Spontaneous Formation of Bilayers and Vesicles in Mixtures of Single-Chain Alkyl Carboxylates: Effect of pH and Aging and Cytotoxicity Studies N. Vlachy, † C. Merle, † D. Touraud, † J. Schmidt, ‡ Y. Talmon, ‡ J. Heilmann, § and W. Kunz* ,† Institute of Physical and Theoretical Chemistry and Department of Pharmaceutical Biology, UniVersity of Regensburg, D-93040 Regensburg, Germany, and Department of Chemical Engineering, Technion-Israel Institute of Technology, 32000 Haifa, Israel ReceiVed March 6, 2008. ReVised Manuscript ReceiVed June 15, 2008 We report the observation of bilayer fragments, some of which close to form vesicles, over a large range of pH at room temperature from mixtures of single-chain biocompatible commercially available nontoxic alkyl carboxylic surfactants after neutralization with HCl. The pH at which the morphological transitions occur is varied only by changing the ratio between two surfactants: the alkyloligoethyleneoxide carboxylate and sodium laurate. The effect of aging of the mixed surfactant systems in the pH region desired for dermatologic application (4.5 < pH < 7) is also studied. Finally, we show results of cytotoxicity studies on the surfactant mixtures. Introduction There is growing interest in the field of synthetic surfactants used for dermatologic purposes. These surfactants exhibit a wide range of structures: particularly useful is the formation of vesicles, which can be used as delivery systems. For cosmetic reasons, the surfactants forming vesicles must be skin compatible (nonirritant), easy to manufacture, and the vesicle region must be stable within the range of physiological pH at room temperature. A simple geometrical characterization of chain packing can be used to analyze trends in surfactant phase behavior. 1,2 The geometric properties of surfactants depend on the ratio between the cross-sectional area of the hydrocarbon part and that of the headgroup. Low packing parameters (around 1/3) are found for single-chain surfactants with a strongly polar headgroup. These systems tend to form spherical micelles, whereas a packing parameter value above 0.5 favors the formation of vesicles. When the packing parameter is increased even further (P ≈ 1) lamellar plates form. An increase in the packing parameter can be obtained by adding a second chain, which doubles the hydrocarbon volume. Double-chain surfactants, 3,4 two surfactants of opposite charge, 5-8 or the association of a surfactant and a cosurfactant, 9-15 can be used. In the latter two cases, a pseudo-double-chain surfactant is obtained by either an ion-pair formation between the anionic and cationic surfactant or association of the two different molecules via hydrogen bonds. As cationic surfactants and short alcohols are undesirable in cosmetic formulations due to their toxicity, and because long- chain alcohols (>C 12 ) exhibit high melting points, monoalkyl carboxylates were chosen for our study. Fatty acids form a range of aggregates depending on the acid concentration and the ionization degree of the terminal carboxylic group. 16,17 The formation of vesicles from monocarboxylic acids has long been known. Gebicki and Hicks 18 first observed the formation of vesicles from unsaturated, long-chain fatty acids. Later, Har- greaves and Deamer also showed that saturated fatty acids can form vesicles. 19 A vesicle phase is spontaneously formed when short- or middle-chain (<C 12 ) fatty acids are neutralized with HCl; 19,20 two types of amphiphiles are then present in solution: the protonated and the ionized forms. The ratio between the two determines the aggregation morphology. Despite the simplicity of the mechanism of vesicle formation, possible applications of fatty acid vesicles in cosmetics remain largely unexplored. 20 This may be a consequence of two obstacles: (1) the high solubility temperature of the long-chain carboxylates and (2) the generally too basic pH necessary for the solubilization of the carboxylate. The problem of the solubility temperature of the alkyl carboxylates has been discussed by Hargreaves et al. 19 Below 25 °C, only alkyl carboxylates with short alkyl chains are water-soluble. However, these are inappropriate due to their skin irritating properties. The solubility temperature of sodium laurate is reported to be equal to or above the room temperature, depending on the † Institute of Physical and Theoretical Chemistry and Department of Pharmaceutical Biology, University of Regensburg. ‡ Technion-Israel Institute of Technology. § Department of Pharmaceutical Biology, University of Regensburg. (1) Tanford, C. J. Phys. 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