Polyethylene glycol (PEG) finds application in paints, detergents, soap, the paper-making industry, defoamers, lubricants, softeners and adhesives, and, due to its ability to resist cell and protein adhesion and recognition from the immune system it is also a widely used and accepted biomaterial. Mixed with water a homogeneous fluid is observed at low temperatures, but on increasing the temperature liquid-liquid separation is seen above a lower critical solution temperature, further increase of the temperature above an upper critical solution temperature leads again to a homogenous phase. In the temperature-composition plane the result is a closed region of liquid-liquid phase separation usually referred to as a closed loop. In this contribution we study the closed-loop phase behaviour of water + PEG binary mixtures using the statistical associating fluid theory for potentials of variable range [1,2]. A molecular model of the mixture is developed which takes into account water-water, water-PEG and PEG-PEG hydrogen bonding interactions as well as repulsion and dispersion contributions. A fully-transferable model is proposed which allows the study of any water +PEG binary mixture with the sole input of the molecular weight of the polymer. The model leads to an excellent description of the liquid-liquid phase behaviour of these systems and can be used in a predictive sense; mixtures involving shorter polymers are used to determine the binary interaction parameters, and with this, the phase behaviour of mixtures of larger molecular weight are predicted in good agreement with experimental data. The high-pressure (GPa) phase behaviour of the liquid phase in these systems is also studied. The region of closed-loop immiscibility is seen to become less extensive with an initial increase in pressure. For intermediate molecular weights of the polymer at high enough pressure the liquid phase becomes homogeneous, and at very high pressures a second region of liquid-liquid separation is observed. These dome shaped regions of liquid-liquid immiscibility are reminiscent of the pressure-temperature denaturation boundaries found in protein systems which are thought to be governed by the corresponding increase in water solubility into the hydrophobic core. For high molecular weight polymers an hour-glass type of phase diagram with liquid-liquid separation persisting for all pressures considered is predicted.

References

[1] A. Gil-Villegas, A. Galindo, P.J. Whitehead, S.J. Mills, G. Jackson, A.N. Burgess, J. Chem. Phys. 106 (1997) 4168–4186.

[2] A.Galindo, L.A. Davies, A. Gil-Villegas, G. Jackson, Mol. Phys. 93 (1998) 241–252.