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.

The proposed research investigates how different properties of the components of the system affect the inhibition of liquid-liquid phase separation.  For systems composed of organic compounds and salts, the properties that we propose to investigate are the identity of the salt, the molecular weight of the organic compound, and the organic fraction.  For systems composed of phase separating polymers, we plan to investigate the role of polymer architecture and temperature.  These studies will allow us to tune the size dependence of the morphology, where small particles are well mixed and larger particles are phase separated.

The second major component of the proposal concerns the dynamics of liquid-liquid phase separation in submicron particles composed of organic compounds and salts.  Many systems undergo liquid-liquid phase separation as the water content of the system decreases in response to decreasing relative humidity surrounding the aerosol particle.  The relative humidity at which phase separation occurs is the separation relative humidity.  From our current work, we have found that the separation relative humidity for submicron particles tends to be lower than that for particles 10s to 100s of micrometers in diameter and spans a much wider range of relative humidities.  In this project, we will further explore these phenomena by characterizing the separation relative humidity in a wider variety of systems to understand the physical chemistry underlying the differences we have observed between systems.  In addition, the onset of phase separation for different organic fractions will allow us to develop an experimental phase diagram for submicron particles as a function of particle size.  These phase diagrams will inform the theory of liquid-liquid phase separation in the submicron size regime.  Our primary tools are cryogenic-transmission electron microscopy and optical microscopy, with many complementary techniques.