Ionic Liquids IIIA: Fundamentals, Progress, Challenges, and

Chapter 14

Activity Coefficients and Heats of Dilution in Mixtures Containing Ionic Liquids 1

2

Andreas Heintz , Wojciech Marczak , and Sergey P . Verevkin

1

Downloaded by UNIV OF ARIZONA on January 22, 2013 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch014

1

Department of Physical Chemistry, University of Rostock, D-18055 Rostock, Germany Institute of Chemistry, University of Silesia, PL-40006, Katowice, Poland 2

Vapor-liquid equilibria of the mixtures of high boiling solvents (aldehydes, ketones, esters, ethers, amines) with the ionic liquid 1-methyl-3-ethyl-imidazolium bis(trifluoromethyl-sulfonyl) imide [emim][ntf ] were studied by using the transpiration method, which allows to determine the composition of the vapor phase of high boiling mixtures and to calculate activity coefficients in the liquid phase. The measurements were carried out over the whole concentration range at different temperatures between 298 Κ and 323 K . Enthalpies of solution of 6 organic solutes in the [emim][ntf ] have been measured at 298.15 Κ in the range of low concentrations using titration calorimetry. Results at infinite dilution are compared with indirectly obtained data from activity coefficients in infinite dilution. 2

2

© 2005 American Chemical Society

187

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

188 The unique properties of ionic liquids open up a wide range for various applications. Ionic liquids have proved to be viable reaction media for numerous types of reactions. Separation of the reaction products from the ionic liquid requires the knowledge of the thermodynamic properties of such mixtures as well as of properties of the pure individual compounds. This work continues our study of thermodynamic properties of mixtures containing ionic liquids [1-7] Activity coefficients and data of mixing enthalpies, which are needed for many design calculations, are still unavailable for systems containing ILs. Vapor-liquid equilibrium of the mixtures of high boiling solvents (aldehydes, ketones, esters, ethers, amines) with the ionic liquid [emim][ntf ] - l-methyl-3-ethyl-imidazolium bis(trifluoromethyl-sulfonyl) imide (CgHuSaC^FeNa) were studied by using the transpiration method. The measurements were carried out over the whole concentration range at different


2

temperatures between 298 Κ and 323 K. Activity coefficients y

t

of these

solvents in the ionic liquid have been derived and from their temperature dependence partial molar excess enthalpies Hf

of the solutes in the ionic

liquid have been estimated. 1. Activity coefficients of high boiling solutes in ionic liquid in the whole concentration range Vapor-liquid equilibrium experiments carried out by dynamic recirculation stills, static method and headspace methods have traditionally been used to obtain information about activity coefficients in the liquid phase and a substantial body of data [8] has been accumulated in this area on the solution behaviour of a large number of mixtures. However all these traditional methods have generally been applied for investigating relatively low boiling componds or providing results for high-boiling compounds at elevated temperatures above 373 K. For the development of separation technologies the knowledge of thermodynamic properties of the mixtures containing high-boiling compounds with with ILs at ambient temperatures is of the crucial importance. One of the suitable methods used for this purpose is the so-called transpiration method. This method is based on using an inert carrier gas stream being loaded with the equilibrium vapor of the vapor-liquid system. When the volume and the molar mass of the entrained vapor and appropriate amount of the carrier gas is known, the vapor pressure in the system can be derived from the ideal gas law provided the vapor pressure is low enough. The method has been successfully applied in our laboratory [9] for measuring vapor pressures of pure compounds and has proved to give results which are in excellent agreement with other established techniques for determining vapor pressures in the range of 0.005 to


189 10000 Pa. Taking into account that ILs have vanishing vapor pressures, we decided to apply this method for the investigation of the mixtures containing a high-boiling compound (solute) and an IL (solvent). In this case, knowing the starting composition of the liquid phase x» the composition of the vapor phase y is governed only by the volatile solute. By isothermal measurements of compositions of the vapor phase and by screening the compositions of the liquid phase, i.e. mole fraction of the solute in the mixture, and additionally having established the vapor pressure of the pure solute activity coefficients in the mixture of solute in IL can be derived. From the temperature dependence of activity coefficients partial molar heats of solution are obtained. Details of the technique can be found elsewhere (4,7). Values of ysoiute have been obtained according to Downloaded by UNIV OF ARIZONA on January 22, 2013 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch014

%

solute '

P.

(1)

χ Λ

* io solute

where Ρ is the measured pressure of the solute at the molefractionx i and P is the vapor pressure of die pure solute, this is the value of Ρ at x iute 1. Test measurements including thermodynamic consistency tests with the mixture (npentanol + decane) have been made showing excellent agreement of the results of y with literature data obtained by a different method (10). Measurements of ysolute covering the whole range of concentration of solute + ionic liquid mixtures have been performed. A series of aldehydes, ketones, esters, ethers, amines mixed with ionic liquid has been studied. As an example Fig. 1 and Fig 2. show the results of nonanal where the pressure as well as the activity coefficients y i of the solute are presented as function of the molefractionof the solute. so ute

io

=

so

t

so ute

2. Enthalpies of solution of organic solutes in the ionic liquid [emim][ntf ] 2

Recently we have reported measurements of activity coefficients at infinite dilution γ" of 38 solutes at different temperatures (1-3). The temperature dependence of yΓ allows to determine enthalpies of solution at infinite dilution Η according to Εϋ0

Eoo

( dlnrr

(2)

R In this work, values of the heat of dilution, defined as partial molar excess enthalpy H* of a series of solutes i (methanol, t-butanol, 1-hexanol, chloroform, toluene, and ethylene glycol) in the ionic liquid: [emim][ntf ] were 2


190 350

350 Ο ο Δ V 0 0

300 250

CO

7=298.65 Κ 7=303.55 Κ 7= 308.55 Κ 7= 313.55 Κ 7= 318.55 Κ 7= 323.55 Κ NRTL

300 250

200


ο.

Figure /. Partial pressure data of nonanal in the mixture with [emim][ntfj as function ofxj (nonanal) 2.5

2.5

7= 298.65 Κ 7= 303.55 Κ 7= 308.55 Κ 7= 313.55 Κ 7= 318.55 Κ 7= 323.55 Κ

Λ •Λ

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

c

0.0 0.0

0.2

0.4

0.6

0.8

0.0 1.0

Χ, Figure 2. Values of Ιηγι of nonanal in the mixture with femimjfntfj as function ofxj (nonanal)


191 measured at 298.15 Κ by using a titration calorimeter 2277 Thermal Activity Monitor (Thermometries, Sweden). Extrapolation of values of H* as function of solute concentration to infinite dilution allows to determine //, ° and to make a comparison with indirectly determined values of H>*~ obtained by Eq. (2). The heat effect of the solute injection into the ionic liquid (Qi) was recalculated into the molar enthalpy of solution (i.e. the partial molar excess enthalpy of the solute, Η ) by the following formula: Eo

Ε

dW

(3)


an,-

where and nn (given in moles) denote the content of the solute and the ionic liquid in the solution, respectively, ff is the enthalpy of mixing and Δη/ is the drop size (in moles). The experimental procedure was described elswere (11). Results are presented in Table 1. Comparison of heats of solution of different solutes in the ionic liquid [emim][ntf ] obtained by a direct calorimetric method could be made with values for the same systems obtained by the indirect method and already presented previously where the temperature dependence of activity coefficients of the solute in the ionic liquid was used. Considering experimental errors involved into both methods the agreement is satisfactory even though thermodynamic consistency was not confirmed in all cases. This indicates that the estimation of experimental errorsmade in ref. (1-3) was too low. 1

2

Conclusions Systematic investigation of activity coefficients in mixtures containing ionic liquids allow to develop reliable methods of predicting solubilities of gases and vapors in ionic liquid where direct experimental data are not available. The knowledge of heats of mixing enable scientists working on the application of ionic liquids in chemical and separation processes to determine the temperature dependence of solubility data.


192 Table 1. Limiting partial molar excess enthalpies Hi of organic solutes in [emim][ntf ] at 298.15 Κ obtained calorimetrically (this work) and by gasliquid chromatography (1-3) 2


Solute methanol t-butanol 1-hexanol chloroform toluene ethylene glycol

(1) (2) (3) (4) (5) (6) (7)

(8) (9) (10) (11)

1

//^(J-mol ) This work 5639 ±54 7039 ±47 10076 ±616 -3255 ± 15 -1018 ±8 11535 ±200

Literature (1-3) 7787 ±360 7240 ±340 10812 ±450 -1722 ±50 -683.5 ±31

References Heintz, Α.; Kulikov, D. V.; Verevkin, S. P. J. Chem. Eng. Data 2001, 46, 1526-1529. Heintz, Α.; Kulikov, D. V.; Verevkin, S. P. J. Chem. Eng. Data 2002, 47, 894-899. Heintz, Α.; Kulikov, D. V.; Verevkin, S. P., J. Chem. Thermodyn. 2002, 34, 1341-1347 Verevkin, S. P.; Vasiltsova, T. V.; Bich, E.; Heintz, Α., Fluid Phase Equil. 2004, 218, 165-175. Heintz, Α.; Lehmann, J. K.; Wertz, C. J. Chem. Eng. Data 2003, 48, 472-474. Heintz, Α.; Klasen, D.; Lehmann, J.K. J. Sol. Chem. 2002, 31, 467476. Heintz, Α.; Lehmann, J. K.; Verevkin, S. P. Thermodynamic properties of Liquid Mixtures Containing Ionic Liquids in "Ionic Liquids as Green Solvents: Progress & Prospects, "ACS Symposium Series 856, American Chemical Society, Washington DC, 2003, 134-151. Gmehling, J.; Onken, U.; Arlt, W. Vapor-Liquid Equilibrium Data Collection. DECHEMA, Frankfurt, 1977. Kulikov, D. V.; Verevkin, S. P.; Heintz, A . Fluid Phase Equil. 2001, 192, 187-202. Treszczanowicz T.; Treszczanowicz, J. Bull. Acad. Pol. Sci. 1979, 27, 689. Marczak, W.; Verevkin, S. P.; Heintz, A. J. Sol. Chem. 2003, 32, 519526.


Ionic Liquids IIIA: Fundamentals, Progress, Challenges, and

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