Alcohol partition in a water-in-oil microemulsion from small-angle neutron scattering

It is often necessary to add a cosurfactant, typically an alcohol of medium chain length, to water oil, and surfactant for a microemulsion to form. To gain information on the role of alcohols, we have measured small-angle neutron scattering (SANS) from microemulsions comprised of n-hexadecane, potassium oleate, and water, plus several concentrations of 1-pentanol, 1-hexanol, and 1-octanol, in two regions of the phase diagram. Several models, including polydisperse spheres interpreted by recently developed equations for the structure function, were tested. Experimental data could be fit successfully with the core-plus-shell monodisperse-oblate-ellipsoids model previously used for similar compositions. Variation of the alcohol-to-surfactant molar ratios, particle dimensions increase with alcohol chain length in both regions of the phase diagrams. The water phase volume can reasonably by assumed constant, and estimates of the amount of alcohol at the water-surfactant interface were made from the parameters obtained from the fits. At fixed alcohol-to-surfactant molar ratios, we found a lower fraction of larger alcohols at the interface, in agreement with the increase of hydrophobicity. For a given alcohol, increasing the alcohol-to-surfactant ratio increased the amount of alcohol at the interface, although the fraction of total alcohol in the particles decreases. 32 refs., 14 figs., 9 tabs.