Phase equilibrium (solid-liquid-gas) in polyethyleneglycol-carbon dioxide systems

Phase equilibria (solid-liquid-gas) for polyethyleneglycol (with molar masses 1500, 4000, 8000 and 35 000 g mol⁻¹) and carbon dioxide mixtures were determined. It was found that the S-L-V lines in P-T projection for all polyethyleneglycols studied show a temperature minimum and a temperature maximum. These phenomena have not been reported previously in the literature. If a reduced temperature (obtained by dividing the temperature of the solid-liquid transition under pressure by the melting point of PEG at 1 bar) is introduced, the S-L transition of all binary systems are represented by one line. The solubility of CO₂ in liquid polyethyleneglycols is influenced by pressure and temperature, and is almost independent of the molecular mass of polyethyleneglycol. In a P-T projection, the vapour-liquid equilibrium is represented by a straight line if isopleths are plotted as a function of the reduced temperature. By combining the P-T_red projections of the S-L and L-V transitions, one master diagram can obtained which is valid for all binary mixtures of polyethyleneglycol and CO₂ in the pressure and temperature range investigated.