Wetting of selected fluorite surfaces by water

The interfacial chemistry features, particularly the wetting characteristics of selected fluorite (CaF₂) surfaces (111, 110 and 100 crystal surfaces), were examined under nonequilibrium conditions (unsaturated solution), in the absence of reagents, including water contact angle measurements and interfacial water structure. Of special interest was the examination of the effect of crystal surface structure on the wetting characteristics of interfacial water molecules at CaF₂ surfaces. Different contact angles were found for the three CaF₂ surfaces, which have been attributed to the differences in water interaction at these crystallographic surfaces. Specifically, the low-energy (111) surface of CaF₂ (the cleavage plane) has a contact angle of ~20°, whereas the (100) and (110) surfaces are completely hydrophilic, with a zero contact angle in each case. Detailed analysis of interfacial water structure was conducted under nonequilibrium conditions in order to understand the wetting characteristics of water molecules at the molecular level using molecular dynamics simulation as well as sum-frequency vibrational spectroscopy. For equilibrium conditions, when water is saturated with calcium fluoride, it appears that recrystallization occurs and the low-energy (111) surface state exists at all surfaces as revealed from contact angle measurements.