Water structure and dynamics at the solid/liquid interface from ab initio simulations

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This project aims to a microscopic understanding of water structure and relaxation dynamics at the solid/water interface. Our goal is to provide a molecular understanding on how the ordering and dynamics of interfacial water changes as function of the pH, namely of the surface protonation and charge state. As a further step we also aim to understand the role of different ions at the water solid interface. We will focus on a selected system, namely the CaF2/water interface for which a parallel simulation and experimental analysis will be conducted. In particular, we will perform ab initio simulations based on Density Functional Theory, while the experimental group of Dr. E. Backus at the Max Planck Institute for Polymer Research in Mainz will perform the SFG vibrational measurements.An innovative aspect here is the use of ab initio simulations to directly interpret the experiments at a molecular level. The first principles simulations are particularly suitable for modelling interfaces, since they naturally include polarization effects (including electronic polarization) which occur at a heterogeneous environment. In addition they can naturally include temperature effects and sampling of both enthalpic and entropic effects. Owing to recent progress in optimization of codes and the availability of supercomputing resources the time required for the simulations reduced drastically. Hence, the data acquisition in the experiments and simulation require a similar amount of time.The questions we would like to address include:(I) Solvent ordering, with particular emphasis on how far it extends and how it is influenced by the surface protonation state and surface charge density. What are the solvent vibrational properties at the interface and how they can be deduced from velocity-velocity (and surface specific) correlation functions?(II) The electrostatic potential at the interfaces. How does the potential depend on pH and how does it compare to experiments? (III) Description of the energy relaxation at the interface starting from localized vibrational excited normal modes.(IV) Influence of ions at the interface. In particular we would like to address how different ions (simple halides versus molecular ions) localize at the interface. How does an anion in direct proximity of the surface water molecule or an anion in the diffuse ion cloud influence the water ordering and modify the vibrational properties of water at the interface? In particular two prototype cases will be considered, namely Cl- and SO42-. Also in this case we will look at dynamics and energy relaxation. The key question here would be how the solvent energy relaxation is influenced by the presence of ions.