BAND PROFILE ANALYSIS OF THE RAMAN-SPECTRA OF SULFATE-IONS IN AQUEOUS-SOLUTIONS

A Raman spectroscopic study of aqueous solutions of sulphates with some mono- and divalent cations was performed. The dynamic properties of the sulphate ion were analysed in the wavenumber domain and in the time domain using Fourier transform methods. Special interest is devoted to those systems for which the nu1(SO42-) band shows an asymmetric profile. This asymmetry has been interpreted in two ways: (a) as arising from different S-O oscillators (associated and non-associated); their relative intensity is calculated using classical fitting methods, although the band parameters are obtained from a self-deconvolution method performed in the time domain; and (b) as arising from a perturbation of the water molecules in the vicinity of the sulphate ion. This perturbation is modelled in the time domain through a generalized correlation function. This latter represents a perturbed distribution of oscillators which undergo mutual interactions. These interactions are modelled by a feedback mechanism with two main parameters, the mutual coupling energy and the fraction of the distribution having mutual interaction. The correlation function thus modelled leads in the frequency domain to an asymmetric bandshape. The fitting of the experimental correlation function by this model gives essentially the same information as the fitting in the wavenumber domain for all the cases studied. Moreover, in the case of Li and Mg sulphates, both methods give the same value for the spectroscopically measured association enthalpy, 14 kJ mol-1 for Li and 21 kJ mol-1 for Mg.