In this talk, we apply first principles DFT-based molecular dynamics simulations (DFT-MD) to investigate the microscopic organization of solid-liquid water interfaces. We are more specifically interested in silica-liquid water interfaces, neat interfaces, and including electrolytes.Experimentally, the structure and dynamics of such interfaces is investigated via a powerful nonlinear spectroscopic tool: the vSFG � vibrational Sum Frequency Generation Spectroscopy. However no general agreement has been reached yet about the interpretation of experimental results in terms of direct relationship between the spectroscopic features and the associated water structures. In other words, there is no general consensus about the assignment of the two main vibrational bands in the O-H stretching region i.e. the ice-like domain at 3200 cm-1 ?and the liquid-like domain at 3400 cm-1. This lack of consensus is even more crucial when electrolytes are added at the interface.As a follow up to the theoretical works on our model system, the fully hydroxylated (0001) a-quartz/water interface, we investigate single ions or ion pairs effects on water and add comparisons with our results for water on a slab of amorphous silica. The study consists first in providing a microscopic picture of water interactions and locations in presence of singly-charged ions found close to the solid part and then in tempting to clarify SFG experiments. On one hand, we focus on the changes induced on the interactions between the adsorbed water layer and the surface hydroxyl groups i.e. very local modifications directly at the interface. A detailed Vibrational Density Of States (VDOS) analysis is provided in order to assign O-H signatures to specific regions in the vibrational spectra and to identify the most relevant differences with respect to the neat interfaces. Apart from this local approach, we consider on the other hand the whole liquid part to follow ions effects in the subsequent water layers. These effects appear in the dispersion of O-H covalent distances, molecular dipole moments as well as in atomic density profiles. This thus indicates that the water response to the vicinity of ions can have short and long-range consequences which were not observed at neat silica interfaces.

How water layers are affected by adsorption of singly-charged ions at silica/water interfaces: A DFT-MD study

248th National Meeting (2014)

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