900
Ind. Eng. Chem. Res. 2008, 47, 900-909
GENERAL RESEARCH Correlations of Low-Pressure Carbon Dioxide and Hydrocarbon Solubilities in Imidazolium-, Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids. Part 1. Using Surface Tension Prem K. Kilaru, Ricardo A. Condemarin, and Paul Scovazzo* Department of Chemical Engineering, UniVersity of Mississippi, UniVersity 134 Anderson Hall, UniVersity, Mississippi 38677
This paper examines the dependence of gas solubility on the surface tension of room-temperature ionic liquids (RTILs). The solubility phenomena of the CO2, ethylene, propylene, 1-butene, and 1,3-butadiene in five imidazolium-, four phosphonium-, and eight ammonium-based RTILs are explained using the Hansen RTIL solubility parameters that were calculated from surface tension measurements. The Hansen solubility parameters were estimated from the surface tensions of the respective RTILs with a proportionality constant that is a function of the ratio of the number of nearest-neighbor interactions in the bulk liquid to those on the surface. The calculated solubility parameters are almost independent of temperature. The surface tension solubility parameter model explains the effect of variation of alkyl chain lengths in the cations and the variation of the RTIL anions on gas solubility within an error of (15%. The analysis may indicate that the CO2-anion interaction is not the dominant factor that determines the relative CO2 solubility between RTILs. Introduction The proliferating research on the room-temperature ionic liquids (RTILs) as environmentally benign1 gas processing agents not only requires a profound understanding of gas-RTIL mixture properties but also needs accurate gas solubility predictive models. The advantage of the RTILs as alternative solvent replacements2 for mundane organics is their immeasurable vapor pressures. This permits a complete recovery of the solvent without any significant loss of the RTILs. The use of isothermal and isobaric mixing of solute with solvents in many of the gas absorption and stripping processes3 was the impetus for thermodynamic models to estimate solubility of various gaseous solutes in the RTILs. In addition, the plethora of ionic liquids (200+)4 that recently have been claimed as liquids at room temperature requires a quick preliminary screening of the RTILs for the separation of interest. The objective of this paper is to continue the development of regular solution predictive models of gas solubility in a wide range of RTILs. The original Hildebrand and Scatchard solubility parameter theory of nonpolar and nonassociating liquids requires modifications to account for the polar and hydrogen bonding interactions.5 The presence of multiple force fields in RTILs (such as dispersion forces due to the presence of alkyl chains, ion-induced polar forces, and hydrogen bonding interactions due to the presence of free electrons) results in the solubility of the certain gases (such as CO2) being higher than those in conventional organic solvents.6 There are many theories to explain the high CO2 solubility in RTILs, such as Lewis acid-base complexations of gaseous species with anions.6-8 Scovazzo et al.9 and Camper et al.10 used RTIL melting-point temperatures to explain CO2 solubility through regular solution theory in four imidazolium * To whom correspondence should be addressed. Tel.: (662)-9155354. Fax: (662)-915-7023. E-mail address:
[email protected].
RTILs and one phosphonium RTIL, using Trounton’s rule; however, the model was limited by the lack of RTIL melting points, because many of the RTILs found in the literature have only glass-transition temperatures.4 Camper et al.11 used lattice energies and molar volumes of the RTILs to correlate lowpressure Henry’s law constants of CO2, CO, alkanes, and alkenes in imidazolium-based RTILs. The calculation of interion distances with spherical symmetrical for lower-alkyl-chain imidazolium ion pairs may not work well for the RTILs that have linear alkyl chains of g4 carbon units.11 In addition, the failure of group contribution methods12 to estimate interaction energies of strongly associated solvents such as electrolytes and ionic liquids made us search for alternative ways to represent ion-pair interaction energies in the RTILs. Therefore, the work presented below is focused on the development of moreuniversal gas solubility regular solution predictive models in 1-alkyl-3-methylimidazolium-, quaternary phosphonium-, and quaternary ammonium-based RTILs. We use RTIL surface tensions as a direct measure of all the surface polarities for the estimation of interaction energies of ion pairs that are involved in the RTILs. Materials The materials used in this work are presented in Table 1, along with CAS registration numbers, abbreviations, and supplier. Schematic depictions of the 1-alkyl-3-methyl imidazolium, quaternary phosphonium, and ammonium cations are shown in Figure 1. Kilaru et al.21 have given details on the synthesis methods for the in-house manufactured RTILs. Table 2 lists the halide content of the RTILs used in this study, and Table 3 gives the gas solubilities in the RTILs, in terms of both mol/mol-atm (Table 3a) and mol/L-atm (Table 3b).
10.1021/ie070834r CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007
Imidazolium 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide 1-ethyl-3-methylimidazolium trifluromethanesulfonate 1-butyl-3-methylimidazolium hexafluorophosphate 1-hexyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide 1-diethylsulfonyl-3-methylimidazolium trifluromethanesulfonate Phosphonium trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101) trihexyl(tetradecyl)phosphonium dicyanamide (Cyphos IL 105) trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide (Cyphos IL 109) tributyl(ethyl)phosphonium diethylphosphate (Cyphos IL 169) Ammonium dimethyl(butyl(i-propyl))ammonium bis((trifluoromethyl)sulfonyl)imide dimethyl(hexyl(i-propyl))ammonium bis((trifluoromethyl)sulfonyl)imide dimethyl(decyl(i-propyl))ammonium bis((trifluoromethyl)sulfonyl)imide trioctyl(methyl)ammonium bis((trifluoromethyl)sulfonyl)imide trimethyl(butyl)ammonium bis((trifluoromethyl)sulfonyl)imide trimethyl(hexyl)ammonium bis((trifluoromethyl)sulfonyl)imide trimethyl(decyl)ammonium bis((trifluoromethyl)sulfonyl)imide triethyl(hexyl)ammonium bis((trifluoromethyl)sulfonyl)imide
RTIL
[N(4)113][Tf2N] [N(6)113][Tf2N] [N(10)113][Tf2N] [N(1)888][Tf2N] [N(4)111][Tf2N] [N(6)111][Tf2N] [N(10)111][Tf2N] [N(6)222][Tf2N]
[P(14)666][Cl] [P(14)666][DCA] [P(14)666][Tf2N] [P(2)444][DEP]
[emim][Tf2N] [emim][TfO] [bmim][PF6] [C6mim][Tf2N] [desmim][TfO]
abbreviation
460092-03-9
258864-54-9
145022-44-2 174501-64-5
CAS number
410.2 438.2 494.2 634.2 396.0 424.4 480.0 354.6
519.3 549.0 764.0 384.0
391.3 260.2 284.2 447.43 352.1
molecular weight, MW
Table 1. Room-Temperature Ionic Liquid (RTIL) Materials Used in This Work, Along with Their Abbreviations, CAS Numbers, and Suppliers
Oak Ridge National Laboratories (ORNL) Oak Ridge National Laboratories (ORNL) Oak Ridge National Laboratories (ORNL) Oak Ridge National Laboratories (ORNL) Ionic Liquid Technologies, Germany Ole Miss Ole Miss Ole Miss
Cytec Canada Cytec Canada Cytec Canada Cytec Canada
Covalent Associates, Inc. EMD Chemicals Sigma-Aldrich Covalent Associates, Inc. University of South Alabama
supplier
Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 901
Figure 1. Two-dimensional (2D) structures of cations and anions used in this study.
Theory
The vapor-liquid equilibrium (VLE) of a gas-RTIL system is represented, in terms of the fugacity of a gaseous solute,5 as
f G2 ) y2φ2P ) x2γ2f 02
0 1 f 2 γ2 ) x2 P
-ln x2 ) ln
()
f 02 + ln γ2 P (1)
where y2 is the mole fraction of solute in the gas phase, φ2 the gas-phase fugacity coefficient of the solute, f G2 the fugacity of the solute in the gas mixture, P the gas pressure, x2 the mole fraction of solute in the RTIL solution, γ2 the activity coefficient of solute in the solution, and f 02 the fugacity of the solute, at a hypothetical liquid state, at solution temperature and pressure. Because RTILs have no measurable vapor pressures, the mole fraction of RTIL in the gaseous phase is approximately zero (y1 ≈ 0) and, assuming an ideal gas, y2 )1 and φ2 ) 1. Therefore, the fugacity of component 2 is equal to the partial pressure, which is equal to the total pressure of gas (f G2 ) P; the partial pressure of RTIL in the gas phase is zero). Therefore, eq 1 can be rearranged as
(2)
The application of natural logarithms yields the following expression:
(3)
The activity coefficient in eq 3 can be estimated from the Hildebrand-Scatchard regular solution equation5 that relates the
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Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008
Table 2. Physical Properties of RTILs of This Study at 30 °C RTIL Imidazolium [emim][Tf2N] [emim][TfO] [bmim][PF6] [desmim][TfO] [C6mim][Tf2N] Phosphonium [P(14)666][Cl] [P(14)666][DCA] [P(14)666][Tf2N] [P(2)444][DEP] Ammonium [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(1)888][Tf2N] [N(6)222][Tf2N]
densitya (g/mL)
molar volume (cm3/mol)
surface tensionb (dyn/cm)
halide content (chloride wt %)c
reference
1.5483 1.3572 1.3443 1.5420 1.3455
252.7 191.7 211.4 241.6 332.5
40.5 47.6 47.2 53.7 36.4
