Liquid–liquid phase separation and evaporation of a laser-trapped organic–organic airborne droplet using temporal spatial-resolved Raman spectroscopy

Chemical reactions in aerosol particles can occur between the reactive components of the particle or between the particle and its surrounding media. The fate of atmospheric aerosols depends on the environment, the composition and the distribution of components within a particle. It could be very interesting to see how a liquid aerosol particle behaves in ambient air if the particle is composed of mixed chemicals. Do the chemical components remain homogeneously mixed within a particle or separate as the mixed liquid is aerosolized? How do the chemicals within a droplet separate and interact with the air? In this paper, a single microdroplet formed from an organic–organic mixture of diethyl phthalate (DEPh) and glycerol was investigated using laser-trapped position-resolved temporal Raman spectroscopy. For the first time, we were able to directly observe the gradient distributions of the two chemicals at different positions within such an airborne droplet, their time-resolved processes of liquid–liquid phase-separation, and changes of the physical microstructure and chemical micro-composition in the droplet. The results revealed that DEPh migrated to the surface and formed an outer layer and glycerol was more concentrated in the interior of the droplet, DEPh evaporated faster than glycerol, and both organic chemicals within the mixed droplet evaporated faster than either of them within their pure droplets. This technique also provides a new method for studying the fine structure and chemical reactions of different molecules taking place inside a particle and at the interface of a particle with the surrounding microenvironment.