Development of a New Surfactant: <i>N</i> -Hexadecyl- <i>N</i> , <i>N</i> -Dimethylammonium Acetate, and its Oil-in-Water Gel’s Longitudinal Stability

A more environmentally benign surfactant, N-Hexadecyl-N, N-Dimethylammonium Acetate (HDAA) was synthesized first. The surfactant formed an oil gel in the organic solvent octane. In an experimental vial, an octane gel (gel-emulsion) was obtained by adding surfactant HDAA and 50 times the equivalent amount of water (mole) to octane. The resulting gel had weak retention power and transformed into a sol in ten days, as a time-dependent longitudinal study showed. It was further reported that the addition of a small amount of highly biodegradable surfactant Sodium Hexadecyl Sulfate (SHDS) as an additive (0.05–0.5 times of HDAA) formed octane gel with less amount of the main HDAA surfactant. The mixed gel had a strong retention power and maintained a gel state for more than 30 days. The formation of fiber-aggregated structure by mixed SHDS-HDAA surfactants and its coexistence with the emulsion led to the improvement in retention power. Visual phase observation, rheological measurement and Confocal Laser Scanning Microscopy (CLSM) were applied to study the formation and stability of the above gel-sol matrices.