Abstract
A new approach for improving the accuracy of infinite dilution activity coefficient of organic solutes in ionic liquids using COSMO-SAC was investigated in this research. The accuracy of Infinite Dilution Activity Coefficient (IDAC) of solutes in ionic liquids determined by the original COSMO-SAC model has been improved by modifying the model and fitting the adjustable parameters with experimental data from National Institute of Standards and Technology (NIST). The exchange energy expression was modified by including segment interaction parameters C1 and C2 to account for the charges on cations and anions, as well as the non-electrostatic contribution Cne to ascertain its effect on the systems. The standard segment surface area, aeff, standard surface area in Staverman–Guggenheim equation, q, standard volume in Staverman–Guggenheim equation, r, segment interaction parameters C1 and C2, and the non-electrostatic contribution, Cne, were fitted to 414 experimental data points using least square curve fitting program in Matlab. The data points were composed of 39 solutes, 10 cations and 11 anions in a temperature range from 288.15 to 395. 95K. The optimal values for the parameters aeff, q, and r in the modified model from this works are 7.649 Å2, 236.61Å2 and 3.111 Å3, respectively. Values of the hydrogen bonding interaction coefficient and hydrogen bonding interaction cut-off charge, Chb and σhb remain 85580 kcalmol−1 Å4 e-2 and 8.4 × 10-3 0.0084 eÅ−2 respectively, as in the original model. The values of parameters C1, C2 and Cne have been determined to be 3.415 × 10-14, 1.278 and 5.06 × 10-2, respectively. The calculated IDAC results are in agreement with the experimental data within the Average Relative Deviation (ARD) deviation of 20.71%, hence can be used for screening purposes of ionic liquids. The model from this work performs best for solutes with low polarity. Among the solutes used in this work, the model performed best for Alkynes, with ARD of 5.91%, while the poorest result was recorded for Water with ARD of 98.75%. The ARD of IDAC from 29 data points which were not included in the parameter optimization was computed using the modified model to be 18.07%. In terms of Henry’s law constant calculation for CO2 in ionic liquids, the ARD of COSMO-SAC model from this work is 17.2% for 17 data points, as against that from COSMO-SAC 2010 which is 9.8%. Henry’s law constant for CO2 in the ionic liquids 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TF2N]) and in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TF2N]) was also in good agreement with the experimental values at temperatures below the critical temperature of CO2.

Key words: Ionic Liquid; Infinite Dilution Activity Coefficient; COSMO-SAC; Exchange Energy; Segment Interaction Parameter