Surface Tension Alteration on Calcite, Induced by Ion Substitution

The interaction of water and organic molecules with mineral surfaces controls many processes in nature and industry. The thermodynamic property, surface tension, is usually determined from the contact angle between phases, but how does one understand the concept of surface tension at the nanoscale, where particles are smaller than the smallest droplet? We investigated the energy required to exchange Mg 2+ and SO 4 2– from aqueous solution into calcite {10.4} surfaces using density functional theory. Mg 2+ substitution for Ca 2+ is favored but only when SO 4 2– is also present and MgSO 4 incorporates preferentially as ion pairs at solution–calcite interfaces. Mg 2+ incorporation weakens organic molecule adhesion while strengthening water adsorption so Mg 2+ substitution renders calcite more water wet. When Mg 2+ replaces 10% of surface Ca 2+, the contact angle changes dramatically, by 40 to 70°, converting a hydrophobic surface to a mixed wet surface or rendering a mixed wet surface hydrophilic. This increase in water wettability decreases affinity for organic compounds. An important outcome is that we can now explain why oil recovery from carbonate reservoirs is enhanced when both Mg 2+ and SO 4 2– are present in the pore water. Incorporation of MgSO 4 into calcite, which is energetically favored, decreases surface tension and releases polar oil compounds.