Confocal Raman study on ZnSO₄ droplets in the efflorescence process

Confocal Raman technique was used to investigate the structure changes of the ZnSO₄ droplets deposited on Teflon substrate in the efflorescence process at the molecular level. Upon decreasing the relative humidity (RH), the symmetric stretching band (v₁-SO₄^2-) of ZnSO₄ droplet blue shifted from 981 cm^-1 gradually to 987, 998 and 1008 cm^-1, respectively, indicating that the free SO₄^2- ion associates with Zn²⁺ ion as the solution concentration increases and forms various contact ion pairs (CIPs), i. e., monodentate, bidentate and complicated multidentate chain-structured or web-structured CIPs. Accordingly, two obvious transition points were also observed in the hygroscopic curve of ZnSO₄ droplet, implying the structure transformations in the droplet. In order to obtain the information about the contents and evolutions of various components in the droplets, curve fitting technique was used to extract the components at positions of 981, 987, 998 and 1008 cm^-1 in the spectral range of 945-1065 cm^-1 representing the free SO₄^2- ion, monodentate, bidentate and complicated CIPs, respectively. The results indicated that the main component in droplets was free hydrated SO₄^2- ion at high RHs. With decreasing the RH, monodentate and bidentate CIPs were formed by Zn²⁺ and SO₄^2- and achieved their highest contents at RHs of 66% to 34%, respectively. Further decreasing the RH, the monodentate and bidentate CIPs gradually changed into multidentate chain-structured CIPs or complicated CIPs webs, which became the main components in supersaturated ZnSO₄ droplets as the RH decreased below 34%.