Structural characterization of NaOH aqueous solution in the glass and liquid states

Using the technique of hydrogen and deuterium substitution, the structure of water in concentrated NaOH solution (10 M) is explored. It is found that major changes in water structure occur both in the liquid phase at T=300 K and in the glassy phase at T=173 K. In particular the 4.4 Angstrom peak in the OO pair correlation function of pure water, which is normally viewed as indicating tetrahedral short-range coordination in water, is totally absent in the NaOH solution at room temperature, and shows up only as a small feature in the NaOH solution in the glassy state. Corresponding changes occur in the OH and HH correlation functions: The hydrogen bond peak position is shifted from 1.85 Angstrom in pure water to 1.65 Angstrom for both the liquid and glassy NaOH, with a reduced number of hydrogen bonds in the glassy phase. The intramolecular HH distance, 1.5 Angstrom, of the water molecule is unaffected by the presence of the solute, but the positions of the peaks in the HH function at 2.4 and 3.8 Angstrom, due to the orientational correlation between neighboring pure water molecules, are respectively, shifted to 2.15 and 3.5 Angstrom. The above findings indicate that ions in aqueous solutions induce a change in water structure equivalent to the application of high pressures. (C) 2001 American Institute of Physics.