Effect of chain lengths of n-alcohol on the formation of single-phase microemulsions in n-heptane/n-alcohol/sodium dodecyl sulfate/water systems

To elucidate the effects of chain lengths of alcohols (1-butanol, 1-hexanol, and 1-octanol) as cosurfactants on the formation of a single-phase microemulsion (L-region) in n-heptane/n-alcohol/sodium dodecyl sulfate (SDS)/water systems, the phase diagrams of pseudoternary systems were determined under a fixed SDS/alcohol weight ratio (= 0.5). Conductivity, viscosity and light scattering intensity of the L-region were measured at 30 degrees C, and the results discussed from the microstructure of these systems. Using the SDS-free model system, the distribution of alcohol and the interfacial tension between oil (heptane) and water phases were measured in the presence of alcohol. The difference in distribution of alcohols resulting from differences in the chain lengths was found to be one of the key factors determining the extensions of the region of microemulsion and the microstructures. The L-regions in the hexanol and octanol systems were much smaller than in the butanol system, The continuous phases of these systems were the oil-rich medium (water-in-oh-type) throughout the L-region with increasing the water content. The L-region in the butanol system, on the other hand, was larger and the continuous phases of this system changed gradually from the oil-rich to the water-rich medium (oil-in-water-type) through the bicontinuous medium (bicontinuous-type) with increasing the water content.