Comprehensive study on the impact of the cation alkyl side chain length on the solubility of water in ionic liquids

abstract

A comprehensive study on the phase behaviour of two sets of ionic liquids (ILs) and their interactions with water is here presented through combining experimental and theoretical approaches. The impact of the alkyl side chain length and the cation symmetry on the water solubility in the asymmetric [C-N (-) (1)C(1)im][NTf2] and symmetric [C-N (-) (1)C(1)im][NTf2] series of Rs (N up to 22), from 288.15 K to 318.15 K and at atmospheric pressure, was studied. The experimental data reveal that the solubility of water in ILs with an asymmetric cation is higher than in those with the symmetric isomer. Several trend shifts on the water solubility as a function of the alkyl side chain length were identified, namely at [C(6)C(1)im][NTf2] for asymmetric ILs and at [C(4)C(4)im][NTf2] and [C(7)C(7)im]INTf2] for the symmetric ILs. To complement the experimental data and to further investigate the molecular-level mechanisms behind the dissolution process, density functional theory calculations, using the Conductor-like Screening Model for Real Solvents (COSMO-RS) and the electrostatic potential-derived CHelpG, were performed. The COSMO-RS model is able to qualitatively predict water solubility as a function of temperature and alkyl chain lengths of both symmetric and asymmetric cations. Furthermore, the model is also capable to predict the somewhat higher water solubility in the asymmetric cation, as well as the trend shift as a function of alkyl chain lengths experimentally observed. Both COSMO-RS and the electrostatic potential-derived CHelpG show that the interactions of water and the IL cation take place on the IL polar region, namely on the aromatic head and adjacent methylene groups that explains the differences in water solubility observed for cations with different chain lengths. Furthermore, the CHelpG calculations for the isolated cations in the gas phase indicates that the trend shift of water solubility as a function of alkyl chain lengths and the difference of water solubility in symmetric may also result from the partial positive charge distribution/contribution of the cation. (C) 2015 Elsevier B.V. All rights reserved.

keywords

MUTUAL SOLUBILITIES; COSMO-RS; THERMOPHYSICAL PROPERTIES; MOLECULAR-DYNAMICS; BINARY-SYSTEMS; SYMMETRY; PREDICTION; BIS(TRIFLUOROMETHYLSULFONYL)IMIDE; EXTRACTION; MIXTURES

subject category

Chemistry; Physics

authors

Kurnia, KA; Neves, CMSS; Freire, MG; Santos, LMNBF; Coutinho, JAP

Groups

G4 - Renewable Materials and Circular Economy
G5 - Biomimetic, Biological and Living Materials
G6 - Virtual Materials and Artificial Intelligence

Projects

Centre for Research in Ceramics and Composite Materials - CICECO (LA0011: 2013-2014) (PEst-C/CTM/LA0011/2013)

Igy Technology: A Purication Platform using Ionic-Liquid-Based Aqueous Biphasic Systems (IGYPURTECH)

acknowledgements

This work was financed by national funding from the Fundacao para a Ciencia e a Tecnologia (FCT, Portugal), European Union, QREN, FEDER and COMPETE by the projects PEST-C/CTM/LA0011/2013 and PTDC/ACC-AMB/119172/2010. C.M.S.S. Neves and K.A. Kurnia also thank FCT for the doctoral SFRH/BD/70641/2010 and postdoctoral grants SFRH/BPD/88101/2012, respectively. M.G. Freire acknowledges the European Research Council (ERC) for the Grant ERC-2013-StG-337753.