Predicting the high-pressure phase equilibria of binary aqueous solutions of 1-butanol, n-butoxyethanol and n-decylpentaoxyethylene ether (C10E5) using the SAFT-HS approach

The phase equilibria of water+n-butan-1-ol is characterized by the presence of so-called closed-loop curves which represent regions of liquid-liquid immiscibility in the phase diagram. The system exhibits lower critical solution temperatures (LCSTs), which denote the lower limit of immiscibility together with upper critical solution temperatures (UCSTs). This behaviour can be explained in terms of the competition between the incompatibility of the water with the alkyl chain and the hydrogen bonding between water and the hydroxyl group. To study the phase equilibria we have used a simplified version of the statistical associating fluid theory (SAFT), which is based on the thermodynamic perturbation theory of Wertheim for associating fluids: the original SAFT-LJ equation of state treats the molecules as chains of Lennard-Jones segments, while the simplified SAFT-HS equation treats molecules as chains of hard-sphere repulsive segments with van der Waals interactions. The water molecules are modelled as hard spheres with four association sites to treat the hydrogen bonding; the dispersion forces are treated at the van der Waals mean-field level. The l-butanol molecules are modelled as chains of hard-sphere segments with two/three bonding sites to treat the terminal hydroxyl group; the dispersion forces also are treated at the mean-held level. The work has been extended to examine water+n-butoxyethanol (C4E1) and water+n-decylpentaoxyethylene ether (C10E5), which are n-alkyl polyoxyethylene ethers (C4E1,). Both of these systems exhibit closed-loop behaviour. These longer chain molecules also are modelled as chains of hard spheres with bonding sites to treat the terminal hydroxyl group, but with an additional three sites per oxyethylene group. For appropriate choices of the intermolecular parameters, the SAFT-HS approach predicts cloud curves with both a UCST and an LCST. The critical temperatures and the extent of immiscibility are in very good agreement with experimental data. The high-pressure critical behaviour is also well described by the theory, including the critical dome for H2O + C4E1.