Article pubs.acs.org/jced
Conductance and Ionic Association of Imidazolium-Based Ionic Liquids in N,N‑Dimethylacetamide Agnieszka Boruń* and Adam Bald University of Łódź, Department of Physical Chemistry, 90-236 Łódź, Pomorska 163, Poland S Supporting Information *
ABSTRACT: The electrical conductivity of dilute solutions of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4] and 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] in N,N-dimethylacetamide (DMA) have been measured over the temperature range from (283.15 to 318.15) K. The conductance data was analyzed using the low concentration Chemical Model (lcCM) in terms of the ionic association constant, KA, the limiting molar conductivities, Λo, and distance parameters, R. Slight ionic association was found for the ionic liquids in DMA within the investigated temperature range. From the temperature dependence of the limiting molar conductivities the Eyring activation enthalpy of charge transport was designated. The thermodynamic parameters such as Gibbs free energy, entropy, and enthalpy for the process of ion pair formation were estimated from the ion association constants at various temperatures. The obtained results were compared with those in N,N-dimethylformamide.
1. INTRODUCTION This paper is a part of a series of studies on thermophysical properties of imidazolium based ionic liquids (ILs) in different protic and aprotic molecular solvents.1−5 The aim of these studies is to obtain original and highly accurate conductivity data for 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4] and 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] in order to explore the ionic association and solvation behavior in various solvents as a function of temperature. Study of transport properties are of great importance to allow a better understanding of the behavior of the electrolytes in solution because we obtain information about the nature of the interactions between the cation and anion of the ionic liquid and with the other compounds of the solution. The information obtained allows us to show how physical properties of the molecular solvents affect their interaction with the ionic liquids. In this paper, we present for the first time the results of the electrical conductivity studies of dilute solutions of [emim][BF4] and [bmim][BF4] in N,N-dimethylacetamide as a function of ILs concentration and the temperature. From the experimental data the values of the limiting molar conductivities, Λo, and the association constants, KA, have been obtained for the investigated mixtures. The Gibbs free energy, ΔGoA, enthalpy, ΔHoA, and entropy,ΔSoA, of ion pair formation as well as the Eyring activation enthalpy of charge transport, ΔH‡λ , have been evaluated and discussed. A comparison of all our previous data and literature data for the investigated ILs in acetonitrile, dichloromethane, methanol, water, N,N-dimethylformamide, dimethyl sulfoxide, and 1propanol was presented in our earlier paper.5 This time, the © 2016 American Chemical Society
comparison with results obtained previously in N,N-dimethylformamide (DMF)1 will be presented. N,N-dimethylformamide and N,N-dimethylacetamide are aprotic donor solvents having similar solvent properties, such as relative permittivity (36.71 and 37.78 for DMF and DMA,6 respectively) or donor and acceptor numbers. Therefore, on the basis of physicochemical properties, DMA might be expected to very closely resemble DMF as an solvent. It is well-known that polar aprotic solvents, (e.g., DMA, DMF, and DMSO) solvate strongly alkali ions, especially Li+ and Na+, while the anion−solvent interactions are weak because of the impossibility of forming hydrogen bonds.7−10 Ion solvation is thus expected to be a dominating factor for the properties of electrolyte solutions in these solvents. The polar nature of N,N-dimethylacetamide enables it to act as a combined solvent and reaction catalyst in many reactions. DMA is industrially important solvent commonly used in the production of pharmaceuticals, plasticizers, adhesives, films, fibers, synthetic leathers, and surface coating.11 2. Experimental Section. The specifications of used chemicals are summarized in Table 1. The specific conductivity, κ, of DMA was 2 × 10−7 S·cm−1 at 298.15 K, which is in good agreement with available data.12,13 Conductance measurements were carried out in a threeelectrode cell with the use of a Precision Component Analyzer 6430B (Wayne-Kerr, UK) under argon atmosphere and at the different frequencies, ν, (0.2, 0.5, 1, 1.5, 2, 3, 5, 10, 20) kHz. Received: April 29, 2016 Accepted: October 7, 2016 Published: October 21, 2016 3788
DOI: 10.1021/acs.jced.6b00354 J. Chem. Eng. Data 2016, 61, 3788−3793
Journal of Chemical & Engineering Data
Article
molality, m, (moles of electrolyte per kilogram of solvent) and temperature and the density gradients, b, are summarized in Table 3. The electrical conductivities, κ, of the investigated systems are presented in Supporting Information (Table S1). Molar conductivities, Λ, versus square root of the molar concentration, c1/2, for ILs mixtures monotonically decrease as shown in Figures 1 and 2. The conductivity data, like the previous one,1−5 was analyzed in the framework of the low concentration Chemical Model (lcCM)24 using the equations
Table 1. Specification of Chemical Samples source
initial mass fraction purity
purification method
final water mass fraction
DMAa [emim] [BF4]c
Aldrich Fluka
0.998 0.990
none none
