Effect of Alkyl Chain Branching on Physicochemical Properties of Imidazolium-Based Ionic Liquids

Abstract

The branched ionic liquids (ILs) 1-(iso-alkyl)-3-methylimidazolium bis­[(trifluoromethane)­sulfonyl]­amide ([(N – 2)­mC N‑1C1im]­[NTf2] with N = 3–7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(n-alkyl)-3-methylimidazolium bis­[(trifluoromethane)­sulfonyl]­amide ([C N C1im]­[NTf2] with N = 3–7). For N = 4–7, the density of the branched IL [(N – 2)­mC N–1C1im]­[NTf2] is the same as that of its linear analogue [C N C1im]­[NTf2] within the standard uncertainty of the measurements. In the case of the N = 3 [1mC2C1im]­[NTf2]/[C3C1im]­[NTf2] pair, the density of the branched IL is 0.13% higher than that of the linear IL. For a branched/linear IL pair with a given N, the glass transition temperature T g, melting temperature T m, and viscosity η are higher for the branched IL than for the linear IL. [2mC3C1im]­[NTf2] is an exception in that its T m is lower than that of [C4C1im]­[NTf2]. Moreover, the viscosity of [2mC3C1im]­[NTf2] is anomalously higher than what would be predicted based on the trend of the other branched ILs. These trends in the viscosities of the linear and branched ILs are consistent with recent molecular dynamics simulations. Thermal gravimetric analysis indicates that linear ILs are thermally more stable than branched ILs. Pulsed-gradient spin–echo (PGSE) NMR diffusion measurements show that the self-diffusion coefficients of the ions vary inversely with the viscosities according to the Stokes–Einstein (SE) equation. The hydrodynamic radii of the cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs.

Synthesis and Characterization of Bromoaluminate Ionic Liquids

“…Figure a shows the densities of the synthesized RTILs. The densities of imidazolium based ILs are decreasing with increasing length of the alkyl chain . As expected from these results, the densities of the ammonium ILs decrease in the order [N 1123 ] + >[N 1224 ] + >[N 1134 ] + >[N 2225 ] + and thus with increasing cation volume V m + .…”

“…The melting point ( T m ) of a molecule is determined by the strength of its crystal lattice, which is influenced by three factors: molecular symmetry, conformational degrees of freedom, and intermolecular forces . For ILs, the effect of varying branching, length, and position of alkyl chains, as well as the influence of the symmetry of the anions and cations on T m was investigated . In general, combining a cation with large size, low symmetry and flexible substituents and an anion with low symmetry and good charge distribution results in an IL with a low melting point.…”

“…[40] For ILs, the effect of varying branching, length, and positiono fa lkyl chains, as wella st he influenceo ft he symmetry of the anions and cations on T m was investigated. [34,[41][42][43][44][45][46][47][48][49][50] In general,c ombining ac ation with large size, low symmetry and flexible substituents and an anion with low symmetry and good charge distribution results in an IL with al ow meltingp oint. Ideally,i ts hould be liquid at room temperature and have aw ide liquid range for electrolyte applications.F igure3 shows T m of the synthesized salts, several of which are solid at room temperature.…”

COSMO‐RS based ionic liquid screening for the separation of acetonitrile and ethanol azeotropic mixture

“…Therefore, imidazolium‐based ILs are used in our work due to their higher selectivity and lower viscosity. Since the selectivity of imidazolium‐based ILs is inversely proportional to their viscosity which is proportional to the length of cation side chain, [BMIM][Ac] is suggested as the solvent to separate the mixture of acetonitrile and ethanol. In order to verify the outstanding separation performance of [BMIM][Ac], the vapor–liquid equilibrium (VLE) data of the acetonitrile + ethanol +[BMIM][Ac] were determined at 101.3 kPa in the following section.…”

Effect of amino acid addition on the micelle formation of the surface‐active ionic liquid 1‐tetradecyl‐3‐methylimidazolium bromide in aqueous solution

“…These ILs, generally named as surface‐active ILs (SAILs), have been extensively studied. It has been demonstrated that the length of the alkyl chain, type of group head, and the nature of counterions affect the self‐aggregation, leading to modification in critical micelle concentrations (cmc), size, shape, and aggregation number ( N agg ), among others …”