Activity coefficients in ionic liquids

Thermodynamic activity coefficients can be determined from the phase equilibrium measurements, and they are a measure of deviations from Raoult s law. Data of the activity coefficients covering the whole range of liquid composition of IL + molecular solvent mixtures have been reported in the literature and discussed in sections 1.6,1.7, and 1.8 as the values obtained from the SLE, LLE, and VLE data. When a strong interaction between the IL and the solvent exists, negative deviations from ideality should be expected with the activity coefficients lower than one. 

Activity coefficients at infinite dilution, of organic solutes in ILs have been reported in the literature during the last years very often [1,2,12,45,64, 65,106,123,144,174-189]. In most cases, a special technique based on the gas chromatographic determination of the solute retention time in a packed column filled with the IL as a stationary phase has been used [45,123,174-176,179,181-187]. An alternative method is the dilutor technique [64,65,106, 178,180]. A lot of y 3 (where 1 refers to the solute, i.e., the organic solvent, and 3 to the solvent, i.e., the IL) provide a useful tool for solvent selection in extractive distillation or solvent extraction processes. It is sufficient to know the separation factor of the components to be separated at infinite dilution to determine the applicability of a compound (a new IL) as a selective solvent. 

The plot of the activity coefficients, y 3, against alkyl chain length, n, in the cation [C Cilm][Tf2N] is shown for n-hexane, hex-l-ene, benzene, and cyclohexane. The increase of the alkyl chain at the cation decreases the activity coefficient for every kind of hydrocarbon in the IL. 

The selectivity is the ratio of the activity coefficients at the infinite dilution and is given by 

It can be seen that the best IL for the economic realization of this process is [C4CiIm][Tf2N]. 

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One of the obstacles in this aim is the lack of experimental thermodynamic data for activity coefficients in ionic liquids, which could be a basis for such solvent selection. In the past years several groups have started to measure such data however, there is a lack of data because the number of suitable anions and cations, and even more the number of ionic liquids, are rapidly increasing compared to the rate (or speed) of measurements. Reliable inter- and extrapolation schemes and group contribution methods are still missing. Thus the search for an appropriate ionic liquid for a certain task can, at present, only be made randomly or by systematic measurements. 

The application of COSMO-RS to the calculation of infinite-dilution activity coefficients in ionic liquids was surprisingly successful. As shown in Fig. 8.5, the activity coefficients of neutral compounds in ionic liquids are very well described. This was achieved without any special adjustment of COSMO-RS, which was developed and parameterized for neutral solvents, just by describing the ionic liquid as a 50 50 mixture of anions and cations. We only needed to take into account the convention of chemical engineers of counting a pair of an anion and a cation as... 

Standard potentials Ee are evaluated with full regard to activity effects and with all ions present in simple form they are really limiting or ideal values and are rarely observed in a potentiometric measurement. In practice, the solutions may be quite concentrated and frequently contain other electrolytes under these conditions the activities of the pertinent species are much smaller than the concentrations, and consequently the use of the latter may lead to unreliable conclusions. Also, the actual active species present (see example below) may differ from those to which the ideal standard potentials apply. For these reasons formal potentials have been proposed to supplement standard potentials. The formal potential is the potential observed experimentally in a solution containing one mole each of the oxidised and reduced substances together with other specified substances at specified concentrations. It is found that formal potentials vary appreciably, for example, with the nature and concentration of the acid that is present. The formal potential incorporates in one value the effects resulting from variation of activity coefficients with ionic strength, acid-base dissociation, complexation, liquid-junction potentials, etc., and thus has a real practical value. Formal potentials do not have the theoretical significance of standard potentials, but they are observed values in actual potentiometric measurements. In dilute solutions they usually obey the Nernst equation fairly closely in the form ... 

Diedenhofen, M., Eckert, F., Mamt, A. Prediction of infinite dilution activity coefficients of organic compounds in ionic liquids using COSMO-RS. J. Chem. Eng. Data 2003, 48, 475 79. 

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

About the same time Beutier and Renon (11) also proposed a similar model for the representation of the equilibria in aqueous solutions of weak electrolytes. The vapor was assumed to be an ideal gas and < >a was set equal to unity. Pitzer s method was used for the estimation of the activity coefficients, but, in contrast to Edwards et al. (j)), two ternary parameters in the activity coefficient expression were employed. These were obtained from data on the two-solute systems It was found that the equilibria in the systems NH3+ H2S+H20, NH3+C02+H20 and NH3+S02+H20 could be represented very well up to high concentrations of the ionic species. However, the model was unreliable at high concentrations of undissociated ammonia. Edwards et al. (1 2) have recently proposed a new expression for the representation of the activity coefficients in the NH3+H20 system, over the complete concentration range from pure water to pure NH3. it appears that this area will assume increasing importance and that one must be able to represent activity coefficients in the region of high concentrations of molecular species as well as in dilute solutions. Cruz and Renon (13) have proposed an expression which combines the equations for electrolytes with the non-random two-liquid (NRTL) model for non-electrolytes in order to represent the complete composition range. In a later publication, Cruz and Renon (J4J, this model was applied to the acetic acid-water system. 

Recently, there have been a number of significant developments in the modeling of electrolyte systems. Bromley (1), Meissner and Tester (2), Meissner and Kusik (2), Pitzer and co-workers (4, ,j5), and" Cruz and Renon (7j, presented models for calculating the mean ionic activity coefficients of many types of aqueous electrolytes. In addition, Edwards, et al. (8) proposed a thermodynamic framework to calculate equilibrium vapor-liquid compositions for aqueous solutions of one or more volatile weak electrolytes which involved activity coefficients of ionic species. Most recently, Beutier and Renon (9) and Edwards, et al.(10) used simplified forms of the Pitzer equation to represent ionic activity coefficients. 

Other references in Table in discuss applications in precipitation of metal.compounds, gaseous reduction of metals from solution, equilibrium of copper in solvent extraction, electrolyte purification and solid-liquid equilibria in concentrated salt solutions. The papers by Cognet and Renon (25) and Vega and Funk (59) stand out as recent studies in which rational approaches have been used for estimating ionic activity coefficients. In general, however, few of the studies are based on the more recent developments in ionic activity coefficients. 

Krummen, M., Wasserscheid, R, and Gmehling, J., Measurements of activity coefficients at infinite dilution in ionic liquids using the dilutor technique, /. Chem. Eng. Data, 47, 1411, 2002. 

Bike, D.M., Brennecke, J.F., and Maginn, E.J., Rredicting infinite-dilution activity coefficients of organic solutes in ionic liquids, Ind. Eng. Chem. Res., 43, 1039, 2004. 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

However, both model solutes for the n- and n-electron dispersion forces, toluene and n-heptane, respectively, show a clear trend. With increasing alkyl chain length, the activity coefficient decreases by factor 2.0 in the case of toluene, and 2.5 in the case of n-heptane, indicating stronger interactions in ionic liquids bearing longer alkyl chains. 

GG measurements have also been used to derive infinite dilution activity coefficients (y ) for a range of potential solutes in ionic liquids. For an ideal mixture, y = 1 if y > 1 solvent-solute interactions are less favorable than the solvent-solvent interactions and if y < 1 solvent-solute interactions are more favorable than the solvent-solvent interactions. Nonpolar solutes such as alkanes, alkenes, and simple... 

The large dataset of partition coefficients (or activity coefficients at infinite dilution) published in the literature may be used to present a general behaviour of solutes in ionic liquids. The values of activity coefficients at infinite dilution () for the -alkanes increase with an increase in carbon number. In most ionic liquids, the high y" values observed with n-alkanes indicate their low solubility in ionic liquids. The values of n-alkanes are higher than the values obtained with cyclohexane, alkenes, alkynes and aromatics. Introduction of a double or triple bond in the n-alkanes decreases the values. 

Krummen, M. Wassersccheid, P. Gmehling, J. (2002). Measurement of Activity Coefficients at Infinite Dilution in Ionic Liquids Using the Dilutor Technique. J. Chem. Eng. Data 47,1411-1417. 

Systems that are near to ideality can be described satisfactorily with Equation 4.4-4, but the equation does not work very well in systems that are far from thermodynamic ideality, even if the self-diffusion coefficients and activities are known. Since systems with ionic liquids show strong intermolecular forces, there is a need... 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

It would appear from Eq. (3.2.8) that the pH, i.e. the activity of a single type of ion, can be measured exactly. This is not, in reality, true even if the liquid junction potential is eliminated the value of Eref must be known. This value is always determined by assuming that the activity coefficients depend only on the overall ionic strength and not on the ionic species. Thus the mean activities and mean activity coefficients of the electrolyte must be employed. The use of this assumption in the determination of the value of Eref will, of course, also affect the pH value found from Eq. (3.2.8). Thus, the potentiometric determination of the pH is more difficult than would appear at first glance and will be considered in the special Section 3.3.2. 

The potentiometric measurement of physicochemical quantities such as dissociation constants, activity coefficients and thus also pH is accompanied by a basic problem, leading to complications that can be solved only if certain assumptions are accepted. Potentiometric measurements in cells without liquid junctions lead to mean activity or mean activity coefficient values (of an electrolyte), rather than the individual ionic values. 

A wide variety of data for mean ionic activity coefficients, osmotic coefficients, vapor pressure depression, and vapor-liquid equilibrium of binary and ternary electrolyte systems have been correlated successfully by the local composition model. Some results are shown in Table 1 to Table 10 and Figure 3 to Figure 7. In each case, the chemical equilibrium between the species has been ignored. That is, complete dissociation of strong electrolytes has been assumed. This assumption is not required by the local composition model but has been made here in order to simplify the systems treated. 

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