title: 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 [CN-1C1im][NTf2] and symmetric [CN-1CN-1im][NTf2] series of ILs (Nup 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 [C6C1im][NTf2] for asymmetric ILs and at [C4C4im][NTf2] and [C7C7im][NTf2] 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 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 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 what 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 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.
authors.affiliation: Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: Kiki A
authors.initials: KA
authors.lastname: Kurnia
authors.affiliation: Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: Catarina M S S
authors.initials: CM
authors.lastname: Neves
authors.affiliation: Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: Mara G
authors.initials: MG
authors.lastname: Freire
authors.affiliation: Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre 687, P-4169-007 Porto, Portugal.
authors.forename: Luís M N B F
authors.initials: LM
authors.lastname: Santos
authors.affiliation: Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: João A P
authors.initials: JA
authors.lastname: Coutinho
keywords: Asymmetry; COSMO-RS; Imidazolium-based; Ionic liquid; Symmetry; Trend shift; Water solubility; ab initio