Article pubs.acs.org/jced
Density, Surface Tension, and Refractive Index of Ionic Liquids Homologue of 1-Alkyl-3-methylimidazolium Tetrafluoroborate [Cnmim][BF4] (n = 2,3,4,5,6) Wei-Guo Xu, Long Li, Xiao-Xue Ma, Jie Wei, Wen-Bin Duan, Wei Guan,* and Jia-Zhen Yang Key Laboratory of Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China ABSTRACT: The values of density, surface tension, and refractive index were measured for a classic series of air and water stable ionic liquids (ILs) based on tetrafluoroborate, [Cnmim][BF4](n = 2,3,4,5,6)(1-alkyl-3-methylimidazolium tetrafluoroborate) in the temperature range of (298.15 to 338.15 ± 0.02) K. In terms of Glasser’s theory, the standard molar entropy and lattice energy of the ILs were calculated. Using Kabo’s method, the molar enthalpy of vaporization of the IL, ΔlgHm0 (298 K), was estimated. According to the interstice model, the thermal expansion coefficient of ILs [Cnmim][BF4] (n = 2,3,4,5,6), α, were calculated, and in comparison with experimental value, their magnitude order is in good agreement. To test this new concept of ionic parachor, [BF4]− was chosen as a reference ion and its individual value of ionic parachor was determined in terms of extrathermodynamic assumption. Then, using ionic parachor of [BF4]−, the ionic parachors for all corresponding imidazolium cations, [Cnmim]+, were calculated and in good agreement with those obtained by Guan’s estimation, and the 17 anion ionic parachors were also obtained. In terms of ionic parachor, the surface tension and refractive index of the investigated ionic liquids were estimated, and the estimated values correlate quite well with their matching experimental values. values of ionic parachors for [BF4]− and [Cnmim]+, the parachor and surface tension of some ionic liquids were estimated. (4) Using the above estimated surface tension and parachor, the values of refractive index, nD, could be estimated and compared to experimental values. (5) The interstice model was applied to calculate the thermal expansion coefficient, α, of ILs [Cnmim][BF4] (n = 2,3,4,5,6) and the magnitude order of its value calculated was the same as the experimental one.
1. INTRODUCTION As the most common of the ionic liquid [Cnmim][BF4] (n = 2,3,4,5,6), they are promising compounds for being used for batteries, organic synthesis, extractions, and alloy electrodeposition.1−6 In recent years, there has been a developing trend in the literature toward estimation of the physicochemical properties for compounds by semiempirical methods, in particular, for ILs.7−9 Although the estimated result cannot be regarded as accurate physicochemical data, it is to be commended because it provides valuable insight into the origins of the behavior of materials. Among all the semiempirical methods, parachor is the simplest.10−12 Not long ago, Guan et al.13 proposed a new concept, ionic parachor, and determined the average value of the ionic parachor for reference ion [OAc]− in terms of the extrathermodynamic assumptions so that the values of the ionic parachor for all corresponding imidazolium cations, [Cnmim]+, were obtained. Using the new concept, ionic parachor, estimations of the properties of ionic liquids become easier. As a continuation of our previous investigation,14,15 in this article, the following contents were reported: (1) the values of density, surface tension, and refractive index for [Cnmim][BF4] (n = 2,3,4,5,6) were measured at (298.15 ± 0.02) K. (2) In terms of Glasser’s theory,16 the lattice energy and standard entropy of the ILs were estimated. Using Kabo’s method,17 the molar enthalpy of vaporization, ΔlgHm0 (298 K), was estimated. (3) If [BF4]− was chosen as a reference ion, its particular value of ionic parachor could be determined in terms of the extrathermodynamic assumption. In order to compare with the previous article, the values of ionic parachor for the corresponding cations, [Cnmim]+, in the [Cnmim][BF4] were calculated with the reference value of [BF4]−; then, using the © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Chemicals. Pure ILs [Cnmim][BF4] (n = 2,3,4,5,6) were purchased from Lanzhou Institute of Chemical Physics, the purity is more than 99%. The purity of ILs and the mass fraction contribution of the impurities are listed in Table 1. Table 1. Purity of ILs and Mass Fraction Contribution of the Impurities mass fraction purity
purity of ILs
1methylimidazole
halogen
water
[C2mim][BF4] [C3mim][BF4] [C4mim][BF4] [C5mim][BF4] [C6mim][BF4]
≥0.99 ≥0.99 ≥0.99 ≥0.99 ≥0.99
