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.

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

The 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.

The nature of surfactant mixing at interfaces and in bulk solution is key to understanding and optimising the diverse industrial, technological, biological and domestic applications of surfactants. The use of neutron reflectivity, NR, and small angle neutron scattering, SANS, in combination with isotopic substitution, has transformed the ability to quantify and understand the nature of surfactant mixing at the air-water interface and in self-assembled aggregates or micelles in solution. The accuracy and scope of the compositional data from NR, the application of recent developments in the pseudo phase approximation, PPA, and the availability of complementary critical micelle concentration, cmc, and micelle composition data, enables a detailed thermodynamical quantification of the mixing properties to be made. The NR data in particular, and the SANS data to a lesser extent, provides constraints on the thermodynamical analysis which reveals important properties and trends about the bulk phase which are not available from the analysis of data such as the variation in the cmc alone. The importance and impact of this approach is illustrated with an overview of a range of mixed surfactant examples from the recent literature, and which encompass mixtures with different degrees of departure from ideality.

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.

Inactivation of influenza A virus by radiofrequency (RF) energy exposure at levels near Institute of Electrical and Electronics Engineers (IEEE) safety thresholds has been reported. The authors hypothesized that this inactivation was through a structure-resonant energy transfer mechanism. If this hypothesis is confirmed, such a technology could be used to prevent transmission of virus in occupied public spaces where RF irradiation of surfaces could be performed at scale. The present study aims to both replicate and expand the previous work by investigating the neutralization of bovine coronavirus (BCoV), a surrogate of SARS-CoV-2, by RF radiation in 6-12 GHz range. Results showed an appreciable reduction in BCoV infectivity (up to 77%) due to RF exposure to certain frequencies, but failed to generate enough reduction to be considered clinically significant.

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.

Emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its current worldwide spread have caused a pandemic of acute respiratory disease COVID-19. The virus can result in mild to severe, and even to fatal respiratory illness in humans, threatening human health and public safety. The spike (S) protein on the surface of viral membrane is responsible for viral entry into host cells. The discovery of methods to inactivate the entry of SARS-CoV-2 through disruption of the S protein binding to its cognate receptor on the host cell is an active research area. To explore other prevention strategies against the quick spread of the virus and its mutants, non-equilibrium molecular dynamics simulations have been employed to explore the possibility of manipulating the structure–activity of the SARS-CoV-2 spike glycoprotein by applying electric fields (EFs) in both the protein axial directions and in the direction perpendicular to the protein axis. We have found out the application of EFs perpendicular to the protein axis is most effective in denaturing the HR2 domain which plays critical role in viral-host membrane fusion. This finding suggests that varying irradiation angles may be an important consideration in developing EF based non-invasive technologies to inactivate the virus.

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.

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.

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.

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.

There is an increased interest in the use of natural surfactant as replacements for synthetic surfactants due to their biosustainable and biocompatible properties. A category of natural surfactants which are attracting much current interest is the triterpenoid saponins; surface active components found extensively in a wide range of plant species. A wide range of different saponin structures exist, depending upon the plant species they are extracted from; but regardless of the variation in structural details they are all highly surface active glycosides. Greater exploitation and application requires a characterisation and understanding of their basic adsorption and self-assembly properties. HYPOTHESIS:Glycyrrhizic acid, extracted from Licorice root, is a monodesmosidic triterpenoid saponin. It is widely used in cosmetic and pharmaceutical applications due to its anti-inflammatory properties, and is an ingredient in foods as a sweetener additive. It has an additional attraction due to its gel forming properties at relatively low concentrations. Although it has attracted much recent attention, many of its basic surface active characteristics, adsorption and self-assembly, remain relatively unexplored. How the structure of the Glycyrrhizic acid saponin affects its surface active properties and the impact of gelation on these properties are important considerations, and to investigate these are the focus of the study. EXPERIMENTS:In this paper the adsorption properties at the air-water interface and the self-assembly in solution have been investigated using by neutron reflectivity and small angle neutron scattering; in non-gelling and gelling conditions. FINDINGS:The adsorption isotherm is determined in water and in the presence of gelling additives, and compared with the adsorption behaviour of other saponins. Gelation has minimal impact on the adsorption; apart from producing a rougher surface with a surface texture on a macroscopic length scale. Globular micelles are formed in aqueous solution with modest anisotropy, and are compared with the structure of other saponin micelles. The addition of gelling agents results in only minimal micelle growth, and the solutions remain isotropic under applied shear flow.

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.

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 (A(CMC)) of the zwitterionic C12LC adsorbed layer at the air/water interface was found to be 46 angstrom(2) from surface tension and neutron reflection, smaller than the values of C12PC, C12E5, DTAB, C(12)C(4)betaine and C(12)C(8)betaine 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 angstrom(2), 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 angstrom(2), 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 angstrom and a shell thickness of 10.5 angstrom. 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. (C) 2021 Published by Elsevier Inc.