Modeling solution vapor equilibria with solvation and solute assembly

Abstract

By combining solvation (e.g. hydration) and solute assembly (e.g. ion pairing), solution vapor–liquid equilibrium (VLE) data was effectively modeled without the use of solute activity coefficients. The method assumes that solvent activity is governed by Raoult's law ideality and corresponds to the concentration of free solvent (solvent not involved in solvation). Within the model, solution behavior is dictated by stochastically consistent speciation (solvated solute and solute assembles). Solvation is modeled as a linear decline from infinite dilution solvation (hydration) values to empirically determined saturation solvation (hydration) values. Ion pairing is modeled as a conventional chemical equilibrium to address conditions where the solvent activity residual is smaller than the anhydrous solute concentration (while also considering solvation). These three parameters, (i) infinite dilution hydration values, (i) saturation hydration values, and (iii) ion-association equilibria are held constant across a solute’s full range of concentration and fall within consistent, expected bounds. These speciation states describe solutes and solvent systems that model as ideal, thus avoiding the requirement of a solute activity model. This speciation-based solution model was successfully applied to 24 common 1–1 and 2–2 salts.