Molecular dynamics simulation of NaCl solutions in methanol-water mixtures. Intramolecular vibrations of the solvent components

Intramolecular vibrations of methanol and water in binary solvents and ternary solutions containing NaCl have been investigated by the molecular dynamics simulation. Methanol and water molecules have been described using three site flexible models. Special attention has been paid to an influence of the mixture composition and the NaCl addition on the OH stretching frequencies of both solvent components. The increase of the methanol concentration shifts the OH frequencies of water and methanol to lower wavenumbers. The red-shifts reflect a strengthening of the hydrogen bonds. In NaCl solutions the additional red-shifts of the OH frequencies are observed. For methanol the red-shift is almost independent of the solvent composition, whereas for water it increases with decreasing concentration of water. Such a behaviour can be attributed to a cooperative effect resulting from the interactions of the water molecules from the hydration shells of cations with the methanol molecules in the bulk.