Specific ion effects of salt solutions at the CaF2/water interface

Calcium fluoride is a slightly soluble compound commonly extracted from ores via flotation at elevated pH, where surfactant molecules bind with hydroxylated surface sites. The addition of F-(aq) suppresses surfactant adsorption by replacing these sites. In this paper, we look at the effects of aqueous Cl-, Br-, F-, and SO4(2-) on the water structure at the CaF2/H2O interface at a pH where surface hydroxylation has not yet occurred. Using static and time-resolved vibrational sum-frequency spectroscopy (VSFS), we find that aqueous Cl- and Br- have only electrostatic screening effects on the interface and do not perturb the interfacial water or surface structure. Sulfate, which we find to be strongly attracted to the interface, affects the interfacial water more than Cl- or Br-. This is in contrast to F- ions that directly interact with the surface and alter the water structure and bonding at the CaF2 surface in addition to screening the surface charge.