbs analysis for determining the limiting coverage at the CMC, especially for SDS. For SDS, adsorption increases above the CMC to a value of 10 × CMC, which is about 25% greater than at the CMC and about the same as at the CMC in the presence of 0.1 M NaCl. In contrast, the adsorption of SLES reaches a limit at the CMC with no further increase up to 10 × CMC, but the addition of 0.1 M NaCl increases the surface excess by 20-25%. The results for SDS are combined with earlier NR results to generate an adsorption isotherm from 2 to 100 mM. The NR results for SDS are compared to the definitive surface tension (ST) measurements of EM, and the surface excesses agree over the range where they can safely be compared, from 2 to 6 mM. This confirms that the anomalous decrease in the slope of EM's σ - ln c curve between 6 mM and the CMC at 8.2 mM results from changes in activity associated with a significant width of micellization. This anomaly shows that it is impossible to apply the Gibbs equation usefully from 6 to 8.2 mM (i.e., the lack of knowledge of the activity in this range is the same as above the CMC (8.2 mM)). It was found that a mislabeling of the original data in EM may have prevented the use of this excellent ST data as a standard by other authors. Although NR and ST results for SDS in the absence of added electrolyte show that the discrepancies can be rationalized, ST is generally shown to be less accurate and more vulnerable to impurities, especially divalent ions, than NR. The radiotracer technique is shown to be less accurate than ST-Gibbs in that the four radiotracer measurements of the surface excess are consistent neither with each other nor with ST and NR. It is also shown that radiotracer results on aerosol-OT are likely to be incorrect. Application of the mass action (MA) model of micellization to the ST curves of SDS and SLES through and above the CMC shows that they can be explained by this model and that they depend on the degree of dissociation of the micelle, which leads to a larger change in the mean activity, and hence the adsorption, for the more highly dissociated SDS micelles than for SLES. Previous measurements of the activity of SDS above the CMC were found to be semiquantitatively consistent with the change in mean activity predicted by the MA model but inconsistent with the combined ST, NR, and Gibbs equation results.

Spontaneously amplified homochiral organic-inorganic nano-helix complexes via self-proliferation

Most spiral coiled biomaterials in nature, such as gastropod shells, are homochiral, and the favoured chiral feature can be precisely inherited. This inspired us that selected material structures, including chirality, could be specifically replicated into the self-similar populations; however, a physicochemical understanding of the material-based heritage is unknown. We study the homochirality by using calcium phosphate mineralization in the presence of racemic amphiphilic molecules and biological protein. The organic-inorganic hybrid materials with spiral coiling characteristics are produced at the nanoscale. The resulted helixes are chiral with the left- and right-handed characteristics, which are agglomerated hierarchically to from clusters and networks. It is interesting that each cluster or network is homochiral so that the enantiomorphs can be separated readily. Actually, each homochiral architecture is evolved from an original chiral helix, demonstrating the heritage of the matrix chirality during the material proliferation under a racemic condition. By using the Ginzburg-Landaue expression we find that the chiral recognition in the organic-inorganic hybrid formation may be determined by a spontaneous chiral separation and immobilization of asymmetric amphiphilic molecules on the mineral surface, which transferred the structural information from the mother matrix to the descendants by an energetic control. This study shows how biomolecules guide the selective amplification of chiral materials via spontaneous self-replication. Such a strategy can be applied generally in the design and production of artificial materials with self-similar structure characteristics.

Effect of polymer molecular weight and solution pH on the surface properties of sodium dodecylsulfate-poly(ethyleneimine) mixtures

The effect of polymer molecular weight and solution pH on the surface properties of the anionic surfactant sodium dodecylsulfate, SDS, and a range of small linear poly(ethyleneimine), PEI, polyelectrolytes of different molecular weights has been studied by surface tension, ST, and neutron reflectivity, NR, at the air-solution interface. The strong SDS-PEI interaction gives rise to a complex pattern of ST behavior which depends significantly on solution pH and PEI molecular weight. The ST data correlate broadly with the more direct determination of the surface adsorption and surface structure obtained using NR. At pH 3, 7, and 10, the strong SDS-PEI interaction results in a pronounced SDS adsorption at relatively low SDS and PEI concentrations, and is largely independent of pH and PEI molecular weight (for PEI molecular weights on the order of 320, 640, and 2000 Da). At pH 7 and 10, there are combinations of SDS and PEI concentrations for which surface multilayer structures form. For the PEI molecular weights of 320 and 640 Da, these surface multilayer structures are most well-developed at pH 10 and less so at pH 7. At the molecular weight of 2000 Da, they are poorly developed at both pH 7 and 10. This evolution in the surface structure with molecular weight is consistent with previous studies, (1) where for a molecular weight of 25,000 Da no multilayer structures were observed for the linear PEI. The results show the importance with increasing polymer molecular weight of the entropic contribution due to the polymer flexibility in control of the surface multilayer formation.

Effect of architecture on the formation of surface multilayer structures at the air-solution interface from mixtures of surfactant with small poly(ethyleneimine)s

The impact of ethyleneimine architecture on the adsorption behavior of mixtures of small poly(ethyleneimines) and oligoethyleneimines (OEIs) with the anionic surfactant sodium dodecylsulfate (SDS) at the air-solution interface has been studied by surface tension (ST) and neutron reflectivity (NR). The strong surface interaction between OEI and SDS gives rise to complex surface tension behavior that has a pronounced pH dependence. The NR data provide more direct access to the surface structure and show that the patterns of ST behavior are correlated with substantial OEI/SDS adsorption and the spontaneous formation of surface multilayer structures. The regions of surface multilayer formation depend upon SDS and OEI concentrations, on the solution pH, and on the OEI architecture, linear or branched. For the linear OEIs (octaethyleneimine, linear poly(ethyleneimine) or LPEI(8), and decaethyleneimine, LPEI(10)) with SDS, surface multilayer formation occurs over a range of OEI and SDS concentrations at pH 7 and to a much lesser extent at pH 10, whereas at pH 3 only monolayer adsorption occurs. In contrast, for branched OEIs BPEI(8) and BPEI(10) surface multilayer formation occurs over a wide range of OEI and SDS concentrations at pH 3 and 7, and at pH 10, the adsorption is mainly in the form of a monolayer. The results provide important insight into how the OEI architecture and pH can be used to control and manipulate the nature of the OEI/surfactant adsorption.

Anionic surfactant ionic liquids with 1-butyl-3-methyl-imidazolium cations: characterization and application

For the first time a series of anionic surfactant ionic liquids (SAILs) has been synthesized based on organic surfactant anions and 1-butyl-3-methyl-imidazolium cations. These compounds are more environmentally friendly and chemically tunable as compared to other common ionic liquids. A detailed investigation of physicochemical properties highlights potential applications from battery design to reaction control, and studies into aqueous aggregation behavior, as well as structuring in pure ILs, point to possible uses in electrochemistry.

Adsorption of aerosol-OT to sapphire: lamellar structures studied with neutrons

The adsorption of sodium bis 2-ethylhexyl sulfosuccinate, NaAOT, to a sapphire surface from aqueous solution has been studied by neutron reflection at concentrations above the critical micelle concentration (cmc). Complementary measurements of the bulk structure were made with small-angle neutron scattering and grazing incidence small-angle neutron scattering. At a concentration of about 1% wt (10 × cmc), lamellar phase NaAOT was observed both at the surface and in the bulk. The structure seen at the interface for a solution of 2% wt NaAOT is a 35 ± 2 Å thick bilayer adsorbed to the sapphire surface at maximum packing density, followed by an aligned stack of fluctuating bilayers of thickness 51 ± 2 Å and with an area per molecule of 40 ± 2 Å(2). Each bilayer is separated by a water: at 25 °C, this layer is 148 ± 2 Å. A simple model for the reflectivity from fluctuating layers is presented, and for 2.0% wt NaAOT the fluctuations were found to have an amplitude of 25 ± 5 Å. The temperature sensitivity of the structure at the surface was investigated in the range 15-30 °C. The effect of temperature was pronounced, with the solvent layer becoming thinner and the volume occupied by the NaAOT molecules in a bilayer increasing with temperature. The amplitude of the fluctuations, however, is approximately temperature independent in this range. The adsorption of NaAOT at the sapphire surface resembles that previously found at hydrophilic and hydrophobic silica surfaces. The coexisting bulk lamellar phase has a spacing of layers similar to that observed at the surface. These observations are an indication that the major driving force for adsorption is self-assembly, independent of the chemical nature of the interface.

Anionic surfactants and surfactant ionic liquids with quaternary ammonium counterions

Small-angle neutron scattering and surface tension have been used to characterize a class of surfactants (SURFs), including surfactant ionic liquids (SAILs). These SURFs and SAILs are based on organic surfactant anions (single-tail dodecyl sulfate, DS, double-chain aerosol-OT, AOT, and the trichain, TC) with substituted quaternary ammonium cations. This class of surfactants can be obtained by straightforward chemistry, being cheaper and more environmentally benign than standard cationic SAILs. A surprising aspect of the results is that, broadly speaking, the physicochemical properties of these SURFs and SAILs are dominated by the nature of the surfactant anion and that the chemical structure of the added cation plays only a secondary role.

Effect of concentration and addition of ions on the adsorption of aerosol-OT to sapphire

Aerosol-OT (sodium bis 2-ethylhexyl sulfosuccinate or NaAOT) adsorbs to hydrophilic sapphire solid surfaces. The structure of the formed bilayer has been determined over the concentration range 0.2-7.4 mM NaAOT. It was found that the hydrocarbon tails pack at maximum packing limit at very low concentrations, and that the thickness of the bilayer was concentration-independent. The adsorption was found to increase with concentration, with the surfactant molecules packing closer laterally. The area per molecule was found to change from 138 ± 25 to 51 ± 4 A(2) over the concentration range studied, with the thickness of the layer being constant at 33 ± 2 A. Addition of small amounts of salt was found to increase the surface excess, with the bilayer being thinner with a slightly larger area per molecule. Addition of different salts of the same valency was found to have a very similar effect, as had the addition of NaOH and HCl. Hence, the effects of adding acid or base should be considered an effect of ionic strength rather than an effect of pH. Adsorption of NaAOT to the sapphire surface that carries an opposite charge to the anionic surfactant is similar in many respects to the adsorption reported previously for hydrophilic and hydrophobic silica surfaces. This suggests that the adsorption of NaAOT to a surface is driven primarily by NaAOT self-assembly rather than effects of electrostatic attraction to the interface.

Surface and solution properties of anionic/nonionic surfactant mixtures of alkylbenzene sulfonate and triethyleneglycol decyl ether

The surface adsorption behavior and the solution microstructure of mixtures of the C(6) isomer of anionic surfactant sodium para-dodecyl benzene sulfonate, ABS, with nonionic surfactant monodecyl triethyleneglycol ether, C(10)E(3,) have been investigated using a combination of neutron reflectivity, NR, and small-angle neutron scattering, SANS. In solution, the mixing of C(10)E(3) and ABS results in the formation of small globular micelles over most of the composition range (100:0 to 20:80 ABS/C(10)E(3)). Planar aggregates (lamellar or unilamellar vesicles, ULV) are observed for solution compositions rich in the nonionic surfactant (>80 mol % nonionic). Prior to the transition to planar aggregates, the micelle aggregation number increases with increasing nonionic composition. The lamellar-phase region is preceded by a narrow range of composition over which mixtures of micelles and small unilamellar vesicles coexist. The variation in surface absorption behavior with solution composition shows a strong surface partitioning of the more surface-active component, C(10)E(3). This pronounced departure from ideal mixing is not readily explained by existing surfactant mixing theories. In the presence of Ca(2+) ions, a more complex evolution of solution phase behavior with solution composition is observed. The lamellar-phase region occurs over a broader range of solution compositions at the expense of the small-vesicle phase. The phase boundaries are shifted to lower nonionic compositions, and the extent to which the solution-phase diagrams are modified increases with increasing calcium ion concentration. The SANS data for the large planar aggregates are consistent with large polydisperse flexible unilamellar vesicles. In the presence of Ca(2+) ions, the surface adsorption patterns become more consistent with ideal mixing in the nonionic-rich region of the surface-phase diagram. However, in the ABS-rich regions the surface behavior is more complex because of the spontaneous formation of more complex surface microstructures (bilayers to multilayers). Both in water and in the presence of Ca(2+) ions the variations in the surface adsorption behavior and in the solution mesophase structure do not appear to be closely correlated.

Nonequilibrium phase transformations at the air-liquid interface

A theoretical model is presented for the formation of an ordered phase close to the air-liquid interface of an open binary aqueous solution. The chemical potential of water in the liquid phase is, in general, not equal to the chemical potential of water in the ambient atmosphere. There are therefore nonequilibrium conditions close to the air-liquid interface. There is also a gradient in the chemical potential of water, which could lead to the formation of a new interfacial phase. The formation of an interfacial phase is analyzed in terms of the equilibrium phase behavior corresponding to the local water chemical potential. The possibility of forming an interfacial phase is strongly dependent on the ambient conditions, bulk composition, and diffusive transport properties of the phases in question. Explicit calculations are presented for the formation of a lamellar liquid-crystalline phase close to the air-liquid interface of an isotropic surfactant solution with parameters chosen from the sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water system. We consider the relevance of the model to neutron reflectivity studies of the interface between air and surfactant/water systems, as well as to surfactant/polymer/water systems.

Determination of the Interfacial Characteristics of a Series of Bolaamphiphilic Poly(fluorooxetane) Surfactants through Molecular Dynamics Simulation

Constant surface tension (NgammaT) and constant volume (NVT) molecular dynamics simulations have been conducted on a series of bolaamphiphilic alpha,varpi-(diammonium disulfato)poly(fluorooxetane)s and on a typical "long-chain" anionic fluorosurfactant used to improve the flow-and-leveling characteristics of aqueous coatings, to compare their behavior at a water/air interface. Recent research has shown that the poly(fluorooxetane) surfactants considered in this paper could serve as an effective substitute for traditional fluorosurfactants used in flow-and-leveling applications.(1) From molecular dynamics simulation, we have determined the saturated interfacial area per surfactant, interfacial area per surfactant as a function of surface tension, density profiles, the degree of hydration for various atoms in each surfactant, the degree of counterion binding, and order parameters. Our results for saturated interfacial area per surfactant molecule are greater than what has been obtained by other researchers through parametric fitting of interfacial area from experimental surface tension data using the Davies isotherm. Possible explanations for this difference are discussed. The low interfacial areas occupied by each poly(fluorooxetane) at the water/air interface are the result of their ability to adopt a "looped" conformation, in which the carbon and oxygen backbone of each surfactant and the attached perfluoroalkyl chains are forced into the air phase. A geometrically defined penetration parameter was calculated from the density profiles, which reveals that each poly(fluorooxetane) surfactant is more effective at separating the air and water phases than the "long-chain" anionic fluorosurfactant. The degree of hydration measured for different atoms in poly(fluorooxetane) during simulation confirms that a "looped" conformation is adopted in which the surfactant backbone and the perfluoroalkyl chains are lifted away from the water surface. Calculation of order parameters revealed a much lower degree of ordering for the perfluoroalkyl side chains in each bolaamphiphile than in the "long chain" anionic fluorosurfactant. When viewed in the context of the penetration parameter analysis, the density profiles and hydration data suggest why each poly(fluorooxetane) is capable of significantly reducing surface tension when other fluorosurfactants with similarly short perfluoroalkyl moieties provide inadequate surface tension reduction for practical flow-and-leveling applications.

Monolayer of Aerosol-OT Surfactants Adsorbed at the Air/Water Interface: An Atomistic Computer Simulation Study

An atomistic molecular dynamics (MD) simulation has been carried out to investigate the structural and dynamical properties of a monolayer of the anionic surfactant sodium bis(2-ethyl-1-hexyl) sulfosuccinate (aerosol-OT or AOT) adsorbed at the air/water interface. The simulation is performed at room temperature and at a surface coverage corresponding to that at its critical micelle concentration (78 A(2)/molecule). The estimated thickness of the adsorbed layer is in good agreement with neutron reflection data. The study shows that the surfactants exhibit diffusive motion in the plane of the interface. It is observed that the surfactant monolayer has a strong influence in restricting both the translational and reorientational motions of the water molecules close to the interface. A drastic difference in the dipolar reorientational motion of water molecules in the aqueous layer is observed with a small variation of the distance from the surfactant headgroups. It has been observed that the water molecules in the first hydration layer (region 1) form strong hydrogen bonds with surfactant headgoups. This results in the slower structural relaxation of water-water hydrogen bonds in the first hydration layer compared to that in the pure bulk water. Most interestingly, we notice that the water molecules present in the layer immediately after the first hydration layer form weaker hydrogen bonds and thus relax faster than even pure bulk water.

Surface ordering in dilute dihexadecyl dimethyl ammonium bromide solutions at the air-water interface

At elevated temperatures and in dilute solution, we have observed lamellar surface ordering at the air-water interface of dihexadecyl dimethylammonium bromide, DHDAB, in the presence of electrolyte. With increasing temperature, the onset in ordering is observed between 35 and 40 degrees C. At 40 degrees C, there is an abrupt change in the lamellar spacing, from approximately 33 to approximately 40 A. Furthermore, in the presence of the cosurfactant benzyl alcohol, the ordering occurs at a lower temperature, between 20 and 25 degrees C. The change in lamellar spacing with temperature is attributed to a surface-induced transition, similar to the Lbeta to Lalpha phase transition observed in bulk lamellar dispersions.