Stewart D J, Cai C, Nayler J, Preston T C, Reid J P, Krieger U K, Marcolli C, Zhang Y H
†School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
‡The Institute of Chemical Physics, Key Laboratory of Cluster Science, Beijing Institute of Technology, Beijing 100081, People's Republic of China.
J Phys Chem A. 2015 May 7;119(18):4177-90. doi: 10.1021/acs.jpca.5b01658. Epub 2015 Apr 27.
Direct measurements of the phase separation relative humidity (RH) and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented. Single droplets of mixed phase composition are captured in a gradient force optical trap, and the evolving size, refractive index (RI), and morphology are characterized by cavity-enhanced Raman spectroscopy. Starting at a RH above the phase separation RH, the trapped particle is dried to lower RH and the transition to a phase-separated structure is inferred from distinct changes in the spectroscopic fingerprint. In particular, the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol (PEG-400)/ammonium sulfate and a mixture of C6-diacids/ammonium sulfate are probed, inferring the RH from the RI of the droplet immediately prior to phase separation. The observed phase separation RHs occur at RH marginally higher (at most 4%) than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging. Clear evidence for the formation of phase-separated droplets of core-shell morphology is observed, although partially engulfed structures can also be inferred to form. Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur. For droplets that are repeatedly cycled through the phase separation RH, the water activity at phase separation is found to be remarkably reproducible (within ±0.0013) and is the same for the 1-phase to 2-phase transition and the 2-phase to 1-phase transition. By contrast, larger variation between the water activities at phase separation is observed for different droplets (typically ±0.02).
本文介绍了对由液态有机域和无机水域组成的气溶胶颗粒的相分离相对湿度（RH）和形态的直接测量。混合相组成的单个液滴被捕获在梯度力光学阱中，其不断变化的尺寸、折射率（RI）和形态通过腔增强拉曼光谱进行表征。从高于相分离相对湿度的相对湿度开始，将捕获的颗粒干燥至较低的相对湿度，并根据光谱指纹的明显变化推断向相分离结构的转变。特别是，探测了由聚乙二醇（PEG - 400）/硫酸铵水溶液以及己二酸混合物/硫酸铵组成的液滴的相分离相对湿度，通过紧接相分离之前液滴的折射率推断相对湿度。观察到的相分离相对湿度出现在比先前通过明场成像研究沉积在疏水表面上的颗粒所报告的相对湿度略高（最多4%）的情况下。观察到了核壳形态的相分离液滴形成的明确证据，尽管也可以推断形成了部分被吞没的结构。相分离不同光谱特征之间的转变表明形态可能会发生波动。对于反复循环通过相分离相对湿度的液滴，发现相分离时的水活度具有显著的可重复性（在±0.0013范围内），并且对于1相到2相转变和2相到1相转变是相同的。相比之下，不同液滴在相分离时的水活度之间观察到更大的变化（通常为±0.02）。
