Study of contact ion pairs of supersaturated magnesium sulfate solutions using Raman scattering of levitated single droplets

Significant retardation of the evaporation rate of levitated aqueous MgSO₄ droplets has been found at high concentrations using an electrodynamic balance. Raman spectroscopy was used to study the structural changes, in particular, the formation of contact ion pairs, in supersaturated aqueous MgSO₄ droplets at ambient temperatures. As the relative humidity (RH) decreases, single levitated droplets lose water and become supersaturated. A molar water-to-solute ratio as low as 1.54 was obtained, facilitating the study of contact ion pairs of unhydrated Mg²⁺ and SO₄^2- ions in MgSO₄ solutions. The characteristics of the ν₁-SO₄^2- band change as a function of the water-to-solute ratio. Overall, a frequency shift from 983 to 1007 cm^-1 and an increase of the full width at half-height from 12 to 54 cm^-1 of the ν₁-SO₄^2- band were observed when the water-to-solute molar ratio decreased from 17.29 to 1.54. Most of the changes occur at a ratio smaller than 6, instead of at the saturation ratio of 15.60. These changes are attributed to the formation of contact ion pairs with different structures. A chain structure based on the contact ion pair of bidentate was proposed. Formation of close contact ion pairs and chain structures can explain the retardation of evaporation of supersaturated MgSO₄ droplets observed in previous experiments. On the basis of the comparisons of the Raman spectra of MgSO₄ and (NH₄)₂SO₄, we assigned the shoulder appearing at 995 cm^-1 in bulk MgSO₄ studies to the contact ion pairs instead of the solvent-separated ion pairs.