Surfactant Penetration into Human Skin from Sodium Dodecyl Sulfate and Lauramidopropyl Betaine Mixtures.

Surfactant mixtures are used in a variety of personal care and cosmetic applications but are known to be harsh on the skin. The purpose of this study was to examine anionic surfactant penetration into human skin from nonideal surfactant mixtures under short-time exposure conditions that are relevant to realistic exposure scenarios. This was done by measuring the penetration of a radiolabeled probe (14C-SDS) into human cadaver skin in Franz diffusion cells in vitro from the mixtures of sodium dodecyl sulfate (SDS) and lauramidopropyl betaine (LAPB). Monomer and micelle concentrations in the SDS/LAPB/14C-SDS mixtures were predicted using a regular solution theory approximation. We confirmed that the mixtures of SDS and LAPB exhibit nonideal behavior with a net attraction between the two surfactants. Penetration of 14C-SDS into excised human skin from the mixtures of SDS and LAPB was found to decrease in a log-linear manner with increasing mole fraction of LAPB in the bulk solution (R2 = 0.97, p < 0.001). Additionally, the penetration of 14C-SDS into excised human skin from the mixtures of SDS and LAPB was found to correlate linearly and strongly with the predicted values of 14C-SDS monomer concentration in SDS/LAPB/14C-SDS mixtures (R2 = 0.95, p < 0.01). 14C-SDS penetration from the mixed surfactant composition could be quantitatively reconciled with that from an SDS-only composition by postulating a secondary, positive contribution from LAPB related to its own penetration and binding to skin components that increased SDS penetration at low concentrations. This research therefore supports a monomer penetration theory of surfactant penetration into the skin, combined with a measurable impact of favorable surfactant interactions within the tissue.

The effect of prolonged exposure on sodium dodecyl sulfate penetration into human skin.

The purpose of this study was to examine the effect of prolonged surfactant exposure on mechanisms of anionic surfactant penetration into human skin. A radiolabeled probe (14-carbon sodium dodecyl sulfate (14C-SDS)) was used to trace the penetration of a model anionic surfactant, sodium dodecyl sulfate (SDS), into excised human skin and into an inert membrane composite in vitro. SDS dose varied from 0.03 to 15 mg/cm2, mimicking the exposure of a rinse-off cleanser on skin. Two surfactant exposure lengths were tested, 2 min and 5 h. SDS penetration into excised human skin was constant from 50 to 600 mM for skin samples exposed to SDS for 2 min. For skin samples exposed to SDS for 5 h, SDS penetration into skin increased log-linearly with increasing SDS concentration. SDS penetration into the inert membrane composite was constant from 50 to 600 mM SDS regardless of length of surfactant exposure. Penetration of the radiolabeled probe into skin and into the inert membrane correlated well with the monomeric concentration of the radiolabeled probe in the applied surfactant solution. These results support that monomer concentration is the driving force for initial SDS penetration into upper layers of the stratum corneum over a wide range of concentrations. With prolonged exposure, SDS penetrates the skin in a dose-dependent manner due to surfactant-induced damage to the skin.

Mechanisms of anionic surfactant penetration into human skin: Investigating monomer, micelle and submicellar aggregate penetration theories.

OBJECTIVE: Once penetrated into the stratum corneum, anionic surfactants bind to and denature stratum corneum proteins as well as intercalate into and extract intercellular lipids. With repeated exposures, this leads to skin dryness and irritation, compromising barrier function and skin health. The mechanisms of anionic surfactant penetration into the skin, however, are still widely debated. The objective of this study was to evaluate current theories of surfactant penetration into human skin. METHODS: A test set comprising 15 anionic surfactant systems and one non-ionic surfactant, all having either dodecyl or lauryl alkyl chains, was tested for surfactant penetration into split-thickness human cadaver skin in vitro using radiolabelled sodium dodecyl sulphate (14 C-SDS). Select physical properties of these formulations thought to be associated with skin penetration including critical micelle concentration, micelle diameter, filtrate concentration and zeta potential were also measured. RESULTS: 14 C-SDS penetration into human cadaver skin from surfactant systems in vitro was found to correlate well with CMC (R2  = 0.34, P < 0.05), filtrate concentration (R2  = 0.36, P < 0.05) and zeta potential (R2  = 0.76, P < 0.001), but poorly with micelle diameter (R2  = 0.12). Furthermore, the latter measure correlated inversely with penetration compared to what would be expected based on the micelle penetration theory. CONCLUSION: Neither monomer nor micelle penetration theories are sufficient to explain anionic surfactant penetration into human skin. Submicellar (or premicellar) aggregate penetration theory is difficult to defend at relevant surfactant concentrations. We propose a new hypothesis for this mechanism in which short-term penetration is based on monomer concentration and longer term penetration is based on surfactant-induced damage to the skin barrier.

OBJECTIF: Une fois pénétrés dans la couche cornée, les tensioactifs anioniques se lient aux protéines de la couche cornée, le dénaturent, s'intercalent dans les lipides intercellulaires et les extraient. Avec des expositions répétées, ceci entraîne un assèchement et une irritation de la peau, compromettant ainsi la fonction de barrière et la santé de la peau. Les mécanismes de pénétration du tensioactif anionique dans la peau restent toutefois largement débattus. L'objectif de cette étude est d'évaluer les théories actuelles de la pénétration de surfactant dans la peau humaine. MÉTHODES: Un ensemble de contrôle comprenant 15 systèmes de tensioactifs anioniques et un tensioactif non ionique, tous possédant des chaînes dodécyles ou lauryle alkyles, a été testé vis-à-vis de la pénétration du tensioactif dans la peau d'un cadavre humain partiellement épaisse in vitro en utilisant du sulfate de sodium dodécyl radiomarqué (14 C-SDS). Certaines propriétés physiques de ces formulations supposées être associées à la pénétration dans la peau, notamment la concentration critique micellaires, le diamètre des micelles, la concentration de filtrat, et le potentiel zêta, ont également été mesurées. RÉSULTATS: La pénétration du 14 C-SDS dans la peau de cadavre humain provenant de systèmes de tensioactifs in vitro était bien corrélée avec la CMC (R2  = 0,34, P < 0,05), la concentration de filtrat (R2  = 0,36, P < 0,05) et le potentiel zêta (R2  = 0,76, P < 0,001), mais faiblement avec un diamètre de micelle (R2  = 0,12). De plus, cette dernière mesure était inversement corrélée à la pénétration par rapport à ce que l'on pouvait attendre de la théorie de la pénétration des micelles. CONCLUSION: Ni les théories de pénétration de monomères ni de micelles ne sont suffisantes pour expliquer la pénétration de tensioactif anionique dans la peau humaine. La théorie de la pénétration des agrégats submicellaires (ou prémicellaires) est difficile à défendre aux concentrations pertinentes de surfactant. Nous proposons une nouvelle hypothèse pour ce mécanisme dans laquelle la pénétration à court terme est basée sur la concentration en monomères et la pénétration à plus long terme est basée sur les dommages à la barrière cutanée induits par les tensioactifs.

Recent Advances in Mild and Moisturizing Cleansers

Mild and moisturizing cleanser technologies, and the science underpinning them, have progressed significantly over the past decade. This includes introduction of amino-acid based milder surfactants into the cleansing arena, a deeper understanding of the roles of stratum corneum lipids and proteins in their interaction with cleanser surfactants, the role of pH in skin cleansing, and the development of improved methodologies for predicting skin irritation and drying potential of cleansers. In this paper, the recent advances in these areas as well as newer technologies are reviewed, and the future directions are outlined. J Drugs Dermatol. 2019;18(1 Suppl):s80-88

Quantification of Lipid Phase Order of In Vivo Human Skin Using Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy and Multivariate Curve Resolution Analysis.

A new analysis methodology utilizing multivariate curve resolution (MCR) has been successfully combined with Fourier transform infrared (FT-IR) measurement of in vivo human skin to resolve lipid phase constituents in the spectra relative to high and low chain ordering. A clinical study was performed to measure lipid order through different depths of stratum corneum of human subjects. Fourier transform IR spectra were collected through the top 10 layers of the skin on four sites on the left and right forearm of 12 individuals. Depth profiling was achieved by tape stripping to remove layers of skin with 10 successive tapes from each site. In vivo ATR FT-IR spectra were collected after removing each tape. Three isolated spectral regions were analyzed, centered around 2850 cm-1, 1460-1480 cm-1, and 730 cm-1, corresponding to stretching, scissoring, and rocking -CH2 vibrational modes, respectively. Both traditional lipid conformation analysis and MCR analysis were performed on the same spectral data. The lipid order ratio, expressed as the fraction of highly ordered orthorhombic (OR) lipids to the total lipids content (orthorhombic + hexagonal [HEX] + liquid crystal [LC]), was assessed as function of depth. Lipid order depth profiles (LODP) show an increase in order with the stratum corneum depth which can be adequately described by an exponential function for the data obtained in this study. The LODP derived from the three vibrational modes show very similar trends, although the absolute order ratios are somewhat different. The variance of the skin LODP across individuals is much greater than between sites within the same individual. The higher arm sites closer to the elbow on the left and right arm show no statistically significant difference and are recommended for use in comparative studies. The scissoring mode shows the highest sensitivity for determination of LODP value.

Modified Corneosurfametry as a new accelerated high-throughput ex vivo methodology for predicting cleanser effects towards human skin.

BACKGROUND: Corneosurfametry (CSM) was originally developed as a tool to predict irritation potential of cleansers. In this method, surface skin stripped using cyanoacrylate is contacted with surfactants/products, rinsed and stained with toluidine blue and basic fuschin dyes. The intensity of staining increases with increases in irritation potential of surfactant. Our objective was to modify the CSM technique to achieve better control of the tape stripping process. Another objective was to correlate the modified CSM (MCSM) with a traditional in-vivo forearm controlled application test (FCAT) for mildness and to explore its utility to assess the state of corneum after a clinical test. METHODS: Surface skin cells were tape stripped from forearms of volunteers with D-Squame Adhesive Discs. Discs were treated with a 10% solution of the product in a 96-plate well for 10 min, rinsed, dried and treated with basic fuschin-toluidine blue dye solution, rinsed and dried again. Forearm Controlled Application Test (FCAT) was based on a published protocol. Tape strips obtained after product treatment were also analyzed by the MCSM procedure without additional product treatment. RESULTS: Mildness/barrier damage assessed from in-vivo FCAT showed a similar ranking to the MCSM results. MCSM, TEWL and Erythema analysis of between-treatment differences showed a good correlation indicating that barrier damage seen in in-vivo studies can be predicted from ex-vivo MCSM studies. MCSM analysis of tape strips after the FCAT study showed that the damage decreased with increase in tape strip number. A moisturizing body wash (MBW) with mild surfactants showed the least damage in all layers. In contrast, harsh dish washing liquid showed significantly higher damage down to several layers. Another MBW with petrolatum in a harsher base showed damage almost similar to that of the harsh dish washing liquid in the surface layers. Thus, the MCSM was able to show underlying damage which would have been normally masked by the deposited petrolatum. CONCLUSION: The MCSM assay was shown to be a valuable tool for accelerated high throughput evaluation of mildness of surfactants and fully formulated products. MCSM can also be used to assess the state of the corneum after a product treatment.

A comparison between interactions of triglyceride oil and mineral oil with proteins and their ability to reduce cleanser surfactant-induced irritation.

OBJECTIVE: Skin irritation in personal cleansing has been correlated with surfactant binding with stratum corneum proteins. Polar and non-polar oils are increasingly being used in cleansing formulations which contain high (10-15%) level of anionic and non-ionic surfactants. However, the effects of oils in modulating skin damage from a cleansing product have not been studied in any detail. The objectives of this study are to determine whether low-viscosity polar and non-polar oils differ in their ability to reduce surfactant-induced skin irritation and, if so, how it might be related to their interactions with proteins. METHODS: Surfactant-induced skin irritation was measured by a 14-day in vivo cumulative patch irritation test. The methodology was similar to the well-known soap chamber test. Surfactant interactions with the water-soluble protein, bovine serum albumin (BSA), in the presence of oils were measured by conductometric titration. The effects of low-viscosity polar and non-polar oils on stratum corneum protein dynamics in the sulfhydryl group region were studied by electron paramagnetic resonance (EPR) using the covalently bound spin-label 3-maleimido-proxyl (5-MSL). EPR measurements were performed with stratum corneums obtained from discarded skins of 3- to 4-week-old female pigs. Simulation of the complex spectra provided insights on the environment and mobility of the protein-bound spin label. RESULTS: Addition of 1% polar sunflower seed oil (viscosity 42 centipoise) reduced in vivo irritation of 1% sodium lauryl ether sulphate with two ethoxylate/cocamidopropyl betaine (SLES/CAPB) by 20%, whereas 1% non-polar mineral oil (viscosity 15 centipoise) had no effect. Polar oil glyceryl trioleate (a major component in sunflower seed oil) at 10% level reduced surfactant binding to BSA protein in water by 40%, whereas the non-polar oil dodecane (a major component of mineral oil) at a similar level did not have any effect. The mobility of the spin label in a dry corneum was very low and was increased significantly with the addition of water and glycerol trioleate but less so with mineral oil. CONCLUSION: Sunflower seed oil reduces surfactant-induced in vivo skin irritation more than mineral oil. This is possibly due to stronger interaction of polar oil with proteins, thus protecting it from surfactant binding.

Effect of mixed surfactants on stratum corneum: a drying stress and Raman spectroscopy study.

OBJECTIVE: Stratum corneum (SC) lipids are known to play an important role in barrier properties of skin by maintaining the optimal hydration levels. The disruption of SC lipids by cleanser surfactants is believed to lead to dry skin damage which can be a precursor to other skin disorders. The purpose of this study is to investigate the effects of commonly used anionic and zwitterionic surfactants sodium lauryl ether sulphate (SLES) and cocoamidopropyl betaine (CAPB) on the generation of drying stresses in SC and the role played by lipids. METHODS: Stratum corneum separated from pig skin was treated with various surfactants (SDS, SLES and CAPB) their mixtures and solvents. The tensile response to these treatments was measured by using a dynamic mechanical thermal analyzer. A Raman spectroscopy study of the treated samples was performed to investigate the effects of lipid modification (lipid chain conformational order and lipid removal) on stress generation in SC. RESULTS: The effects of commonly used anionic and zwitterionic surfactants on the generation of drying stresses in SC were studied. Although known to be milder in comparison with SDS, both SLES and CAPB generated high drying stresses individually. In mixtures, SLES-CAPB at 4 : 1 ratio leads to lower drying stress as compared to water alone. A Raman spectroscopic study of surfactant-treated SC shows changes in lipid chain conformational order as well as a decrease in lipid-protein ratio in SC. A chloroform-methanol 2 : 1 treatment leads to the highest drying stress as well delipidization of SC. CONCLUSION: The results show a correlation between generation of drying stress in SC and extent of lipid modification. We propose that the changes in lipid conformational order and removal of lipid components affect the stress relaxation properties of SC leading to high drying stresses.

Effect of detergents on the physicochemical properties of skin stratum corneum: a two-photon excitation fluorescence microscopy study.

OBJECTIVE: Understanding the structural and dynamical features of skin is critical for advancing innovation in personal care and drug discovery. Synthetic detergent mixtures used in commercially available body wash products are thought to be less aggressive towards the skin barrier when compared to conventional detergents. The aim of this work is to comparatively characterize the effect of a mild synthetic cleanser mixture (SCM) and sodium dodecyl sulphate (SDS) on the hydration state of the intercellular lipid matrix and on proton activity of excised skin stratum corneum (SC). METHOD: Experiments were performed using two-photon excitation fluorescence microscopy. Fluorescent images of fluorescence reporters sensitive to proton activity and hydration of SC were obtained in excised skin and examined in the presence and absence of SCM and SDS detergents. RESULTS: Hydration of the intercellular lipid matrix to a depth of 10 µm into the SC was increased upon treatment with SCM, whereas SDS shows this effect only at the very surface of SC. The proton activity of SC remained unaffected by treatment with either detergent. CONCLUSION: While our study indicates that the SC is very resistant to external stimuli, it also shows that, in contrast to the response to SDS, SCM to some extent modulates the in-depth hydration properties of the intercellular lipid matrix within excised skin SC.

A Novel Glycinate-based Body Wash: Clinical Investigation Into Ultra-mildness, Effective Conditioning, and Improved Consumer Benefits.

OBJECTIVE: To assess the properties of a novel body wash containing the mild surfactant glycinate. DESIGN: Biochemical and clinical assays. SETTING: Research laboratories and clinical sites in the United States and Canada. PARTICIPANTS: Women 18 to 65 years of age (cleansing efficacy); male and female subjects 26 to 63 years of age with mild or moderate dryness and erythema (leg-controlled application test); subjects 5 to 65 years of age with mild-to-moderate eczema (eczema compatibility); and women 18 to 64 years of age (home use). MEASUREMENTS: Assessments across studies included colorimetric dye exclusion to assess skin damage potential (corneosurfametry), efficacy of cosmetic product removal from skin, change from baseline in visual dryness, change from baseline in Eczema Area and Severity Index, and self-perceived eczema attributes and self-reported product preference. RESULTS: The glycinate-based cleanser demonstrated mildness to skin components when evaluated in a corneosurfametry assay. Short-term use under exaggerated wash conditions in subjects with dryness scores <3 and erythema scores <2 (both on a 0-6 scale) indicated an initial reduction in visual dryness. In subjects with eczema, normal use resulted in significant improvements (p<0.05) at Week 4 compared with baseline in skin dryness (change from baseline = -0.73), rash (-0.56), itch (-0.927), tightness (-0.585), and all eczema (-0.756). The glycinate-based body wash removed 56 percent of a long-lasting cosmetic foundation from skin compared with less than 30 percent removed by two competitive products tested. The glycinate-based body wash was preferred over a competitive mild cleansing product overall. CONCLUSION: The patented glycinate-containing body wash demonstrated better product mildness and patient-preferred attributes and clinical benefits.

Efficient control of the rheological and surface properties of surfactant solutions containing C8-C18 fatty acids as cosurfactants.

Systematic experimental study is performed about the effects of chain length (varied between C8 and C18) and concentration of fatty acids (FAc), used as cosurfactants to the mixture of the anionic surfactant SLES and the zwitterionic surfactant CAPB. The following properties are studied: bulk viscosity of the concentrated solutions (10 wt % surfactants), dynamic and equilibrium surface tensions, surface modulus, and foam rheological properties for the diluted foaming solutions (0.5 wt % surfactants). The obtained results show that C8-C10 FAc induce formation of wormlike micelles in the concentrated surfactant solutions, which leads to transformation of these solutions into viscoelastic fluids with very high apparent viscosity. The same FAc shorten the characteristic adsorption time of the diluted solutions by more than 10 times. In contrast, C14-C18 FAc have small effect on the viscosity of the concentrated solutions but increase the surface modulus above 350 mN/m, which leads to higher friction inside sheared foams and to much smaller bubbles in the formed foams. The intermediate chain C12 FAc combines some of the properties seen with C10 FAc and other properties seen with C14 FAc. These results clearly demonstrate how appropriate cosurfactants can be used for efficient control of the rheological properties of concentrated surfactant solutions and of some important foam attributes, such as bubble size and foam rheology.

Stratum corneum fatty acids: their critical role in preserving barrier integrity during cleansing.

Stratum corneum (SC) bilayer lipids, specifically fatty acids, ceramides and cholesterol, contribute to the permeability barrier function of the skin. Normal skin cleansing is associated with damage to the SC lipids because cleanser surfactants, in addition to providing the desired effect of solubilizing and facilitating the removal of sebum and skin soils, have a propensity to disrupt bilayer lipids by extracting endogenous skin lipids or intercalating into the bilayer. Disrupted SC lipids are associated with a variety of pathological skin conditions, as well as with dry skin induced by harsh cleansing. In an attempt to preserve the barrier and mitigate the damage caused by frequent normal cleansing, the incorporation of physiologically relevant lipids into skin cleansers has become common in leading cleansing products. It has been noted that fatty acids are more susceptible to surfactant-induced removal than other lipids (eg, ceramides), an observation that may form the basis for a critically important strategy for replenishing SC lipids. This review will focus on the role of fatty acids in the structure and function of the SC, and the rationale for incorporation of stearic acid into moisturizing body cleansers to minimize their extraction by surfactants and replenish lost fatty acids to promote skin barrier preservation.

A global perspective on caring for healthy stratum corneum by mitigating the effects of daily cleansing: report from an expert dermatology symposium.

A symposium was held in October 2011 on skin cleansing and its role in basic skin care. The meeting was attended by an international group of leading dermatologists.

Effect of cationic polymers on foam rheological properties.

We study the effect of two cationic polymers, with trade names Jaguar C13s and Merquat 100, on the rheological properties of foams stabilized with a mixture of anionic and zwitterionic surfactants (sodium lauryloxyethylene sulfate and cocoamidopropyl betaine). A series of five cosurfactants are used to compare the effect of these polymers on foaming systems with high and low surface dilatational moduli. The experiments revealed that the addition of Jaguar to the foaming solutions leads to (1) a significant increase of the foam yield stress for all systems studied, (2) the presence of consecutive maximum and minimum in the stress vs shear rate rheological curve for foams stabilized by cosurfactants with a high surface modulus (these systems cannot be described by the Herschel-Bulkley model anymore), and (3) the presence of significant foam-wall yield stress for all foaming solutions. These effects are explained with the formation of polymer bridges between the neighboring bubbles in slowly sheared foams (for inside foam friction) and between the bubbles and the confining solid wall (for foam-wall friction). Upon addition of 150 mM NaCl, the effect of Jaguar disappears. The addition of Merquat does not noticeably affect any of the foam rheological properties studied. Optical observations of foam films, formed from all these systems, show a very good correlation between the polymer bridging of the foam film surfaces and the strong polymer effect on the foam rheological properties. The obtained results demonstrate that the bubble-bubble attraction can be used for efficient control of the foam yield stress and foam-wall yield stress, without significantly affecting the viscous friction in sheared foams.

Control of Ostwald ripening by using surfactants with high surface modulus.

We describe results from systematic measurements of the rate of bubble Ostwald ripening in foams with air volume fraction of 90%. Several surfactant systems, with high and low surface modulus, were used to clarify the effect of the surfactant adsorption layer on the gas permeability across the foam films. In one series of experiments, glycerol was added to the foaming solutions to clarify how changes in the composition of the aqueous phase affect the rate of bubble coarsening. The experimental results are interpreted by a new theoretical model, which allowed us to determine the overall gas permeability of the foam films in the systems studied, and to decompose the film permeability into contributions coming from the surfactant adsorption layers and from the aqueous core of the films. For verification of the theoretical model, the gas permeability determined from the experiments with bulk foams are compared with values, determined in an independent set of measurements with the diminishing bubble method (single bubble attached at large air-water interface) and reasonably good agreement between the results obtained by the two methods is found. The analysis of the experimental data showed that the rate of bubble Ostwald ripening in the studied foams depends on (1) type of used surfactant-surfactants with high surface modulus lead to much slower rate of Ostwald ripening, which is explained by the reduced gas permeability of the adsorption layers in these systems; (2) presence of glycerol which reduces the gas solubility and diffusivity in the aqueous core of the foam film (without affecting the permeability of the adsorption layers), thus also leading to slower Ostwald ripening. Direct measurements showed that the foam films in the studied systems had very similar thicknesses, thus ruling out the possible explanation that the observed differences in the Ostwald ripening are due to different film thicknesses. Experiments with the Langmuir trough were used to demonstrate that the possible differences in the surface tensions of the shrinking and expanding bubbles in a given foam are too small to strongly affect the rate of Ostwald ripening in the specific systems studied here, despite the fact that some of the surfactant solutions have rather high surface modulus. The main reason for the latter observation is that the rate of surface deformation of the coarsening bubbles is extremely low, on the order of 10(-4) s(-1), so that the relaxation of the surface tension (though also slow for the high surface modulus systems) is still able to reduce the surface tension variations down to several mN/m. Thus, we conclude that the main reason for the reduced rate of bubble Ostwald ripening in the systems with high surface modulus is the low solubility and diffusivity of the gas molecules in the respective condensed adsorption layers (which have solid rather than fluid molecular packing).

Formation and rheology of viscoelastic "double networks" in wormlike micelle-nanoparticle mixtures.

We present a systematic study of thermodynamics, structure, and rheology of mixtures of cationic wormlike micelles and like-charged nanoparticles. Structural and thermodynamic measurements in dilute surfactant-nanoparticle mixtures show the formation of micelle-nanoparticle junctions that act as physical cross-links between micelles. The presence of these junctions is shown to build significant viscosity and viscoelasticity in dilute and semidilute WLMs, even in cases where the fluid is Newtonian in the absence of nanoparticles. Increases in viscosity, shear modulus, and relaxation time, as well as decreases in entanglement concentration, are observed with increasing particle concentration. As such, nanoparticle addition gives rise to a so-called "double network" comprised of micellar entanglements and particle junctions. A simple model for such networks is proposed, where the elasticity can be tuned through two energetic scales, the micellar end-cap energy and micelle-nanoparticle adsorption energy. As a practical application, the results demonstrate that nanoparticle addition provides formulators a unique method to tailor surfactant solution rheology over a wide range of conditions.

Breakup of bubbles and drops in steadily sheared foams and concentrated emulsions.

This experimental study is focused on the process of bubble breakup in steadily sheared foams, at constant shear rate or constant shear stress. Two different types of surfactants were used and glycerol was added to the aqueous phase, to check how the bubble breakup depends on the surface modulus and on bulk viscosity of the foaming solutions. The experiments show that bubble breakup in foams occurs above a well defined critical dimensionless stress, tau[over]CR identical with(tauCRR/sigma) approximately 0.40, which is independent of surfactant used, solution viscosity, and bubble volume fraction (varied between 92 and 98%). Here tauCR is the dimensional shear stress, above which a bubble with radius R and surface tension sigma would break in sheared foam. The value of the critical stress experimentally found by us tau[over]CR approximately 0.40, is about two orders of magnitude lower than the critical stress for breakup of single bubbles in sheared Newtonian liquids, tau[over]CR approximately 25. This large difference in the critical stress is explained by the strong interaction between neighboring bubbles in densely populated foams, which facilitates bubble subdivision into smaller bubbles. A strong effect of bubble polydispersity on the kinetics of bubble breakup (at similar mean bubble size) was observed and explained. Experiments were also performed with hexadecane-in-water emulsions of drop volume fraction 83%

Theoretical model of viscous friction inside steadily sheared foams and concentrated emulsions.

In a recent Letter [N. D. Denkov, Phys. Rev. Lett. 100, 138301 (2008)] we calculated theoretically the macroscopic viscous stress of steadily sheared foam or emulsion from the energy dissipated inside the transient planar films, formed between neighboring bubbles or drops in the shear flow. The model predicts that the viscous stress in these systems should be proportional to Ca 1/2, where Ca is a capillary number and n=1/2 is the power-law index. In the current paper we explain our model in detail and develop it further in several aspects: First, we extend the model to account for the effects of viscous friction in the curved meniscus regions, surrounding the planar films, on the dynamics of film formation and on the total viscous stress. Second, we consider the effects of surface forces (electrostatic, van der Waals, etc.) acting between the surfaces of the neighboring bubbles or drops and show that these forces could be important in emulsions, due to the relatively small thickness of emulsion films (often comparable to the range of action of surface forces). In contrast, the surface forces are usually negligible in steadily sheared foams, because the dynamic foam films are thicker than the extent of surface forces, except for foams containing micrometer-sized bubbles and/or at very low shear rates. Third, additional consideration is made for bubbles or drops exhibiting high surface viscosity, for which we demonstrate an additional contribution to the macroscopic viscous stress, created by the surface dissipation of energy. The new upgraded model predicts that the energy dissipation at the bubble or drop surface leads to power-law index n<1/2 , whereas the contribution of the surface forces leads to n>1/2 , which explains the rich variety of foam or emulsion behaviors observed upon steady shear. Various comparisons are made between model predictions and experimental results for both foams and emulsions, and very good agreement is found.

Viscous friction in foams and concentrated emulsions under steady shear.

We present a model for the viscous friction in foams and concentrated emulsions, subject to steady shear flow. First, we calculate the energy dissipated due to viscous friction inside the films between two neighboring bubbles or drops, which slide along each other in the flow. Next, from this energy we calculate the macroscopic viscous stress of the sheared foam or emulsion. The model predictions agree well with experimental results obtained with foams and emulsions.

Interactions of a hydrophobically modified polymer with oppositely charged surfactants.

The interaction of a hydrophobically modified anionic polymer (PMAOVE) with a cationic surfactant (DTAB) was studied using a multi-technique approach: turbidity, surface tension, and viscosity measurements, as well as EPR (5-doxyl stearic acid) and fluorescence (pyrene) probe techniques were used. In the investigated pH range (4-10), the cationic surfactant headgroups interact with the anionic carboxylic groups of the polymer backbone. In addition, nonpolar interactions of the surfactant chains with the n-octyl chains of PMAOVE stabilize the PMAOVE-DTAB complexes. Charge neutralization of the anionic polymer by the cationic surfactant leads to precipitation of the PMAOVE-DTAB complex at a certain DTAB concentration range. Further addition of DTAB causes a charge reversal of the complex and, subsequently, resolubilization of the precipitate. At an acidic pH (pH = 4), a second precipitation was observed, which is probably caused by conformational changes in the PMAOVE-DTAB complex. This second precipitate can be resolubilized by further addition of surfactant. At a neutral and basic pH, this second precipitation is absent. EPR analysis indicates that the surfactants form an ordered structure at the extended polymer chain at a neutral and basic pH, whereas at an acidic pH, a less ordered surfactant layer is formed on the coiled polymer with more hydrophobic microdomains.