Electrokinetic comparison of CaF2 and AgCl from a MDS perspective

Abstract

CaF2 has limited electrokinetic sensitivity to fluoride ions in solution and the positive CaF2 surface charge is not reversed at high fluoride ion concentration. Similar insensitivity of surface charge to high fluoride ion concentration has been observed for other alkali and alkaline earth fluoride salts. On the contrary, chloride ions act as potential determining ions for AgCl and reverse the surface charge, as expected. FTIR-IRS results indicate that the AgCl/water interface has a water structure-breaking tendency, whereas the CaF2/water interface has a structure-making tendency. It is speculated that the strong hydrogen bonding between fluoride ions and water prevents the accommodation of excess fluoride ions at surface lattice sites of fluorite, and therefore explains the low sensitivity of the fluorite surface charge sign and magnitude to fluoride ion concentration. However, this speculation has not been examined at the molecular level and is evaluated by molecular dynamics simulations (MDS) in the research presented in this paper. Higher hydrogen bond density at the CaF2 mineral/water interface confirms that the water structure has stronger hydrogen bonding at both the CaF2 mineral surface and with the F− ion in solution. In this way, the interaction of the F− ion with the CaF2 surface is inhibited.