Water in and on ionic materials: Structure, energetics, and vibrations

  • Datum:
  • Plats: Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
  • Doktorand: Kebede, Getachew
  • Om avhandlingen
  • Arrangör: Strukturkemi
  • Kontaktperson: Kebede, Getachew
  • Disputation

Many chemical and physical phenomena in nature, in industrial processes, and in our daily lives take place at water/solid interfaces. The aim of this thesis is to further our knowledge of such processes at a molecular level. Here theoretical calculations can provide new insights about molecular bonding, structure and dynamics and how these respond to the perturbations from the surroundings.

Calculations can also yield for example vibrational spectra to be directly compared with experimental ones and help in the interpretation. This thesis describes the results of quantum-mechanical and quantum-dynamical studies of water properties on ionic surfaces [NaCl(001), MgO(001) and CaO(001)] and in ionic hydrates [e.g Na2CO3∙10H2O, MgSO4∙11H2O, Al(NO3)3∙9H2O] with especial emphases on surface and interface systems. In particular, calculations of binding energies, OH stretching frequencies, in situ electric field, dipole moments and intra/intermolecular OH distances were performed and analyzed to probe the strength of the water–environment interplay and to disentangle the components of the perturbation. Furthermore, validation of a range of dispersion-inclusive DFT methods for binding energies of interface water and structure and vibrational properties of water in condensed systems also constitutes part of the thesis.

Two correlations among the investigated properties were established and extensively explored: (i) OH stretching frequency vs. H-bond distance to characterize the H-bond strength and patterns on the surfaces and (ii) OH stretching frequency vs. local electric field to understand the effect of the water/hydroxide environment on the calculated gas-to-bound OH frequency shift behaviour. It was found that both the intact and dissociated water molecules on MgO(001) and CaO(001) follow essentially the same frequency-distance correlations. However, if the frequency is instead correlated against the in situ electric field from the environment, water and hydroxide ion follow different “frequency vs. field” curves. Both water and hydroxide curves, however, can be described by the same model, namely by an electrostatic dipole model presented in the thesis. The gas-to-surface frequency shifts can be traced back to the competition between the signs and magnitudes of the permanent and induced dipole derivatives along the stretching coordinate. Furthermore, the “frequency vs. field” model offers useful insights into the frequency shifts of various surface H-bond motifs on the H2O/MgO interface induced by the adsorption of multilayer cold water.