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

摘要

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 thewater structure by changing the relative abundance of fully hydrogen-bondedwater (FHW)and partially hydrogen-bondedwater (PHW). Specifically, NaCl induced blue shifts of the Raman main peak above 3390 cm~(-1) (v_(PHW)) at temperatures below 433 K, which could be attributed to the strengthened H_2O-Cl~- interaction (HCI) and theweakened H_2O-H_2O interaction (HHI) in the hydration shell of Cl~-. Theredshifts of v_(PHW) at temperatures above 433 K could be explained by the strengthenedHHI andweakened HCI.NaCl also showed contrary effects on Raman intensity ratio of O-H stretching vibration of FHWand 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 fromFHWto PHWwas determined by Gaussian fitting. Itwas found that ΔH (kJ·mol~(-1)), ΔS (J·mol~(-1)·K-1) and salinity (C, mass%) were linearly correlated with the following relationships: ΔH=-0.14C+7.01, ΔS=-0.22C+27.64 (T≤430±9 K, depending on salinity) and ΔH=-0.35C+13.43, ΔS=-0.70C+42.34 (T N 430 ± 9 K), respectively from 273 to 573 K.