Fatty acid-asphaltene interactions at oil/water interface

Abstract

Asphaltenes have been shown to stabilize water-in-oil or oil-in-water emulsions by forming a viscoelastic interfacial film via molecular aggregation at oil-water interfaces. Natural carboxylic acids (“naphthenic acids”) and their anions present in crude oil are able to compete with asphaltenes to adsorb at the crude oil-water interface, decreasing significantly the crude oil-water interfacial tension. In this study, we designed a group of experiments to systematically probe the molecular interactions of naphthenic acid with asphaltenes at the oil-water interface by studying the Langmuir interfacial isotherms.Stearic acid as a representative naphthenic acid alone was not able to form rigid films at the toluene-water interface, in contrast to rigid interfacial films of asphaltenes. Upon mixing of asphaltenes with stearic acids, non-ideal (non-additive) behavior of interfacial isotherms was observed. Stearic acid was found to associate strongly with asphaltene molecules at the interface and render the films more expanded and flexible. The reduction in rigidity of interfacial film was found to be directly proportional to the amount of the stearic acid present in the system. Washing experiments by replacement of the top phase with fresh solvent showed irreversible adsorption of both asphaltenes and stearic acids at the toluene-water interface. The softening of interfacial film by stearic acid led to reduced compression energy of the interface, measurable quantitatively by defining the excess work of compression (Wexcess) as a measure of molecular interactions at the oil-water interface. The calculated Wexcess was found to increase with increasing stearic acid concentrations in the mixture as a result of softened interfacial films. In-situ Brewster angle microscopy visualization revealed a progressive reduction of molecular aggregates at the toluene-water interface with increasing addition of stearic acids.