Raman spectral studies on ionic interaction in aqueous alkali sulfate solutions

Raman spectra were measured for aqueous solutions of alkali sulfates, Li2SO4, Na2SO4, K2SO4, Rb2SO4 and Cs2SO4 at various concentrations and al room temperature. The changes in the band parameters and are in the band profiles of the v(1)(SO42-) mode were studied as a function of the salt concentration. The band maximum nu(max) shifts almost linearly with concentration for all the salts. The slope of the lines plotted nu(max) vs. concentration decreases in the order of Li+ > Na+ > K+ > Rb+ > Cs+, and the former two have positive slope, while the last two have a negative one. For the case of K+ the line is almost horizontal. The intercept of the lines converges to a single point of 980.8 +/- 0.2 cm(-1). The width measured at the half height and the integrated band width given by beta = A/I(nu(max)), where A denotes the integrated area of the band and I(nu(max)) the intensity at the band maximum; increased with concentration for all the studied salts. Their behaviour is non linear and a polynomial fit on the data shown an intercept at zero concentration of 6.2 +/- 0.1 cm(-1) for the width measured at the half height and of 7.2 +/- 0.1 cm(-1) for the integrated band width. The band profiles which have been measured by several methods found an asymmetry in the high wavenumber side of the band for Li+ and Na+ while for the other salts the profile remain symmetrical. The changes in the band parameters and band profiles observed for nu(1)(SO42-) mode in the present work are interpreted in terms of two main factors (a) the changes in the local field experienced by sulfate surrounded by the water-screened cations and (b) the perturbation arising from the different dynamic behaviour of hydration water molecules of the cations. In particular, these results are found to be consistent with the concepts of positive and negative hydration proposed by Samoilov. (C) 1997 Elsevier Science B.V.