Raman Spectral Studies of the Effects of Temperature on Water and Electrolyte Solutions

Abstract

All known intermolecular Raman bands of water, viz., the hydrogen-bond bending and stretching bands, and the librational bands, decrease rapidly in intensity with temperature rise. In contrast, the librational intensities of water in electrolyte solutions exhibit very small variations with temperature. The intensity decreases observed for pure water indicate that hydrogen bonds are broken by increase of temperature, but the near constancy observed for solutions indicates that primary hydration is not greatly affected, even at temperatures near the normal boiling points of some of the solutions studied. Integrated Raman intensities of the hydrogen-bond-stretching vibrations of pure water at 152–175 cm−1 were redetermined in the temperature range of −6.0° to 94.7°C. The new intensity data, which are more accurate than the old [cf., J. Chem. Phys. 40, 3249 (1964)], yield the values ΔH°=5.6 kcal/mole and ΔS°≈19 cal/deg·mole for the process B→U, where B refers to water molecules which contribute intensity to the 152–175-cm−1 Raman band, and U refers to molecules which make very little or no contribution. Interpreted in terms of non hydrogen-bonded monomeric defects in a tetrahedral liquid lattice, the above ΔH° yields a value of 2.8 kcal/mole H bond in resonable agreement with Scatchard's value of 3.41 kcal/mole H bond. The value of ΔS° for the process B→U also leads to interesting comparisons with known entropies, but the calculated heat capacity of water is only in fair agreement with accepted values. The observed insensitivity of the solution librational intensities to changes of temperature indicates that primary hydration is involved almost exclusively. This conclusion complements the previous observations involving linearity of librational intensity with electrolyte concentration [cf., J. Chem. Phys. 36, 1035 (1962)], since both observations can be explained by primary hydration. In addition, molar librational intensities (obtained from the temperature studies) confirm the large anionic effects reported previously, with Br−>Cl−, but they also indicate that the effects produced by NH4+ are much smaller than those arising from Li+, Na+, and K+. It is apparent, therefore, that at least some cationic effects can be observed in the Raman spectra.