A temperature-dependent effect of Na2CO3 on water structure investigated from Raman spectroscopy

The hydrating structure of CO32- is investigated based on the solute-correlated (SC) OD/OH stretch bands obtained using Multivariate Curve Resolution on the Raman spectra of D2O/H2O-Na2CO3 solutions at temperatures up to 573 K. At temperatures below 413 K, the SC OD/OH stretch bands display a bimodal profile by the boundary at ∼2360 cm-1/∼3190 cm-1. As temperature increases over 433 K, the ∼2290 cm-1/∼3060 cm-1 peak is weakened and the bands get sunken around 2610 cm-1/3590 cm-1 above ∼473 K, making the 2670 cm-1/3635 cm-1 mode a separate peak at 573 K. The SC OH stretch main peak shifts by ∼120 cm-1 when temperature rises over 493 K but the SC OD stretch main peak remains at a nearly unchanged position because more HCO3- formed in H2O than DCO3- in D2O leads to less CO32--H2O interactions than CO32--D2O interactions above 453 K. These spectral features support a temperature-dependent hydrating water structure around CO32- balanced by the electrostatic, CO32-⋯H/D-O·and O-H/D⋯O hydrogen bonding interactions. Due to the strong electrostatics from CO32-, the dominated hydrogen bonding configuration among CO32- hydrated water is tetrahedral instead of single donor (SD) and single hydrogen-bond water (SHW), in contrast to the situations for SO42- and Cl-. CO32- changes from water structure breaker to water structure maker as temperature rises above ∼353 K where the tetrahedrality of hydrating water exceeds that in bulk water.