Alcohol Partition in a Water-in-Oil Microemulsion:  Small-Angle Neutron-Scattering Contrast Measurements

We have earlier reported estimates of alcohol distribution between a water-in-oil microemulsion and the continuous hydrocarbon phase, based on the core surface area, the dimensions being determined by small-angle neutron scattering from the microemulsions with D2O cores. To make a more direct determination, we have carried out contrast measurements on three of the compositions with other components, except for the surfactant, potassium oleate, deuterated: water, hexadecane, and the alcohol 1-heptanol or 1-octanol. Patterns were obtained with individual components isotopically labeled and with combinations of components labeled. Of the models tested, oblate ellipsoids gave better fits to all contrasts for a given composition than prolate ellipsoids or polydisperse spheres. The parameters were also more plausible. A somewhat higher proportion of alcohol was found to be distributed in the particles than earlier estimated from surface-area arguments.