SELF-DIFFUSION IN NONIONIC SURFACTANT WATER-SYSTEMS

The self-diffusion behavior of the micellar phase in some aqueous solutions of the nonionic dodecyloligoethylene oxide surfactants C12E5 and C12E8 was analyzed according to the cell-diffusion model. Our calculations show that the hydration number per ethylene oxide (EO) group is independent of the surfactant concentration. The maximum extension of the headgroup shell for a C12E8 micelle was estimated to be around 18 angstrom at both 5 and 66-degrees-C, corresponding to an average mean water concentration of 8-9 water molecules per EO group. By comparing the water diffusion within the micellar headgroup shell with the water diffusion in poly(ethylene oxide)-water solutions of the same concentration expressed in water molecules per ethylene oxide group, our calculations show that the micellar water content decreases mainly from the innermost ethylene oxide groups at increasing temperatures. This approach further allows an analysis of the surfactant concentrated regime, where no "free" water exists. The experimental data are then found to be consistent with a water continuous structure up to at least 80 wt % C12E8 at 70-degrees-C. We also present a model that accounts for the retardation of the micellar diffusion constant due to obstruction between prolate- and oblate-shaped aggregates. With this model, in combination with the results obtained from the cell-diffusion model, we find the observed surfactant diffusion data to be consistent with rodlike micelles with an axial ratio of 1:40 for C12E5 at 25-degrees-C and an axial ratio of 1:10 for C12E8 at 70-degrees-C over a large concentration interval.