Micro-Raman and FTIR spectroscopic observation on the phase transitions of MnSO₄ droplets and ionic interactions between Mn²⁺ and SO₄^2-

Micro-Raman and FTIR spectroscopy have been used to investigate the micrometer-sized MnSO₄ droplets. The concentration of solute within the droplet is controlled accurately by decreasing the relative humidity (RH) of the surroundings. According to the Raman spectra of MnSO₄ droplets, when the RH decreased from ∼94% to ∼76%, the full width of the peak at half-maximum (fwhm) of the v₁-SO₄^2- band at 983 cm^-1 initially increased from 95.7 to 104.2 cm^-1. Two shoulders at ∼993 and ∼1002 cm^-1 occurred in the v ₁-SO₄^2- band at ∼76% RH, while the v ₂-SO₄^2- band at 450 cm^-1 split into two bands at 442 and 462 cm^-1, indicating the formation of monodentate and bidentate contact ion pairs (CIPs) in supersaturated MnSO ₄ droplets. From component band analysis of the v₁-SO ₄^2- band, four peaks at 983, 993, 1002, and 1010 cm ^-1 were identified and assigned to the free SO₄ ^2-, monodentate CIPs, bidentate CIPs, and more complex ion aggregates, respectively. Signatures of monodentate and bidentate CIPs reached their maximum values at ∼76% and 60% RH, respectively. With further decreasing the RH, great abundance of various ion pairs presented in the droplet, corresponding the v₁-SO₄^2- band steadily blue-shifted, and the fwhm increased continuously until it went through a plateau at ∼60% RH. When the RH was between ∼44% and 13%, the intensity of the signature from four species of ion pairs was almost invariant, corresponding to the formation of an amorphous phase of MnSO ₄•2.8H₂O. Further decreasing the RH below 13% RH, monodentate CIPs changed to bidentate ones. In the meantime, MnSO ₄•2.8H₂O was deduced to transform into another amorphous phase with a stoichiometry of MnSO₄•1.7H ₂O. This transition was also supported by the observation of the FTIR spectra according to a sharp increase of the fwhm of the v₃-SO ₄^2- band (at ∼1091 cm^-1) because of the appearance of a shoulder at 1132 cm^-1.