Abstract:

Ionic liquids (ILs) have become one of the most rapidly growing new research areas of ionic liquid and have attracted considerable attention from industry and the academic community since they are derived from natural ions and are heralded a new "natural ILs" or "bio-ILs". ILs can be expected to find application in all of the biological, medical, and pharmaceutical sciences. However, the fundamental physical properties of an IL are extremely important in determining if a particular IL is appropriate for a given application. Recently, there is a developing trend in the literature towards estimation of thermodynamic properties for IL's, which is to be commended because it provides valuable insight into the origins of the behaviour of ILs. Acidic ionic liquids have been applied in desulfuration process widely. Phosphoric acid IL is one kind of acidic ILs, which are also stable and economical efficiency. ILs [C_nmim][H₂PO₄] (n= 3, 4, 5, 6; 1-alkyl-3-methylimidazolium phosphonate) were synthesized and characterized. An on-line solution-reaction isoperibol calorimeter was constructed. It consists of a water thermostat, a 200 mL pyrex-glass plated silver Dewar, a 4 mL glass sample cell, a calibration heater, a glass-sheathed thermistor probe, an amplifier, a circuit used as an A/D converter and a personal computer for data acquisition and processing. The performance and accuracy of the calorimetric system was tested by measuring the molar enthalpy of solution of KCl in water and THAM [tris-(hydroxymethyl) aminomethane] in 0.1 mol∙dm⁻³ HCl(aq) at 298.15 K. The molar enthalpies of solutions of the ILs [C_nmim][H₂PO₄] (n= 3, 4, 5, 6) at various molalities in water were measured by using a solution-reaction isoperibol calorimeter at 298.15 K respectively. According to the Pitzer's electrolyte solution theory, the molar standard enthalpy of the solution of [C_nmim][H₂PO₄] (n = 3, 4, 5, 6), Δ_solH_m⁰, and Pitzer's parameters, β_MX^(0)L, β_MX^(1)L, and C_MX^ϕ, were calculated and deduced. The standard molar enthalpies of solution of [C_nmim][H₂PO₄] (n = 3, 4, 5, 6) decreases with increasing alkyl chain length. When we make a plot with the standard molar enthalpy of solution vs carbon number in alkyl chains of the ionic liquid, a straight line was obtained with a correlation coefficient 0.99 and a slope of 1.54. The mean contribution per methylene (―CH₂―) group to the standard molar enthalpies of the solution of [C_nmim][H₂PO₄] (n = 3, 4, 5, 6) was then obtained, which will give reliable estimation of other IL properties.

Key words: Ionic liquid, Isoperibol calorimeter, Solution enthalpy, Pitzer's equation