Statistical Thermodynamic Isotherm-Based Model for Activity Coefficients in Complex Aqueous Solutions with Atmospheric Aerosol Applications

Abstract

Aqueous aerosol particles are nearly ubiquitous in the atmosphere and yet there remain large uncertainties in their formation processes and ambient properties. The uncertainty is in part due to the complex nature of the individual particle microenvironment, which can involve a myriad of chemical components and multiple phases. The calculation of gas-liquid-solid equilibrium partitioning of the water, electrolyte, and soluble organic components is critical to accurate determination of atmospheric chemistry properties and processes such as new particle formation and activation to cloud condensation nuclei. Previously, a transformative model for capturing thermodynamic properties of multicomponent aqueous solutions over the entire concentration range (Dutcher et al. J. Phys. Chem 2011, 2012, 2013) was developed using statistical mechanics and multilayer adsorption isotherms. That model needed only a few adsorption energy values to represent the solution thermodynamics of each solute. In the current work, we posit that the adsorption energies are due to dipole-dipole electrostatic forces in solute-solvent and solvent-solvent interactions. This hypothesis was tested in aqueous solutions on (a) thirty-seven 1:1 electrolytes, over a range of cation sizes, from H+ to tetrabutylammonium, for common anions including Cl-, Br-, I-, NO3-, OH-, ClO4-, and (b) twenty water soluble organic molecules including alcohols and polyols. For both electrolytes and organic solutions, the energies of adsorption can be calculated with the dipole moments of the solvent, molecular size of the solvent and solute, and the solvent-solvent and solvent-solute intermolecular bond lengths. Many of these physical properties are available in the literature, with the exception of the solute-solvent intermolecular bond lengths. For those, predictive correlations developed here enable estimation of solute and solvent solution activities for which there are little or no activity data. The model was successfully validated using thirty-seven 1:1 electrolytes and twenty non-dissociating organic solutions (Ohm et al. J. Phys. Chem. 2015). However, careful attention is needed for weakly dissociating semi-volatile organic acids. Dicarboxylic acids such as malonic and glutaric acid are treated here as a mixture of non-dissociated organic species (HA) and dissociated organic species (H+ + A-). It was found that the apparent dissociation was greater than that predicted by known dissociation constants alone, emphasizing the effect of dissociation on activity coefficient predictions. To avoid additional parameterization from the mixture approach, an expression was used to relate the Debye-H�ckel hard-core collision diameter to the adjustable solute-solvent intermolecular distance. This work results in predictive correlations for estimation of solute and solvent solution activities for which there are little or no activity data.