Structure, Dynamics and Vibrational Spectroscopy of Interfacial Alkali Nitrate and Alkali Halide Aqueous Solutions from ab initio Molecular Dynamics

Abstract

The interfacial structure and dynamics of solutions containing alkali nitrates and alkali halides have been studied by density functional theory-based molecular dynamics (DFTMD) simulations. We have presented a detailed analysis of the hydrogen bond (HB) structure at the interface and calculated the interface vibrational sum-frequency generation (VSFG) spectra to provide a molecular interpretation of the available experimental data.

Both the measured and the calculated VSFG spectra of the alkali nitrate solution show a reduced intensity in the lower frequency portion of the stretching band, compared with the water/vapor interface. This reduction is attributed to the hydrogen (H-) bonds established between nitrate and the surrounding water molecules at the interface. This spectral feature is only related to the presence of nitrate at the water surface and is not influenced by the alkali metal ions. We have also shown that, to provide a microscopic interpretation of the spectra, realistic models of the interface are required beyond simple cluster models.

Heavier halide anions such as iodine have an effect similar to that of nitrate on the structure and dynamics of the water/vapor interface. From the results of the simulations and the calculation of the nonlinear susceptibilities, we conclude that water molecules at the interfaces of LiI, NaI, and KI solutions are participating in weaker H-bonds, compared with those at the water/vapor interface. This feature originates from the unique distribution of iodide ions and alkali metal cations, which form a double layer over a thickness of about 5--10 Angstroms.

The second aspect we have investigated is the HB dynamics. To analyze HB dynamics at interfaces, we determined the instantaneous interface based on spatial density, and proposed a statistical scheme based on the interfacial HB (IHB) population for identifying the instantaneous interface. Combining the IHB method with interfacial molecule sampling (IMS), we obtained a method to determine the thickness of the water/vapor interface. The IHB method has also been extended to the solvation shells of ions in aqueous solutions.