Heat of mixing and liquid–liquid-equilibrium of water + polypropylene glycol (PPG) with different molecular weights and water + propylene glycol dimethyl ether

•Simultaneously modeling of LLE of aqueous solution of polypropylene glycol.•New experimental data related to heat of mixing.•Detailed association model including self- and cross associations.•Study of the critical points as function of molecular weight.•New experimental data for 1,2-dimethoxypropane and water related to demixing.

Polypropylene glycols (PPG) are widely used in a large variety of different fields, e.g. polyurethane formulations, rheology, surfactant, wetting agent, dispersant in leather finishing, pharmacy, and tissue engineering, where polymers with different molecular weights are employed. Therefore, the phase behavior in aqueous solutions plays an important role. For thermodynamic models able to describe the phase behavior, especially the liquid–liquid equilibria (LLE), the occurring association phenomena are very significant. The purpose of this work is the analysis of detailed association models taking into account self-association of water, self-association of polymer and cross-associations. Starting from a general association scheme of hydrogen bonds two models (model 2 and model 3) were derived and investigated with regard to the performance of correlating LLE data as well heat of mixing data, simultaneously. In model 2 identical associations between water to ether-oxygen and water to hydroxyl end-group are considered and in model 3 identical associations between water to hydroxyl end-group and hydroxyl end-group to water are established. The extrapolation power related to phase equilibria or heat of mixing data not involved in the parameter fitting procedure of the two models were analyzed carefully. Finally, the prediction power were studied by application of the adjusted parameters to a slightly different system, namely the LLE of 1,2-dimethoxypropane and water mixtures. It turns out, that the model can predict the LLE of 1,2-dimethoxypropane and water in fair agreement with new data measured in this work.