Electrolytes at the Hydroxylated (0001) α-Quartz/Water Interface: Location and Structural Effects on Interfacial Silanols by DFT-Based MD

Abstract

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...

Hydrogen bonding interactions of H2O and SiOH on a boroaluminosilicate glass corroded in aqueous solution

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Measuring surface charge: Why experimental characterization and molecular modeling should be coupled

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