Xi Zhang, Xiandong Liu, Yingchun Zhang, Xiancai Lu
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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.
期刊介绍:
ACS Applied Bio Materials is an interdisciplinary journal publishing original research covering all aspects of biomaterials and biointerfaces including and beyond the traditional biosensing, biomedical and therapeutic applications.
The journal is devoted to reports of new and original experimental and theoretical research of an applied nature that integrates knowledge in the areas of materials, engineering, physics, bioscience, and chemistry into important bio applications. The journal is specifically interested in work that addresses the relationship between structure and function and assesses the stability and degradation of materials under relevant environmental and biological conditions.