Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium Cation

Abstract

The alkyl chain length of 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Rmim][(CF(3)SO(2))(2)N], R = methyl (m), ethyl (e), butyl (b), hexyl (C(6)), and octyl (C(8))) was varied to prepare a series of room-temperature ionic liquids (RTILs), and the thermal behavior, density, viscosity, self-diffusion coefficients of the cation and anion, and ionic conductivity were measured over a wide temperature range. The self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity change with temperature following the Vogel-Fulcher-Tamman equation, and the density shows a linear decrease. The pulsed-field-gradient spin-echo NMR method reveals a higher self-diffusion coefficient for the cation compared to that for the anion over a wide temperature range, even if the cationic radius is larger than that of the anion. The summation of the cationic and anionic diffusion coefficients for the RTILs follows the order [emim][(CF(3)SO(2))(2)N] > [mmim][(CF(3)SO(2))(2)N] > [bmim][(CF(3)SO(2))(2)N] > [C(6)mim][(CF(3)SO(2))(2)N] > [C(8)mim][(CF(3)SO(2))(2)N], which greatly contrasts to the viscosity data. The ratio of molar conductivity obtained from impedance measurements to that calculated by the ionic diffusivity using the Nernst-Einstein equation quantifies the active ions contributing to ionic conduction in the diffusion components, in other words, ionicity of the ionic liquids. The ratio decreases with increasing number of carbon atoms in the alkyl chain. Finally, a balance between the electrostatic and induction forces has been discussed in terms of the main contribution factor in determining the physicochemical properties.

Prediction of Macroscopic Properties of Protic Ionic Liquids by Ab Initio Calculations

“…All PILs show approximately the same curvature with a minimum T m for an alkyl chain length of 1-3 methylene units. A maximum value of ∆S, and consequently a minimum in T m , for the ethyl side chain length in 1-alkyl-3-methyl-imidazole bis(trifluoromethanesulfonyl)imide aprotic ILs have recently been reported by Tokuda et al 48 in agreement with our T m data.…”

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…This finding shows that the ionicity of ILs is dominated by the subtle balance between coulombic and molecular interactions (dispersive and induced forces), and the latter interactions dominate over coulombic interactions in determining the ionicity. 20,23 Such behavior is not observed in inorganic high-temperature molten salts and is characteristic of the nature of organic ionic liquids. Figure 2 also indicates that the ionicity significantly decreases when the number of C atoms in the alkyl chain changes from 2 to 4.…”

“…17 ILs having high ionicity are categorized as "good ILs", whereas those having low ionicity are classed as "poor ILs". 17 By using PGSE-NMR, we systematically examined the ionicity of a variety of ILs [19][20][21][22][23][24][25] having different anions with the same cation, 19 different cations with the same anion, 22 and different alkyl chain length on imidazolium cation with the same anion. 20 As shown schematically in Fig.…”

“…2. 20 Because the density the ILs decreases while their formula weight increases with increasing alkyl chain length, the average distance between the cations and anions increases, reducing the coulombic interactions. Nevertheless, the ionicity decreases with the alkyl chain length.…”

“…Figure 2 also indicates that the ionicity significantly decreases when the number of C atoms in the alkyl chain changes from 2 to 4. 20 Lopes et al conducted molecular dynamics simulations of ILs having different alkyl chain lengths on imidazolium cations paired with the same anion. Their results indicated that nanophase separation between the ionic and nonpolar domains occurs.…”