Electrokinetic and adsorption properties of asphaltenes

Abstract

The electrophoretic mobilities of asphaltenes were measured in an aqueous medium and an attempt was made to measure their electrokinetic properties in non-aqueous solvents. The asphaltenes were crushed to fine particles and suspended in the liquid medium. The electrophoretic mobilities of the asphaltenes were determined as functions of pH, ionic strength and ionic composition. These asphaltenes carried a positive or negative charge in aqueous solutions and the magnitude of the charge was a function of the pH, ionic strength and the amount of hardness in the electrolyte solution. A charge reversal was observed for some cases especially in the presence of multivalent cations at high pH. Attempts to measure the electrophoretic mobility of asphaltenes dispersed in a crude oil were unsuccessful due to the low permittivity of the oil phase. However, it was possible to measure the electrophoretic mobility of asphaltenes in a non-aqueous solvent for example, nitromethane. The electrophoretic mobilities of some rock minerals were not altered by the adsorption of asphaltenes. The adsorption of asphaltenes and other heavy ends in the oil on to the rock surface is greatly influenced by the surface charge on asphaltenes as well as that on the rock minerals. The adsorptions of asphaltenes from toluene solution on to various rock minerals were determined. Langmuir Type 1 adsorption isotherms were obtained with all minerals indicating monolayer adsorption.