Aiming at scrutinizing the soluting effect occurring in alcoholic systems composed of two polymers (polymer 1 + polymer 2 + alcohol systems), the systematic isopiestic measurements of the vapor-liquid equilibrium behavior were carried out on some ternary ethanolic, propanolic, or butanolic solutions of two polymers at 298.1 K. In order to cover a wide range of hydrophilic and lipophilic behaviors, the polymers: polyethylene glycol 400 (PEG400), polyethylene glycol dimethyl ether 250 (PEGDME250), polyethylene glycol dimethyl ether 500 (PEGDME500), polypropylene glycol 400 (PPG400), polypropylene glycol 1000 (PPG1000) and polyvinylpyrrolidone 10000 (PVP10000) were selected. Deviations of iso-solvent activity lines from the linear isopiestic relation (LIR) were considered as a benchmark to identify the soluting effects of polymer 1 on polymer 2 in the systems under investigation. It was found that the investigated PVP – polymer solutions show the soluting-in effect (positive deviation from the linear isopiestic relation), and PEG-PPG, PEG-PEGDME, PPG-PEGDME, PEG-PEG, PPG-PPG and PEGDME-PEGDME systems show the semi-ideal (linear isopiestic relation) behavior. The effect of alcohol type, polymer type, and polymer molar mass on the solvent activity of binary alcohol-polymer solutions as well as on the deviations of ternary systems from the semi-ideal behavior was studied. Finally, the obtained solvent activity data were used to calculate the vapor pressure of solutions as a function of concentration, and the segment-based local composition Wilson model was used to correlate the experimental solvent activity data.
