Effects of the Interaction between Ionic Liquids and Redox Couples on Their Reaction Entropies

The temperature dependence of the potential, dE/dT, of some redox couples was determined using an isothermal cell with ferrocenium / ferrocene couple as a calibration redox couple in different ionic liquids, BMPTFSA, EMITFSA, PP13TFSA, BMP-BETA and EMI[BF4] (BMP+ = 1-butyl-1-methypyrrolidinium, EMI+ = 1-ethyl-3-methylimidazolium, PP13(+) = 1-methyl-1-propylpiperidinium, TFSA(-) = bis(trifluoromethylsulfonyl)amide and BETA(-) = bis(perfluoroethylsulfonyl)amide). The reaction entropies of the redox couples, Delta S-rc degrees, were calculated from the dE/dT values. It is suggested that the coulombic interaction between the redox couples and the ions with the opposite sign of charge to those of the redox couples is a dominant factor determining the Delta S-rc degrees value and that the absolute Delta S-rc degrees value for the redox couple increases with an increase in the charge density of the ions with the opposite charge to the redox couple. However, the Delta S-rc degrees values are considered to be affected not only by the charge density of the ions but also by the polarization and/or steric shape of the ions. (C) 2013 The Electrochemical Society. All rights reserved.