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 1H NMR study on the system soybean lecithin/water/perdeuterated cyclohexane is presented. The self-diffusion coefficient of water, Dw, was measured as a function of the water content (W0) and was found to show a bell-shaped trend. The composition at which the maximum in Dw 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 W0. 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.