We report a first principles molecular dynamics (FPMD) study of surface acid chemistry of the two growth surfaces of quartz, (100) (including Alpha and Beta terminations) and (101) facets. The interfacial hydration structures are characterized in detail and the intrinsic pK_as of surface silanols are evaluated using the FPMD based vertical energy gap method. The calculated acidity constants reveal that every surface termination shows a bimodal acid–base behavior. It is found that all doubly-protonated forms (i.e. SiOH₂) on the three terminations have pK_as lower than −2.5, implying that the silanols hardly get protonated in a common pH range. The pK_as of surface silanols can be divided into 3 groups. The most acidic silanol is the donor SiOH on the (100)-beta surface (pK_a = 4.8), the medium includes the germinal silanol on (100)-alpha and the outer silanol on (101) (pK_a = 8.5–9.3) and the least acidic are inner silanols on the (101)-facet, acceptor SiOH on (100)-beta, and the secondly-deprotonated OH (i.e. Si(O–)(OH)) on (100)-alpha (pK_a > 11.0). With the pK_a values, we discuss the implication for understanding metal cations complexing on quartz surfaces.

Montmorillonite layer edge surfaces have pH-dependent properties, which arises from the acid-base reactivity of their surface functional groups. Edge surface acidity (with intrinsic reaction equilibrium constant, pK_a) is a chemical property that is affected by crystal structure. While a cis-vacant structure predominates in natural montmorillonites, prior molecular-level studies assume a centrosymmetric trans-vacant configuration, which potentially leads to an incorrect prediction of montmorillonite acid-base surface properties. We computed intrinsic acidity constants of the surface sites of a montmorillonite layer with a cis-vacant structure using the first-principles molecular dynamics-based vertical energy gap method. We evaluated pK_a values for both non-substituted and Mg-substituted layers on common edge surfaces (i.e., surfaces perpendicular to [010], [01̅0], [110], and [1̅1̅0] crystallographic directions). The functional groups ≡Si(OH), ≡Al(OH₂)₂/≡Al(OH)(OH₂), and ≡SiO(OH)Al sites on surfaces perpendicular to [010] and [01̅0] and ≡Si(OH)^U, ≡Si(OH)^L, ≡Al(OH₂), and ≡Al(OH₂)₂ on surfaces perpendicular to [110] and [1̅1̅0] determine the proton reactivity of non-substituted cis-vacant edge surfaces. Moreover, the structural OH sites on edge surfaces had extremely high pK_a values, which do not show reactivity at a common pH. Meanwhile, Mg²⁺ substitution results in an increase in pK_a values at local or adjacent sites, in which the effect is limited by the distance between the sites. A surface complexation model was built with predicted pK_a values, which enabled us to predict surface properties as a function of pH and ionic strength. Edge surface charge of both trans- and cis-vacant models has little dependence on Mg²⁺ substitutions, but the dependence on the crystal plane orientation is strong. In particular, at pH below 7, edge surfaces are positively or negatively charged depending on their orientation. Implications of these findings on contaminant adsorption by smectites are discussed.