Micellization and adsorption properties of novel Zwitterionic surfactants

A series of novel zwitterionic surfactants each with two hydrophilic and tw o hydrophobic groups in the molecule (so-called heterogemini surfactants) h as been synthesized. One of the hydrophilic groups is a phosphodiester anio n and the other is a quaternary ammonium salt. Two methylene groups separat e the two headgroups. The critical micelle concentration values of the surf actants were determined using du Nouy tensiometry and steady-state fluoresc ence 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 pac ked. NMR self-diffusion measurements gave information about the micelle siz e distribution. Abroad distribution of self-diffusion coefficients was obse rved and indicated that the time scale of monomer-aggregate and/or aggregat e-aggregate exchange is slow compared to the NT M time scale used (100 ms). A mean aggregate size of about 55 nm is obtained for one sample. Adsorptio n 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, indica ting a very tight packing of surfactant molecules. At higher surfactant con centrations all gemini surfactant gave very high adsorbed amount, most prob ably due to formation of aggregates at the surface.