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Alkylammonium-Based Protic Ionic Liquids. II. Ionic Transport and Heat-Transfer Properties: Fragility and Ionicity Rule Mérièm Anouti,* Magaly Caillon-Caravanier, Corinne Le Floch, and Daniel Lemordant Laboratoire CIME (EA2098), UniVersite´ Franc¸ois Rabelais, Parc de Grandmont 37200, Tours, France ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: May 16, 2008
The physicochemical characterization of six alkylammonium-based protic ionic liquids (PILs) is presented. These compounds were prepared through a simple and atom-economic neutralization reaction between a tertiary amine and a Brønsted acid, HX, where X- is HCOO-, CH3COO-, HF2-. The temperature dependency and the effect of added water on properties such as density, viscosity, ionic conductivity, and the thermal comportment of these PILs were measured and investigated. The results allowed us to classify them according to a classical Walden diagram and to appreciate their great “fragility”. PILs have applicable perspectives in replacements of conventional inorganic acids for fuel cell devices and thermal transfer fluids. 1. Introduction An ionic liquid (IL) is usually defined as a substance comprised of ions, typically with one ion being an organic species, with a melting temperature of less than 100 °C. ILs have been developed and studied extensively during the last 10 years.1,2 Room-temperature ionic liquids (RTILs) are a “subcategory” of ILs which are liquid at temperatures less than 25 °C.3 The first IL extensively studied was 1-ethyl-3-methylimidazolium chloroaluminate (EMImAlCl4).4 However, chloroaluminate ionic liquids are hydrolyzed by traces of water, which limits their use. More recently, ILs consisting of large organic, asymmetrical ions, such as 1,3-dialkylimidazolium, 1-alkylpyridinium, 1-alkylpyrazolium, tetraalkylammonium, or tetraalkylphosphonium cations and BF4-, PF6-, CF3SO3-, or N(SO2CF3)2anions have been developed.1,5 The intrinsic properties of ILs can be varied by simply combining different cations or anions. Thus, specific demands for a synthesis or chemical process can be met by optimally tailoring the ionic liquid. The most advantages attributed to ILs are (i) a thermal stability which allows the use of these solvents in a larger temperature range than conventional solvents; a wide range liquid phase (up to 300 or 400 °C in some cases) is able to facilitate processes; (ii) a high electrochemical stability and a large electrochemical window opens the whole range of electrochemical applications; (iii) low vapor pressure reduces the loss of liquid into the environment (“green chemistry”); (iv) adjustable solubility permits us to design hydrophobic as well hydrophilic ionic liquids. Among the most important properties in terms of potential application as electrolytes in electrochemical devices is their conductivity. A great number of studies concern ILs’ specific conductivity, in their pure liquid state as well as solutions in molecular solvents or in a mix with polymers.6–10 Typical ionic liquids have aprotic structures and do not have chemically active protons,11–15 but proton-transfer salts obtained from Brønsted acid/base pairs are additional candidates for the preparation of protic ionic liquids (PILs) and proton conductors.16–20 This work is a continuation of a previous study,21 which is devoted to the preparation and physicochemical characterizations of alkylammonium-based PILs prepared through a simple and atom-economic neutralization reaction between a tertiary amine * Corresponding author. E-mail:
[email protected]. Fax: (33)247367360. Tel.: (33)247366951.
SCHEME 1: Preparation of PILs
and a Brønsted acid, HX, where X- is HCOO-, CH3COO-, HF2-. The temperature dependency and the effect of added water on some properties such as density, viscosity, ionic conductivity, and the thermal comportment of these PILs have been measured and investigated in detail. The results allowed us to classify them according to a classical Walden diagram and to appreciate their great “fragility”. PILs have applicable perspectives in replacements of conventional inorganic acids for fuel cell devices and thermal transfer fluids. 2. Experimental Section 2.1. Materials. The bases N-diisopropylethylamine DIEA (99+%) and N-diisopropylmethylamine DIMA (99+%) were obtained from Aldrich Chemical Co. Anhydrous acetic acid (HAc, 99+%), anhydrous formic acid (HFm, 98%), and hydrofluoric acid 50% in water were obtained from Fluka. All chemicals were used as received. Water is purified with a Milli-Q 18.3 MΩU water system. 2.2. General Procedures for the Preparation of N-Alkylamine-Based PILs. All ionic liquids used in this study are synthesized according to the procedure described in part 1 of this article,21 by neutralization of tertiary amines with suitable acids. The preparation is easy by just mixing these two components equimolarly which gave salts composed of protonated ammonium cations and corresponding acid anions (see Scheme 1). The preparation of N-alkylamine-based PILs and the abbreviations used to design them are indicated, respectively, in Scheme 1 and Table 1. 3. Measurements Density values were determined by measuring the weight of samples filling a 5 mL picnometer volumetric flask at 25 °C. Before each measurement, the flasks were maintained at 25 °C in a thermostatted JULABO circulating water bath for half an hour until the temperature was steady. A series of measurements with distilled water is used to calibrate the picnometer. The accuracy is better than 0.5%. Viscosities of pure PILs and their
10.1021/jp803489n CCC: $40.75 2008 American Chemical Society Published on Web 07/17/2008
Fragility and Ionicity of Protic Ionic Liquids
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Figure 3. Walden plots for protic ionic liquids (PILs).
Figure 1. Arrhenius plot for the ionic conductivities (a) and viscosities (b) for some ionic liquids presented in this work.
TABLE 1: PIL Names and Abbreviations Used in This Work
mixture with water are measured using a TA instrument rheometer (AR 1000) with conical geometry at temperatures ranging from 25 to 80 °C. Ionic conductivities are measured using a Crison (GLP 31) digital conductimeter at several frequencies. The temperature control (from 20 to 200 °C) is
ensured by a JULABO thermostatted oil bath. Differential scanning calorimetry (DSC) is carried out on a Perkin-Elmer DSC6 under a N2 atmosphere. Samples for DSC measurements are sealed in Al pans. Thermograms are recorded during heating (from 10 to 400 °C) with a scan rate of 10 °C · min-1. All PILs-water mixtures were prepared by mass using a balance with a sensitivity of 0.1 mg. Mixtures were prepared by diluting the pure PILs by a known amount of Milli-Q water in order to obtain the required concentration. 3.1. Effect of Temperature on Conductivities and Viscosities of PILs. The ionic conductivities of the six ionic liquids studied are determined in the range of 20-120 °C. The results, which are summarized in Table 2, show that PILs possess good ionic conductivity at 25 °C (1.3 mS · cm-1 (1b) < σ < 8.2 mS · cm-1 (2a)) and higher better values are obtained at 100 °C (10.0 mS · cm-1 (1b) < σ < 42.9 mS · cm-1 (2c)). The best conductivities are obtained when at the same time the cation and the anion are of small size (2c). This evolution of conductivity can be attributed to several factors, such as the geometrical and electronic structure of the cation and anion, hydrogen-bond interactions, diffusion coefficient of the proton, and lower viscosity of the PILs [18 cP (1a) < η
