Structural features of interfacially adsorbed acyl-l-carnitines.

HYPOTHESIS: Acyl-l-carnitines (CnLCs) are potentially important as biosurfactants in drug delivery and tissue engineering due to their good biocompatibility. However, little is currently known about the basic interfacial behavior underlying their technological applications. Following our previous characterization of their solution aggregation and adsorption at the air/water interface, this work examines how they adsorb at the hydrophilic solid/liquid interface. EXPERIMENTS: As the SiO2/water interface has served as the model substrate for many interfacial adsorption studies, so it has been used in this work as the solid substrate to facilitate dynamic adsorption by spectroscopic ellipsometry (SE) and structural determination of the adsorbed layers by neutron reflection (NR) under different conditions at the SiO2/water interface from a group of CnLC (n = 12, 14, and 16). FINDINGS: CnLC surfactants are zwitterionic at neutral pH. They reached saturated adsorption above their critical micellar concentrations (CMCs) and formed a sandwich bilayer with a head-tail-head structure at the hydrophilic SiO2/water interface. The total thicknesses of the adsorbed layers at CMC were found to be 33 ± 2, 35 ± 2, and 37 ± 2 Å for C12LC, C14LC, and C16LC, respectively, with their inner and outer head layers remaining similar but the thickness of the interdigitated middle layer increasing with acyl chain length. As the solution becomes acidic, the carboxyl groups become protonated and the l-carnitine heads are net positively charged, resulting in increased repulsion between the head groups. In this situation, the CnLC surfactants are adsorbed as distinct aggregates to reduce repulsive interaction, resulting in reduced surfactant volume fraction and layer thickness. However, a high ionic strength can screen the repulsive interaction and enhance the adsorbed amount, effectively diminishing the impact of pH. This information provides a useful basis for exploring the technological applications of CnLCs involving a solid substrate.

In-Membrane Nanostructuring of Cationic Amphiphiles Affects Their Antimicrobial Efficacy and Cytotoxicity: A Comparison Study between a De Novo Antimicrobial Lipopeptide and Traditional Biocides.

Cationic biocides have been widely used as active ingredients in personal care and healthcare products for infection control and wound treatment for a long time, but there are concerns over their cytotoxicity and antimicrobial resistance. Designed lipopeptides are potential candidates for alleviating these issues because of their mildness to mammalian host cells and their high efficacy against pathogenic microbial membranes. In this study, antimicrobial and cytotoxic properties of a de novo designed lipopeptide, CH3(CH2)12CO-Lys-Lys-Gly-Gly-Ile-Ile-NH2 (C14KKGGII), were assessed against that of two traditional cationic biocides CnTAB (n = 12 and 14), with different critical aggregation concentrations (CACs). C14KKGGII was shown to be more potent against both bacteria and fungi but milder to fibroblast host cells than the two biocides. Biophysical measurements mimicking the main features of microbial and host cell membranes were obtained for both lipid monolayer models using neutron reflection and small unilamellar vesicles (SUVs) using fluorescein leakage and zeta potential changes. The results revealed selective binding to anionic lipid membranes from the lipopeptide and in-membrane nanostructuring that is distinctly different from the co-assembly of the conventional CnTAB. Furthermore, CnTAB binding to the model membranes showed low selectivity, and its high cytotoxicity could be attributed to both membrane lysis and chemical toxicity. This work demonstrates the advantages of the lipopeptides and their potential for further development toward clinical application.

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.

Strong synergistic interactions in zwitterionic-anionic surfactant mixtures at the air-water interface and in micelles: The role of steric and electrostatic interactions.

HYPOTHESIS: The milder interaction with biosystems makes the zwitterionic surfactants an important class of surfactants, and they are widely used in biological applications and in personal care formulations. An important aspect of those applications is their strong synergistic interaction with anionic surfactants. It is anticipated that the strong interaction will significantly affect the adsorption and self-assembly properties. EXPERIMENTS: Surface tension, ST, neutron reflectivity, NR, and small angle neutron scattering, SANS, have been used here to explore the synergistic mixing in micelles and at the air-water interface for the zwitterionic surfactant, dodecyldimethylammonium propanesulfonate, C12SB, and the anionic surfactants, alkyl ester sulfonate, AES, in the absence and presence of electrolyte, 0.1 M NaCl. FINDINGS: At the air-water interface the asymmetry of composition in the strong synergistic interaction and the changes with added electrolyte and anionic surfactant structure reflect the relative contributions of the electrostatic and steric interactions to the excess free energy of mixing. In the mixed micelles the synergy is less pronounced and indicates less severe packing constraints. The micelle structure is predominantly globular to elongated, and shows a pronounced micellar growth with composition which depends strongly upon the nature of the anionic surfactant and the addition of electrolyte.

How do chain lengths of acyl-l-carnitines affect their surface adsorption and solution aggregation?

HYPOTHESIS: l-carnitines in our body systems can be readily converted into acyl-l-carnitines which have a prominent place in cellular energy generation by supporting the transport of long-chain fatty acids into mitochondria. As biocompatible surfactants, acyl-l-carnitines have potential to be useful in technical, personal care and healthcare applications. However, the lack of understanding of the effects of their molecular structures on their physical properties has constrained their potential use. EXPERIMENTS: This work reports the study of the influence of the acyl chain lengths of acyl-l-carnitines (CnLC) on solubility, surface adsorption and aggregation. Critical micellar concentrations (CMCs) of CnLC were determined by surface tension measurements. Neutron reflection (NR) was used to further examine the structure and composition of the adsorbed CnLC layer. The structural changes of the micellar aggregates under different concentrations of CnLC, pH and ionic strength were determined by dynamic light scattering (DLS) and small angle neutron scattering (SANS). FINDINGS: C12LC is fully soluble over a wide temperature and concentration range. There is however a strong decline of solubility with increasing acyl chain length. The adsorption and aggregation behavior of C14LC was therefore studied at 30 °C and C16LC at 45 °C. The solubility boundaries displayed distinct hysteresis with respect to heating and cooling. The CMCs of C12LC, C14LC and C16LC at pH 7 were 1.1 ± 0.1, 0.10 ± 0.02 and 0.010 ± 0.005 mM, respectively, with the limiting values of the area per molecule at the CMC being 45.4 ± 2, 47.5 ± 2 and 48.8 ± 2 Å2 and the thicknesses of the adsorbed CnLC layers at the air/water interface increasing from 21.5 ± 2 to 22.6 ± 2 to 24.2 ± 2 Å, respectively. All three surfactants formed core-shell spherical micelles with comparable dimensional parameters apart from an increase in core radius with acyl chain length. This study outlines the effects of acyl chain length on the physicochemical properties of CnLCs under different environmental conditions, serving as a useful basis for developing their potential applications.

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.

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.

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.

Surface adsorption and solution aggregation of a novel lauroyl-l-carnitine surfactant.

HYPOTHESIS: l-carnitine plays a crucial role in the cellular production of energy by transporting fatty acids into mitochondria. Acylated l-carnitines are amphiphilic and if appropriate physical properties were demonstrated, they could replace many currently used surfactants with improved biocompatibility and health benefits. EXPERIMENTS: This work evaluated the surface adsorption of lauroyl-l-carnitine (C12LC) and its aggregation behavior. The size and shape of the aggregates of C12LC surfactant were studied at different temperatures, concentrations, pH and ionic strength by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Surface tension measurements were carried out to determine the critical micellar concentration (CMC) of C12LC. Combining with the Gibbs equation, the surface excess at different concentrations could be determined. Neutron reflection (NR) was used to determine the structure of the adsorbed layer at the air/water interface with the help of isotopic contrast variations. FINDINGS: At pH 7, the limiting area per molecule (ACMC) of the zwitterionic C12LC adsorbed layer at the air/water interface was found to be 46 Å2 from surface tension and neutron reflection, smaller than the values of C12PC, C12E5, DTAB, C12C4betaine and C12C8betaine but close to that of SDS. A pronounced surface tension minimum at pH 2 at the low ionic strength was linked to a minimum value of area per molecule of about 30 Å2, indicating the competitive adsorption from traces of lauric acid produced by hydrolysis of C12LC. As the concentration increased, area per molecule reached a plateau of 37-39 Å2, indicating the dissolution of the more surface-active lauric acid into the micelles of C12LC. DLS and SANS showed that the size and shape of micelles had little response to temperature, concentration, ionic strength or pH. The SANS profiles measured under 3 isotopic contrasts could be well fitted by the core-shell model, giving a spherical core radius of 15.7 Å and a shell thickness of 10.5 Å. The decrease of pH led to more protonated carboxyl groups and more positively charged micelles, but the micellar structures remained unchanged, in spite of their stronger interaction. These features make C12LC potentially attractive as a solubilizing agent.

α-Sulfo alkyl ester surfactants: Impact of changing the alkyl chain length on the adsorption, mixing properties and response to electrolytes of the tetradecanoate.

HYPOTHESIS: The α-sulfo alkyl ester, AES, surfactants are a class of anionic surfactants which have potential for improved sustainable performance in a range of applications, and an important feature is their enhanced tolerance to precipitation in the presence of multivalent counterions. It is proposed that their adsorption properties can be adjusted substantially by changing the length of the shorter alkyl chain, that of the alkanol group in the ester. EXPERIMENTS: Surface tension and neutron reflectivity have been used to investigate the variation in the adsorption properties with the shorter alkyl chain length (methyl, ethyl and propyl), the impact of NaCl on the adsorption, the tendency to form surface multilayer structures in the presence of AlCl3, and the effects of mixing the methyl ester sulfonate with the ethyl and propyl ester sulfonates on the adsorption. FINDINGS: The variations in the critical micelle concentration, CMC, the adsorption isotherms, the saturation adsorption values, and the impact of NaCl illustrate the subtle influence of varying the shorter alkyl chain length of the surfactant. The non-ideal mixing of pairs of AES surfactants with different alkanol group lengths of the ester show that the extent of the non-ideality changes as the difference in the alkanol length increases. The surface multilayer formation observed in the presence of AlCl3 varies in a complex manner with the length of the short chain and for mixtures of surfactants with different chains lengths.

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.

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. Region B extends from the onset of bulk PE-S binding to the end of cooperative binding, in which the slow variation of surfactant activity with cooperative binding means that the ST changes relatively little, although adsorption may be significant. This leads to an approximate plateau, which may be at high or low ST. Region C starts where the binding in the bulk complex loses its cooperativity leading to a rapid change of surfactant activity with the total concentration. This, combined with significant adsorption, often leads to a sharp drop in ST. Region D is where precipitation and redissolution of the bulk PE-S complex occur. ST peaks may arise in region D because of loss of the solution complex that matches the value of the preferred surface stoichiometry, which seems to have a well-defined value for each system. The analysis is applied to the experimental systems, sodium polystyrene sulfonate-alkyltrimethylammonium bromides and poly(diallyldimethyl chloride)-sodium alkyl sulfates, with and without the added electrolyte, and includes data from bulk binding isotherms, phase diagrams, aggregation behavior, and direct measurements of the surface excess and stoichiometry of the surface. The successful fits of the Gibbs equation to the data confirm that the surfaces in these systems are largely equilibrated.

Multivalent electrolyte induced surface ordering and solution self-assembly in anionic surfactant mixtures: Sodium dodecyl sulfate and sodium diethylene glycol monododecyl sulfate.

The formation of surface multilayer structures, with the addition of multivalent electrolytes, has been observed in a range of different anionic surfactants; and notably the sodium oxyethylene glycol alkyl sulfate, SAES, and alkyl ester sulfonate, AES, surfactants. The addition of increasing amounts of AlCl3 results in increasing surface layering, with a transition from monolayer to bilayer to ultimately more extended multilayer structures at the interface. The headgroup structures of these SAES and AES surfactants and their hydrophilic / hydrophobic balance give a degree of tolerance to the precipitation induced by multivalent counterions. This was considered to be important factor associated with the multivalent counterion induced surface layering. In this paper the impact of sodium dodecyl sulfate, SDS, an anionic surfactant more susceptible to precipitation in the presence of multivalent counterions, on the surface multilayer formation and solution self-assembly of sodium diethylene glycol monododecyl sulfate, SLES, is explored using surface tension, neutron reflectivity and small angle neutron scattering. The results show that SDS exhibits a similar progressive evolution in surface structures with increasing AlCl3 concentrations, as observed in SLES and related SAES surfactants, and in MES, sodium methyl ester dodecyl sulfonate surfactant. However in the SLES / SDS mixtures the structural evolution is different, and more complex pattern with increasing AlCl3 concentration is observed. The initial transition from monolayer to bilayer / trilayer structures exists, but the surface at higher AlCl3 concentration reverts to monolayer adsorption before extended multilayer structures are formed. Complementary small angle neutron scattering measurements indicate a more complex evolution in the micelle structure which broadly correlates with the surface behaviour. The results illustrate how subtle changes in headgroup structure and packing affect relative counterion binding and hence the surface and solution structures. The results reinforce and extend the observations of related structures on different SAES and AES surfactants, and highlight the opportunity for manipulating surface adsorption behaviour with surfactant mixtures.

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. The results show that in the absence of electrolyte small globular micelles form, and in the presence of NaCl, and AlCl3 relatively modest micellar growth occurs before the onset of precipitation. The micellar growth is more pronounced for the longer unbranched and branched alkyl chain lengths. Whereas changing the headgroup geometry from methyl ester to ethyl ester has in general a less profound impact. The study highlights the importance of relative counterion binding strengths and shows how the surfactant structure affects the counterion binding and hence the micelle structure. The results have important consequences for the response of such surfactants to different operational environments.

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.

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. Neutron reflectivity measurements were made in aqueous solution and in the presence of NaCl, CaCl2, MgCl2 and AlCl3, for a range of alkyl ester sulfonate surfactants, in which the headgroup and alkyl chain geometries were manipulated. In the regions of monolayer adsorption changing the AES headgroup and alkyl chain geometries results in an increased saturation adsorption and in a more gradual decrease in the adsorption at low concentrations, consistent with a greater adsorption efficiency. Changing the AES headgroup and alkyl chain geometries also results in changes in the transition from monolayer adsorption to more ordered surface structures with the addition of AlCl3 and mixed multivalent electrolytes. A more limited surface layering is observed for the ethyl ester sulfonate, EES, with a C14 alkyl chain. Replacing the C14 alkyl chain with a C18 isostearic chain results in only monolayer adsorption. The results demonstrate the role and importance of the surfactant molecular structure in determining the nature of the surface adsorption in the presence of different electrolytes, and in the tendency to form extended surface multilayer structures.

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 Al3+ 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 Ca2+ 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. The α-methyl ester sulfonate surfactants are a promising class of anionic surfactants which have much potential for improved performance in many applications, greater tolerance to extreme solvent conditions such as water hardness, biocompatibility and sustainable production. Hence in this study we have used neutron reflectivity to extend previous studies on the surface ordering of the α-methyl ester sulfonate surfactant, sodium tetradecanoic 2-sulfo 1-methyl ester, in the presence of electrolyte to investigate the role of binary mixtures of electrolytes, AlCl3/CaCl2, and AlCl3/MgCl2. In the mixed electrolytes the evolution of the surface structure, from monolayer to multilayer with increasing AlCl3 concentration, is observed. It is broadly similar to that reported for the addition of only AlCl3. However with increasing CaCl2 concentration the structural evolution is shifted progressively to higher AlCl3 concentrations. Similar observations occur for the AlCl3/MgCl2 mixtures. However the presence of the MgCl2 results in an additional phenomenon; the partial co-adsorption of a more compact lamellar structure which exists until the highest AlCl3 concentrations. The results demonstrate the importance of the competitive adsorption of different counterions in driving and controlling the formation of surface multilayer structures with anionic surfactants. Furthermore it offers a facile route to the manipulation of these surface structures.

Markov Chain Modeling of Surfactant Critical Micelle Concentration and Surface Composition.

A Markov chain (MC) model has been used to model the following binary surfactant mixtures: linear alkylbenzenesulfonate (LAS4)/octaethylene glycol monododecyl ether (C12E8) at 10 and 25 °C, LAS6/acidic sophorolipid (AS), C12Betaine/C12Maltoside, sodium lauryl ether sulfate (SLES2)/C12E8, and rhamnolipid (R1)/LAS6. The critical micellar concentration and the composition of the adsorbed layer, for each system, can be modeled using the same monomer reactivity ratio values, g1 and g2. This implies that the interactions between the surfactants in the bulk solution and at the interface are the same, within error. For the LAS4/C12E8 system at 25 °C, the ranges of g1 and g2 values which can model both sets of data are within 0.03-0.05 and 1.55-2.10, respectively; g1 ≪ g2 implies that C12E8 is significantly more surface active than LAS4. The MC model indicates a negative change in the free energy upon mixing for all of the surfactant systems, consistent with the literature. The interfacial mixing behavior of LAS4/SLES2 is inferred from the results of the MC analysis of the LAS4/C12E8 and SLES2/C12E8 systems, which share a common surfactant partner in C12E8, and the prediction is in line with the published data.

Thermodynamics of the Air-Water Interface of Mixtures of Surfactants with Polyelectrolytes, Oligoelectrolytes, and Multivalent Metal Electrolytes.

A full comparison of results from binding isotherms and surface tension (ST) measurements on polyelectrolyte (PE)-surfactant (S) mixtures, especially the polymer dependence, shows up clear distinctions between the behavior of two representative PE-S systems, poly(sodium styrenesulfonate) (NaPSS) with dodecyltrimethylammonium bromide (C12TAB) and poly(dimethyldiallylammonium chloride) (PDMDAAC) with sodium dodecylsulfate (SDS) in 100 mM NaCl. The surfactant-monomer binding constant in NaPSS-C12TAB is an order of magnitude greater than that in the PDMDAAC-SDS-NaCl system. This results in the ST behavior being dominated largely by non-cooperativity in the former and by cooperativity in the latter. This leads to the ST in PDMDAAC-SDS-NaCl being at its lowest when the average bound fraction is low but increasing rapidly as saturation approaches. A full analysis is also given of how this is altered in the mixture of PDMDAAC-SDS-NaCl with the nonionic surfactant hexethylene glycol monododecyl ether. In contrast, the much stronger interaction in NaPSS-C12TAB leads to high ST complexes at low bound fractions, whereas the lowest ST occurs near the maximum bound fraction, close to or at precipitation. In the PDMDAAC-SDS-NaCl system, the ST drops to a low value at low surfactant concentrations but then increases to a high value just before precipitation occurs, which, combined with increasing surface adsorption of the free surfactant, results in a sharp ST peak. In the NaPSS-C12TAB system, the ST does not drop until the system is at or close to precipitation and it then stays on a plateau until the point at which free surfactant takes over. Thermodynamics does not allow large step changes in the ST, and it is suggested that where these have been observed they are the result of self-depletion of PE or PE-S complexes on the finely divided precipitate of the complex and are therefore not representative of full equilibrium.

Temperature Resistant Binary SLES/Nonionic Surfactant Mixtures at the Air/Water Interface.

Surface compositions of adsorbed monolayers at the air/water interface, formed from binary surfactant mixtures in equilibrium, have been studied using neutron reflectivity at three discrete temperatures: 10, 25, and 40 °C. The binary compositions studied are sodium lauryl dodecyl ether sulfate (SLES EO3)/C12E n, where n = 6 and 8, at a fixed concentration of 2 mM with and without the addition of 0.1 M NaCl. Without NaCl, the nonionic surfactant dominates at the interface and nonideal mixing behavior is observed. This is modeled using the pseudophase approximation with a quadratic expansion of the free energy of mixing. The addition of 0.1 M NaCl screens the charge interaction between the surfactants and drives the surface composition of each system closer to that of the bulk composition. However, model fits to both the micelles and surface layers suggest that nonideal mixing is still taking place, although it is difficult to establish the extent of nonideality due to the limited data quality. The effect of temperature changes on the surface adsorption and composition of the surfactant mixtures is minimal and within error, with and without NaCl, but the critical micelle concentrations are significantly affected. This indicates the dominant influence of steric hindrances and surfactant charge interactions in determining interfacial behavior for these surfactants, relative to the temperature changes. The study also highlights the delicate effect of a relatively small change in the number of EO groups on mixing behavior.