pK_a at Quartz/Electrolyte Interfaces

Morgane Pfeiffer-Laplaud (1) , Marie-Pierre Gaigeot (1) , Marialore Sulpizi (2)

1 LAMBE - Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement
2 JGU - Johannes Gutenberg - Universität Mainz = Johannes Gutenberg University

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 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.

Domaines: Chimie-Physique [physics.chem-ph]; Sciences du Vivant [q-bio]

Morgane Pfeiffer-Laplaud, Marie-Pierre Gaigeot, Marialore Sulpizi. pK_a at Quartz/Electrolyte Interfaces. Journal of Physical Chemistry Letters, 2016, 7 (16), pp.3229-3234. ⟨10.1021/acs.jpclett.6b01422⟩. ⟨hal-02065252⟩