Interfacial turbulence and spontaneous emulsification in alkali—acidic oil systems

Abstract

The phenomenon of spontaneous emulsification, which takes place between an alkaline solution and an oil containing an organic acid, has both scientific interest in the modeling of mass transfer processes and practical applications of industrial importance in processes such as the enhanced recovery of crude oil. We have conducted an experimental investigation in order to determine the mechanisms responsible for the spontaneous emulsification observed in reacting alkali—acidic oil systems with and without added preformed surfactant. Both model oils (i.e. high and low viscosity) and a crude oil are used in this study, along with a petroleum sulfonate as a preformed surfactant. The effects of pH, sodium concentration, and preformed surfactant are studied on the spontaneous emulsification phenomenon. Interfacial turbulence as a means for spontaneous emulsification has been established by direct microscopic observation of roll cells in both the model and crude-oil systems. We have observed that the roll-cell size and formation time depend strongly on the pH and ionic strength of the alkaline solution. For a particular roll-cell size, the addition of preformed surfactant causes the cells to take longer to form, causing an interfacial resistance to mass transfer and making the interface more rigid. We show that interfacial turbulence is a necessary but not sufficient condition for spontaneous emulsification. Low interfacial tension is also a necessary condition, along with cellular rotational velocity, which is found to increase with pH. The effect of pH can be explained in terms of the concentration gradient of unionized acid in the oil phase, the sensitivity of interfacial tension to preformed surfactant concentration, and the rate of acid ionization. Moreover, these systems appear ideal for investigating the interfacial turbulence phenomenon.