Surface acidity of 2:1-type dioctahedral clay minerals from first principles molecular dynamics simulations

Abstract

We report a first principles molecular dynamics (FPMD) study of the acid chemistry of 2:1-type dioctahedral phyllosilicates. Using the FPMD based vertical energy gap method, we computed intrinsic acidity constants of phyllosilicate edge sites. The investigated models include both neutral and charged frameworks (i.e., Mg for Al replacement in octahedral sheets and Al for Si replacement in tetrahedral sheets) and the common edge surface types (i.e., (010) and (110)). The result of the neutral framework agrees with the experiment of pyrophyllite. For charged frameworks, it is found that Al(OH) sites in T-sheets (i.e., Al-sub) and Mg(OH2) sites (i.e., Mg-sub) have extremely high pKas and thus they all keep protonated. Both types of substitutions increase pKas of the apical oxygen sites on (110) surfaces and Mg substitution also increases the pKas of neighboring silanol sites. With the calculated pKas, we explore the mechanism of heavy metal cations complexation on edge surfaces. As a quantitative basis, the results in this work can be used in future modeling and experimental studies for understanding acid reactivity of phyllosilicates.