R. Angelico et al., WATER DIFFUSION AND HEADGROUP MOBILITY IN POLYMER-LIKE REVERSE MICELLES - EVIDENCE OF A SPHERE-TO-ROD-TO-SPHERE TRANSITION, JOURNAL OF PHYSICAL CHEMISTRY B, 102(16), 1998, pp. 2883-2889
A pulsed gradient spin-echo FT H-1 NMR study on the system soybean lec
ithin/water/perdeuterated cyclohexane is presented. The self-diffusion
coefficient of water, D-w, was measured as a function of the water co
ntent (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 o
f a water diffusion inside wormlike reverse micelles which increase th
eir length upon increasing W-0. On the contrary, the descending part i
ndicates a rod-to-sphere evolution of the aggregate's shape. This inte
rpretation is supported by measurement on the headgroup rotation by di
electric spectroscopy. The composition at which a structural transitio
n 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.