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 Mg2+ and SO42– from aqueous solution into calcite {10.4} surfaces using density functional theory. Mg2+ substitution for Ca2+ is favored but only when SO42– is also present and MgSO4 incorporates preferentially as ion pairs at solution–calcite interfaces. Mg2+ incorporation weakens organic molecule adhesion while strengthening water adsorption so Mg2+ substitution renders calcite more water wet. When Mg2+ replaces 10% of surface Ca2+, 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...