Review and new insights into the application of molecular-based equations of state to water and aqueous solutions

Abstract

Water is a fascinating compound, essential to our life and present in most of the biological and industrial systems. In spite of its importance, the particular features of water and the complex interactions formed when mixed with other compounds make most of the models developed up to date unable to accurately describe the phase and interfacial behavior of aqueous mixtures, unless a vast amount of experimental data are available to fit the model to particular conditions. As a consequence, several theoretical approaches have been developed over the years to reproduce the physics of water solutions with different degrees of success, depending on the underlying accuracy of the models. Among them, equations of state which explicitly account for hydrogen-bonding interactions, such as the Statistical Association Fluid Theory (SAFT) and similar approaches, are built to provide a better picture of water-systems behavior, although there is still room for improvement.With the occasion of the 25th anniversary of the SAFT creation, we summarize here some association theories developed in the last 25 years including the association term proposed by Wertheim. Given the vast amount of works published during these years, the review focuses on the application of SAFT equations, Cubic-plus-Association (CPA) and Group Contribution plus Association (GCA) to aqueous solutions of systems with large impact at the chemical and energy industry today: binary mixtures of water with hydrocarbons, CO2, alkanols, amines and ionic liquids. Different molecular models and approaches are revised in detail. In addition, we present new modeling data using the soft-SAFT equation, to highlight the advantages of explicitly including hydrogen bonds when building the equation, even if done at a coarse-grained level, and the remaining challenges and opportunities for improvement.