Droplet polydispersity and shape fluctuations in AOT [bis(2-ethylhexyl)sulfosuccinate sodium salt] microemulsions studied by contrast variation small-angle neutron scattering.

Microemulsions consisting of AOT water, and decane or iso-octane are studied in the region of the phase diagram where surfactant covered water droplets are formed. The polydispersity and shape fluctuations of the microemulsion droplets are determined and compared in the two different alkane types. Conductivity measurements show that there is a pronounced dependence of the temperature behavior of the microemulsion on the type of alkane used. In both cases the microemulsion droplets start to form larger aggregates when the temperature increases. But in the system with decane this aggregation temperature occurs at a temperature about 10 degrees C lower than in a similar system with iso-octane. Aggregation phenomena are avoided and the two systems are at approximately the same reduced temperature with respect to the aggregation temperature when the temperature of the AOT/D(2)O/decane microemulsion is 10 degrees C and the temperature of the AOT/D(2)O/iso-octane microemulsion is 20 degrees C. Contrast variation small-angle neutron scattering measurements are performed at these temperatures on systems with volume fractions of 5% D2O+AOT by varying the scattering length density of the alkane. The small-angle scattering for 11 different contrasts evenly distributed around the match points are studied for each sample. The scattering data for the different contrasts are analyzed using a molecular constrained model for ellipsoidal droplets of water covered by AOT, interacting as polydisperse hard spheres. All contrasts are fitted simultaneously by taking the different contrast factors into account. The analysis show that at the same reduced temperature with respect to the aggregation temperature the droplet size, polydispersity index, the size of the shape fluctuations are similar in the two systems. A polydispersity index (sigma/R of the Gaussian size distribution) of 16% and an average axis ratio of the droplets of 1.56 is found in the AOT/D(2)O/decane microemulsion. In the AOT/D(2)O/iso-octane system the polydispersity index is also 16% while the axis ratio is 1.72. The bending elastic constant kappa and the Gaussian bending elastic constant kappa; can be estimated from these numbers. For AOT/D(2)O/decane we find kappa=3.4 k(B)T and kappa=-5.9 k(B)T and for AOT/D(2)O/iso-octane we find kappa=2.35k(B)T and kappa=-3.8k(B)T, where k(B) is the Boltzmann constant and T is the absolute temperature.