Conductometric studies of aqueous ionic liquids, 1-alkyl-3-methylimidazolium halide, solutions at T=298.15-328.15 K

Electrical conductances of aqueous ionic liquids, 1-butyl-3-methylimidazolium bromide, [BMlm]Br, 1-pentyl-3-methylimidazolium bromide, [PnMlm]Br, 1-hexyl-3-methylimidazolium bromide, [HMlm]Br, 1-butyl-3-methylimidazolium chloride, [BMlm]Cl, and 1-butyl-3-methylimidazolium iodide, [BMlm]I solutions at low concentrations of ionic liquids have been measured at (T = 298.15-328.15 K). The ion association constants, K-A, limiting molar conductivities, Lambda(0) and distance parameters, R were obtained using low concentration Chemical Model (IcCM) conductivity equation for the investigated systems. The results show that the K-A values decrease with lengthening alkyl chain and larger size of anion; i.e. Cl- < Br- < l(-). Therefore, there is slight ion association effect in [BMlm]Cl at all experimental temperatures. Furthermore, the K-A and Lambda(0) values for the aqueous solutions of ionic liquids increase as the temperature increases. Thermodynamic functions such as Gibbs free energy, enthalpy and entropy of ion pair formation were calculated using K-A values. Results also show that Delta H-A(0) values exhibit weak dependence on the temperature this shows that ion pair formation of ionic liquids in water is the entropy driven process. (C) 2009 Elsevier B.V. All rights reserved.