Micellization and adsorption properties of novel zwitterionic surfactants

A series of novel zwitterionic surfactants each with two hydrophilic and two hydrophobic groups in the molecule (so-called heterogemini surfactants) has been synthesized. One of the hydrophilic groups is a phosphodiester anion and the other is a quaternary ammonium salt. Two methylene groups separate the two headgroups. The critical micelle concentration values of the surfactants were determined using du Nouy tensiometry and steady-state fluorescence and are of the order of 10^-5 M. Very low surface areas per molecule were observed suggesting that the monolayer formed is extremely tightly packed. NMR self-diffusion measurements gave information about the micelle size distribution. A broad distribution of self-diffusion coefficients was observed and indicated that the time scale of monomer-aggregate and/or aggregate-aggregate exchange is slow compared to the NMR time scale used (100 ms). A mean aggregate size of about 55 nm is obtained for one sample. Adsorption of the gemini surfactants at hydrophilic and at hydrophobized silica was studied by reflectometry. The more symmetrical gemini surfactants gave very low values of surface area per molecule on the hydrophobic surface, indicating a very tight packing of surfactant molecules. At higher surfactant concentrations all gemini surfactant gave very high adsorbed amount, most probably due to formation of aggregates at the surface.