Observation of the Repartitioning of Malonic Acid in Levitated Malonic Acid/NaNO3/H2O Droplets by Optical Tweezers
LÜ Xi-juan1, ZHANG Yun-hong2*
1. School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China
2. School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China
Abstract：Exploring the repartitioning process of semi-volatile organic compounds (SVOCs) between particles is important for understanding the growth and shrinkage of atmospheric particulates. Optical tweezers-stimulated Raman spectroscopy, compared with other suspension techniques, can not only suspend droplets, but also obtain the chemical composition, structure of droplets and other information according to the conventional Raman scattering spectra of droplets. The radius and refractive index of droplets can be calculated according to stimulated Raman-Mie scattering resonance. The advantages of optical tweezers stimulated Raman spectroscopy are that particle radius can be accurately measured, chemical composition, phase and shape can be controlled, and long-term observation can be realized. In this study, optical tweezers coupled with cavity-enhanced Raman spectroscopy was used to observe the repartitioning of malonic acid between a levitated malonic acid/NaNO3/H2O droplet and surrounding malonic acid /NaNO3/H2O droplets deposited on the inner wall of a chamber with different organic to inorganic molar ratios (OIRs). It was found that when the OIR was 1∶1, no NaNO3 crystallization occurred both for levitated and deposited dropletsin the whole relative humidity (RH) range, the radius of the levitated droplet decreased slowly due to the evaporation of malonic acid from the levitated droplet at constant RHs. When the OIRs were 1∶2 and 1∶3, the radius of the suspension droplets did not decrease but increased gradually when the RH was lower than 52.5% and 58% respectively. This indicated that the repartitioning of malonic acid occurred. The reason was that NaNO3 in the deposited droplets was crystallized when RH was lower than 52.5% for 1∶2 or 58% for 1∶3. So the vaporpressure of malonic acid at the surface of deposited droplets was higher than that of the levitated droplet which always remained as liquid droplet without NaNO3 crystallization, resulting in the transfer of malonic acid from the deposited ones to the levitated one. This is a good model for explaining the repartitioning process of SVOC among different phase particles states.
[1] Kulmala H V M, Petäjä T, Dal Maso M, et al. Journal of Aerosol Science，2004，35：143.
[2] LÜ Xi-juan，GAO Xiao-yan，MA Jia-bi，et al(吕席卷，高晓艳，马嘉壁，等). Spectroscopy and Spectral Analysis(光谱学与光谱分析)，2019，39(5)：1648.
[3] Lv X J, Wang Y, Cai C, et al. Spectrochimica Acta Part A, Molecular and Biomolecular Spectroscopy，2018，200：179.
[4] Marshall F H, Miles R E H, Song Y C, et al. Chemical Science，2016，7：1298.
[5] Wang L N, Cai C, Zhang Y H. Journal of Physical Chemistry B，2017，121：8551.
[6] Gao Xiaoyan, Ma Jiabi，Zhang Yunhong. Royal Society Open Science，2018，5：170819.
