Structural, Vibrational and Thermodynamic Characteristics of Nacl Single-Molecule Dissolution in Water: A DFT Study

Abstract

The solvation of a single NaCl molecule in water molecules is discussed using density functional theory. As we add H2O molecules, the distance between Na and Cl ions increases. Six H2O molecules are enough to make both the distances between Na and Cl ions with nearest O atoms (in H2O molecule) less than Na-Cl distance. Natural bond analysis (NBO) shows that absolute charges on Na and Cl ions decrease as we add H2O molecules with the absolute charge on Na ion slightly less than Cl ion. The energy gap increases and approaches the experimental energy gap of liquid water at 6.9 eV. Both Gibbs free energy and enthalpy are negative indicating that the reaction is spontaneous and exothermic. The enthalpy converges to the value of the solvation of both Na and Cl ions in water. The negative sign of the entropy indicates order reaction. The Raman vibrational spectra changes from the single vibrational mode at 341.93 cm−1 of NaCl molecule and three H2O molecule vibrational modes at 1584, 3863.9, and 3778 cm−1 to three multi bands centred at 500, 1600 and 3400 cm−1 for NaCl with 9 H2O molecules with the diminishing of Na-Cl vibration intensity.