Pressure influence on true cosolvency. Measured and calculated solubility limits of polystyrene in mixtures of acetone and diethylether

The phenomenon of “true cosolvency”, which has only recently been observed in the system acetone/diethyl ether/polystyrene (AC/DEE/PS), is investigated with regard to its pressure dependence and compared with the thermodynamic behaviour of the corresponding binary sub‐systems. For the determination of the limits of complete solubility, a pressure apparatus was used which had been constructed, in order to measure the turbidity of fluids in the temperature range of −70 to +500°Cand in the pressure range of 1 to 4000 bar (10 5 to 4·10 8 Nm −2 ). The critical line observed for the system DEE/PS ( M PS =20400) looks similar to that reported for AC/PS in the literature, i.e. the upper critical solution temperatures are shifted towards lower and the lower critical solution temperatures towards higher values when the pressure is increased. In the case of true cosolvency ( M PS =110000), the critical line for atmospheric pressure encircles the range of temperature and composition of the mixed solvent within which the solutions stay homogeneous irrespective of polymer concentration. With increasing pressure, this range expands all around, thus creating a trumpet‐like surface. This experimental result is well reproduced theoretically by the “single‐liquid approximation” of Scott , where the thermodynamic behaviour of the binary sub‐systems is described on the basis of the Prigogine‐Patterson theory.