Raman spectral investigations on the phase transition behaviors of MgSO₄·7H₂O crystals in different efflorescence processes

In the present paper, the in situ confocal Raman spectra of MgSO₄·7H₂O crystal in different efflorescence processes were obtained through changing relative humidities in differernt ways. Thus, detailed investigation of phase transition of MgSO₄·7H₂O crystal was achieved. At 3% RH, the phase transition of MgSO₄ crystal from MgSO₄·7H₂O to MgSO₄·3H₂O both occurred in the fast-efflorescence and slow-efflorescence processes. However, in the deliquescence process, distinct phase transition behaviors were found between the MgSO₄·3H₂O crystals which formed in different efflorescence processes. the MgSO₄·3H₂O crystals formed in fast-efflorescence process transformed to MgSO₄·6H₂O crystals at 70% RH in the deliquescence process. On the contrary, the MgSO₄·3H₂O crystals formed in slow-efflorescence process deliquesced to supersaturated MgSO₄ droplet. The studies indicated that there is a correlation between the phase transition behaviors and ways of changing RH for the different MgSO₄ crystal hydrates.