硫酸镁微液滴水和重水交换动力学的微区拉曼研究

Micro-Raman studies on the kinetics of the exchange between H_2O and D_2O of MgSO_4 droplets

本文利用微区拉曼技术,研究硫酸镁液滴水和重水交换的动力学。在低湿度时,由接触离子对连接形成的链状结构使硫酸镁液滴表面形成胶态结构,阻碍其与环境之间的水交换,造成表面和内部的结构差异。拉曼光谱的高空间分辨能力为观测这一特殊的表面结构提供了便利。沉积在聚四氟乙烯疏水基底上的硫酸镁重水液滴呈球形,可以实现对液滴表面和中心的两次聚焦,为研究水从液滴表面逐步扩散到中心的动力学过程提供了可能。同时,利用拉曼光谱的时间分辨能力,可以观测硫酸镁微液滴中水和重水的交换过程,从而揭示传质受阻硫酸镁液滴中水的扩散机制。

In this paper, we will reveal the kinetics of evaporation of MgSO4/D20 droplets and the exchange between H20 and D20 in the mass transfer-limited MgSO4 droplets by micro-Raman spectroscopy. The gel layer causes a delay of the droplet in response to the change of ambient relative humidity, and results in the structural difference between the surface and the centre of MgSO4 droplets. Micro-Raman technique provides a good ap- proach for detecting the surface structure in virtue of its high spatial resolution. In parti- cular, as droplets are likely to maintain a spherical shape on a hydrophobic Teflon sub- strate, the laser beam can be tightly focused twice on the surface and in the center of the typically spherical droplets, thus the Raman spectra of the surface and the centre of the droplet can be obtained respectively and the diffusion of water in the droplet can be moni- tored. On the other hand, with an accumulation time of less than one second in a static mode, micro-Raman spectra of droplets can be obtained with high signal-to-noise ratio. The temporal scale allows the observation on the exchange of HzO and D20 in MgSO4 droplets with a slow exchanging rate, which is critical to understand the kinetics of the e- vaporation and condensation of water in mass transfer-limited MgSO4 droplet.