Solvent systems composed of ionic liquid (IL) + poly( ethylene glycol) (PEG) provide "hybrid green" systems where "green meets green." In this work, the density and viscosity of binary mixtures of 1-butyl-3methylimidazolium tetrafluoroburate ([bmim][BF4]) and PEG with different number average molecular weights (i.e. 200, 400, 600, and 1000) were measured and analyzed as a function of temperature and composition. Excess molar properties of these mixtures were calculated and interpreted. A synergistic effect or super-viscosity was observed in the mixtures of [bmirn][BF4] with PEG400, PEG600, and PEG1000 in which the mixture viscosity is higher than the viscosity of both pure components. By adding PEG to [bmim][BF4]in these mixtures, the decrease in the columbic attractive and van der Waals interactions is overcompensated by the formation of extensive H bonding interactions which is responsible for observing the synergistic effect in the viscosity behavior of these mixtures. This effect can also influence on the excess properties. The FTIR spectroscopy was used for better analysis of molecular interactions. The variation in the positions and intensities of the peaks of FTIR spectra may be attributed to the possibility of H-bonding between C2 hydrogen of [bmim](+) and terminus -OH as well as ethoxy -O- of PEG. (C) 2016 Elsevier B.V. All rights reserved.
