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J. Chem. Eng. Data 2007, 52, 2306-2314
Density and Surface Tension Measurements of Imidazolium-, Quaternary Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids: Data and Correlations Prem Kilaru,† Gary A. Baker,‡ and Paul Scovazzo*,† Department of Chemical Engineering, University of Mississippi, 134 Anderson Hall, University, Mississippi 38677, and Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6110
Presented are experimental densities and surface tensions of imidazolium-, quaternary ammonium-, and phosphonium-based room-temperature ionic liquids (RTILs) in the temperature range of (298 to 350) K. Densities of the RTILs decrease slightly with temperature in the studied range (298 to 333) K. At 298 K, the densities of the phosphonium-based RTILs ranged from (0.88 to 1.05) g‚cm-3, while those of the ammonium-based RTILs ranged from (1.08 to 1.37) g‚cm-3. The volume expansivities of phosphonium and ammonium RTILs at 1 atm and 298 K are in the range of (5.5 to 6.5)‚10-4 K-1. Surface tension and temperature relationships were established using the van der Waals-Guggenheim equation, σ ) Es(1 - T/Tc)n, where n ≈ 1, for the RTILs studied. The Macleod-Sugden-Wright (MSW) equation was used to correlate surface tension with the respective molar volume of the various RTILs. The developed equation can accurately correlate surface tension for this grouping of RTILs with a maximum estimated error of 0.15 % within the temperature ranges considered.
Introduction Room-temperature ionic liquids (RTILs) are three-dimensional networks of ions,1 with hydrocarbon tails on the cationic structures, which remain in their molten state at or below 150 °C.2 In contrast to their more mundane organic solvent counterparts, RTILs have a very low vapor pressures and are hence treated as green solvents for many organic reactions,3 liquid-liquid extractions,4 gas-liquid absorption,5 membranebased bioseparations,6 gas separations,7-9 and other areas. RTILs are generally thermally stable (> 250 °C),10 nontoxic, nonflammable materials with tunable physiochemical properties such as density, viscosity, surface tension, thermal conductivity, and solubility of various liquid and gaseous solutes.11-13 There has been a considerable growth of research on RTILs as possible replacements for conventional organic solvents. Some of this research focused on understanding the effects of structural variation on the physiochemical properties of RTILs,12-19 in addition to contributing new experimental data to the RTIL property databank. In this direction, we focus in this report on density and surface tension measurements of linear alkyl quaternary phosphonium- and ammonium-based RTILs. Our interest in surface tension measurements lies in the application of RTILs as supported liquid membranes (SLMs)6-9 of relevance in energy-efficient membrane-based gas separations.7-9 While the negligible vapor pressure allows RTILs to be immobilized within the pores of a polymeric membrane support, the surface tension determines the hydrodynamic stability of the entrapped liquid. That is, the operating gas pressure on the membrane is directly related to the surface tension and inversely related to the pore size. To design a hydrodynamically stable membrane, the cross membrane pressure (∆Pcm) should not exceed the equilibrium pressure (∆Peq) exerted on the surface of the liquid meniscus. Therefore, the design of efficient RTIL* Corresponding author. E-mail:
[email protected]. Phone: (662)915-5354. Fax: (662)-915-7023. † University of Mississippi. ‡ Oak Ridge National Laboratory.
based membrane systems requires an accurate database of surface tension data for the immobilized liquids. In this work, we report on the density and surface tension measurements of five phosphonium and eight ammonium RTILs. Physical property data for the phosphonium and ammonium classes of RTILs are scarce because most of the literature results to date are focused on the 1-alkyl-3-methylimidazolium [Rmim] RTILs. For ease of comparison, we, therefore, also include surface tension results of four [Rmim] RTILs.
Materials and Procedures In the following, the purity levels mentioned in the parentheses are specified by the supplier. Our analyses of halide content appear in Table 2. The basic structures of the cations and anions studied in this work are in Figure 1. 1-Ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide [emim][Tf2N] (≈100 % purity) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C6mim][Tf2N] (99 % pure) were obtained from Covalent Associates, Inc. 1-Ethyl-3methylimidazolium trifluoromethane sulfonate [emim][TfO] (CAS # 145022-44-4) (≈100 % pure by mass fraction) was obtained from EMD chemicals, and 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] (CAS #174501-64-51) with > 96 % purity was purchased from Fluka Chemica, SigmaAldrich, Germany. The tetraalkylphosphonium RTILs used were sourced from Cytec Canada (Niagara Falls, ON) and include: trihexyl(tetradecyl)phosphonium chloride [P(14)666][Cl] (CAS # 258864-54-9) (96 % to 97 % purity with (0.1 to 0.4) % traces of tetradecene isomers, (0.1 to 0.5) % HCl, and (0.1 to 1.2) % [PR3H]+Cl-); trihexyl(tetradecyl)phosphonium dicynamide [P(14)666][DCA] (residual chloride of 0.5 %); trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide [P(14)666][Tf2N] (CAS# 460092-03-9) (> 97 % pure, < 0.1 % water, and < 0.1 % chloride content);20 tributyl(ethyl)phosphonium diethylphosphate [P(2)444][DEP]; and tributyl(tetradecyl)phosphonium dodecylbenzenesulfonate [P(14)444][DBS]. The
10.1021/je7003098 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007
Journal of Chemical and Engineering Data, Vol. 52, No. 6, 2007 2307
Figure 2. Temperature-dependent densities of quaternary ammonium RTILs. ), [N(4)111][Tf2N]; +, [N(4)113][Tf2N]; ×, [N(6)111][Tf2N]; 0, [N(6)113][Tf2N]; -, [N(6)222][Tf2N]; *, [N(10)111][Tf2N]; ∆, [N(10)113][Tf2N]; and O, [N(1)888][Tf2N]. Error bars are smaller than the data point symbols. Figure 1. Chemical structures of cations and anions employed in this study.
alkylammonium RTILs dimethyl(butyl(i-propyl))ammonium bis((trifluoromethyl)sulfonyl)imide [N(4)113][Tf2N], dimethyl(hexyl)(i-propyl) bis((trifluoromethyl)sulfonyl)imide [N(6)113][Tf2N],dimethyl(decyl(i-propyl))bis((trifluoromethyl)sulfonyl)imide [N(10)113][Tf2N], and trioctyl(methyl)ammonium bis((trifluoromethyl)sulfonyl)imide [N(1)888][Tf2N] were synthesized at Oak Ridge National Laboratory and stored in sealed septacapped bottles. Trimethyl(butyl)ammonium bis(trifluoromethylsulfonyl)imide [N(4)111][Tf2N] was purchased from Ionic Liquid Technologies, Germany. The other salts, trimethyl(hexyl)ammonium bis(trifluoromethylsulfonyl)imide [N(6)111][Tf2N], trimethyl(decyl)ammonium bis(trifluoromethylsulfonyl)imide[N(10)111][Tf2N],andtriethyl(hexyl)ammoniumbis(trifluoromethylsulfonyl)imide [N(6)222][Tf2N] were synthesized at The University of Mississippi. The cation sources for the trimethyl(alkyl)ammonium RTILs were obtained from TCI America as bromides in 99+ % purity. Triethyl(hexyl)ammonium iodide was synthesized at the University of Mississippi. Lithium trifluoromethanesulfonimide (CAS #90076-65-6) with 99.95 % purity was obtained from Sigma-Aldrich. The numerals presented in the square brackets for phosphonium and ammonium RTILs indicate the number of carbons in each chain with the longest chain length usually provided in parentheses; this notation is used throughout this manuscript. Synthesis of [N(6)111][Tf2N] and [N(10)111][Tf2N]. For [N(6)111][Tf2N], equimolar aqueous solutions of trimethyl(hexyl)ammonium bromide and lithium bis(trifluoromethylsulfonyl)imide were mixed in a separation funnel and gently shaken for several minutes to assist the metathesis reaction. The immiscible RTIL phase was separated from the aqueous phase and washed three times with water to remove excess halide content and dried under vacuum (< 0.013 atm) at 363 K for 48 h. The synthesis of [N(10)111][Tf2N] followed the same procedure as [N(6)111][Tf2N] with the substitution of trimethyl(n-decyl)ammonium bromide for trimethyl(n-hexyl)ammonium bromide. Synthesis of [N(6)222]I and [N(6)222][Tf2N]. Triethylamine (1.25 mol) was reacted with 1 mol of 1-iodohexane under reflux at 343 K with nitrogen bubbling into the reaction vessel. The reaction was carried out for (1 to 2) days with periodic addition of hexane. A white precipitate of triethyl(hexyl)ammonium iodide was separated using gravity filtration. Synthesis of [N(6)222][Tf2N] was similar to that of [N(6)111][Tf2N] and [N(10)-
111][Tf2N] with the use of the [N(6)222]I in place of the quaternary ammonium bromide. Synthesis of [N(n)113][Tf2N] (n ) 4, 6, 10) and [N(1)888][Tf2N]. The ionic liquids, [N(n)113][Tf2N] (for n ) 4, 6, and 10), were prepared from dimethyl(isopropyl)amine by following recently reported synthetic methods.21,22 In the case of [N(1)888][Tf2N], the precursor salt [N(1)888]Br was first treated with aqueous decolorizing charcoal at 338 K for 48 h and then filtered to remove carbon.23 This decolorizing step was repeated twice prior to metathesis. Density Measurements. Densities of the phosphonium- and ammonium-based RTILs were measured in the temperature ranges from (298 to 333) K by weighing in a standard 5.0 cm-3 specific gravity bottle in a temperature-controlled nitrogen environment. At each temperature increment, the sample was allowed (20 to 30) min to attain thermal equilibrium and four to five readings were averaged. The standard deviation of all the measurements and the relative uncertainty in temperature do not exceed ( 0.0005 g‚cm-3 and 0.1 °C, respectively. Surface Tension Measurements. A PC-controlled KSV Sigma 70 Tensiometer (KSV Instruments Ltd., Finland) was used for surface tension measurements. Built-in auto-correction software is a major advantage of using this instrument to account for the correction factors as suggested in the literature.24,25 The Du´Nou´y ring method was used for air-RTIL surface tension measurements of the phosphonium-based RTILs using a procedure found elsewhere.26 Surface tension measurements for the quaternary ammonium RTILs were made under a dry nitrogen atmosphere to eliminate frequent drying of RTILs during the experiment. The tensiometer was calibrated by measuring the surface tension of deionized water at 293 K; measured values agreed to within 0.1 % of the literature value of 72.8 mN‚m-1 at 293 K. The same instrumental settings were used for measuring surface tension for all the RTILs to avoid the use of scaling methods to account for discrepancies. Surface tensions were measured against temperature in the range between (295 and 350) K for phosphonium RTILs and between (295 and 333) K for ammonium RTILs. The temperature was maintained using an external water circulating bath (EX 7 Digital One; Thermo Neslab). The RTILs were thermally equilibrated for about (20 to 30) min at each temperature prior to recording a measurement. Four to five readings were taken at each temperature, and the results were averaged. We report the averages, all of which all had standard deviations e 0.05 mN‚m-1.
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Journal of Chemical and Engineering Data, Vol. 52, No. 6, 2007
Table 1. Multitemperature Specific Gravity Data for Quaternary Phosphonium and Ammonium RTILsa
a
t (°C)
25
30
35
[P(14)666][Cl] [P(14)666][DCA] [P(14)666][Tf2N] [P(2)444][DEP] [P(14)444][DBS]
0.8826 0.9209 1.0501 0.9978 0.9384
0.8801 0.9173 1.0483 0.9957 0.9371
0.8779 0.9163 1.0452 0.9928 0.9337
[N(4)113][Tf2N] [N(6)113][Tf2N] [N(10)113][Tf2N] [N(4)111][Tf2N] [N(6)111][Tf2N] [N(10)111][Tf2N] [N(6)222][Tf2N] [N(1)888][Tf2N]
1.3483 1.2846 1.2007 1.3747 1.3106 1.2263 1.2793 1.0823
1.3457 1.2816 1.1977 1.3686 1.3078 1.2222 1.2754 1.0803
1.3421 1.2775 1.1942 1.3656 1.3040 1.2186 1.2719 1.0773
40
45
50
55
60
Phosphonium 0.8754 0.9131 1.0420 0.9897 0.9311
0.8731 0.9101 1.0392 0.9858 0.9284
0.8694 0.9075 1.0356 0.9826 0.9254
0.8669 0.9050 1.0329 0.9795 0.9228
0.8644 0.9024 1.0296 0.9758 0.9201
Ammonium 1.3378 1.2732 1.1908 1.3614 1.2995 1.2147 1.2676 1.0738
1.3341 1.2699 1.1869 1.3573 1.2957 1.2106 1.2644 1.0738
1.3297 1.2660 1.1869 1.3537 1.2921 1.2063 1.2608 1.0672
1.3258 1.2660 1.1838 1.3497 1.2879 1.2047 1.2574 1.0639
1.3215 1.2585 1.1760 1.3439 1.2839 1.2003 1.2530 1.0611
Uncertainties in measurements do not exceed ( 0.0005.
Table 2. Volume Expansivities of Phosphonium and Ammonium RTILs Estimated at 1 atm and 298 K from Equation 2, along with Halide Content of Tested RTILsa volume expansivity,b R‚104 K-1
halide content (chloride Wt %)c
stock sample water content (ppm, mg‚kg-1)
[P(14)666][Cl] [P(14)666][DCA] [P(14)666][Tf2N] [P(2)444][DEP] [P(14)444][DBS]
5.96 5.69 5.71 6.43 5.77
Phosphonium N/A N/A
