Asphaltene-silica interactions in aqueous solutions: Direct force measurements combined with electrokinetic studies

An atomic force microscope (AFM) was used to study the interaction between asphaltenes and silica surfaces in aqueous solutions directly. Electrokinetic measurements were performed on similar systems to complement the AFM results. Asplialtene-silica interactions in aqueous solutions exhibited a time-dependent characteristic, A repulsive force component developed with increasing sample incubation time, suggesting molecular rearrangements at the asphaltene-water interface. Upon the addition of 1.0 mM KCl, the repulsive force was suppressed, and the measured force became overall attractive, implying an electrostatic nature for this repulsive force. An increase of the solution pH in the same system eliminated the attractive force regime through the development of a stronger repulsive force component. The increase in the range and magnitude of the asphaltene-silica interaction with increasing solution pH clearly indicates the presence of pH-dependent ionizable groups on the asphaltene surface. The measured long-range force can be very well accounted for by the Poisson-Boltzmann (PB) equation using a constant Stern layer potential boundary condition. The fitted Stern layer potential,; of asphaltene surfaces (psi(as)) became more negative with increasing pH, In the case of asphaltenes at high solution pH values, a satisfactory agreement was obtained between the fitted Stern layer potential based on the AFM force measurements and the electrokinetic potential measured using the electrophoresis method. The swelling/stretching of surface asphaltene molecules at high solution pH and the sensitivity of such stretched layers to added salt were also evident from the force-distance data. The measured adhesion force correlated well with the long-range force behavior.