Viscosity and self-diffusivity of ionic liquids with compressed hydrofluorocarbons: 1-Hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide and 1,1,1,2-tetrafluoroethane

Highlights

• First viscosity and self-diffusivity data of a hydrofluorocarbon gas/ionic liquid system.

• Viscosity can dramatically decrease with the presence of a dissolved gas.

• Diffusivity significantly increases.

• Diffusivity and Viscosity closely follow Stokes-Einstein behavior.

Abstract

The viscosity and self-diffusivity of mixtures of the ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][Tf₂N]), and the compressed gas, 1,1,1,2-tetrafluoroethane (R-134a), were measured at three different temperatures (298.15 K, 323.15 K, 343.15 K) and pressures to 21 bar. The high-pressure vapor-liquid equilibrium (VLE) of the ionic liquids with R-134a was measured to calculate the composition of the liquid phase at the various pressures encountered in the viscosity and diffusivity measurements. The viscosity of the ionic liquid decreases significantly with increased composition of R-134a at VLE. The “excess” viscosity demonstrates positive values that are believed to represent relatively strong intermolecular forces of the polar R-134a and [HMIm][Tf₂N]. The self-diffusivity of mixtures of [HMIm][Tf₂N] and liquefied R-134a (298.15 K and 6.7 bar) was measured and increases significantly with increases in the composition of R-134a. The self-diffusivity of [HMIm][Tf₂N] in vapor-liquid equilibrium with compressed R-134a gas was measured and indicates similar increases in diffusivity with composition of R-134a. The diffusivity demonstrates close Stokes-Einstein behavior using the measured mixture viscosity data and ambient pressure self-diffusivity.

Introduction

Coupling ionic liquids with compressed gases have been demonstrated in diverse applications in chemistry and engineering. The extremely low to immeasurable volatility of most ionic liquids inhibits them from entering a gas phase. However, ILs often have large solubility of certain gases in the ionic liquid-rich phase. Coupling ILs with compressed CO₂ forms an interesting platform for combining catalyzed reactions and separations [1], [2]. For instance, we have investigated the effect of CO₂ pressure/composition on the range of kinetic and mass-transport limited regimes in rhodium-catalyzed olefin hydrogenation and hydroformylation in biphasic IL/CO₂ systems [3], [4]. These IL/compressed gas systems are a subset of what are currently called gas-expanded liquids (GXLs) or CO₂-expanded liquids (CXLs) [5], [6]. There is currently a tremendous amount of research to use ionic liquids for pre- or post-combustion CO₂ capture [7], [8], [9], [10].

In addition to CO₂, ionic liquids can be involved with a variety of other gases. ILs have been proposed as lubricants in gas compressors which would bring any number of gases in contact with the ionic liquid lubricant [11], [12]. Ionic liquids can improve the efficiency of absorption refrigeration processes where the IL absorbs the refrigerant gas (usually hydrofluorocarbons) in one stage and releases the high pressure gas with heat in another stage to produce a high pressure gas without the need for a mechanical compressor [13], [14]. Here the liquid solvent needs to have a high solubility of the gas at lower temperatures and no volatility. Current liquid solvents for absorption refrigeration are not optimal as often bulky equipment is needed to remove their contamination of the high-pressure gas. Non-volatile ionic liquids in these systems with refrigerants may help solve these problems.

While a considerable amount of research has involved the phase equilibrium thermodynamics of gas/ionic liquid systems, the transport properties of these mixtures, such as viscosity and diffusivity, are equally as important to understand some of these vast applications in chemical dynamics to mass transfer. We [15] and other groups such as Harris and Kanakubo [16], [17], and Tomida [18], [19] have measured the high-pressure (>1000 bar) viscosity of pure ionic liquids. Several studies have appeared in the literature for the viscosity of ionic liquids saturated with compressed CO₂. We have measured the viscosity of three n-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ([R-MIm][Tf₂N]) ionic liquids saturated with compressed CO₂ at temperatures of 298.15 K, 323.15 K, and 343.15 K and pressures to 287 bar [20]. The dissolution of CO₂ significantly reduces the viscosity of the ionic liquid. Liu et al. [21] measured the viscosity of mixtures of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF₆]), methanol, and CO₂ using the falling ball viscometer. Laurenczy and Dyson [22] measured the viscosity of 1-butyl-3-methylimidazolium [Tf₂N] ([BMIm][Tf₂N]) and [BMIm][PF₆] under subcritical CO₂ pressure with a falling ball method. Tomida et al. [23], [24] measured the viscosity of non-saturated CO₂ mixtures with 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIm][BF₄]) and [BMIm][PF₆] by a rolling ball viscometer. However, the viscosity of ionic liquids saturated with compressed hydrofluorocarbons could not be found in the literature despite possible applications in absorption refrigeration [14], etc.

The diffusivity of gases has mainly been measured for CO₂ at low and high pressures into ionic liquids. All current reports represent the so-called Fickian diffusion, or more precisely the inter-or mutual-diffusion for binary systems [25], of gases into ionic liquids in a chemical potential (fugacity) gradient, i.e. away from saturation. This is opposed to self- or intra-diffusion measured in this study at phase equilibrium, i.e. constant chemical potential (composition) [26]. Some examples are demonstrated from the groups of Shiflett at DuPont [27], Noble [28], Baltus [29], [30], [31], Kroon [32], Rodriguez [33], [34], Jalili [35], [36], Li [37], [38], [39], Scovazzo [40], [41], etc. However for fluorocarbon gases, only Shiflett et al. [42], [43] also measured and correlated the inter-diffusivity of several hydrofluorocarbons into [BMIm][PF₆] and [BMIm][BF₄] using a gravimetric microbalance. There are currently no known self-diffusivity measurements for fluorocarbon gases and ionic liquids.

In the present work, the viscosity, self-diffusivity, and phase equilibrium of the ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][Tf₂N]), with compressed 1,1,1,2-tetrafluoroethane (its refrigerant designation is R-134a) (see Fig. 1) were measured at three different temperatures (298.15 K, 323.15 K, and 343.15 K) and pressures to approximately 21 bar. These temperatures allow an understanding of the temperature effects of the IL/gas system. These viscosity and self-diffusion coefficients are some of the first for a hydrofluorocarbon and ionic liquid. [HMIm][Tf₂N] is a common ionic liquid in the literature and has been selected by an IUPAC ionic liquid committee to be used as a model and standard ionic liquid [44], [45].