Elsevier

Fluid Phase Equilibria

Volume 242, Issue 2, 25 April 2006, Pages 220-232
Fluid Phase Equilibria

Solubility and diffusivity of 1,1,1,2-tetrafluoroethane in room-temperature ionic liquids

https://doi.org/10.1016/j.fluid.2006.01.026Get rights and content

Abstract

The solubility and diffusivity of 1,1,1,2-tetrafluoroethane (R-134a) in seven room-temperature ionic liquids (RTILs) are presented. Among them, five of the RTILs were prepared for the first time with three new fluorocarbon sulfonate anions, and two were commercially available (1-butyl-3-methylimidazolium hexafluorophosphate was previously studied with R-134a). The gas absorption measurements were made using a gravimetric microbalance. Four isotherms (283.15, 298.15, 323.15, and 348.15 K) were measured at pressures from 0.01 to 0.35 MPa. Two of the newly synthesized ionic liquids, tetradecyl(trihexyl)phosphonium 1,1,2-trifluoro-2-(perfluoroethoxy)ethanesulfonate and tributyl(tetradecyl)phosphonium 1,1,2,3,3,3-hexafluoropropanesulfonate had the strongest interaction (negative deviations from Raoult's Law) with R-134a. Experimental gas solubility data were successfully correlated with the nonrandom two-liquid (NRTL) solution model. The time-dependent absorption data was used to calculate diffusivities that were analyzed using a model based on a modified Stokes–Einstein equation. The derived molecular size for R-134a is 2–3 times larger than the known size. Magnitudes in the observed diffusion coefficients are 10−10 to 10−11 m2 s−1, which are about 10–100 times lower than typical values, found in various organic liquids.

Introduction

The phase behavior of ionic liquids with hydrofluorocarbon gases is necessary to assess the feasibility of their use for separations and as new absorption cooling fluids. Many papers have been published on the solubility and diffusivity of non-fluorocarbon gases in ionic liquids [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], but until our work little has been known about the interaction of hydrofluorocarbons and ionic liquids. In our previous work [12], we studied solubilities of hydrofluorocarbons (HFCs), which included trifluoromethane (R-23), difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1,2-tetrafluoroethane (R-134a), 1,1,1-trifluoroethane (R-143a), and 1,1-difluoroethane (R-152a), in 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], and show that R-32 had the highest gas solubility (R-32 > R-152a > R-23 > R-134a > R-125 > R-143a) in the solvent-rich side. The trend in solubility did not correlate with the HFCs dipole moment as expected; however, the unique H-bonding capability (Hsingle bondFsingle bondH) of HFCs is believed to be involved. In this work, we continue our investigations to understand the gas solubility and diffusivity of R-134a, which is the most commonly used hydrofluorocarbon. R-134a is primarily used in mobile air-conditioning systems. In addition, because R-134a is nonflammable it can be blended with other flammable refrigerants such as R-32, R-143a, and R-152a to produce nonflammable refrigerant mixtures such as R-404A (44 wt.% R-125, 52 wt.% R-143a, 4 wt.% R-134a) and R-407C (23 wt.% R-32, 25 wt.% R-125, and 52 wt.% R-134a). Therefore, we have investigated how R-134a interacts with RTILs.

Two commercially available ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and 1-ethyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)imide [emim][BEI]) and five ionic liquids synthesized by DuPont (1-butyl-3-methylimidazolium 1,1,2,3,3,3-hexafluoropropanesulfonate [bmim][HFPS], 1-butyl-3-methylimidazolium 1,1,2-trifluoro-2-(perfluoroethoxy)ethanesulfonate [bmim][TPES], 1-butyl-3-methylimidazolium 1,1,2-trifluoro-2-(trifluoromethoxy)ethanesulfonate [bmim] [TTES], tetradecyl(trihexyl)phosphonium 1,1,2-trifluoro-2-(perfluoroethoxy)ethanesulfonate [6,6,6,14-P][TPES], and tributyl(tetradecyl) phosphonium 1,1,2,3,3,3-hexafluoropropanesulfonate [4,4,4,14-P][HFPS]) were included in this study. Two of the RTILs, [6,6,6,14-P][TPES] and [4,4,4,14-P][HFPS], were prepared for the first time and details of the synthesis are provided. Details for the synthesis of the other three RTILs ([bmim][HFPS], [bmim][TPES], [bmim][TTES]) can be found in our previous work [13]. In addition to our own work, only two literature references are known regarding the interaction (electrical conductivity) of R-134a in an ionic liquid [14], [15]. The present study is the first systematic investigation of the solubility and diffusivity of R-134a in RTILs. Similar to our previous work [12], [13], we analyze the observed solubility data with the NRTL solution model and correlate the diffusivity behavior with a simple semi-theoretical model.

Section snippets

Samples and synthesis

R-134a was obtained from DuPont Fluoroproducts with a minimum purity of 99.9%. A molecular sieve trap was installed to remove any trace amounts of water from the gas. Table 1 provides the chemical name, CAS registry number, source, abbreviation, structure, and the molecular weight of the seven RTILs that were tested. The samples obtained from Fluka Chemika have stated purities of >97%. The samples synthesized by DuPont have purities >98% based on elemental analysis. The cation salts were

Solubility model

In this section, we analyze the experimental solubility (T, P, x) data with the existing solution models for non-electrolyte solutions, which may also be applied even for electrolyte solutions [12], [13], [22], [23], [24], [25]. In general, low-and-medium pressure vapor liquid equilibria (VLE) for an N-component system can be described by [26]:yiPΦi=xiγiPis,(i=1,,N).where yi is the vapor phase mole fraction for ith species, xi the liquid phase mole fraction for ith species, P pressure, Pis

Results

The present solubility and diffusivity (T, P, x, D) data are summarized in Table 3, Table 4, Table 5. Fig. 1 shows an example for the comparison of isothermal Px plots using the R-134a/[emim][BEI] system. The binary interaction parameters used in Fig. 1 are τ12(1)=993.44 and τ21(1)=523.03K in Eq. (9), respectively. Standard deviations in the pressure fit are 0.0021 MPa. All observed solubility behaviors in the present ionic solutions have been well correlated using these methods described in

Discussion

In this paper, we have investigated the solubility of R-134a with seven new RTILs including [bmim][PF6] that we have studied, in our previous work [12]. Five of the RTILs contain imidazolium cations ([emim] and [bmim]) with fluorinated anions ([PF6], [BEI], [HFPS], [TPES], [TTES]). The other two newly synthesized RTILs contain phosphonium cations ([6,6,6,14-P] and [4,4,4,14-P]) with fluorinated anions ([TPES] and [HFPS]). The thermal stability of the synthesized RTILs was measured by TGA. The

Conclusions

We have reported the solubility and diffusivity of an important HFC (R-134a) in seven RTILs for temperatures from 283.15 to 348.15 K and pressures up to 0.35 MPa. Two of these RTILs ([6,6,6,14-P][TPES] and [4,4,4,14-P][HFPS]) which were synthesized for the first time, had the largest negative-deviation behavior from Raoult's law in R-134a. Although the mechanism of the solubility difference is not clear at a molecular level, hydrogen–fluorine interactions between the HFC and the anion are

Acknowledgments

The authors thank Mr. Brian L. Wells for conducting the microbalance experiments and Mr. Seun S. Solesi for his assistance in calculating the diffusivity data. We also would like to thank Dr. Richard A. Maynard, Linda M. Williams and Mr. Michael J. Logue for making the density, viscosity, and water titration measurements, respectively. DuPont Central Research and Development supported the present work.

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