Ab initio investigation on ion-associated species and association process in aqueous Na2SO4 and Na2SO4/MgSO4 solutions

In the present paper, the possible ion associated species in pure Na2SO4 and mixed Na2SO4/MgSO4 aqueous solutions are investigated via the ab initio method at the HF/6-31+G* level. The vibrational nu(1)-SO42- band is analyzed. For the unhydrated species, when the number of metal ions around the SO42- ion is less than 3, the dominating effect to the nu(1)-SO42- band is the polarization of the cations, while the M-O bonding will be dominating as the number is equal to or more than 3. For the hydrated species, the coordinated structures of the Na+ ion in all ion pairs are not stable due to the strong effect of the SO42- ion but relatively stable in the triple ion (TI) clusters since there are fewer vacant hydration sites around the SO42-. The nu(1)-SO42- frequencies are close to that of the hydrated SO42- ion in the ion pairs and larger in both Na2SO4 and Na2SO4/MgSO4 TI clusters. On the basis of our calculated results, the evolvement of Raman spectra in the Na2SO4/MgSO4 droplet with the molar ratio of 1:1 is explained. (C) 2011 American Institute of Physics. [doi:10.1063/1.3622859]