Interaction Forces between Asphaltene Surfaces in Organic Solvents

TLDR

The colloidal interactions between asphaltene surfaces in heptol, a mixture of n‑heptane and toluene, were studied for the first time by colloidal force measurements using an atomic force microscope (AFM). Asphaltenes were deposited on silica wafers and silica spheres via Langmuir‑Blodgett upstroke and their forces measured with an AFM. The study found that varying the toluene‑to‑heptane ratio in heptol alters solvent quality, with pure toluene producing steric long‑range repulsion described by polymer‑brush scaling, while decreasing toluene below 20 % reduces this repulsion to a weak attraction that in pure heptane is governed by van der Waals forces, thereby promoting asphaltene aggregation and precipitation and offering insight into asphaltene behavior in crude oils.

Abstract

The colloidal interactions between asphaltene surfaces in heptol, a mixture of n-heptane and toluene, were studied for the first time by colloidal force measurements using an atomic force microscope (AFM). Asphaltenes were deposited on silica wafers and silica spheres using the Langmuir-Blodgett upstroke technique. The results showed that the ratio of toluene to heptane can significantly change solvent quality in terms of the ability to solubilize asphaltenes and hence the nature and the magnitude of the interaction forces between asphaltene surfaces. In pure toluene, there is a steric long-range repulsion which can be well fitted by the scaling theory of polymer brushes. As toluene volume fraction in heptol (Phi(T)) is gradually decreased from Phi(T) = 1 (pure toluene) to Phi(T) = 0 (pure n-heptane), the steric repulsion reduced and changed to weak attraction when Phi(T) < 0.2. The attraction in heptane can be fitted by van der Waals forces alone which are thus believed to promote asphaltene aggregation, leading to asphaltene precipitation. The results obtained in this study provide an insight into interactions that determine asphaltene behavior in an organic medium and hence in crude oils.