Individual alkali (Na+, K+) and halide (Cl–, I–) ion effects have been characterized at the fully hydroxylated (0001) α-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.201281037; J. Phys.: Condens. Matter201224124106). 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.

International audienceUnderstanding the microscopic origin of the acid base behavior of mineral surfaces in contact with water is still a challenging task, for both the experimental and the theoretical communities. Even for a relatively simple material, such as silica, the origin of the bimodal acidity behavior is still a debated topic. In this contribution we calculate the acidity of single sites on the humid silica surface represented by a model for the hydroxylated amorphous surface. Using a thermodynamic integration approach based on ab initio molecular dynamics, we identify two different acidity values. In particular, some convex geminals and some type of vicinals are very acidic (pK(a) = 2.9 and 2.1, respectively) thanks to a special stabilization of their deprotonated forms. This recalls the behavior of the out-of-plane silanols on the crystalline (0001) alpha-quartz surface, although the acidity here is even stronger. On the contrary, the concave geminals and the isolated groups present a quite high pK(a) (8.9 and 10.3, respectively), similar to the one of silicic acid in liquid water

Structural properties of NaCl, KCl, and NaI electrolytes forming an electrical double layer (EDL) at the fully hydroxylated (0001) α-quartz/liquid water interface have been investigated by means of first-principles molecular dynamics simulations (FPMD). Cations are found in inner-sphere conformations, directly bonded on two in-plane silanol groups that replace water molecules that would be present in the first solvation shell of the aqueous cations. Anions are located within the second/third water layer above the surface, fully solvated as in pure liquid water, and cation−anion adopt rather flexible solvent separated ion-pair (SSIP) geometries in the EDL. While the individual solvation shells of the aqua-ions are only slightly affected by ion-pairing at the interface, the silanols at the quartz surface are strongly perturbed. The presence of the electrolytes in the EDL affects more deeply the silanols' geometrical properties than single ions do (J. Phys. Chem. C 2016, 120, 4866−4880): the silanol−silanol intrasurface H-bonding that was observed at the neat interface is extremely weakened by the presence of the electrolytes. Further disordering of the surface silanols is characterized by large changes in their orientation and covalent bond-lengths, regardless of their in-plane (IP) or out-of-plane (OP) orientations. Such structural changes of the surface silanols are tentatively correlated here with an increase in the basicity of all surface sites. ■ INTRODUCTIONIons from electrolytic solutions play key roles in interfacial water/mineral properties. Their adsorption behaviors typically drive pollutant transport in groundwater, 1,2 mineral dissolution, 3 or clay swelling, 4 to name a few. Electrolytes can strongly affect the binding of organic molecules because of competitive adsorption. 5 Even non-adsorbed electrolyte ions can influence adsorption of biomolecules like peptides or DNA 6−9 through conformational changes or ion-pairing.Cations and/or anions from electrolytes get distributed at aqueous solid interfaces so as to form an electrical double layer (EDL). EDLs are known to control the chemical reactivity of the surface, and varying the ionic strength tunes the EDL depth or Debye length. Ionic strength effects can be probed at water/ mineral interfaces via sum frequency generation (SFG) spectroscopy through the indirect knowledge of the interfacial water structural response to the presence of the ions. 10−12 Of particular interest are the electrolyte-induced perturbations on the "ice-like" and "liquid-like" bands in the O−H stretching region. It is well accepted that electrolytes bring interfacial disorder, as evidenced by the intensity shrink of the ice-like vibrational signatures, which in turn shows the disappearance of the strong H-bonds formed by interfacial water molecules. At the fused quartz/water interface, it has been shown that SFG intensities depend on the nature of both cations 13 and anions. 14 From the spectroscopic results, cations with higher surface charge densities induce the largest SFG i...

Acidity of silanol sites at the crystalline quartz/aqueous electrolyte (NaCl, NaI, KCl) interfaces are calculated from ab initio molecular dynamics simulations. pKa's are found to follow a combination of the cationic and anionic Hofmeister series in the order pKa(neat solution) < pKa(NaCl) < pKa(NaI) < pKa(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 pKa 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.

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