A temperature-dependent hydrating water structure around chlorine anion

Abstract

The hydrating water structure in NaCl aqueous solution has long been a controversial topic. We employ the Multivariate Curve Resolution method to get the solute-correlated (SC) OD/OH stretch bands from the Raman spectra of D2O/H2O–NaCl solutions at temperatures from 293 to 573 K. The largely narrowed SC bands remain a nearly constant wavenumber of the main peak with increasing temperature but get sunken around 2611 cm−1/3591 cm−1 to let the ~2660 cm−1/~3635 cm−1 modes be more prominent above a pivoting temperature (TP, ≈453 K/433 K for OD/OH bands). These spectral features incline to support a more flexible water structure in the Cl− hydration shell (CHS) balanced by the water–water hydrogen bond and water–Cl− bond. Most hydrating water molecules are engaged in hydrogen bonding configurations of single donor (SD) and single hydrogen-bond water (SHW). As temperature rises, the SD/SHW ratio in CHS first decreases below TP because of the SD → SHW transition but then increases above TP owning to the rapid increase of the contact ion pairs. Moreover, we ascribe the differences between the SC OD and OH bands primarily to the stronger D2O–D2O hydrogen bond and D2O–Cl− bond.