Solubility, volumetric properties and viscosity of the sustainable systems of liquid poly(ethylene glycol) 200 with imidazolium- and phosphonium-based ionic liquids: Cation and anion effects

• Important sustainable systems (liquid PEG200 + ionic liquids) were studied.
• The studied systems show qualitative differences in thermodynamic behavior.
• The results were used to scrutinize the interactions in the studied systems.
• Entropic factor is crucial to interpret the differences in behavior of the studied systems.

In this work, solubility, volumetric and viscosity behavior were studied for the systems containing the environmentally acceptable compounds: liquid poly(ethylene glycol) (PEG200) and three ionic liquids: 1-butyl-3-methylimidazolium dicyanamide ([C4mim][dca]), trihexyltetradecyl phosphonium dicyanamide ([P6,6,6,14][dca]) and 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ([C6mim][NTf2]). The studies were performed in a temperature range (288.15 to 328.15) K and at a pressure of 0.1 MPa. For the only system that evidenced limited miscibility, namely (PEG200 + [P6,6,6,14][dca]), the temperature-composition phase diagram at 0.1 MPa was determined, mapping the existing one- and two-phase regions. In the homogeneous region of this diagram, densities and viscosities were measured and the excess molar volumes, as well as deviations in viscosity were calculated. For the other two systems, as they are always homogeneous in the temperature ranges of the present work, these measurements and calculations were performed in the full range of compositions. The molecular interactions in the studied systems were scrutinized using the obtained excess molar volumes, deviations of viscosity, as well as Kamlet–Taft parameters of PEG200 and the ionic liquids. In addition, the excess molar Gibbs free energies of activation of viscous flow and the related enthalpies and entropies were calculated and introduced to take into consideration the differences in size of the molecules.