Predicting liquid-liquid phase separation in ternary organic-organic-water mixtures

Liquid-liquid phase separation (LLPS) affects the water uptake of aerosol particles in the atmosphere through Kelvin and Raoult effects. This study investigates LLPS in ternary mixtures containing water and two organic compounds, using a conductor-like screening model for real solvents (COSMO-RS). COSMO-RS found LLPS in all of the studied mixtures containing water and proxies for primary and secondary organic aerosol (POA and SOA, respectively), due to the limited solubility of the hydrophobic POA proxies in water. The computations predict additional three-phase states in some of the SOA-POA-water mixtures at relative humidity (RH) close to 100%, which was not observed in experiments, likely due to the relatively low RH (90%) used in the experiments. A computational method, such as COSMO-RS, allows for the estimation of new information on mixing states and mixtures that cannot be accessed experimentally. Comparison with experiments can also provide insight into which types of compounds may be present in SOA. Additionally, the possibility of LLPS can be assessed faster with rough estimates rather than by computing the whole phase diagram.