pKa at Quartz/Electrolyte Interfaces

Acidity of silanol sites at the crystalline quartz/aqueous electrolyte (NaCl, NaI, KCl) interfaces are calculated from ab initio molecular dynamics simulations. pK(a)'s are found to follow a combination of the cationic and anionic Hofmeister series in the order pK(a)(neat solution) < pK(a)(NaCl) < pK(a)(NaI) < pK(a)(KCl), in agreement with experimental measurements. Rationalization of this ranking is achieved in terms of the microscopic local solvation of the protonated silanols and their conjugated bases, the silanolates SiO-. The change in the pK(a) is the result of both water destructuring by alkali Si si halides, as well as of the specific cation/SiO- interaction, depending on the electrolyte. Molecular modeling at the atomistic level is required to achieve such comprehension, with ab initio molecular dynamics being able to model complex inhomogeneous charged interfaces and the associated interfacial chemical reactivity.