Diffusion in Surfactant Systems Studied by Forced Rayleigh Scattering

The technique of forced Rayleigh scattering (FRS) was used to obtain information on the microstructure of two surfactant systems: i.e. bis(2-ethylhexyl) sulfosuccinate (AOT)/octanol/water and tetradecyldimethylammonium oxide (TDMAO)/tetradecyltrimethylammonium bromide (TTMAB)/hexanol/water. Diffusion coefficients of the dye molecules congo red and methyl red, and of surfactant aggregates labeled with these dye probes, have been determined. In the L 2 (microemulsion) phase of AOT/octanol/water stained with congo red, diffusion coefficients have been found to decrease with increasing water content. In contrast, at the transition from the L 2 to the cubic phase, a continuous increase was observed. Although submicroscopically bicontinuous structures may exist in the microemulsion, the interconnectivity of the rodlike aggregates is not sufficient to enable free diffusion of the water soluble dye over the distance range investigated by FRS. Thus the determined diffusion coefficients reflect the motion of dynamic structural units. The situation is different in the cubic phase: here the long range order makes it possible for the dye molecules to diffuse in the water channels. Compared to unhindered motion in aqueous solution, the diffusion coefficient is reduced to about 20% by steric obstruction effects. The continuous increase of the diffusion coefficient at the phase transition reflects the close structural relationship between the two phases near the phase boundary, whilst there is a remarkable difference in the L 2 phase further away. For the system TDMAO/TTMAB/hexano1/water the transition from the micellar L 1 to the vesicular L α phase with increasing alcohol content has been investigated. The lipophilic dye methyl red is anchored in the surfactant layers, as is verified by polarization microscopy. In the L 1 phase there is a transition from spherical to rodlike micelles. Self-diffusion coefficients of these spheres and rods have been measured, and the results have been compared to theoretical values, calculated by the model of Edwards and Doi for concentrated solutions of rodlike molecules. In the L α phase the vesicles are densely packed, so that the diffusion coefficients originate from the motion of the dye probe within the surfactant bilayers and from an exchange between different bilayers. In complementary experiments, the mobility of the vesicles in an electric field has been probed.