Adsorption of Singly Charged Ions at the Hydroxylated (0001) α-Quartz/Water Interface

Individual alkali (Na+, K+) and halide (Cl-, I-) ion effects have been characterized at the fully hydroxylated (0001) a quartz water interface by means of ab initio molecular dynamics simulations in the framework of the electronic DFT representation (DFT-MD). We particularly focus our analyses on the ion adsorption and solvation Structures (made by water and by surface silanols), as well as on perturbations undergone by the silanol surface sites when comparing the charged interfaces (present work) to the neat interface (our previous works, J. Chem. Theory. Comput. 2012, 8, 1037; J. Phys.: Condens. Matter 2012, 24, 124106). Both sodium and potassium cations are found adsorbed in an inner-sphere configuration, while chloride and iodide are found in between inner- and outer-sphere. Cation adsorption at the interface is found to induce more perturbation on interfacial properties than anions do. In particular, we show in details how and why the orientation of out-of-plane and in-plane surface silanols found at the neat interface are modified by inner-sphere cations at the charged interfaces, with also consequences on the silanol-silanol intrasurface hydrogen bond network. All this detailed analysis provides a clear picture of a reduction of acidity of the surface silanols at the quartz/water interface in the presence of the alkali/halide salts.