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Pyrene and pyrene-labeled poly(acry1ic acid) were used as fluorescence probes to investigate the interactions between poly(acry1ic acid) and dodecyltrimethylammonium bromide. The polarity dependence of the vibrational structure of the pyrene emission spectrum indicated the formation of micelle-like surfactant aggregates at concentrations that are significantly below the critical micelle concentration (cmc). The extent of excimer formation of the polymeric probe was found to depend significantly upon the surfactant concentration. These results were interpreted in terms of polymer conformational changes induced by surfactant as well as the static versus dynamic nature of excimer formation.

SynopsisStratum 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. R esum eLes lipides des bicouchesdu stratum corneum (SC), les acides gras en particulier, les c eramides, le cholest erol contribuent a la fonction de barri ere de perm eabilit e de la peau. Le nettoyage normal de la peau est associ e a des dommages aux lipides du SC parce que les tensioactifs nettoyants, en plus de fournir l'effet d esir e de solubilisation et d' elimination du s ebum et des souillures de la peau, ont une propension a perturber les lipides des bicouches par l'extraction de lipides endog enesde la peau ou ceux intercal es dans les bicouches. La perturbation des lipides du SC est associ ee a une vari et e d'affections cutan ees pathologiques, ainsi qu'avec la peau s eche induite par le nettoyage agressif. Dans une tentative pour pr eserver la barri ere et att enuer les d egâts caus es par le nettoyage fr equent normal, l'incorporation de lipides physiologiquement pertinents dans les nettoyants pour la peau est devenu habituel pour les produits de nettoyage de de qualit e. Il a et e not e que les acides gras sont plus sensibles a l' elimination induite par les tensio-actifs que d'autres lipides (par exemple, les c eramides), une observation qui pourrait constituer la base d'une strat egie cruciale pour la reconstitution des lipides du SC. Cet examen portera sur le rôle des acides gras dans la structure et la fonction du SC, et la justification de l'incorporation de l'acide st earique dans les nettoyants hydratants pour le corps afin de minimiser leur extraction par des agents tensioactifs et de reconstituer les acides gras perdus pour promouvoir la pr eservation de la barri ere cutan ee.

Alpha-hydroxy acids are effective agents for the treatment of skin xerosis and it is known that, following treatment with lotions containing D,L-lactic acid, the stratum corneum prevents xerosis more effectively. To date, the relative efficacy of the different isomers of lactic acid has not been evaluated and the mode of action of lactic acid in improving stratum corneum resilience is not known. The objective of the present studies was to determine the effects of lactic acid isomers on keratinocyte ceramide biosynthesis, stratum corneum barrier function and the resistance of the stratum corneum to the appearance of skin xerosis. In vitro, lactic acid enhanced the production of ceramides by keratinocytes. L-Lactic acid was more effective than the D isomer (300% increase vs 100% increase). Carbon label from lactic acid was incorporated into all keratinocyte lipid species and a greater incorporation of label into ceramides was achieved with L-lactate than with D-lactate. In vivo, lactic acid increased the levels of stratum corneum ceramides. Whereas, lotions containing L-lactic acid resulted in the greatest increase (48% increase) followed by D,L-lactic acid (25% increase), D-lactic acid had no effect on stratum corneum ceramide levels. The increases in stratum corneum ceramide levels following lactic acid treatment also led to improvements in stratum corneum barrier function, measured by transepidermal water loss following a challenge to the skin with SLS and in the regression phase of a moisturization efficacy study. Significant improvements in barrier function and resistance to the appearance of skin xerosis were observed following L-lactic acid and D,L-lactic acid, but not following D-lactic acid treatment. From these results we believe that lactic acid, particularly the L isomer, stimulates ceramide biosynthesis leading to increased stratum corneum ceramide levels which results in superior lipid barrier and a more effective resistance against xerosis.

Alpha-hydroxy acids are effective agents for the treatment of skin xerosis and it is known that, following treatment with lotions containing D,L-lactic acid, the stratum corneum prevents xerosis more effectively. To date, the relative efficacy of the different isomers of lactic acid has not been evaluated and the mode of action of lactic acid in improving stratum corneum resilience is not known. The objective of the present studies was to determine the effects of lactic acid isomers on keratinocyte ceramide biosynthesis, stratum corneum barrier function and the resistance of the stratum corneum to the appearance of skin xerosis. In vitro, lactic acid enhanced the production of ceramides by keratinocytes. L-Lactic acid was more effective than the D isomer (300% increase vs 100% increase). Carbon label from lactic acid was incorporated into all keratinocyte lipid species and a greater incorporation of label into ceramides was achieved with L-lactate than with D-lactate. In vivo, lactic acid increased the levels of stratum corneum ceramides. Whereas, lotions containing L-lactic acid resulted in the greatest increase (48% increase) followed by D,L-lactic acid (25% increase), D-lactic acid had no effect on stratum corneum ceramide levels. The increases in stratum corneum ceramide levels following lactic acid treatment also led to improvements in stratum corneum barrier function, measured by transepidermal water loss following a challenge to the skin with SLS and in the regression phase of a moisturization efficacy study. Significant improvements in barrier function and resistance to the appearance of skin xerosis were observed following L-lactic acid and D,L-lactic acid, but not following D-lactic acid treatment. From these results we believe that lactic acid, particularly the L isomer, stimulates ceramide biosynthesis leading to increased stratum corneum ceramide levels which results in superior lipid barrier and a more effective resistance against xerosis.

Three nitroxide spin probes (5-, 12-, and 16-doxylstearic acid) were used to investigate the structure of sodium dodecyl sulfate (SDS) micelles and the interfacial layer between water and alumina formed by SDS (hemimicelles of SDS). It was found that whereas the rotational correlation times of the three probes differ in SDS micelles, the effective microviscosities of the probes are the same. In contrast, the microviscosity within the SDS hemimicelles varies according to the distance from the alumina surface: the nitroxide sees a more ordered environment as it is placed closer to the alumina. This represents the first report of variations in flexibility (microviscosity) within a hemimicelle.

Our results demonstrated that there are characteristic differences in the barrier property between normal and cosmetic dry skin. In comparison to the normal skin, the stratum corneum of the cosmetic dry skin is considerably thicker, however, with a lower barrier quality. The results also showed that the amount of the SC removal in sequential tape-stripping is generally non-uniform. Therefore, the number of tape strips is not a good indicator for the tape-stripping depth.