Aggregation\nCharacteristics of Biobased Anionic Surfactant,\nHydroxy Alkane Sulfonate in Aqueous CaCl<sub>2</sub> Solutions: Vesicle\nand Supported Bilayer Formation

Vesicles are known to spontaneously adsorb onto solid–liquid\ninterfaces and to form supported bilayers in aqueous solution. Cationic\nsurfactants have typically been used to generate supported bilayers\nbecause solid surfaces in water are often negatively charged. The\npresent study investigated the aggregation behavior of an anionic\nsurfactant, hydroxy alkane sulfonate having a C18 alkyl chain (C18HAS)\nin aqueous CaCl<sub>2</sub> solutions. These assessments were performed\nby acquiring data related to equilibrium surface tension, the solubilization\nof an oil-soluble dye, UV–visible transmittance, pyrene fluorescence\nand dynamic light scattering together with freeze-fracture transmission\nelectron microscopy observations. The results suggest that C18HAS\ncan form vesicles in aqueous CaCl<sub>2</sub> solutions under certain\nsurfactant concentrations. Specifically, this aggregation behavior\nis greatly affected by C18HAS/CaCl<sub>2</sub> molar ratio. At the\nC18HAS/CaCl<sub>2</sub> molar ratio is less than an equivalence point\n(that is, less than 2:1), phase separation occurs with the formation\nof a vesicle above solubility limit of the C18HAS Ca salt. On the\nother hand, in the case that the C18HAS/CaCl<sub>2</sub> molar ratio\nis above an equivalence point (that is, above 2:1), the Na salt of\nC18HAS forms micelles above the critical micelle concentration (cmc),\ncausing solubilization of vesicles. Analyses by high-speed atomic\nforce microscopy demonstrated that the C18HAS vesicles can spontaneously\nform a supported bilayer on a negatively charged mica surfaces, similar\nto the behavior of cationic surfactant vesicles, even though C18HAS\nis an anionic surfactant. These results suggest that C18HAS could\nserve as a detergent component but also as a surface modifier when\nthe C18HAS/CaCl<sub>2</sub> molar ratio is optimized.