DEPENDENCE ON CONCENTRATION AND TEMPERATURE OF THE DYNAMICS OF SO42- IN LI2SO4, NA2SO4 AND K2SO4 AQUEOUS-SOLUTIONS STUDIED BY RAMAN-SPECTROSCOPY

The Raman spectra of the nu(1)(SO42-) band in aqueous solutions of Li2 SO4, Na2SO4 and K2SO4 as a function of concentration at room temperatu re and for fixed concentrations in the temperature range 5-85 degrees C were studied. It was found that the molar scattering coefficient for SO42- is virtually independent of the concentration and temperature i n the studied range. Nevertheless, the nu(1)(SO42-) band shows some as ymmetry on the high-wavenumber side for Li and Na and on the low-waven umber side for K, which for these last two cases has not previously be en reported. Moreover, the band position seems to be correlated with t he sign and amount of the asymmetry as the concentration increases. On the other hand, temperature does not have a great effect on the band profile, In the case of Na and K, there is little narrowing of the ban d, whereas for Li there is slight broadening and an increase in the as ymmetry. Quantitative analysis of these changes was performed using th e vibrational correlation function. This was modelled considering the distribution of sulphate oscillators in the aqueous media as a convolu tion product of an unperturbed distribution by a perturbation function . This last function assumes a feedback mechanism in which the main pa rameters are the coupling energy and the fraction of the distribution having interactions. The dependence of these parameters and of the Gau ssian and Lorentzian contributions to the band profile with concentrat ion and temperature are discussed.