DIFFUSION IN SURFACTANT SYSTEMS STUDIED BY FORCED RAYLEIGH-SCATTERING

The technique of forced Rayleigh scattering (FRS) was used to obtain i nformation on the microstructure of two surfactant systems: i.e. bis(2 -ethylhexyl) sulfosuccinate (AOT)/octanol/water and tetradecyldimethyl ammonium oxide (TDMAO)/tetradecyltrimethylammonium bromide (TTMAB)/hex anol/water Diffusion coefficients of the dye molecules congo red and,m ethyl red, and of surfactant aggregates labeled with these dye probes, have been determined. In the L(2) (microemulsion) phase of AOT/octano l/water stained with congo red, diffusion coefficients have been found to decrease with increasing water content. In contrast, at the transi tion from the L(2) to the cubic phase, a continuous increase was obser ved. 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 situa tion is different in the cubic phase: here the long range order makes it possible for the dye molecules to diffuse in the water channels. Co mpared to unhindered motion in aqueous solution, the diffusion coeffic ient is reduced to about 20% by steric obstruction effects. The contin uous increase of the diffusion coefficient at the phase transition ref lects the close structural relationship between the two phases near th e phase boundary, whilst there is a remarkable difference in the L(2) phase further away. For the system TDMAO/TTMAB/hexanol/water the trans ition from the micellar L(1) to the vesicular L(alpha) phase with incr easing alcohol content has been investigated. The lipophilic dye methy l red is anchored in the surfactant layers, as is verified by polariza tion microscopy. In the L(1) phase there is a transition from spherica l to rodlike micelles. Self-diffusion coefficients of these spheres an d rods have been measured, and the results have been compared to theor etical values, calculated by the model of Edwards and Doi for concentr ated solutions of rodlike molecules. In the L(alpha) phase the vesicle s are densely packed, so that the diffusion coefficients originate fro m the motion of the dye probe within the surfactant bilayers and fi om an exchange between different bilayers. In complementary experiments, the mobility of the vesicles in an electric field has been probed.