Statistical Thermodynamic Model for Surface Tension of Organic and Inorganic Aqueous Mixtures

The surface composition and tensions of aqueous aerosols govern a set of processes that largely determine the fate of particles in the atmosphere. Predictive modeling of surface tension can provide significant contributions to studies of atmospheric aerosol effects on climate and human health. A previously derived surface tension model for single solute aqueous solutions used adsorption isotherms and statistical mechanics to enable surface tension predictions across the entire concentration range as a function of solute activity. Here, we extend the model derivation to address multicomponent solutions and demonstrate its accuracy with systems containing mixtures of electrolytes and organic solutes. Binary model parameters are applied to the multicomponent model, requiring no further parametrization for mixtures. Five ternary systems are studied here and represent three types of solute combinations: organic-organic (glycerol-ethanol), electrolyte-organic (NaCl-succinic acid, NaCl-glutaric acid), and electrolyte-electrolyte (NaCl-KCl and NH₄NO₃-(NH₄)₂SO₄). For the NaCl-glutaric acid system, experimental measurements of picoliter droplet surface tension using aerosol optical tweezers show excellent agreement with the model predictions.