Theory of phase equilibria and closed-loop liquid–liquid immiscibility for model aqueous solutions of associating chain molecules: Water +alkanol mixtures

Abstract

In this work we examine the phase equilibria exhibited by a number of model binary mixtures of water+alkanols. The water molecules are modeled as hard spheres with four off‐center square‐well hydrogen‐bonding sites representing the two hydrogen atoms and the two electron lone pairs on the oxygen atom. Dispersion forces are included and are treated within the mean‐field approximation. Each alkanol is modeled as a chain molecule formed from fused hard spheres with dispersion forces and two hydrogen‐bonding sites. Thus water–water, water–alkanol, and alkanol–alkanol association is allowed. A simple thermodynamic perturbation theory is used to develop an augmented van der Waals equation of state which treats the asymmetry in the attractions which arise because of the directional hydrogen‐bonding sites, and the asymmetry in the repulsions caused by the molecular shape of the chains. The effects of chain length and association on the global phase equilibria are studied. As well as the usual gas–liquid phase separation, liquid–liquid and gas–gas phase separation, positive, negative, and double azeotropes, and Bancroft points are found. Closed‐loop liquid–liquid immiscibility corresponding to the type VI classification of Scott and van Konynenburg is seen for water+alkanol mixtures with intermediate chain lengths and bonding energies. We also discuss our serious reservations about the type VI behavior claimed to have been found in some recent work. Although the potential parameters used in our work were not chosen to model any specific systems, it is gratifying to see that the theory can reproduce the types of phase behavior exhibited by aqueous solutions of alkanols.