Surface Tension Modeling of Secondary Organic Aerosol Chemical Mimics

Surface properties of secondary organic aerosol particles are an important diagnostic of atmospheric aerosol behavior because of their effect on particle interactions with the ambient environment, such as condensation of water vapor, radical species uptake, and particle morphology. Previously, a predictive model was developed using a statistical mechanical approach for surface tension of both electrolyte and non-electrolyte aqueous solutions across the entire solute concentration range (Wexler and Dutcher, J. Phys. Chem. Lett., 2013). While the adjustable model parameters had statistical mechanical interpretations, in practice they remained largely empirical. In this talk, the parameters in this surface tension model are related to solute molecular properties in aqueous solutions, reducing the number of free parameters down to one for both organic and electrolyte solutions (Boyer and Dutcher, J. Phys. Chem. Lett., 2015). For organics, sorption tendencies suggest a strong relation with molecular size and functional group spacing. For electrolytes, surface adsorption of ions follows the simulations of Pegram and Record, J. Phys. Chem. B 2007. Next, the model approach is extended to multi-component aqueous solutions by identifying surface partition functions for solutes of varied molecular size where competitive adsorption between solute species is expected at the surface. Parameter-free predictions for surface tension of aqueous systems of multiple solutes are presented using binary model parameter values. The multi-component model is applied to organic acid aqueous solutions. Excellent agreement has been found between the model predictions and experimental data obtained both from literature and by using a microfluidic tensiometry method adapted by our group to study atmospheric aerosol chemical mimics. This improved, multi-component, competitive adsorption predictive model will eventually lead to improved treatment of aerosol mixing state in climate modeling and aerosol particle measurement interpretation.