Probing the Effects of Ca2+, Mg2+, and SO42-on Calcite-Oil Interactions by "soft Tip" Atomic Force Microscopy (AFM)

The interactions of crude oil with calcite surface are influenced by the Ca2+, Mg2+, and SO42- ions in the aqueous solution in the context of wettability alteration and enhanced oil recovery; however, their effects are not well understood from the fundamental perspective. The force spectroscopy techniques that measure the attractive and repulsive forces between oil droplets and solid surfaces are crucial in developing this understanding. In this study, we investigated the effects of Ca2+, Mg2+, and SO42- ions on calcite-oil interactions by immobilizing a droplet of crude oil to a tipless cantilever and used this "soft tip"to probe the calcite surface in different salt solutions with atomic force microscopy (AFM). The force behaviors at approach curves and the adhesion values at retract curves showed that the attractive calcite-oil interactions in the NaCl solution were enhanced by Mg2+ ions while the attractive interactions were mitigated by Ca2+ ions. In contrast, the calcite-oil interactions became repulsive in the presence of SO42- ions with an observation of small adhesions in the Na2SO4 solution. The adhesion works were correlated to the "effective contact angles"to evaluate the effects of Ca2+, Mg2+, and SO42- ions on calcite wettability. The results suggested that the Mg2+ ions made the calcite surface more oil-wet but the Ca2+ and SO42- ions made the calcite surface more water-wet compared to indifferent ions.