共聚焦拉曼光谱研究ZnSO_4微液滴的风化过程

通过共聚焦拉曼原位光谱技术在分子水平上研究了沉积在聚四氟乙烯基底上的ZnSO_4微液滴的风化过程.根据获得的高信噪比的Raman光谱,详细探讨了湿度降低时ZnSO_4微液滴中Zn~(2+)和SO_4~2离子的缔合行为,即经由单齿、双齿最终转化为复杂的链状或网状接触离子对.利用曲线拟合技术对其光谱的ν1-SO_4~2峰进行分峰,获得了不同湿度下ZnSO_4微液滴中各组分的量化信息:高湿度时,液滴中主要包含自由的水合SO_4~(2-);随着湿度降低,自由SO_4~(2-)逐渐向单齿、双齿接触离子对转化,并在湿度为66%和34%时分别达到其最大值;继续降低湿度,复杂的链状或网状接触离子对逐渐成为液滴中的主要成分.

Confocal Raman Study on ZnSO_4 Droplets in the Efflorescence Process

Confocal Raman technique was used to investigate the structure changes of the ZnSO_4 droplets deposited on Teflon substrate in the efflorescence process at the molecular level. Upon decreasing the relative humidity (RH), the symmetric stretc-hing band (ν1-SO_4~(2-)) of ZnSO_4 droplet blue shifted from 981 cm~(-1) gradually to 987,998 and 1 008 cm~(-1) , respectively, indicating that the free SO_4~(2-) ion associates with Zn~(2+) 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 ZnSO_4 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 fit-ting technique was used to extract the components at positions of 981, 987, 998 and 1 008 cm~(-1) in the spectral range of 945-1 065 cm~(-1) representing the free SO_4~(2-) ion, monodentate, bidentate and complicated CIPs, respectively. The results indicated that the main component in droplets was free hydrated SO_4~(2-) ion at high RHs. With decreasing the RH, monodentate and biden-tate CIPs were formed by Zn~(2+) and SO_4~(2-) and achieved their highest contents at RHs of 66% to 34%, respectively. Further de-creasing 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 ZnSO_4 droplets as the RH decreased below 34%.