Dependence on concentration and temperature of the dynamics of SO 42− in Li2SO4, Na2SO4 and K2SO4 aqueous solutions studied by Raman spectroscopy

The Raman spectra of the v1(SO) band in aqueous solutions of Li2SO4, Na2SO4 and K2SO4 as a function of concentration at room temperature and for fixed concentrations in the temperature range 5—85°C were studied. It was found that the molar scattering coefficient for SO is virtually independent of the concentration and temperature in the studied range. Nevertheless, the v1(SO) band shows some asymmetry on the high-wavenumber side for Li and Na and on the low-wavenumber side for K, which for these last two cases has not previously been reported. Moreover, the band position seems to be correlated with the 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 band, whereas for Li there is slight broadening and an increase in the asymmetry. Quantitative analysis of these changes was performed using the vibrational correlation function. This was modelled considering the distribution of sulphate oscillators in the aqueous media as a convolution product of an unperturbed distribution by a perturbation function. This last function assumes a feedback mechanism in which the main parameters are the coupling energy and the fraction of the distribution having interactions. The dependence of these parameters and of the Gaussian and Lorentzian contributions to the band profile with concentration and temperature are discussed.