Raman spectroscopic evidence for contact ion pairing in aqueous magnesium sulfate solutions

Abstract

Communications to the Editor 1315 vent in the vicinity of the moving ion is considered to be the same as in the vicinity of a moving uncharged sphere; thus it is assumed not to be affected by the electrostatic interaction of the moving ion with the solvent dipoles. This electrostatic interaction would increase the residence time of oriented solvent dipoles near the ion and, consequently, the friction due to the dielectric relaxation of the solvent would be reduced. The neglect of this electrostatic effect would be particularly important when the electrostatic force is large, i.e., for small and/or polyvalent ions.

Speciation in Aqueous MgSO4Fluids at High Pressures and High Temperatures from ab Initio Molecular Dynamics and Raman Spectroscopy

“…Furthermore, temperature and concentration dependent changes of the symmetric stretching mode of sulfate (ν 1 −SO 2− 4 ) in Raman spectra of MgSO 4 solutions have been related to the coexistence of different sulfate species [13][14][15][16][17] . Although the interpretation of the observed spectra has been somewhat controversial, particularly on the assignment of the ν 1 −SO 2− 4 band component at ∼995 cm −1 to CIPs 14 , most authors agree that these changes are due to increasing concentrations of more associated species, including bidentate ligands, triple ions and larger clusters 15,17 .…”

Theoretical understanding on the v1‐SO band perturbed by the formation of magnesium sulfate ion pairs

“…Some researchers 18,30,31 attributed it to the larger differences in the rotational correlation time of the water near the sulfate ion and the cation. Others 13,17,[22][23][24]32 suggested that the presence of the shoulder at 995 cm 21 was the indicator of the presence of the CIP. Remarkably, Rudolph et al 11 predicted that the shoulder at 995 cm 21 is caused by the monodentated CIP (MgOSO 3 ) by using the dielectric relaxation spectroscopy technique.…”

Isopiestic determination of the osmotic and activity coefficients of {zH2SO4+ (1−z)MgSO4}(aq) atT= 298.15 K. II. Results forz= (0.43040, 0.28758, and 0.14399) and analysis with Pitzer's model

“…Derived fractions of neutral ion pairs present, f = 1−α(MgSO 4 ), either increased regularly with molality to a value of about f = 0.13 at m = 2.87 mol·kg −1 , (26) or initially increased but then became roughly constant at f = (0.11 to 0.13) above ≈ 1 mol·dm −3 . (27) The authors of both studies claim that f reaches a maximum value of f = (0.12 ± 0.01) at high molalities. The authors of these Raman studies disagreed whether solvent-separated (26) or contact ion pairs (27) are present in these solutions.…”

“…(26,27) This was done by deconvolution of the v 1 peak into two overlapping peaks. Derived fractions of neutral ion pairs present, f = 1−α(MgSO 4 ), either increased regularly with molality to a value of about f = 0.13 at m = 2.87 mol·kg −1 , (26) or initially increased but then became roughly constant at f = (0.11 to 0.13) above ≈ 1 mol·dm −3 .…”