Water+1-alkanol systems: Modeling the phase, interface and viscosity properties

In this work, we present a thermodynamic characterization of the water. +. 1-alkanol mixtures, including the description of phase diagrams, interfacial tension and viscosities, by the soft-SAFT equation of state coupled with the Density Gradient Theory and the Free-Volume Theory. A molecular model for water and 1-alkanols is chosen within the soft-SAFT framework with particular attention to the hydrogen-bonding interactions. The cross-association parameters are, in most cases, predicted from the Wolbach-Sandler rules, while the dispersive energy and segment diameter of the mixture are normally fitted to an isotherm/isobar of one mixture and predicted for the rest. Quantitative agreement is found in all cases, with a single set of parameters able to simultaneously describe vapor-liquid and liquid-liquid equilibria. The interfacial tension of these systems is predicted using the Density Gradient Theory without using any adjustment for the crossed influence parameter, finding good agreement with the experimental data. Finally, the viscosity of water and several 1-alkanols is described by the Free-Volume Theory, using the density as an input taken from soft-SAFT. In particular, the viscosity of the water. +. methanol, water. +. ethanol and water. +. 1-propanol mixtures is described with two binary viscosity parameters in order to quantitatively reproduce the viscosity maximum of those systems. The excellent agreement found for all properties represents a step forward to the extension and implementation of molecular-based equations for the accurate design of processes involving these complex mixtures with very modest computational effort.