Experimental Determination And Estimation Of The Polarity For Ionic Liquid

In this paper, four hydrophilic ionic liquids 1-alkyl-3-methylimidazolium acetate [Cnmim][OAc] (n=2,4,5,6) and two hydrophobic ionic liquids 1-alkyl-2,3-dimethylimdazolium-N,N-bis(trifluorosulfonyl)imide [Cnmmim][NTf2] (n=2,4) were prepared by the method in literature, the intermediates and product which were purified were characterized by 1H NMR spectroscopy and differential scanning calorimetry (DSC).The Reichardt’s dye was added to the ionic liquids with the concentration of 5.0×10-4 mol·L. The position of the maximum absorbance peaks were obtained by the Reichardt’s dye at the temperature of 298.15 K. The (30) values of the sythesized ionic liquids were calculated from the position of the maximum absorbance peaks by the giving equation. For convenience these values were normalized to ETN. The ETN values of [C2mim][OAc], [C4mim][OAc], [C5mim][OAc], [C6mim][OAc], [C2mmim][NTf2], [C4mmim][NTf2] were 0.621,0.614,0.612,0.606,0.563,0.551, respectively. It can be seen that, ETN ([C2mim][OAc])>ETN([C4mim][OAc])> ETN([C5mim][OAc]> ETN([C6mim][OAc]), ETN([C2mmim][NTf2])> ETN([C4mmim][NTf2]),ETN([C4imm][OAc]>ETN(C4mim][Pro]). For the same series of ionic liquids, the ETN values were decresing with the increasing length of carbon chain. The N,N-diethyl-4-nitroaniline was added into the ionic liquids with the concentration of 1.0×10-3 mol-L, the π* values of [C2mim][OAc], [C4mim][OAc], [C5mim][OAc], [C6mim][OAc], [C2mmim][NTf2], [C4mmim][NTf2] were calculated from the position of the maximum absorbance peaks which were 1.122,1.027,0.979, 0.957,1.031,1.013, respectively. It can be seen that,π*([C2mim][OAc])> π*([C4mim] [OAc])>π*([C5mim] [OAc]>π*([C6mim] [OAc]),π*([C2mmim] [NTf2])> π*([C4mmim][NTf2]),π*([C4mim][OAc])>π*([C4mim][Pro]). The trend of π* values of the ionic liquid with the different length of carbon chain were as same as the ETN values.According the Hildebrand theory, the scale of polarity for ionic liquids,δμ, were obtained by the data of vaporization enthalpy. It found that,δμ([C2mim][OAc])> δμ([C4mim][OAc])>δμ([C5mim][OAc]>δμ([C6mim][OAc]),δμ([C2mmim][NTf2])> δμ([C4mmim][NTf2]),δμ([C4mim][OAc])>δμ([C4mim][Pro]). The trend of δμ values of the ionic liquid with the different length of carbon chain were as same as the ETN values and π* values. When the anionic were OAc-, Pro-, BF4-, and the cation were all 1-butyl-3-methylimidazolium, the polarity order were [C4mim][OAc]>[C4mim][Pro]> [C4mim][BF4-], as same as the ETN values and π* values. For the ionic liquid [Cnmim][OAc](n=2,4,5,6) and [Cnmmim][NTf2](n=2,4),δμ can prediction the polarity order well.