The understanding of molecular-level mechanisms for the phase separation in polymer-based ABSs and the role of polymer structure are essential in the selection of a suitable ABS in applications such as bioseparation. For this purpose, in this work, a systematic investigation of ternary aqueous solutions containing [Ch][L-Phe] ionic liquid as a green new entrainer and different water soluble polymers (PEG(400), PPG(400), PEGDME(250)) was made by the volumetric, ultrasonic, and viscometric studies at different temperatures. These investigations indicate that PEGDME(250) and PPG(400) are able to form ABSs with ionic liquid; but phase separation cannot be occurred with PEG. This is indeed what we observed experimentally by studying phase diagrams. Unlike PEG the interaction between PEGDME or PPG with the ionic liquid are unfavorable leading to phase separation. Finally, the favorable partitioning of BSA in the ABS containing PPG compared with PEGDME was discussed on the basis of hydrogen bonding capability of the these polymers. (C) 2018 Elsevier B.V. All rights reserved.