Solubility of Aliphatic Hydrocarbons in Piperidinium Ionic Liquids: Measurements and Modeling in Terms of Perturbed-Chain Statistical Associating Fluid Theory and Nonrandom Hydrogen-Bonding Theory
Ionic liquids (ILs) reveal many unique properties which make them very interesting for applications in modern "green" technologies. For that reason, detailed knowledge about correlations between the ions' structure, their combinations, and the bulk properties is of great importance. That knowledge can be accessed by reliable measurements and modeling of systems with ILs in terms of various theoretical approaches. In this paper we report new experimental results on liquid-liquid equilibrium (LLE) measurements of 10 binary systems composed of piperidinium ILs [namely, 1-propyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide and 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide] and aliphatic hydrocarbons (n-hexane, n-heptane, n-octane, cyclohexane, and cycloheptane). Moreover, new results on liquid density of pure 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide are presented. Upper critical solution temperature type of phase behavior for all studied systems was observed. Decrease of solubility of n-alkane with an increase of its alkyl chain length and increase of solubility when changing linear into cyclic structure of hydrocarbon were detected. LLE modeling of investigated systems was performed in terms of two modern theories, namely, perturbed-chain statistical associating fluid theory (PC-SAFT) and nonrandom hydrogen-bonding theory (NRHB). Pure fluid parameters of the models were obtained from fitting of experimental liquid density and solubility parameter data at ambient pressure and tested against high pressure densities. Then literature values of activity coefficients of n-alkanes and cycloalkanes at infinitely diluted mixtures with ILs were used to optimize binary interaction parameters of the models. Finally, the LLE phase diagrams were calculated with average absolute relative deviations of 4.1% and 3.4% of the IL mole fraction for PC-SAFT and NRHB, respectively. The PC-SAFT and NRHB models were both able to capture phase behavior in a qualitative manner. Both models predict the order in which solubility of hydrocarbon in the IL increases, including the effects of chain length of n-alkane as well as chain length of alkyl substituent in piperidinium cation. Moreover, predicted solubility of cycloalkanes is also higher than that of respective n-alkanes. Our results suggest that the presented approach of PC-SAFT and NRHB modeling can be successfully applied to cross-associating systems as well. In summary, we have shown that relatively good results can be obtained for such complex systems by using quite simple molecular models and combining rules. To the best of our knowledge, this is the very first paper in which such equation-of-state modeling has been adopted for systems with ILs.
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