Micro-Raman and FTIR spectroscopic observation on the phase transitions of MnSO4 droplets and ionic interactions...

Micro-Raman and FTIR spectroscopy have been used to investigate the micrometer-sized MnSO(4) 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 MnSO(4) droplets, when the RH decreased from approximately 94% to approximately 76%, the full width of the peak at half-maximum (fwhm) of the v(1)-SO(4)(2-) band at 983 cm(-1) initially increased from 95.7 to 104.2 cm(-1). Two shoulders at approximately 993 and approximately 1002 cm(-1) occurred in the v(1)-SO(4)(2-) band at approximately 76% RH, while the v(2)-SO(4)(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 v(1)-SO(4)(2-) band, four peaks at 983, 993, 1002, and 1010 cm(-1) were identified and assigned to the free SO(4)(2-), monodentate CIPs, bidentate CIPs, and more complex ion aggregates, respectively. Signatures of monodentate and bidentate CIPs reached their maximum values at approximately 76% and 60% RH, respectively. With further decreasing the RH, great abundance of various ion pairs presented in the droplet, corresponding the v(1)-SO(4)(2-) band steadily blue-shifted, and the fwhm increased continuously until it went through a plateau at approximately 60% RH. When the RH was between approximately 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).2.8H(2)O. Further decreasing the RH below 13% RH, monodentate CIPs changed to bidentate ones. In the meantime, MnSO(4).2.8H(2)O was deduced to transform into another amorphous phase with a stoichiometry of MnSO(4).1.7H(2)O. This transition was also supported by the observation of the FTIR spectra according to a sharp increase of the fwhm of the v(3)-SO(4)(2-) band (at approximately 1091 cm(-1)) because of the appearance of a shoulder at 1132 cm(-1).