Excess Partial Molar Absorptivity of Aqueous Solutions of KCl, KBr, CsCl and CsBr: Are There Three Universal Chromophores in the Excess Molar Absorptivity of the ν 2 + ν 3 Band of H2O for Aqueous Salt Solutions?

Abstract

A new analysis methodology of the ν 2 + ν 3 band of H2O developed by us earlier was applied for aqueous solutions of KCl, KBr, CsCl and CsBr. We isolate the effect of inter-molecular interactions among solute ions and solvent molecules on the near infrared spectra as the excess absorptivity, e E, and then take its mole fraction derivative, $$ \varepsilon_{\text{S}}^{\text{E}} $$ (S stands for the solute). $$ \varepsilon_{\text{S}}^{\text{E}} $$ then signifies the effect of solute S on e E, the transition moment of the ν 2 + ν 3 combination band due only to the intermolecular interactions. Together with the earlier similar studies on aqueous NaCl, NaBr, NaI, (CH3)4Cl, and (C2H5)4Cl, we found that e E has apparently three universal bands among all the aqueous salts studied so far; a negative band centered at 4873 cm−1, a positive one at 5123 cm−1, and a negative band at 5263 cm−1. They were attributed to “solid-like”, “liquid-like” and “gas-like” H2O chromophores, respectively, depending on the local and instantaneous completeness of hydrogen bonding. Our recent thermodynamic studies indicated that K+, Cs+, and Cl− ions are “hydration centers” that are hydrated by a number of H2O molecules, but the bulk H2O away from hydration shells is left unperturbed. Br− is a “hydrophile” ion that forms hydrogen bonds directly to the fleetingly and yet permanently existing hydrogen bond network of H2O, but pins down the fluctuations inherent in pure H2O. The distinction between Cl− and Br− are apparent only in the behavior of $$ \varepsilon_{\text{S}}^{\text{E}} $$ at the 5123 cm−1 “liquid-like” chromophore band. The quantitative differences in the hydration numbers among Na+, K+, and Cs+ are not conspicuous in either of these three chromophores. We stress, however, that at the e E level there seem to be three bands at 4873, 5123, and 5263 cm−1 that could be universal among aqueous solutions of electrolyte solutes.