Small angle neutron scattering and small angle X-ray scattering methods were used alongside droplet size analysis to determine the nanoscale effects of the addition of stearate salts of sodium, calcium and magnesium at the interface of a 30:70 oil : water emulsion. The dispersed phase consisted of hydrophobic dodecanoic acid : menthol deep eutectic solvent with up to 2.5 wt.% stearate salt. The continuous phase consisted of an aqueous solution of non-ionic, poloxamer surfactant. The scattering data shows the presence of sharp smooth interfaces, indicated by the Q-4 scattering decay, while undulations in the scattering in the Porod region indicate the presence of aggregated surfactant with a lamellar structure at the interface of the two phases. The nanoscale structural characteristics, such as layer thickness and d-spacing of these surfactant assemblies are modified by the presence of stearate salts. This effect is demonstrated especially with sodium stearate, which improves the emulsion storage stability and reduces the polydispersity of the emulsion droplets. This increase in stability is due to the ability of sodium salts to alter the surfactant structure and promote the formation of gel networks on the emulsion interface.

Quaternary ammonium compounds (QACs) combined with nonionic surfactants have been among the most effective disinfectants for over half a century, leveraging QACs' broad-spectrum antimicrobial activity that targets microbial membranes. However, the specific interactions between QACs and microbial membranes, as well as the role of nonionic surfactants in disinfection, remain unclear. This study investigates these mechanisms using two representative surfactants: the cationic didecyldimethyl ammonium chloride (DDAC) and the nonionic hexaethylene glycol monododecyl ether (C12E6). The antimicrobial activity of these agents, individually and sequentially, was assessed against Gram-negative bacteria through a series of in vitro assays, including outer membrane (OM) permeability, inner membrane (IM) depolarization, and live/dead bacterial imaging. Further insights into membrane interactions were obtained using model lipid bilayers in conjunction with antimicrobial efficacy matrices, FICI (fractional inhibition concentration index), fluorescent liposome leakage, small-angle neutron scattering (SANS), and neutron reflectivity (NR). Results indicate that C12E6 binds to the rough A lipopolysaccharide (RaLPS) head region in the OM, reassembling it into heterogeneous aggregates but with limited penetration to cause IM disruption. Conversely, DDAC induced structural disruptions in both OM and IM, resulting in low inhibitory concentrations and rapid bacterial killing. In mixtures, the C12E6 : DDAC ratio significantly influences antimicrobial efficacy, with higher C12E6 levels inhibiting DDAC's effective membrane interactions.

Small angle neutron scattering (SANS) has been employed to examine the self-assembly behaviour of sodium 5-tetradecyl furan-2-sulfonate (STFS), an emerging anionic surfactant, in bulk dilute solutions. STFS was also studied in binary and ternary mixtures with a common non-ionic (hexaethylene glycol monododecyl ether, C12E6) and anionic (sodium dodecyl sulfate, SDS) surfactant to explore the shape, size and composition of the mixed systems in dilute solution. SANS confirmed that STFS alone formed prolate ellipsoidal micelles and transitioned to small polydisperse spherical core-shell micelles of 55 ± 5 Å diameter for mixed systems. Anionic/non-ionic STFS/C12E6 mixtures exhibited strong synergistic interactions, with a significant reduction in critical micelle concentration (CMC) and preferential incorporation of C12E6 into the micelle compared to bulk solution molar ratios. In contrast, STFS/SDS mixtures showed weaker synergism, with minimal SDS incorporation attributed to electrostatic repulsions. Ternary systems displayed intermediate behaviour, with STFS and C12E6 dominating micelle formation while SDS remained mostly excluded. At higher concentrations (10, 20 mM) in SANS measurements the STFS/C12E6 tended towards more bulk solution molar ratio mixing behaviour whilst SDS stayed minimally incorporated into the micelle, regardless of concentration.

HYPOTHESIS:Cationic surfactants have a wide range of applications, often associated with their affinity for a range of solid surfaces and their anti-microbial properties. Manipulating their adsorption and self-assembly properties is key to most applications, and this is commonly achieved through surfactant mixtures or manipulating their headgroup or alkyl chain structure. Achieving this through adjustments to their headgroup structure is less common in cationic surfactants than in anionic surfactants. Ethoxylation provides the ability to adjust the hydrophilic / hydrophobic balance, as extensively demonstrated in a range of anionic surfactants. EXPERIMENTS:This same approach has been applied here to a range of ethoxylated cationic surfactants in the form of the quaternary ammonium salts, and their tertiary nonionic equivalents before quaternisation. Their adsorption and self-assembly properties are investigated using predominantly the neutron scattering techniques of neutron reflectivity, NR, and small angle neutron scattering, SANS. FINDINGS:The trends in the adsorption at the air-water interface and the self-assembly in aqueous solution demonstrate how the hydrophilic / hydrophobic balance can be adjusted by varying the degree of ethoxylation and the alkyl chain length, and illustrate the degree of interdependence of the different structural changes. The variation in the adsorption and the micelle structure shows how the surfactant conformation / packing changes as the degree of ethoxylation and alkyl chain length increases and how the introduction of charge induces further changes.

Sphingosine, an amphiphilic molecule, plays a pivotal role as the core structure of sphingolipids, essential constituents of cell membranes. Its unique capability to enhance the permeability of lipid membranes profoundly influences crucial life processes. The molecular structure of sphingosine dictates its mode of entry into lipid bilayers and governs its interactions with lipids, thereby determining membrane permeability. However, the incomplete elucidation of the relationship between the molecular structure of sphingosine and the permeability of lipid membranes persists due to challenges associated with synthesizing sphingosine molecules. A series of sphingosine-derived molecules, featuring diverse hydrophobic chain lengths and distinct headgroup structure, were meticulously designed and successfully synthesized. These molecules were employed to investigate the permeability of large unilamellar vesicles, functioning as model lipid bilayers. With a decrease in the hydrophobic chain length of sphingosine from C15 to C11, the transient leakage ratio of vesicle contents escalated from ∼ 13 % to ∼ 28 %. Although the presence of double bond did not exert a pronounced influence on transient leakage, it significantly affected the continuous leakage ratio. Conversely, modifying the chirality of the C-3 hydroxyl group gives the opposite result. Notably, methylation at the C-3 hydroxyl significantly elevates transient leakage while suppressing the continuous leakage ratio. Additionally, sphingosines that significantly affect vesicle permeability tend to have a more pronounced impact on cell viability. Throughout this leakage process, the charge state of sphingosine-derived molecule aggregates in the solution emerged as a pivotal factor influencing vesicle permeability. Fluorescence lifetime experiments further revealed discernible variations in the effect of sphingosine molecular structure on the mobility of hydrophobic regions within lipid bilayers. These observed distinctions emphasize the impact of molecular structure on intermolecular interactions, extending to the microscopic architecture of membranes, and underscore the significance of subtle alterations in molecular structure and their associated aggregation behaviors in governing membrane permeability.

The biogenic amines, polyamines, such as putrescine and cadavarine, are small flexible polycations. They interact with charged biological species and have a range of important biological functions. Their strong interaction with anionic surfactants results in enhanced adsorption at interfaces. Aspects of the impact of the polyamine molecular weight and structure on the pattern of surfactant adsorption at the air–water interface have been explored previously using neutron reflectivity, NR, and surface tension, ST. However there is limited evidence for the extent to which surfactant adsorption can be manipulated and the impact of the diamine structure and surfactant headgroup structure on surfactant adsorption at the air–water interface is reported here.The addition of putrescine [1,4 diaminobutane or butanediamine] over the pH range 3 to 10 enhances significantly the adsorption of sodium dodecyl sulfate, SDS, sodium dodecyl diethylene sulfate, SLES, and sodium tetradecyl methyl ester sulfonate, MES. For SDS and to a lesser extent MES there are regions where surface multilayer adsorption occurs at both pH 3 and 10, and this extends significantly the range of polyamine structures for which this is observed. Modifying the diamine spacer with ethylene oxide groups of varying length still results in enhanced adsorption for SDS and MES, but the enhancement is less pronounced and depends upon the size of the ethylene oxide group. However the incorporation of the ethylene oxide group does suppresses the formation of the more complex surface multilayered structures.

Hypothesis: Saponins are highly surface active glycosides, and are extensively used to stabilise emulsions and foams in beverages, foods, and cosmetics. Derived from a variety of plant species these naturally occurring biosurfactants have wider potential for inclusion in many low carbon and or sustainably sourced products. Although their adsorption at the air-solution and liquid-liquid interfaces has been extensively studied, the nature of their adsorption at solid surfaces is much less clear. The aim of this study was to establish the criteria for and nature of the adsorption of saponins at both hydrophilic and hydrophobic solid surfaces. Experiments: Adsorption at the hydrophilic and hydrophobic solid surfaces was investigated using neutron reflectivity. Measurements were made for the saponins escin, quillaja and glycyrrhizic acid. At the hydrophilic surface measurements were also made for escin / cetyltrimethyl ammonium bromide, C16TAB, mixtures; using deuterium labelling to determine the surface structure and composition. Findings: At a range of solution concentrations, from below to well in excess of the critical micelle concentration, cmc, there was no saponin adsorption evident at either the hydrophilic or hydrophobic surface. This implies an inherent incompatibility between the surface OH- groups at the hydrophilic surface and the saponin sugar groups, and a reluctance for the hydrophobic triterpenoid group of the saponin to interact with the octadecyltrichlorosilane, OTS, hydrophobic solid surface. Above a critical composition or concentration escin / C16TAB mixtures adsorb at the hydrophilic solid surface; with a surface composition which is dominated by the escin, and a structure which reflects the disparity in the molecular arrangement of the two surfactant components. The results provide an important insight into how cooperative adsorption can be utilised to promote adsorption of saponins at the solid- solution interface.

Deep eutectic solvents (DESs) are an emerging class of modern, often "green" solvents with unique properties. Recently, a deep eutectic system based on amphiphilic surfactant N-alkyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C12 & C14 sulfobetaine) and (1S)-(+)-10-camphor-sulfonic acid in the molar ratio 1:1.5 has been reported. Nanostructuring can be expected in this DES due to the nature of the components. In this work, we have investigated the native nanostructure in the DES comprising C12-C18 alkyl chain sulfobetaines with camphor sulfonic acid and how it interacts with polar and nonpolar species, water and dodecane, respectively, using small angle neutron scattering. By using contrast variation to highlight the relative position of the solvent components and additives, we can resolve the structure of the solvent and how it changes upon interaction with water and dodecane. Scattering from the neat DES shows structures corresponding to the self-assembly of sulfobetaines; the size of the structure increases as the alkyl chain length of the sulfobetaines increases. Water and dodecane interact, respectively, with the hydrophilic and hydrophobic moieties in the DES structure, primarily the sulfobetaine, thereby swelling and solvating the entire structure. The extent of the shift of the peak position, and the swelling, depend on concentration of the additive. The solution phase organization and the interaction of polar and nonpolar species as observed here, have the potential to affect the ordering of inorganic or polymeric materials grown in such solvents, paving new avenues for templating applications.

Sphingosine plays crucial roles in various cellular functions. Deuterated molecules are widely used as the important tools to explore the structure, property, and function of biochemical materials. We here report a gram-scale synthesis of sphingosines with different deuterated chains. The synthesis started from different protiated fatty acids, which underwent hydrothermal platinum-catalyzed hydrogen/deuterium (H/D) exchange to produce the deuterated chains with different length. The Horner-Wadsworth-Emmons (HWE) coupling of a chiral amino acid derivative with the deuterated chains generated the unsaturated ketone precursors. The following stereoselective reduction via cyclic Felkin-Anh transition led to the desired anti-amino alcohol geometry. This efficient protocol enabled the gram-scale synthesis of the chain-deuterated sphingosines. The subsequent synthesis of the chain-deuterated derivatives, i. e. ceramide, sphinganine and sphingosine-1-phosphate, is also described. Their deuterium content was simultaneously characterized by H-1 NMR and high-resolution mass spectrum (HRMS).

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. (C) 2022 Elsevier Inc. All rights reserved.

Saponins are surface active glycosides which can be extracted from a range of different plant species. This class of biosurfactants is of much current interest because of their biocompatibility and biosustainability, and a range of other interesting properties. The Quillaja saponin, the focus of this paper, is extensively used in foods, beverages and cosmetics due to their accessibility and agency approval. Aspects of their wider application rests with their use in combination with different synthetic surfactants, to optimise and tailor performance and functionality. Characterising and understanding the self-assembly of such mixtures is an essential requirement for that wider exploitation. The surface activity of a range of different saponins has been extensively reported, but the self-assembly properties of the Quillaja saponin, and its mixture with different synthetic surfactants has not been reported in any detail. Of particular interest is how the bidesmodic structure of the Quillaja saponin and the mixing with the different cosurfactants affects the evolution in the micelle structure, and this is the focus of this study. Small angle neutron scattering, SANS, has been used to characterise the micelle formation of the Quillaja saponin and of its mixtures with the anionic surfactant sodium dodecyl sulfate, SDS, and the nonionic surfactants, dodecaethylene glycol monododecyl ether, C12E12, and pentaethylene glycol monododecyl ether, C12E5. The Quillaja saponins form relatively small globular micelles with a relatively small aggregation number. In mixtures with SDS and the nonionic surfactants the micelles retain their globular structure. In the mixtures the micelle aggregation number evolves towards the value for the pure synthetic surfactant component, as the packing constraints change with the decreasing mole fraction of saponin in the micelle. The evolution in the micelle structure and size reflects the change in the structure and geometry of the associated surfactant components, and is largely dominated by the Quillaja structure. The results provide an interesting insight into the factors controlling the self-assembly in saponin – surfactant mixtures, and how the saponin structure affects the self-assembly.

Hypothesis: Saponins are a class of plant derived surfactants which are widely used in food related foams and emulsions, aerated drinks, and in pharmaceuticals and cosmetics. As a potential biosourced and renewable ingredient in a wider range of surfactant based formulations their potential is intimately associated with their mixing with synthetic surfactants. As such the nature of the mixed saponin-surfactant self-assembly is an important characteristic to investigate and understand. The unconventional structure of the saponins compared to the conventional synthetic surfactants poses some interesting constraints on the structures of the mixed aggregates. Experiments: Small angle neutron scattering, SANS, is used to investigate the structure of the saponin, escin, mixed with a range of nonionic surfactants with different ethylene oxide groups, from triethylene glycol monododecyl ether, C12E3, to dodecaethylene glycol monododecyl ether, C12E12. Findings: The scattering data reveal a complex evolution in the solution self-assembled structure with varying escin / nonionic composition and ethylene oxide chain length. The rich structural development comprises of the evolution from the elongated micelle structure of escin to the micelle structure of the nonionic surfactant. At the intermediate solution compositions the structure is predominantly planar, comprising mostly of planar / micellar mixed phases. The nature of the planar structures depend upon the ethylene oxide chain length and the solution composition, and include lamellar, bilamellar vesicle, multilamellar vesicle, and nanovesicle structures, in common with what is observed in other surfactant mixtures.

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 angstrom 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. (C) 2022 The Authors. Published by Elsevier Inc.

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 degrees C and C16LC at 45 degrees 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 angstrom(2) 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 angstrom, 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. (C) 2021 Published by Elsevier Inc.

Hypothesis: Nonionic alkyl ethoxylate surfactants are widely used in agrochemicals to facilitate the per-meation of systemic herbicides and fungicides across the plant waxy film. Industrial grade surfactants are often highly mixed and how the mixing affects their interactions with pesticides and wax films remains largely unexplored. A better understanding could enable design of mixed nonionic surfactants for herbi-cides and fungicides to maximize their efficiency and reduce wastage whilst controlling their impact on plant wax films. Experiment: In this study, nonionic surfactants with general structure n-oxyethylene glycol monododecyl ether (C12En) were used to form surfactant mixtures with the same average ethoxylate numbers but dif-ferent hydrophilic-lipophilic balance (HLB) values. Their mixed micellar systems were then used to sol-ubilize a herbicide diuron (DN) and a fungicide cyprodinil (CP), followed by plant wax solubilization upon contact with wax films. These processes were monitored by H-1 NMR and SANS. Finding: Pesticide solubilization made surfactant micelles effectively more hydrophobic but subsequent wax dissolution caused pesticide release and the restoration of the micellar amphiphilicity. Nonionic surfactants with lower HLBs form larger nanoaggregates, show enhanced wettability, and have better ability to solubilize and permeate pesticides across the wax film, but may cause significant damage to plant growth. These observations help explain why herbicides applied on weeds would benefit from surfactants with lower HLB values while fungicides require surfactants with HLBs to balance between delivery efficiency and potential phytotoxicity risks. (C) 2022 Published by Elsevier Inc.

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.

Saponins are plant based biosurfactants that are obtained from a wide variety of plant species. They are surface active glycosides with a hydrophobic group, which is commonly a triterpenoid or steroidal group, and saccharide hydrophilic units. Their structures are markedly different from conventional synthetic surfactants, and present interesting challenges in understanding their optimal packing in self-assembled structures such as micelles. Furthermore the wide variety of molecular structures available provide interesting opportunities to optimise the packing in mixed systems. In the current literature there is limited information available on saponin self-assembly, and especially in saponin mixtures. The general view is that globular to more elongated micelle structures form at relatively low concentrations. Here small angle neutron scattering, SANS, is used to investigate and quantify the structure of escin, tea and glycyrrhizic acid micelles and particularly of escin/tea, tea/glycyrrhizic acid and escin/glycyrrhizic acid mixtures at relatively low concentrations. The focus is principally on how the different saccharide headgroup structures affect the self-assembly. Tea saponins form relatively small globular micelles, whereas escin and glycyrrhizic acid form larger more elongated globular micelle structures. The micelle structure and changes observed reflect the packing constraints associated with the aglycone triterpenoid hydrophobic section and the differences in the saccharide headgroups. In the escin/tea and glycyrrhizic acid/tea mixtures the greater intrinsic curvature associated with the tea saponin dominates the micelle structure over much of the composition range explored. For the escin/glycyrrhizic acid mixture the micelle size and aggregation number go through a minimum at an approximately equimolar composition, and this implies an intrinsic difficulty in efficient packing of the two different saponin structures, which results in an increase in the preferred curvature

Saponins are a class of bio-surfactants obtained from a wide variety of plant species. They are surface active glycosides which are used in the stabilisation of foams and emulsions in many food, drink and cosmetic applications. Their wider utilisation and application will involve their mixture with different synthetic and bio-derived surfactants, proteins and polymers. Understanding the mixing properties of saponins with other surfactants at surfaces and in self-assembly is key to their wider exploitation; and the focus here is on triterpenoid saponin escin and the anionic surfactant sodium dodecylsulfate, SDS. Previous analysis of the surface adsorption and critical micelle concentration, cmc, data using the pseudo phase approximation, PPA, indicate that the micelle mixing is more non-ideal than the surface mixing. The non-ideality is associated with the packing constraints arising from the quite different molecular structures and how this impacts upon the mixture self-assembly is probed here. To address specifically the self-assembly properties of saponin-surfactant mixtures, small angle neutron scattering, SANS, has been used to explore the nature of mixed micelles of escin and SDS. The SANS data for the escin / SDS mixtures are modelled as globular ellipsoidal micelles. The micelles exhibit an overall increase in size and aggregation number as the solution becomes richer in escin, and the evolution in micelle size goes through a minimum at relatively rich SDS solutions. The results illustrate the impact of the disparity in the molecular structures of escin and SDS on the self-assembly, and provides an insight into the factors affecting the departure from ideal mixing.

Although surfactants have been widely used in skin care and other related applications, our knowledge about how surfactants interact with stratum corneum (SC) lipids remains limited. This work reports how surfactants interact with a lipid SC model by neutron diffraction and molecular dynamics (MD) simulations, focusing on examining the impact of surfactant molecular architecture. The surfactant-SC mixed membrane was constructed by an equimolar mixture of ceramide/cholesterol/fatty acids and surfactant at 1% molar ratio of total lipids. The arrangements of water and surfactant molecules in the membrane were obtained through neutron scattering length density (NSLD) profiles via contrast variation method, meanwhile, MD simulation clearly demonstrated the mechanism of hydration change in the surfactant-model SC mixed membrane. No drastic difference was detected in the repeating distance of the short periodicity phase (SPP) upon adding surfactants, however, it significantly enhanced the membrane hydration and reduced the amount of phase separated crystalline cholesterol, showing a strong dependence on surfactant chain length, branching and double bond. This work clearly demonstrates how surfactant architecture affects its interaction with the SC membrane, providing useful guidance for either choosing an existing surfactant or designing a new one for surfactant-based transdermal application.

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.