Raman spectroscopic investigation on aqueous NaCl solutions at temperatures from 273 to 573Â K: Effect of NaCl on water structure

The Raman spectra of aqueous NaCl solutions (0-25.0 mass%) were investigated at temperatures from 273 to 573 K and a pressure of approximately 30 MPa. It was known that the dissolution of NaCl could significantly alter the water structure by changing the relative abundance of fully hydrogen-bonded water (FHW) and partially hydrogen-bonded water (PHW). Specifically, NaCl induced blue shifts of the Raman main peak above 3390 cm− 1 (vPHW) at temperatures below 433 K, which could be attributed to the strengthened H2O-Cl− interaction (HCI) and the weakened H2O-H2O interaction (HHI) in the hydration shell of Cl−. The red shifts of vPHW at temperatures above 433 K could be explained by the strengthened HHI and weakened HCI. NaCl also showed contrary effects on Raman intensity ratio of O-H stretching vibration of FHW and PHW below and above the temperature of 513 K, which was due to the further strengthened HHI and weakened HCI in the hydration shell of Cl−, and the intensified ion-ion interaction (III) promoted the formation of ion pairs with rising temperature. The test results also showed a significant decrease of the water structure breaking capability of NaCl with an increasing temperature. The energy change due to the transition from FHW to PHW was determined by Gaussian fitting. It was found that ∆H (kJ·mol− 1), ∆S (J·mol− 1·K− 1) and salinity (C, mass%) were linearly correlated with the following relationships: ∆H = − 0.14C + 7.01, ∆S = − 0.22C + 27.64 (T ≤ 430 ± 9 K, depending on salinity) and ∆H = − 0.35C + 13.43, ∆S = − 0.70C + 42.34 (T > 430 ± 9 K), respectively from 273 to 573 K.