(384g) Thermodynamics of Electrolyte-Containing Solutions and Aerosols

Solute and solvent activity coefficients of complex fluids over a wide range of concentrations govern numerous physical phenomena, from microstuctural transformations seen in plant cells as a function of intracellular water content, to phase partitions seen in atmospheric aerosols as a function of equilibrium relative humidity. Accurate predictions of water and solute activities in, for example, complex atmospheric aerosol mixtures to very low water content are essential for reliable climate predictions. A powerful method has been recently developed (Dutcher et al. JPC C, 2011, 2012) for capturing the thermodynamic properties of multicomponent solutions and aerosols at high solute concentrations by applying the principles of multilayer adsorption to ion hydration found in electrolyte solutions.  In these works, statistical mechanics is used to model adsorption of a solvent on to n energetically distinct layers in the hydration shell surrounding the solute molecule in aqueous mixtures. 

Here, we extend the statistical mechanical model of the adsorption free energy to include the dilute limit, mixed solute charge types, mixed solvents and surface tensions, allowing for a unified thermodynamic treatment of complex electrolyte containing solutions over wide ranges of solvent contents. Gibbs free energy, solvent and solute activity, solute concentration, as well as interfacial energies are derived, with 1-2 adjustable parameters per solute which can be estimated from the physical and electrostatic properties, if available, of the solution constituents or fit directly to osmotic and activity coefficient data. The resultant thermodynamic model is compared to existing activity models that have been extended to electrolyte containing solutions, such as statistically associating fluid theory based models, local composition based models, and aerosol organic/inorganic based models, and found to yield unprecedented agreement of the solute concentration and osmotic coefficients for solutions over the entire solute concentration range.  Finally, the effect of the long- and short-range electrostatics on the mixing relationship is discussed.