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.

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.

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 (Γ), the limiting surface tension (σ), 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.

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 (CPS) and dodecyldimethylammonium carboxybetaine (CCB), and of their mixtures with the cationic dodecyltrimethylammonium bromide (CTAB) 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.

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-CD hydrophobic group with = 8, 12, and 16 at levels of 0.5, 1, 2, 4, and 6 mol %.

Counterion Condensation, the Gibbs Equation, and Surfactant Binding: An Integrated Description of the Behavior of Polyelectrolytes and Their Mixtures with Surfactants at the Air-Water Interface.

By applying the Gibbs equation to the bulk binding isotherms and surface composition of the air-water (A-W) interface in polyelectrolyte-surfactant (PE-S) systems, we show that their surface behavior can be explained semiquantitatively in terms of four concentration regions, which we label as A, B, C, and D. In the lowest-concentration range A, there are no bound PE-S complexes in the bulk but there may be adsorption of PE-S complexes at the surface. When significant adsorption occurs in this region, the surface tension (ST) drops with increasing concentration like a simple surfactant solution.

The structure of alkyl ester sulfonate surfactant micelles: The impact of different valence electrolytes and surfactant structure on micelle growth.

The ester sulfonate anionic surfactants are a potentially valuable class of sustainable surfactants. The micellar growth, associated rheological changes, and the onset of precipitation are important consequences of the addition of electrolyte and especially multi-valent electrolytes in anionic surfactants. Small angle neutron scattering, SANS, has been used to investigate the self-assembly and the impact of different valence electrolytes on the self-assembly of a range of ester sulfonate surfactants with subtly different molecular structures.

Impact of molecular structure, headgroup and alkyl chain geometry, on the adsorption of the anionic ester sulfonate surfactants at the air-solution interface, in the presence and absence of electrolyte.

The transition from monolayer to multilayer adsorption at the air-water interface in the presence of multivalent counterions has been demonstrated for a limited range of anionic surfactants which exhibit increased tolerance to precipitation in the presence of multivalent counterions. Understanding the role of molecular structure in determining the transition to surface ordering is an important aspect of the phenomenon. The focus of the paper is on the alkyl ester sulfonate, AES, surfactants; a promising group of anionic surfactants, with the potential for improved performance and biocompatibility.

The role of competitive counterion adsorption on the electrolyte induced surface ordering in methyl ester sulfonate surfactants at the air-water interface.

The strong binding of Al trivalent counterions to the anionic surfactants sodium polyethylene glycol monoalkyl ether sulfate and α-methyl ester sulfonate results in surface multilayer formation at the air-water interface. In contrast the divalent and monovalent counterions Ca and Na result only in monolayer adsorption. Competitive counterion adsorption has been extensively studied in the context of surfactant precipitation and re-dissolution, but remains an important feature in understanding this surface ordering and how it can be manipulated.

Adsorption and self-assembly in methyl ester sulfonate surfactants, their eutectic mixtures and the role of electrolyte.

The α-methyl ester sulfonate, MES, anionic surfactants are a potentially important class of sustainable surfactants for a wide range of applications. The eutectic-like Kraft point minimum in the C and C-MES mixtures is an important feature of that potential. Understanding their individual adsorption properties and the surface mixing of the eutectic mixtures are key to their wider exploitation.

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 (γ) for all the diamine/SDS mixtures are markedly lower than that of SDS. The shorter diamines reduce γ to a greater extent regardless of the hydrophilicity/hydrophobicity of the diamines.

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 (CE), dodecane-6-p-sodium benzene sulfonate (LAS6), and diethylene glycol monododecyl ether sodium sulfate (SLES), mixed with two biosurfactants, the rhamnolipids l-rhamnosyl-l-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoyl, R2, and l-rhamnosyl-β-hydroxydecanoyl-β-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, G, that depends on the quadratic and cubic terms in the composition.

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 SO introduced from the vapor phase. This was used to synthesize a chain deuterated sample of αCMES, which was then used to measure the surface excess of αCMES 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.

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, CE, 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 CE but addition of electrolyte gradually changes this so that SLES and LAS dominate at higher electrolyte concentrations.

Surface Adsorption in Ternary Surfactant Mixtures above the Critical Micelle Concentration: Effects of Asymmetry on the Composition Dependence of the Excess Free Energy.

The composition of the adsorbed layer of a ternary surfactant mixture at the air-water interface has been studied by neutron reflectivity. The adsorption of the ternary mixture of octaethylene monododecyl ether (CE) sodium dodecyl 6-benzene sulfonate (LAS), and sodium dioxyethylene glycol monododecyl sulfate (SLES), as well as each of the binary mixtures, at solution concentrations greater than the mixed critical micelle concentration is highly nonideal. In the ternary mixture, the surface adsorption is dominated by CE and LAS, and there is little SLES at the interface.

Analysis of the Asymmetric Synergy in the Adsorption of Zwitterionic-Ionic Surfactant Mixtures at the Air-Water Interface below and above the Critical Micelle Concentration.

Surface tension (ST) and neutron reflection (NR) measurements have been made on a series of mixtures of two ionic surfactants, one anionic (SDS) and one cationic (C12TAB), with the two zwitterionic surfactants dodecyldimethylammonium propanesulfonate (C12SB) and dodecyldimethylammonium acetate C12CB. The anionic surfactant SDS interacts equally strongly with both zwitterionics and the C12TAB less strongly. For the SDS-C12SB mixtures simultaneous fitting of ST and NR data made it possible to use the pseudophase approximation with an expansion of the excess free energy, GE, up to and including the quartic term.

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.

Manipulating perfume delivery to the interface using polymer-surfactant interactions.

Enhanced delivery of perfumes to interfaces is an important element of their effectiveness in a range of home and personal care products. The role of polyelectrolyte-surfactant mixtures to promote perfume adsorption at interfaces is explored here. Neutron reflectivity, NR, was used to quantify the adsorption of the model perfumes phenylethanol, PE, and linalool, LL, at the air-water interface in the presence of the anionic surfactant sodium dodecylsulfate, SDS, and the cationic polyelectrolytes, poly(dimethyldiallyl ammonium chloride), polydmdaac, and poly(ethyleneimine), PEI.

Anionic surfactant - Biogenic amine interactions: The role of surfactant headgroup geometry.

Oligoamines and biogenic amines (naturally occurring oligoamines) are small flexible polycations. They interact strongly with anionic surfactants such as sodium dodecyl sulfate, SDS. This results in enhanced adsorption and the formation of layered structures and the formation of layered structures at the air-water interface which depends on surfactant concentration and solution pH.

Impact of biogenic amine molecular weight and structure on surfactant adsorption at the air-water interface.

The oligoamines, such as ethylenediamine to pentaethylenetetramine, and the aliphatic biogenic amines, such as putrescine, spermidine and spermine, strongly interact with anionic surfactants, such as sodium dodecylsulfate, SDS. It has been shown that this results in pronounced surfactant adsorption at the air-water interface and the transition from monolayer to multilayer adsorption which depends upon solution pH and oligoamine structure. In the neutron reflectivity, NR, and surface tension, ST, results presented here the role of the oligoamine structure on the adsorption of SDS is investigated more fully using a range of different biogenic amines.

Enhanced perfume surface delivery to interfaces using surfactant surface multilayer structures.

Enhanced surface delivery and retention of perfumes at interfaces are the keys to their more effective and efficient deployment in a wide range of home and personal care related formulations. It has been previously demonstrated that the addition of multivalent counterions, notably Ca(2+), induces multilayer adsorption at the air-water interface for the anionic surfactant, sodium dodecyl-6-benzenesulfonate, LAS-6. Neutron reflectivity, NR, measurements are reported here which demonstrate that such surfactant surface multilayer structures are a potentially promising vehicle for enhanced delivery of perfumes to interfaces.

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

The adsorption of the proteins β-casein, β-lactoglobulin, and hydrophobin, and the protein mixtures of β-casein/hydrophobin and β-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 β-lactoglobulin, but results in a substantial change in the structure of the adsorbed layer of β-casein.

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.

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.

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.