STRUCTURE OF A COSURFACTANT-FREE MICROEMULSION

Formation of single-phase microemulsions in toluene/water/nonionic surfactant systems

Mixtures of toluene and water from 5 to 50% oil fraction and 5 to 25% surfactant by weight were studied. Winsor Type IV microemulsions were formed in numerous cases. Review of partial ternary phase diagrams for these systems indicated the area of single-phase microemulsion with toluene could be maximized at an hydrophilic-lipophilic balance (HLB) of approximately 14.5. Select single-phase samples were further analyzed by surface tension and dynamic light scattering techniques, which allowed a detailed characterization of the solution equilibrium thermodynamics and size stability. Particle sizes averaged approximately 5 nm and were nearly constant over a wide variety of conditions and for 6-18 months. When benzyl alcohol was used instead of toluene, the optimum HLB for the formation of single-phase systems was found to have a lower limit of 17. Particle sizes in these systems were <30 nm but showed greater variability. The decrease in particle size as surfactant concentration increased was determined to be associated with changes in ethlyene oxide chain conformation. The increase in particle size due to swelling with increased oil concentration was used to determine the surfactant surface area in the oil phase. A detailed comparison of alkylamine ethoxylate to octyl- and nonylphenol ethoxylate surfactants in terms of micelle thermodynamics, size, and stability indicate that the alkylamine-based surfactants are potential candidates for the replacement of nonylphenol-based surfactants in some systems with a more polar oil phase like benzyl alcohol.