Kurnia Kiki A, Neves Catarina M S S, Freire Mara G, Santos Luís M N B F, Coutinho João A P
Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
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.
J Mol Liq. 2015 Oct 1;210(B):264-271. doi: 10.1016/j.molliq.2015.03.040.
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 Cim][NTf] and symmetric [C C im][NTf] series of ILs ( 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 [CCim][NTf] for asymmetric ILs and at [CCim][NTf] and [CCim][NTf] 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.
本文通过结合实验和理论方法，对两组离子液体（ILs）的相行为及其与水的相互作用进行了全面研究。研究了烷基侧链长度和阳离子对称性对不对称[C Cim][NTf]和对称[C C im][NTf]系列离子液体（最高22种）在288.15 K至318.15 K以及大气压下的水溶解度的影响。实验数据表明，不对称阳离子离子液体中的水溶解度高于对称异构体离子液体中的水溶解度。确定了水溶解度随烷基侧链长度变化的几个趋势转变，即不对称离子液体在[CCim][NTf]时，对称离子液体在[CCim][NTf]和[CCim][NTf]时。为了补充实验数据并进一步研究溶解过程背后的分子水平机制，使用真实溶剂的导体类筛选模型（COSMO-RS）和基于静电势的CHelpG进行了密度泛函理论计算。COSMO-RS模型能够定性地预测水溶解度随温度以及对称和不对称阳离子烷基链长度的变化。此外，该模型还能够预测不对称阳离子中稍高的水溶解度，以及实验观察到的随烷基链长度变化的趋势转变。COSMO-RS和基于静电势的CHelpG均表明，水与离子液体阳离子的相互作用发生在离子液体的极性区域，即芳香头部和相邻的亚甲基上，这解释了不同链长阳离子水溶解度的差异。此外，气相中孤立阳离子的CHelpG计算表明，水溶解度随烷基链长度的趋势转变以及对称体中水溶解度的差异也可能源于阳离子的部分正电荷分布/贡献。
