Interfacial structure and acidity of orthoclase (001) surface: understanding the effect of surface metal cation

Surface acid chemistry is central to interfacial properties of orthoclase. In this study, we report a first principles molecular dynamics (FPMD) study of interfacial structures and acid constants (pKa) of orthoclase (001) with the presence of Na+/K+ cation on the surface. Detailed structural analyses show that Na+ and K+ show similar coordination structures on the surface while the exchange of Na+ for K+ hardly changes hydration structures of surface groups. The surface groups (i.e., ≡SiOH, ≡AlOH, and ≡AlOH2) have pKas of 11.5, 18.5, and 7.8 with K+ on the surface and 5.5, 17.7, and 4.3 with Na+ on the surface, respectively. FPMD derived pKas indicate that with K+ on the surface ≡AlOH2 is the only active group in the common pH range while Na+ decreases surface pKas of surface groups, that makes ≡AlOH2 and ≡SiOH active. Based on the pKas, we derive a PZC (point of zero charge) of 9.7 and 4.9 for orthoclase (001) with surface K+ and Na+, respectively. This means that Na+ significantly enhances surface acid reactivity. The implication for understanding geochemical properties of orthoclase was discussed with the focus on surface complexation of metal cations.