Adsorption and Lubrication of Mixtures Consisting of Cationic Surfactant and Long-Chain Alcohol in Aqueous Media.

We studied the adsorption and lubrication of cationic surfactants on silica in the presence of 1-hexadecanol (C16OH). The surfactants employed in this study were quaternary ammonium salts with different alkyl chain lengths (alkyltrimethylammonium chloride, CnTAC, where n is the alkyl chain length of 14, 16, and 18). The mixing of CnTAC, C16OH, and water generally yields the structure of "α-form hydrated crystals" or the structural unit of "lamellar gel networks" at appropriate compositions. In these structures, the alkyl chains of CnTAC and C16OH are packed in the hexagonal arrangement within lamellar bilayers. Quartz crystal microbalance with dissipation monitoring technique suggested that the adsorbed vesicular aggregates of CnTAC and C16OH flattened on the silica surface. The larger vesicular aggregates formed a viscosity-dominant thicker adsorption film in the initial stage, and then experienced the flattening more significantly to yield an elasticity-dominant thin film. At a given size of the vesicular aggregates, the alkyl chain length of CnTAC did not make large impact on the adsorption in their steady state. The adsorption film achieved excellent lubricity, being irrespective on the size of the vesicular aggregates and the alkyl chain length of CnTAC. We anticipate that these findings will be useful in formulating "α-gels" in various personal care applications.