The role of water solubility of the oil phase in the formation of miniemulsions

Abstract

Oil-in-water miniemulsions are prepared at slow agitation by an emulsification process that involves the use of mixed emulsifier solutions of an ionic surfactant (sodium lauryl sulfate) and a long chain fatty alcohol (cetyl alcohol) in low concentrations, 1–2% by weight based on the oil phase. The effect of the water solubility of the oil phase on the ease of miniemulsion formation and degradation was studied. This was done by observing the shelf-life and ultracentrifugation stability, by measuring the conductivity, and by determining the partitioning of cetyl alcohol and sodium lauryl sulfate between the aqueous phase and the dispersed phase. Miniemulsions of oils with relatively high water solubility (> 2 × 10 −3 M) are formed with ease and at a faster rate than those with lower water solubility. The phenomenon of spontaneous emulsification is also observed when an oil phase of a high water solubility is used. However, these miniemulsions also degrade faster during aging experiments at room temperature. Concomitantly, the fraction of sodium lauryl sulfate in the continuous phase increases, but virtually all cetyl alcohol present in the system remains in the oil phase. A formation mechanism is presented, comprising the swelling of large mixed emulsifier structures with solvent and a concurrent break-down of these swollen structures. It is suggested that the driving force for miniemulsion formation is the gain in entropy of mixing during the net transfer of cetyl alcohol from the aqueous phase to the oil phase.