Neutron reflection and the thermodynamics of the air-water interface.

By means of isotopic substitution, measurements of the neutron reflectivity (NR) from a flat water surface generally give model independent measurements of the amount of a chosen solute at the surface irrespective of whether the layer is a mixture or whether there is any aggregation in the bulk solution. Previously, adsorption at air-water interfaces has been determined by applying the Gibbs equation to surface tension (ST) measurements, which requires assumptions about the composition of the surface and about the activity of the solute in the bulk, which, in turn, means that in practice the surface is assumed to consist of the pure solute or of a mixture of pure solutes, and that the activity of the solute in the bulk solution is known. The use of NR in combination with ST-Gibbs measurements makes it possible to (i) avoid these assumptions and hence understand several patterns of ST behaviour previously considered to be anomalous and (ii) to start to analyse quantitatively the behaviour of mixed surfactants both below and above the critical micelle concentration. These two developments in our understanding of the thermodynamics of the air-water interface are described with recent examples.

Surface Activity of Ethoxylate Surfactants with Different Hydrophobic Architectures: The Effect of Layer Substructure on Surface Tension and Adsorption.

A series of nonionic ethoxylate surfactants containing different combinations of alkyl, phenyl, and adamantyl units in nine different arrangements, each combined with penta- and hexa-ethylene glycol groups, were synthesized and purified. The surface properties of all of the surfactants were investigated at the air-water (A-W) interface using surface tension (ST) to determine the limiting surface excess (Γlim), the limiting surface tension (σlim), and the critical micelle concentration (CMC). A smaller selection was investigated at the hydrophilic silica-water interface by neutron reflectometry to obtain the thickness of the adsorbed layer and the total adsorption at the CMC. An unusual and largely unrecognized feature of the ethoxylate group is that it is both hydrophilic and hydrophobic. It was found possible to account for the variation of σlim and Γlim of all of the adsorbed layers in terms of a balance of the estimated STs of the sublayers forming the overall adsorbed layer, including that of the underlying ethoxylate layer. The values of σlim were found to be highest for phenyl- and adamantyl-capped surfactants and lowest mainly when there was more than one methyl group at the surface. However, in terms of the concentration required to reach a given low ST, increasing the number of attached methyl groups was found to be less effective than using a smaller number of better-placed methyl groups. At the solid-liquid interface, adsorption at or above the CMC was in all cases in the form of a fragmented bilayer whose coverage varied approximately linearly with the packing parameter. However, results on the phenyl-capped surfactants showed that the high ST exhibited by these surfactants at the A-W interface becomes a high cohesion energy in the interior of the bilayer and they exhibited significantly higher adsorption than expected from simple packing arguments.

Unusual Maximum in the Adsorption of Aqueous Surfactant Mixtures: Neutron Reflectometry of Mixtures of Zwitterionic and Ionic Surfactants at the Silica-Aqueous Interface.

The adsorption of two zwitterionic surfactants, dodecyldimethylammonium propanesulfonate (C12PS) and dodecyldimethylammonium carboxybetaine (C12CB), and of their mixtures with the cationic dodecyltrimethylammonium bromide (C12TAB) and the anionic sodium dodecylsulfate (SDS) at the silica-water interface has been studied by neutron reflection (NR). The total adsorption, the composition of the adsorbed layer, and some structural information have been obtained over a range of concentrations from below the critical micelle concentration (CMC) to about 30× the mixed CMC. The adsorption behavior has been considered in relation to the previously measured micellar equilibrium of these mixtures in their bulk solutions and their adsorption at the air-water interface. C12CB adsorbs cooperatively close to its CMC to form an almost complete bilayer on its own, whereas C12PS adsorbs more weakly in a fragmented bilayer structure. Although SDS does not normally adsorb at the silica-water interface, SDS adsorbs strongly and cooperatively with C12PS at fractional SDS compositions up to about 0.5. This cooperativity is lost when the adsorbed fraction of SDS rises above about 0.5. At this point, adsorption drops sharply, creating an unusual maximum in the variation of adsorption with a total concentration above the mixed CMC. Neither the increase in cooperativity nor the subsequent decline in adsorption results directly from variations of the independently determined monomer concentrations in the bulk solution. The adsorption maximum is predominantly the effect of strong cooperative interaction, possibly accompanied by partial segregation of SDS within the layer, followed by charge repulsion from the surface. Although the solution aggregation and adsorption at the A-W interface are similar for SDS with C12CB, the addition of SDS to C12CB at the silica-water interface promotes the opposite behavior to that of SDS with C12PS, and SDS simply disrupts the cooperative binding of C12CB. Unlike SDS, the cationic surfactant C12TAB adsorbs on silica. It therefore coadsorbs at the SiO2-W interface with either C12CB or C12PS. However, in neither case is there any pronounced cooperativity and, even though the presence of C12TAB might be expected to favor adsorption, the adsorption is generally unexpectedly low.

Collapsed Structure of Hydrophobically Modified Polyacrylamide Adsorbed at the Air-Water Interface: The Polymer Surface Excess and the Gibbs Equation.

Neutron reflectometry has been used to measure the surface excesses and structures of hydrophobically modified polyacrylamide polymers (HMPAMs) at the air-water (A-W) interface. The HMPAMs were based on a range of commercially available PAM, which were modified by the N-alkylation of the amide group to give an N-CnD2n+1 hydrophobic group with n = 8, 12, and 16 at levels of 0.5, 1, 2, 4, and 6 mol %. A further HMPAM was synthesized in two isotopic forms with either N-CnD2n+1 or N-CnH2n+1 as hydrophobes. For moderate- and high MW species the near surface structure at the A-W interface consists of two layers. All the hydrophobic units are in these two layers as well as a large fraction of backbone units, often amounting to a total volume comparable to that of the hydrophobes. The outer layer next to air contains no water, but the residual volume in the inner layer is filled with water. A further large fraction of the backbone units also form a diffuse third layer extending a substantial distance into the solution. In a low MW HMPAMs there was preferential adsorption of species with higher mol % of hydrophobe and a tendency to form apparently nonequilibrium structures, which in some cases resulted in more complex structures than the simple one characteristic of the large MW polymers. With the exception of this polymer, the variation of the patterns of surface excess and structure with solution concentration suggested that systems containing hydrophobic units at a level of 0.5, 1, and 2 mol % formed equilibrium or near-equilibrium surface layers at bulk concentrations of 0.01-0.35 wt % for C8 to C16 units. However, higher levels of 4 and 6 mol % of the C12 hydrophobe led to much less regular patterns of adsorption, indicating that equilibration is more difficult once the molar fraction of hydrophobe exceeds 2 mol %. The behavior of the surface tension (ST) over the same concentration range as the NR experiments could be accounted for by the Gibbs equation using the directly measured surface excesses and the incorporation of a low charge on the polymers (about 1 charge per 100 backbone units). The presence of such a charge in PAM can arise from hydrolysis of some amide to carboxylate and was known to be present for one of the polymers. The extra structural information obtained by NR on these HMPAMs combined with more recent measurements of the state of ionization in polyacrylates (PAA) allowed us to reinterpret earlier ST and X-ray reflection results on hydrophobically modified HMPAANa containing a similar level of 1 and 2 mol % C12H25 hydrophobes. The Gibbs equation again accounted quantitatively for the ST behavior by using the correct state of ionization of the polymer. Although the adsorption of hydrophobic groups in HMPAANa is about one-tenth of that for the corresponding HMPAM, the ST drops more quickly to lower values for HMPAANa because of its higher level of dissociation, which increases the magnitude of the slope in the Gibbs plot.

Adsorption at the Air-Water Interface in Biosurfactant-Surfactant Mixtures: Quantitative Analysis of Adsorption in a Five-Component Mixture.

The composition of the air-water adsorbed layer of a quinary mixture consisting of three conventional surfactants, octaethylene glycol monododecyl ether (C12E8), dodecane-6-p-sodium benzene sulfonate (LAS6), and diethylene glycol monododecyl ether sodium sulfate (SLE2S), mixed with two biosurfactants, the rhamnolipids l-rhamnosyl-l-rhamnosyl-ß-hydroxydecanoyl-ß-hydroxydecanoyl, R2, and l-rhamnosyl-ß-hydroxydecanoyl-ß-hydroxydecanoyl, R1, has been measured over a range of compositions above the mixed critical micelle concentration. Additional measurements on some of the subsets of ternary and binary mixtures have also been measured by NR. The results have been analyzed using the pseudophase approximation (PPA) in conjunction with an excess free energy, GE, that depends on the quadratic and cubic terms in the composition. The compositions of the binary, ternary, and quinary mixtures could all be fitted to two sets of interaction parameters between the pairs of surfactants, one for micelles and one for adsorption. No ternary interactions or ternary corrections were required. Because the system contains two strongly anionic surfactants, the PPA can be extended, in practice, to ionic surfactants, contrary to the prevailing view. The values of the interaction parameters show that the quinary mixture, SLE2S-LAS6-C12E8-R1-R2, which is known to be a highly effective surfactant system, is characterized by a sequence of strong surface but weak micellar interactions. About half of the minima in GE for the strong surface interactions occur well away from the regular solution value of 0.5.

Impact of Electrolyte on Adsorption at the Air-Water Interface for Ternary Surfactant Mixtures above the Critical Micelle Concentration.

The composition of the air-water adsorbed layer of the ternary surfactant mixture, octaethylene monododecyl ether, C12E8, sodium dodecyl 6-benzenesulfonate, LAS, and sodium dioxyethylene glycol monododecyl sulfate, SLES, and of each of the binary mixtures, with varying amounts of electrolyte, has been studied by neutron reflectivity. The measurements were made above the mixed critical micelle concentration. In the absence of electrolyte adsorption is dominated by the nonionic component C12E8 but addition of electrolyte gradually changes this so that SLES and LAS dominate at higher electrolyte concentrations. The composition of the adsorbed layer in both binary and ternary mixtures can be quantitatively described using the pseudo-phase approximation with quadratic and cubic interactions in the excess free energy of mixing (GE) at both the surface and in the micelles. A single set of parameters fits all the experimental data. A similar analysis is effective for a mixture in which SDS replaces SLES. Addition of electrolyte weakens the synergistic SLES-C12E8 and LAS-C12E8 interactions, consistent with them being dominated by electrostatic interactions. The SLES-LAS (and SDS-LAS) interaction is moderately strong at the surface and is little affected by addition of electrolyte, suggesting that it is controlled by structural or packing factors. Most of the significant interactions in the mixtures are unsymmetrical with respect to composition, with the minimum in GE at the 1:2 or 2:1 composition. There is a small structural contribution to the LAS-C12E8 interaction that leads to a minimum intermediate in composition between 1:2 and 1:1 (LAS:C12E8) and to a significant residual GE in strong electrolyte.

Adsorption of Methyl Ester Sulfonate at the Air-Water Interface: Can Limitations in the Application of the Gibbs Equation be Overcome by Computer Purification?

We describe a new laboratory synthesis of the α-methyl ester sulfonates based on direct sulfonation of the methyl ester by SO3 introduced from the vapor phase. This was used to synthesize a chain deuterated sample of αC14MES, which was then used to measure the surface excess of αC14MES directly at the air/water interface over a wide range of concentration using neutron reflection. The adsorption isotherm could be fitted to an empirical equation close to a Langmuir isotherm and gave a limiting surface excess of (3.4 ± 0.1) × 10-6 mol m-2 in the absence of added electrolyte. The neutron-measured surface excesses were combined with the integrated Gibbs equation to fit the variation in surface tension with concentration (σ-ln C curve). The fit was exact provided that we used a prefactor consistent with the counterion at the surface being an impurity divalent ion, as has previously been found for sodium diethylhexylsulfosuccinate (aerosol OT or AOT) and various perfluorooctanoates. The critical micelle concentration (CMC) was determined from this fit to be 2.4 ± 0.3 mM in the absence of electrolyte. In the presence of 100 mM NaCl, this contamination was suppressed and the σ-ln C curve could be fitted using the integrated Gibbs equation with the expected prefactor of 1. The new data were used to reinterpret measurements by Danov et al. on an unpurified sample of αC14MES for which computer refinement was used to try to eliminate the effects of the impurities.

Effects of length and hydrophilicity/hydrophobicity of diamines on self-assembly of diamine/SDS gemini-like surfactants.

This work studied gemini-like surfactants formed from anionic surfactant sodium dodecyl sulfate (SDS) and cationic charged bola-type diamines with hydrophilic or hydrophobic spacers of different lengths using surface tension, small angle neutron scattering, isothermal titration microcalorimetry and cryogenic transmission electron microscopy. The critical micelle concentrations (CMC) and the surface tension at CMC (γCMC) for all the diamine/SDS mixtures are markedly lower than that of SDS. The shorter diamines reduce γCMC to a greater extent regardless of the hydrophilicity/hydrophobicity of the diamines. Meanwhile, either the hydrophobic diamine with a longer spacer or the hydrophilic diamine with a shorter spacer is more beneficial to decrease CMC and leads to the transition from spherical micelles into rodlike or wormlike micelles. This is principally because of the formation of gemini-like surfactants by the electrostatic binding between SDS and the diamines, where the electrostatic repulsion between the adjacent headgroups of SDS becomes much weaker due to the electrostatic binding of oppositely charged diamine with SDS, and the longer hydrophobic spacer may also bend into the hydrophobic domain of micelles to promote micellar growth. However, the hydrophilic spacers are more compatible with the headgroup region, leading to micelles with a larger curvature. This work contributes to the understanding of the relationship between the properties of constructed gemini-like surfactants and the natures of connecting molecules, and provides guidance to efficiently improve the performance of surfactants.

Unusual Adsorption at the Air-Water Interface of a Zwitterionic Carboxybetaine with a Large Charge Separation.

The structures of layers of three different dodecylcarboxybetaine surfactants adsorbed at the air-water interface have been determined by neutron reflection. The zwitterionic compounds differed in the length of the spacer separating the quaternary ammonium and carboxylate groups, which was (CH2)1, (CH2)4, or (CH2)8. The limiting area per molecule was found to be 45, 52, or 84 Å(2), respectively, and compared reasonably with results from surface tension showing that the Gibbs prefactor is 1 in each case. Isotopic labeling was used to distinguish between the position of the alkyl and spacer groups in the layer. The spacer was found to be well-immersed in water for the (CH2)1 and (CH2)4 spacers but significantly above water for the (CH2)8 spacer. The distribution of the (CH2)8 spacer along the surface normal was found to be similar to that of the dodecyl group; i.e., it projects out of the water, contrary to an earlier hypothesis that it forms a loop. Comparison of the overlap of water with dodecyl and spacer groups also indicates that the (CH2)8 spacer is well out of the water. This in turn suggests that the anionic carboxylic acid group, which is dissociated in solution, is not ionized in the adsorbed layer. A further observation is that the dodecylcarboxybetaine with the (CH2)8 spacer reaches surface saturation at one-tenth of the critical micelle concentration. This is highly unusual and is attributed to the long spacer destabilizing the micelle relative to the surface layer.

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.

Unusual excess free energies of mixing in mixtures of partially fluorinated and hydrocarbon surfactants at the air-water interface: correlation with the structure of the layer.

The air-water interface of three mixtures of partially fluorinated surfactants and hydrocarbon surfactants, C4F9C11H22N(CH3)3Br (fC4hC11TAB) with hexadecyltrimethylammonium bromide (C16TAB), (CF3)2C3F6C10H20N(CH3)3Br (fC5hC10TAB) with C16TAB, and C8F17C6H12N(CH3)3Br (fC8hC6TAB) with C18TAB, have been investigated using surface tension (ST) and neutron reflection (NR). Using the composition of the layer determined by NR, the pseudophase separation model was used to fit the variation of concentration for a specific ST to a free energy of mixing, G(E), that included adjustable quadratic, cubic and quartic terms. In all three cases, G(E) was found to be highly unsymmetrical, being approximately ideal at low surface fractions of hydrocarbon surfactant and repulsive at high fractions with a maximum value of 0.2-0.3RT. The corresponding structure of the layer was also determined by NR and showed that the initial ideal behavior of G(E) probably results from a balance of a gain in energy from a reduced immersion of the fluorocarbon chain, brought about by screening of the fluorocarbon from water by the hydrocarbon surfactant, and a loss from increased fluorocarbon-hydrocarbon repulsion. At higher concentration, there is no space in the layer for further screening and the fluorocarbon-hydrocarbon repulsion leads to the expected positive G(E). The calculated G(E) also indicated that there should be phase separation of the two components in the interface over a bulk composition range of about 60-95% hydrocarbon surfactant. However, experiment indicates no phase separation. It is suggested that there are a number of possible additional negative contributions to G(E) close to a phase transition, which are not possible for a true bulk phase separation, and which prevent surface phase separation unless it is strongly favored.

Multivalent-Counterion-Induced Surfactant Multilayer Formation at Hydrophobic and Hydrophilic Solid-Solution Interfaces.

Surface multilayer formation from the anionic-nonionic surfactant mixture of sodium dodecyl dioxyethylene sulfate, SLES, and monododecyl dodecaethylene glycol, C12E12, by the addition of multivalent Al(3+) counterions at the solid-solution interface is observed and characterized by neutron reflectivity, NR. The ability to form surface multilayer structures on hydrophobic and hydrophilic silica and cellulose surfaces is demonstrated. The surface multilayer formation is more pronounced and more well developed on the hydrophilic and hydrophobic silica surfaces than on the hydrophilic and hydrophobic cellulose surfaces. The less well developed multilayer formation on the cellulose surfaces is attributed to the greater surface inhomogeneities of the cellulose surface which partially inhibit lateral coherence and growth of the multilayer domains at the surface. The surface multilayer formation is associated with extreme wetting properties and offers the potential for the manipulation of the solid surfaces for enhanced adsorption and control of the wetting behavior.

Adsorption of Hydrophobin-Protein Mixtures at the Air-Water Interface: The Impact of pH and Electrolyte.

The adsorption of the proteins ß-casein, ß-lactoglobulin, and hydrophobin, and the protein mixtures of ß-casein/hydrophobin and ß-lactoglobulin/hydrophobin have been studied at the air-water interface by neutron reflectivity, NR. Changing the solution pH from 7 to 2.6 has relatively little impact on the adsorption of hydrophobin or ß-lactoglobulin, but results in a substantial change in the structure of the adsorbed layer of ß-casein. In ß-lactoglobulin/hydrophobin mixtures, the adsorption is dominated by the hydrophobin adsorption, and is independent of the hydrophobin or ß-lactoglobulin concentration and solution pH. At pH 2.6, the adsorption of the ß-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption over the range of ß-casein concentrations studied. At pH 4 and 7, the adsorption of ß-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption at low ß-casein concentrations. At higher ß-casein concentrations, ß-casein is adsorbed onto the surface monolayer of hydrophobin, and some interpenetration between the two proteins occurs. These results illustrate the importance of pH on the intermolecular interactions between the two proteins at the interface. This is further confirmed by the impact of PBS, phosphate buffered saline, buffer and CaCl2 on the coadsorption and surface structure. The results provide an important insight into the adsorption properties of protein mixtures and their application in foam and emulsion stabilization.

Adsorption at air-water and oil-water interfaces and self-assembly in aqueous solution of ethoxylated polysorbate nonionic surfactants.

The Tween nonionic surfactants are ethoxylated sorbitan esters, which have 20 ethylene oxide groups attached to the sorbitan headgroup and a single alkyl chain, lauryl, palmityl, stearyl, or oleyl. They are an important class of surfactants that are extensively used in emulsion and foam stabilization and in applications associated with foods, cosmetics and pharmaceuticals. A range of ethoxylated polysorbate surfactants, with differing degrees of ethoxylation from 3 to 50 ethylene oxide groups, have been synthesized and characterized by neutron reflection, small-angle neutron scattering, and surface tension. In conjunction with different alkyl chain groups, this provides the opportunity to modify their surface properties, their self-assembly in solution, and their interaction with macromolecules, such as proteins. Adsorption at the air-water and oil-water interfaces and solution self-assembly of the range of ethoxylated polysorbate surfactants synthesized are presented and discussed.

Limitations in the use of surface tension and the Gibbs equation to determine surface excesses of cationic surfactants.

Neutron reflection (NR) and surface tension (ST) are used to show that there are serious limitations in applying the Gibbs equation accurately to ST data of cationic surfactants to obtain the limiting surface excess, Γ(CMC), at the critical micelle concentration (CMC). Nonionic impurities in C12TABr and C16TABr have been eliminated by extensive purification to give ST - ln(concentration) (σ - ln c) curves that are convex with respect to the ln c axis around the CMC, which is characteristic of a finite micellization width. Because NR shows that the surface excess often continues to increase at and above the CMC, this finite width makes it impossible to apply the Gibbs equation to obtain Γ(CMC) without knowledge of the effect of aggregation on the activity. NR data made it possible to apply the integrated Gibbs equation to the ST below the onset of the convex region of the σ - ln c curve and show that for C12TABr the micellization width causes the ST to underestimate Γ(CMC) by 12%. Hexadecyltrimethylammonium (C16TA) sulfate is used to show that divalent ion impurities are not a significant problem. For cationic surfactants, further errors are associated with ST methods that rely on complete wetting. Measurements using ring, plate, and bubble shape analyses indicate that with ring and plate incomplete wetting occurs at or above the CMC and may extend to lower concentrations and also causes the ST-Gibbs analysis to underestimate the surface excess. In combination with ion association and preaggregation in cationic gemini surfactants, this can cause errors as large as 100% in Γ(CMC). Comparison of ellipsometry and NR for C16TAX in 0.1 M KX (X = F or Cl) shows that ellipsometry cannot, as yet, be quantitatively modeled accurately enough for surface excess determination independent of NR calibration.

Sodium dodecyl sulfate-ethoxylated polyethylenimine adsorption at the air-water interface: how the nature of ethoxylation affects the pattern of adsorption.

The strong interaction between ionic surfactants and polyelectrolytes of opposite charge results in enhanced surface adsorption at the air-water interface down to low surfactant concentrations and in some cases in the formation of ordered surface structures. A notable example which exhibits such properties is the mixture of polyethylenimine, PEI, and sodium dodecyl sulfate, SDS. However, the electrostatic interaction, around charge neutralization, between the surfactant and polymer often results in precipitation or coacervation. This can be mitigated for PEI-surfactant mixtures by ethoxylation of the PEI, but this can also result in a weaker surface interaction and a significant reduction in the adsorption. It is shown here that by localizing the ethoxylation of the PEI into discrete regions of the polymer precipitation upon the addition of SDS is suppressed, the strong surface interaction and enhanced adsorption of the polymer-surfactant mixture is retained. The adsorption of SDS in the presence of ethoxylated PEI is greatly enhanced at low SDS concentrations compared to the adsorption for pure SDS. The adsorption is equally pronounced at pH 7 and 10 and is largely independent of the degree of ethoxylation. Surface ordering, more than monolayer adsorption, is observed over a relatively narrow range of SDS concentrations and is most pronounced at pH 10 and for the polymers with the lower degree of ethoxylation. The results show that ethoxylated PEI's reported here provide a suitable route to enhanced surfactant adsorption while retaining favorable solution properties in which precipitation effects are minimized.

Ion specific effects in trivalent counterion induced surface and solution self-assembly of the anionic surfactant sodium polyethylene glycol monododecyl ether sulfate.

The effect of different trivalent counterions, Al(3+), Cr(3+), Sc(3+), Gd(3+), and La(3+), on the surface adsorption and Al(3+), Cr(3+), and Sc(3+) for solution self-assembly of the anionic surfactant sodium polyethylene glycol monododecyl ether sulfate has been studied by neutron reflectivity and small angle neutron scattering. The strong binding and complexation between the trivalent counterions and the anionic surfactant result in significant micellar growth and the formation of surface multilayer structures at the air-water interface at relatively low counterion concentrations. Broadly similar surface and solution behaviors are observed for the different trivalent counterions. The evolution in the surface and solution structures in detail depends upon the nature of the counterion, its hydrated radius and its strength of binding. Exceptionally the addition of Cr(3+) counterions have a less pronounced effect. This is attributed to a greater reluctance for exchange within the primary hydration shell for Cr(3+) ions, which results in a shielding of the electrostatic interactions and a reduced surfactant-counterion binding.

Impact of the degree of ethoxylation of the ethoxylated polysorbate nonionic surfactant on the surface self-assembly of hydrophobin-ethoxylated polysorbate surfactant mixtures.

Neutron reflectivity measurements have been used to study the surface adsorption of the polyethylene sorbitan monostearate surfactant, with degrees of ethoxylation varying from 3 to 20 ethylene oxide groups, with the globular protein hydrophobin. The surface interaction between the ethoxylated polysorbate nonionic surfactants and the hydrophobin results in self-assembly at the air-solution interface in the form of a well-defined layered surface structure. The surface interaction arises from a combination of the hydrophobic interaction between the surfactant alkyl chain and the hydrophobic patch on the surface of the hydrophobin, and the hydrophilic interaction between the ethoxylated sorbitan headgroup and the hydrophilic regions on the surface of the hydrophobin. The results presented show that varying the degree of ethoxylation of the polysorbate surfactant changes the interaction between the surfactant and the hydrophobin and the packing, and hence the evolution in the resulting surface structure. The optimal degree of ethoxylation for multilayer formation is over a broad range, from of order 6 to 17 ethylene oxide groups, and for degrees of ethoxylation of 3 and 20 only monolayer adsorption of either the surfactant or the hydrophobin is observed.

The Gibbs and Butler Equations and the Surface Activity of Dilute Aqueous Solutions of Strong and Weak Linear Polyelectrolyte-Surfactant Mixtures: The Roles of Surface Composition and Polydispersity.

In a previous paper, we applied a combination of direct measurements of both surface tension and surface excess in conjunction with the Gibbs equation to explain features of the adsorption and surface tension of mixtures of surfactants and strong linear polyelectrolytes at the air-water interface. This paper extends that model by including (i) the restrictions of the Butler equation for the behavior of the surface tension of mixed systems and (ii) the surface behavior of surfactant and linear weak polyelectrolyte mixtures, for which the inclusion of measurements of the surface excess and composition is shown to be particularly important. In addition, a closer examination of earlier data at higher concentrations provides evidence that the surface layering that is often observed in polyelectrolyte-surfactant systems is also an average equilibrium phenomenon and is driven by particular aggregation patterns that occur in some systems and not in others. Although the successful application of the Gibbs and Butler equations indicates that strong polyelectrolyte-surfactant systems can be described in terms of an average equilibrium over wide ranges of concentration, we have identified two concentration ranges where polydispersity in either polyelectrolyte molecular weight or composition results in significant time dependence of the surface behavior.

The formation of surface multilayers at the air-water interface from sodium diethylene glycol monoalkyl ether sulfate/AlCl3 solutions: the role of the alkyl chain length.

The influence of the alkyl chain length on surface multilayer formation at the air-water interface for the anionic surfactant sodium diethylene glycol monoalkyl ether sulfate, SAE2S, in the presence of Al(3+) multivalent counterions, in the form of AlCl3, is described. In the absence of electrolyte, the saturated monolayer adsorption is determined by the headgroup geometry and is independent of the alkyl chain length. In the presence of Al(3+) counterions, surface multilayer formation occurs, due to the strong SAE2S/Al(3+) binding and complexation. The neutron reflection data show that the alkyl chain length of the surfactant has a significant impact upon the evolution of the surface multilayer structure with surfactant and AlCl3 concentration. Increasing the alkyl chain length from decyl to tetradecyl results in the surface multilayer formation occurring at lower surfactant and AlCl3 concentrations. At the short alkyl chain lengths, decyl and dodecyl, the regions of multilayer formation with a small number of bilayers are increasingly extended with decreasing alkyl chain length. For the alkyl chain lengths of tetradecyl and hexadecyl, the surface behavior is further affected by decreases in the surfactant solubility in the presence of AlCl3, and this ultimately dominates the surface behavior at the longer alkyl chain lengths.