Influence of ions on molecular vibrations and hydrogen bonds in methanol–water mixtures: MD simulation study

Abstract

Solutions of NaCl, MgCl2 and CaCl2 in methanol–water mixtures have been investigated by the molecular dynamics simulations. Special attention has been paid to the OH stretching frequencies and hydrogen bonds of the water and methanol molecules in the subsystems; the coordination shells of the ions and the bulk solvent. The influence of the ions on the properties of the solvent molecules is limited to the first coordination shells and it depends on the ion charge density. Thus all cations affect the intramolecular bonds, valence angles, and the dipole moment and the Cl− ion influences only the length and vibrations of the nearest OH bond of the water molecules, but it does not affect the properties of the methanol molecules. Though the influence of the cations on the solvent molecules is strong, the interactions between the solvent molecules enhance the molecular polarization. In consequence the induced dipole moment and the OH frequencies of the water and methanol molecules in the cation shells depend also on the methanol mole fraction. The shift of the OH stretching frequency correlates linearly with the induced dipole moment, but it does not correlate with the strength of the hydrogen bonds.