Liquid-Liquid Phase Separation in Submicron Aerosol Particles

             M. A. Freedman, The Pennsylvania State University (Principal Investigator)

Liquid-liquid phase separation is a phase transition of interest in atmospheric, biological, and materials chemistry.  This phase transition occurs when two liquid phases demix to form a phase separated particle.  Our focus is on understanding liquid-liquid phase separation in the submicron size regime, due to the importance of this size regime for atmospheric and materials chemistry.  In previous work, we found that liquid-liquid phase separation is inhibited in aerosol particles less than 30 nm in diameter, which has implications for new particle growth, the formation of cloud condensation nuclei, and the synthesis of soft materials.  The inhibition of this phase transition stems from the fact that small particles cannot overcome the activation barrier to form a new phase.  The characterization of the change in physical properties of materials in the nanometer size regime is of ongoing interest to the physical chemistry community.  The study of the inhibition of liquid-liquid phase separation in the submicron regime can give insight into the dynamics of solutions and the processes that form interfaces in solution. Our primary tools are cryogenic-transmission electron microscopy and optical microscopy, with many complementary techniques.  

The proposed research builds off several recent results from my research group.  We have built an instrument that allows for the observation of the process of phase separation.  We find that phase separation occurs at lower relative humidities than expected based on studies of larger droplets.  In addition, the relative humidity at which phase separation occurs is dependent on the time the aerosol particles are given to undergo the nucleation process.  We have used this technique to create an experimental phase diagram for a system that undergoes liquid-liquid phase separation.  In addition, we have investigated how the identity of the components that compose the aerosol particles, both the salt and the organic components, impact the inhibition of phase separation.

This proposal expands our research in several directions.  Organonitrogen compounds are commonly found in aerosol particles, but their impact on liquid-liquid phase separation has not been explored.  We will determine the influence of organonitrogen compounds on the relative humidity at which phase separation occurs.  In a direct extension of previous work, we will continue to investigate the role of the organic oxidation state on the inhibition of phase separation.  In addition, we will characterize the rate at which the phase separation process occurs to estimate the activation energy for this process, which will add to our fundamental understanding of this phase transition.  Finally, we will explore the impact of insoluble particles on liquid-liquid phase separation.  Aerosol particles are often complex mixtures of soluble and insoluble components, and phase transitions have the potential to nucleate on these insoluble components when they are present, which can alter the relative humidity at which liquid-liquid phase separation takes place.