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The key of CAMD for ionic liquids is the selection of an appropriate thermodynamic method to obtain the target properties. For extractive distillation, the most importance of all in the explicit properties is activity coefficient which can be used to calculate relative volatility or selectivity and thus evaluate the possible ionic liquids.

Today, there are two methods commonly used for predicting activity coefficient, i.e. UNIFAC (UNIQUAC functional group activity coefficient) model and MOSCED (modified separation of cohesive energy density) model. The UNIFAC model is the most widely used, and based on the contributions of the constituent groups in a molecule. Since the first version of UNIFAC, several revisions and extensions have been proposed to improve the prediction of activity coefficient. The MOSCED model is an extension of regular solution theory to mixtures that contain polar and hydrogen bonding components. The cohesive energy density is separated into dispersion forces, dipole forced, and hydrogen bonding with small corrections made for asymmetry. However, group parameters of the selected ionic liquid can't be found in the present limited parameter table of UNIFAC and MOSCED models. In this case, COSMO-RS (Conductor-like Screening Model for Real Solvents) model is selected and used as an alternative to UNIFAC and MOSCED models [119, 120],

COSMO-RS model is a novel and efficient method for the priori prediction of thermophysical data of liquids, and has been developed since 1994 [121-123], It is based on unimolecular quantum chemical calculations that provide the necessary information for the evaluation of molecular interactions in liquids. It can be applied to nearly any system for which no group parameters are available in the UNIFAC and MOSCED models. Moreover, COSMO-RS model is open for a large number of qualitative improvements and functional extensions. A correction for misfit charge interactions will improve the accuracy of the electrostatic part and enable calculations for ions.

In COSMO-RS model, the activity coefficient of component i is related with chemical potential and given as follows:

where //, is the chemical potential of component i in the mixture, //," the chemical potential in the pure liquid substance, T the absolute temperature and R gas constant. The chemical potential can be solved by using the exact equation resulting from statistical thermodynamics.

Therefore, it is possible to select the potential ionic liquid by means of CAMD. But it is believed that the ionic liquids must meet the requirement of implicit properties that can't be obtained by calculation. In this case it is better to set up a data bank of commonly used ionic liquids. Tables 22a and 22b show some implicit properties of commonly used ionic liquids [70], which can be input into the CAMD program for screening ionic liquids. By combination of explicit and implicit properties, the best ionic liquid can be found by CAMD.

Table 22a

Some implicit properties of commonly used ionic liquids; adapted from the reference [70]

Ionic liquid"

Color (with impurities)

Density (g ml"1)

Liquid temperature (°C)

[emim]PF() N-butylpyridine / AICU

(CH^NHCl / 2AICI3

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