Atmospheric Aqueous Aerosol Interfaces: Thermodynamic Modeling and Biphasic Microfluidic Flows with Fluid-Fluid Interfaces

Abstract

Surface properties of atmospheric aerosol particles are crucial for accurate assessment of the fates of liquid particles in the atmosphere. Surface tension directly influences predictions of aerosol particle activation to clouds, as well as indirectly acts as a proxy for chemical surface partitioning. Challenges to predicting surface tension are posed by the chemical complexity of particles, which contain mixtures of water soluble compounds of both surface-active organics and inorganic electrolytes. The interface itself is varied in that it may be liquid - vapor, as in the surface of an aerosol particle with the ambient air, or liquid - liquid, as in the interior surfaces that exist in multiphase particles. These surface-based properties and their relevant processes govern atmospheric aerosol particle size, morphology, composition, and growth. This thesis explores aqueous aerosol interfaces through thermodynamic modeling of liquid-vapor surfaces to predict surface tension and biphasic microfluidic measurements of liquid-liquid interfacial tension of atmospheric aqueous aerosols. Using adsorption isotherms and statistically mechanically derived expressions for entropy and Gibbs free energy, predictive modeling of surface tension as a function of concentration for aqueous solutions containing both water-soluble organic species and inorganic electrolytes is demonstrated for a breadth of atmospherically relevant solutes. Alcohols, polyols, sugars and organic acids represent the organic solutes. Nitrates, sulfates and chlorides represent the electrolytes. A unique feature of the model is the surface partition function, where the solvent molecules (waters) represent adsorption sites, and solute molecules can displace more than one waters either positively or negatively, therefore the model implicitly depends on solute size dependence and surface propensity. For binary solutions, model parameters are eliminated through strong correlations with solute properties, such as molar volume for organics and surface-bulk partitioning coefficients for electrolytes. A multicomponent model is derived for an arbitrary number of solutes, using no further parametrization beyond the optimized binary cases. For organic and inorganic aqueous mixtures, model predictions agree excellently with available data, including novel measurements made at supersaturated concentrations using optical tweezers. To further complement model predictions, interfacial tensions were measured for liquid-liquid systems using microfluidics. Microfluidic platforms afford many advantages, including high throughput, rapid prototyping of devices (using soft photolithography), small sample volume and potential for controlled manipulation of thermal, mechanical, and chemical changes. Microfluidics also offers an appropriate lengthscale, where surface forces influence the system far more than gravitational and inertial forces. In this thesis, atmospheric aerosol interfaces are examined using droplet microfluidics, where the droplets chemically represent the aerosol phase dispersed in an immiscible surrounding phase. The droplets consist of either a chemical mimic or a sample obtained from photochemical smog chambers that simulate atmospheric chemistries. Interfacial tensions of numerous individidual droplets are measured with low sample volumes, otherwise unattainable in bulk analogues. Surface and interfacial tensions are applicable to numerous industrial, environmental, and biological engineering areas and this work could be valuable to each of these fields. In this thesis, model development and experimental techniques are reinforced in the context of atmospheric chemistry to facilitate further application to atmospheric processes, such as aerosol-cloud activation.