Calcite wettability in the presence of dissolved Mg²⁺ and SO₄^2-

The wettability of mineral surfaces controls a range of phenomena in natural and industrial processes. In reservoirs, rock wettability determines the effectiveness of oil production; thus, modification of mineral surface properties can lead to enhanced oil recovery. Recent work reports that potential determining ions in seawater, Mg²⁺, Ca²⁺, and SO₄^2-, are responsible for altering the wettability of calcite surfaces. In favorable conditions, e.g., elevated temperature, calcium at the calcite surface can be replaced by magnesium, making organic molecules bind more weakly and water molecules bind more strongly, rendering the surface more hydrophilic. We used atomic force microscopy in chemical force mapping mode to probe the adhesion forces between a hydrophobic CH₃-terminated AFM tip and a freshly cleaved calcite {10.4} surface to investigate wettability change in the presence of Mg²⁺ and SO₄^2- at 75 and 80°C. We made submicrometer scale maps of adhesion force and contact angle and demonstrated that the adhesion force between the hydrophobic tip and calcite decreases when both Mg and SO₄ are present. Surface analysis with X-ray photoelectron spectroscopy showed Mg associated with calcite even after rinsing with CaCO₃-saturated deionized water, suggesting sorption on or in calcite. When the calcite-saturated solution of MgSO₄ was replaced by calcite-saturated NaCl at the same ionic strength, adhesion force increased again, indicating that the effect is reversible and suggesting Mg replacement by Ca. Experiments with solutions of Na₂SO₄ and MgCl₂ suggest that Mg²⁺ uptake is favored when SO₄^2- is also present.