A near-infrared study of the hydration of various ions and nonelectrolytes

Abstract

The differential infrared absorption spectra of H2O at 1200 nm and HOD/D2O at 1450 nm have been recorded as function of temperature between 2 and 55°C. Following earlier investigations in the 1000 nm region,(1,2) the data were interpreted on the basis of a hydrogen-bonding equilibrium invoking two states of the OH oscillators which are found to differ by a van't Hoff heat of formation of 2–3 kcal-mole−1. The spectral changes induced by various non-electrolytes (1.0m) on the 1000 nm band have been recorded at 25°C for the following: tetrahydrofuran, tetrahydropyran, dioxane, dimethoxyethane, dimethyl sulfoxide, acetone, methyl acetate, propylene carbonate, dimethylformamide, dimethylacetamide, tetramethylurea, acetonitrile, and nitromethane. The specific influence of hydrocarbon chains on the near-IR absorption spectrum of water was further studied using a series of homologous sodiumn-alkyl carboxylates (formate to octanoate). The differential solvation spectra of three azoniaspiroalkane bromides have been obtained to compare the relative effects of cyclic and linear alkyl substituents. Finally, the study of ionic hydration effects has been extended to include the following electrolytes: NaPF6, NaClO4, NaBF4, NaClO3, NaNO3, NaBrO3, NaCN, NaSCN, Na2SO4, Na2SO3, Na2CO3, Na2S2O3, MgCl2, CaCl2, BaCl2, and SrCl2. These data are discussed in terms of perturbations of the hydrogen-bonding equilibrium in water originating from one of the following: (1) direct OH... [polar group or anion] interactions, (2) structure-breaking effects due to weak OH... solute interactions, and (3) water-structure-promoting effects by alkyl groups.