A study on D2O/H2O-hydrohalite from Raman spectroscopy

Abstract

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.