Ionic Liquids as Green Solvents - American Chemical Society

Chapter 12

Downloaded by PENNSYLVANIA STATE UNIV on July 18, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch012

Thermodynamic Properties of Liquid Mixtures Containing Ionic Liquids Andreas Heintz, Jochen K . Lehmann, and Sergey P. Verevkin Department of Physical Chemistry, University of Rostock, D-18055 Rostock, Germany

The systematic study of thermodynamic properties of ionic liquids and their mixtures with organic solvents is an important task for understanding the physical chemistry of systems containing ionic liquids. Results of activity coefficients of solutes in three ionic liquids mixed with a series of alkanes, alkylbenzenes, and alcohols are reported as well as liquid-liquid equilibrium curves and excess volumes. The experimental equipment is described and the results are discussed on a qualitative basis.

The study of ionic liquids that are air and moisture stable has become the subject of an increasing number of scientific investigations documented in the literature and the other chapters of this book (1 - 11). Most work has been invested in the elaboration of the synthetic methods, application in catalytic processes, electrochemistry and other more specialized fields. However, the physico-chemical properties of ionic liquids and their mixture with other fluids have not been studied systematically, this holds in particular for thermodynamic properties. For example, ionic liquids are claimed to be potential solvents for many organic, inorganic, and polymeric substances (2), but even the simple

134

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


135 question as to what degree any compound of interest is soluble in any kind of ionic liquid has received almost no answer so far due to the lack of experimental data. Only a very restricted number of investigations of the liquid-liquid equilibrium (7, 12, 13), solubility of gases in ionic liquids (14-16) and the viscosity of mixtures (8) containing ionic liquids are available in the literature. Our interest in mixtures containing ionic liquids is focussed on the following thermodynamic properties; • Activity coefficients of solutes in ionic liquids • Liquid-liquid equilibria in mixtures of ionic liquids and organic solvents • Excess properties such as excess volumes and excess enthalpies (heats of mixing). The systematic knowledge of these properties is important for designing chemical engineering processes such as homogeneous catalysis, biphase catalysis or biocatalysis where ionic liquids are involved. In this chapter experimental methods will briefly be described and the results will be discussed. The ionic liquids 4-methyl-N-butyl-pyridinum tetrafluoroborate ([4-M-nBP][BF ]); 1methyl-3-ethyl-imidazolium bis(trifluoromethyl-sulfonyl) amide ([emim][NTf ]), and 1,2-dimethyl-3-ethyl-imidazolium bis(trifluoromethyl-sulfonyl) amide ([emmim][NTf2]) discussed in this chapter are characterized in Fig. 1. 4

2

1. Activity coefficients and solubilities of solutes in ionic liquids at infinite dilution Activity coefficients y Γ in ionic liquids at infinite dilution of the solute allow to determine the solubility of the solute i expressed by the mole fraction jc, at low pressures according to Henry's law:

P,=H x,

(1)

r

with

H =P I

*

y"

(2)

to ft

where Hi in eq. (1) is Henry's coefficient strictly defined as H. = l i m ^ i -

*-* χ.

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

(3)

136 / is theftigacityin the vapor phase, P is the vapor pressure of the pure liquid solute i and P, is the (partial) pressure of the solute dissolved in the ionic liquid. Eq. (1) is a good approximation as long as values of are small enough. Due to the fact that ionic liquids have no detectable vapor pressure the special technique io

of gas-liquid chromatography is most suitable for determining values of / f i n ionic liquids which are used as stationary phase in the chromatographic process. Chromosorb W/AW-DMCS mesh was used as solid support for the ionic liquid in the GC column. This support material is covered by the ionic liquid of accurately known mass and filled into the GC column. The retention time t of solute samples injected into the carrier gas nitrogen has been measured. t is related to the retention volume V :


r

r

N

ρ

1—

Tco1

V =J-U { -t ) N

0

tr

G

o w

(4)

ρ out Λ

where t is the dead time of the column. U is the volume flow rate, measured by a soap bubble flow meter, T is the temperature of the column, and 7} is the temperature of the flow meter. P is the saturation pressure of water at 7) and P is the pressure at the column outlet. The quantity J in eq. (4) is a correction factor accounting for the influence of the drop of gas pressure P - P inside the column where P is the pressure of the carrier gas nitrogen measured at the inlet of the column. / is given by (17): G

0

col

ow

out

in

out

in

3 (P IP

Ϋ -1

The desired value of γ°° is given by

lny~ =ln

*ILRT

(6) RT

[v.-if)

RT

where n is the number of moles of the ionic liquid in the column used as stationary phase and i ^ i s the vapor pressure of the pure liquid solute. The second and third term in eq. (6) are correction terms containing the second virial coefficient of the solute B and the cross virial coefficient B of the solute and nitrogen. It turns out that these correction terms contribute less than 3 % to the lL

u

l2


137

value of y~obtained with eq. (6) (18). 38 solutes in ionic liquids have been studied at different temperatures so far (18 - 20). Selected results are shown graphically in Figs. 2 - 4 . Values of y °° increase with increasing size of the alkyl rests in all three classes of solutes indicating a decreasing solubility in the


ionic liquids with the number of C-atoms. Generally values of y ~ for alkylbenzenes and alkanols are distinctly lower than for n-alkanes. This indicates a higher molecular affinity of aromatic rings and polar groups such as the OHgroups to ionic liquids than for the unpolar and less polarizable n-alkanes. Similar results are obtained with other solutes (18 - 20). From the temperature dependence of y~ the heat of solution of the liquid solute at infinite dilution in the ionic liquid Hf°* can be obtained: f

\

Selected results are shown in Table 1. Not unexpected the values of Hf°* of alkylbenzenes are lower than those of n-alkanes and in some cases even a negative sign is observed indicating an exothermic solution process. Somewhat surprising is the fact the values of Hf~

of alkanols are relatively

high. Most probably the reason is that hydrogen bonding of pure alkanols in the liquid state has to be broken when alkanols are dissolved in the ionic liquid. Breaking hydrogen bonds exhibits an energy consuming i.e. positive contribution to the solution process. 2. Activity coefficients of solutes in ionic liquids in the whole concentration range Activity coefficients covering the whole range of concentration in a mixture of an ionic liquid with an organic solute can be measured using a recently developed method (21). This method is based on the so-called transpiration technique which is particularly suitable when the vapor pressure of the solute is low. The principle of this method is shown in Fig. 5. Small glass beads are covered with a liquid mixture consisting of the ionic liquid and an organic solute of exactly known composition and are filled into an U-shaped glass tubing which


138

Me—^T^N—Bu

BF" 4

[4-M-nBP][BF ];


4

^N^3N Me

>^

(CFjSO^N'

%

Et

J

^N^N LMe

[emim][NTf ]

γ Me

Ionic Liquids as Green Solvents - American Chemical Society

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