Confocal Raman studies of the evolution of the physical state of mixed phthalic acid/ammonium sulfate aerosol droplets and the effect of substrates.

The confocal Raman spectra of mixed phthalic acid/ammonium sulfate (AS) droplets deposited on a polytetrafluoroethylene (PTFE) substrate and a hydrophilic glass substrate are collected. The evolution of the physical state of the mixed droplet deposited on the PTFE substrate at various relative humidities consists of three states: well-mixed liquid state, liquid-liquid phase-separated state, and crystalline state. When the mixed droplets exist in liquid-liquid phase-separated state, the morphologies of the droplets on a PTFE substrate and on a glass slide are totally reverse, that is, an aqueous AS inner phase surrounded by an organic outer phase and an organic inner phase surrounded by an AS outer phase, respectively. We propose that the salting-out effect may induce the diffusion and formation of the organic phase, thus leading to the generation of liquid-liquid phase separation. The surface tension and hydrophobicity/hydrophilicity of substrates influence the spatial distribution of mixed aerosols. The understanding about the evolution of the physical state of mixed droplets and the effect of the substrates are important for probing the formation of atmospheric aerosols' morphology in the dehumidifying process.