Water diffusion in polymer-like reverse micelles. 2. Composition dependence

A pulsed field gradient (FTH)-H-1 NMR study on the system soybean lecithin/water/perdeuterated cyclohexane, covering a wide range of compositions, is presented. The water diffusion was found to be Gaussian with a well-defined, time-independent, single diffusion coefficient. After the contribution from the diffusion of water dissolved in the cyclohexane has been properly taken into account, all the data follow the same master curve as a function of the water to lecithin molar ratio (W-0) The dominating mechanism for the water diffusion was found to be the motion inside giant wormlike reverse aggregates mediated by an interaggregates exchange with a characteristic time on the order of microseconds. For all the lecithin concentrations investigated, increasing the value of W-0 increases the water diffusion up to W-0 = 15, where a structural transition toward spherical aggregates causes a decrease in the water diffusion.