Water diffusion and headgroup mobility in polymer-like reverse micelles: evidence of a sphere-to-rod-to-sphere transition

A pulsed gradient spin-echo FT H-1 NMR study on the system soybean lecithin/water/perdeuterated cyclohexane is presented. The self-diffusion coefficient of water, D-w, was measured as a function of the water content (W-0) and was found to show a bell-shaped trend. The composition at which the maximum in D-w occurs is the same at which the viscosity is the highest. We rationalize the rising part of the plot in terms of a water diffusion inside wormlike reverse micelles which increase their length upon increasing W-0. On the contrary, the descending part indicates a rod-to-sphere evolution of the aggregate's shape. This interpretation is supported by measurement on the headgroup rotation by dielectric spectroscopy. The composition at which a structural transition occurs and the location of the phase boundary can be predicted by a geometrical model in which the effective packing parameter is equal to 1.58. This value is in agreement with small-angle neutron scattering data in the literature.