A study on D2O/H2O-hydrohalite from Raman spectroscopy

The cryogenic phase behaviors and Raman spectra for D2O/H2O−NaCl solutions were observed and measured at temperatures down to 173.15 K. The (ice + hydrohalite) eutectic temperature of D2O−NaCl solutions is ∼255.95 K, 4 K higher than that of H2O−NaCl solutions. On the Raman spectral patterns, the four characteristic peaks of NaCl⚫2D2O hydrohalite are displayed to be more scattered than those of NaCl⚫2H2O. The third peak is easier to appear for NaCl⚫2D2O than for NaCl⚫2H2O that more cooling-heating runs and higher NaCl concentration are needed for this peak of NaCl⚫2H2O to be prominent, which indicates that NaCl⚫2D2O crystallizes more easily than NaCl⚫2H2O. We assign the 1st peak to the OD/OH stretch vibration of local water, and assign the 2nd, 3rd and 4th peaks to the OD/OH stretch vibrations of coordinating water respectively with D2O/H2O·Na+, Cl−···D2O/H2O·Na+, and (Cl−)2···D2O/H2O·Na+ interaction configurations. The harder crystallization of hydrohalite and lower eutectic temperature for NaCl⚫2H2O than for NaCl⚫2D2O are ascribed to the O–D···Cl− > O–H···Cl− and O–D···O > O–H···O hydrogen bonding interactions.