Studies on the density, heat capacity, surface tension and infinite dilution diffusion with the ionic liquids [C4mim][NTf2], [C4mim][dca], [C2mim][EtOSO3] and [Aliquat][dca]
C. A. Nieto de CastroElisa LangaAna Lucia Ferreira de MoraisManuel Matos LopesMaria José LourençoF. J. V. SantosM. Soledade C.S. SantosJosé N. Canongia LopesH. I. M. VeigaM. MacatrãoJosé M. S. S. EsperançaCarolina S. MarquesLuís Paulo N. RebeloCarlos A. M. Afonso
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The instructive structure-property relationships of ionic liquids (ILs) can be put to task-specific design of new functionalized ILs. The dicyanamide (DCA) ILs are typical CHN type ILs which are halogen free, chemical stable, low-viscous and fuel-rich. The transport properties of DCA ionic liquids are significant for their applications as solvents, electrolytes and hypergolic propellants. This work systematically investigates several important transport properties of four DCA ILs ([C4mim][N(CN)2], [C4m2im][N(CN)2], N4442[N(CN)2], and N8444[N(CN)2]) including viscosity, conductivity, and electrochemical property at different temperatures. The melting points, temperature-dependent viscosities and conductivities reveal the structure-activity relationship of four DCA ILs. From the Walden plots, the imidazolium cations exhibit stronger cation–anion attraction than the ammonium cations. DCA ILs have relatively high values of electrochemical windows (EWs), which indicates that the DCA ILs are potential candidates for electrolytes in electrochemical applications. The cyclic voltammograms of Eu(III) in these DCA ILs at GC working electrode at various temperatures 303–333 K consists of quasi-reversible waves. The electrochemical properties of the DCA ILs are also dominated by the cationic structures. The current intensity (ip), the diffusion coefficients (Do), the charge transfer rate constants (ks) of Eu(III) in DCA ILs all increased with the molar conductivities increased. The cationic structure-transport property relationships of DCA ILs were constructed for designing novel functionalized ILs to fulfill specific demands.
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Despite the recent increase in the number of works addressing the effect of water on the thermophysical properties of ionic liquids (ILs), in particular densities and viscosities, the strong effect of the water content in the region of low water content on these properties requires further study for a deeper understanding of the phenomenon. Densities and viscosities of two imidazolium-based ionic liquids, 1-butyl-3-methylimidazolium dicyanamide [C4mim][N(CN)2] and 1-butyl-3-methylimidazolium tricyanomethane [C4mim][C(CN)3], and their binaries with water, for mole fractions up to 0.6, at atmospheric pressure and temperatures from (278.15 to 363.15) K were determined. The densities and viscosities of the pure ionic liquids and their binaries with water were described successfully by using several correlations. An extension to the Ye and Shreeve group contribution method shows a good agreement with the density data. An Orrick−Erbar-type approach and the Vogel−Tammann−Fulcher method were also applied to the experimental viscosity data, presenting a good agreement and allowing the estimation of new group contribution parameters, extending the applicability of these methods to new ILs. For mixtures, the one-constant Grunberg and Nissan equation was investigated for the correlation of the experimental viscosity binary mixture data of ILs + H2O, providing a good description of the experimental data using a single parameter for each ionic liquid studied.
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Ionic liquids (ILs) have been extensively studied for their use as solvents in separation processes. In this work, we determine the vapor-liquid equilibria (VLE) and diffusion coefficients at infinite dilution of three hydrofluorocarbons (R32 – difluoromethane, R125 – pentafluoroethane, and R134a – 1,1,1,2-tetrafluoroethane) and two hydrofluoroolefins (R1234yf – 2,3,3,3-tetrafluoropropene, and R1234ze(E) – trans-1,3,3,3-tetrafluoropropene) and the ILs 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium dicyanamide ([C2mim][dca] and [C4mim][dca]). Additionally, we study the VLE of R125 and [C2mim][SCN] (thiocyanate) to complete the previous studies with this liquid. These ILs have been selected because other ILs with cyanide moieties in the anion have shown good selectivity to separate fluorinated refrigerant gases and their low viscosity would improve the mass transfer and reduce the pumping costs in a separation process. The experimental data have been determined at temperatures from 283.15 K to 323.15 K and pressures up to 1 MPa, and the nonrandom two-liquid model (NRTL) has been used to fit the VLE. The non-competitive ideal selectivity and the separation factor achieved in an isothermal flash operating at 303.15 K and 5 bar have been evaluated for target mixtures of interest. The use of dicyanamide based ILs leads to the best selectivity results to date, concluding that the studied ILs are promising for the design of separation processes that selectively recover fluorinated refrigerant gases.
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An extensive study of interaction energies in ion pairs of pyrrolidinium and imidazolium ionic liquids is presented. The Cnmpyr and Cnmim cations with varying alkyl chains from Methyl, Ethyl, n-Propyl to n-Butyl were combined with a wide range of routinely used IL anions such as chloride, bromide, mesylate (CH3SO3 or Mes), tosylate (CH3PhSO3 or Tos), bis(trifluoromethanesulfonyl)amide (NTf2), dicyanamide (N(CN)2 or dca), tetrafluoroborate (BF4) and hexafluorophosphate (PF6). A number of energetically favourable conformations were studied for each cation-anion combination. The interaction energy and its dispersion component of the single ion pairs were calculated using a sophisticated state-of-the-art approach: a second-order of Symmetry Adapted Perturbation Theory (SAPT). A comparison of energetics depending on the cation-anion type, as well as the mode of interaction was performed. Dispersion forces were confirmed to be of importance for the overall stabilisation of ionic liquids contributing from 28 kJ mol(-1) in pyrrolidinium ion pairs to 59 kJ mol(-1) in imidazolium ion pairs. The previously proposed ratio of total interaction energy to dispersion components and melting points was assessed for this set of ionic liquids and was found to correlate with their melting points for the anionic series, producing separate trends for the Cnmim and Cmpyr series of cations. Chlorides, bromides and tetrafluoroborates formed close-to-ideal correlations when both types of cations, Cnmim and Cnmpyr, were combined in the same trend. Correlation of the dispersion component of the interaction energy with transport properties such as conductivity and viscosity was also considered. For imidazolium-based ionic liquids strong linear correlations were obtained, whereas pyrrolidinium ionic liquids appeared to be insensitive to this correlation.
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Binary mixtures of the ionic liquids 1-ethyl-3-methylimidazolium dicyanamide and tributyl(methyl)ammonium dicyanamide, [C2C1im]x[N4441](1–x)[N(CN)2], are studied by means of their excess properties (enthalpy and volume), their viscosity, and their Raman spectra. The mixtures exhibit positive values of excess volume VE and excess enthalpy HE. The plot of the logarithm of viscosity as a function of composition is consistent with the finding HE > 0. The excess thermodynamic properties of the ionic liquid mixtures are compared with well-known results for binary mixtures of (high-temperature) molten salts. The asymmetry of the curve HE versus composition is reproduced by considering the volumetric fraction of the [C2C1im]x[N4441](1–x)[N(CN)2] mixtures according to basic results of the theory of regular solutions. The anion totally symmetric stretching mode νs(C≡N) was used to probe the local environment around the anion in a Raman spectroscopy investigation of the [C2C1im]x[N4441](1–x)[N(CN)2] mixtures as a function of the composition. The Raman band shape (peak frequency and bandwidth) of the νs(C≡N) mode changes as expected from the fluctuation of concentration around the probe oscillator according to models for vibrational spectroscopy of liquid mixtures. This thermodynamic and spectroscopic study leads to the classification of the [C2C1im]x[N4441](1–x)[N(CN)2] mixture as a regular solution. The application of the regular solution equation for HE to other ionic liquid binary mixtures is discussed.
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