High-Frequency Vibrational Dynamics of Na+ and Cl– Aquacomplexes: The Role of Nano-Scale Defects in Liquid as Reflected in the Absorption Spectra

The dissolution of salts including NaCl accompanied by stirring promotes the formation of different defects in the H-bond network of water. The defect sizes can vary from parts of a nanometer to hundreds nanometers when nanobubbles are formed in the liquid. The salt ions can be localized both in the bulk phase and close to defects and water–air interfaces, which corresponds to the formation of their bulk and surface hydration structures respectively. Depending on the water : salt molar ratio and the character of the ion hydration, coordination bonds between the ions and closest water molecules are characterized by different effective electronic charges and, hence, different strengths, which is reflected in the vibrational states of the molecules and their infrared absorption spectra in the high-frequency range. Nonempirical simulations carried out for the X(H2O)n clusters (where X = Na+ or Cl– and n ≤ 55) at the DFT-B3LYP/6-31G(d, p) level were used for predicting the absorption spectra of the ionic systems. The results show that the surface and bulk hydration structures of both ions differ in the intensities of spectral signals depending on the frequency subranges, which can be used as indicators of the changing fractions of the bulk and surface hydration structures of the ions particularly reflecting the role of nano-scale defects close to which the ions can be localized in the dynamic structure of the solution.