Bruce Duncan W, Cabry Christopher P, Lopes José N Canongia, Costen Matthew L, D'Andrea Lucía, Grillo Isabelle, Marshall Brooks C, McKendrick Kenneth G, Minton Timothy K, Purcell Simon M, Rogers Sarah, Slattery John M, Shimizu Karina, Smoll Eric, Tesa-Serrate María A
Department of Chemistry, University of York , Heslington, York YO10 5DD, U.K.
Centro de Química Estrutural, IST, Universidade de Lisboa , 1049-001 Lisboa, Portugal.
J Phys Chem B. 2017 Jun 22;121(24):6002-6020. doi: 10.1021/acs.jpcb.7b01654. Epub 2017 Jun 8.
Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure, and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This article presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cmim][TfN] and [Cmim][TfN]). Physical property measurements (viscosity, conductivity, and density) reveal that these IL mixtures are not well described by simple mixing laws, implying that their structure and dynamics are strongly composition dependent. Small-angle X-ray and neutron scattering measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [Cmim][TfN], the bulk of the IL is composed of small aggregates of [Cmim] ions in a [Cmim][TfN] matrix, which is driven by nanosegregation of the long alkyl chains and the polar parts of the IL. As the proportion of [Cmim][TfN] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and nonpolar domains is formed. Reactive atom scattering-laser-induced fluorescence experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, in contrast to previous suggestions, that the [Cmim] ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [Cmim][TfN].
离子液体（IL）混合物具有巨大的潜力，因为通过混合两种常见成分而非合成大量具有不同化学结构的纯离子液体，可以形成具有广泛性质的液体。此外，这些混合物可以表现出一系列取决于其组成的性质和结构组织，这为特定应用中基于组成控制离子液体性质开辟了新的可能性。然而，目前对离子液体混合物的基本性质、结构和动力学了解甚少，这限制了它们更广泛的应用。本文首次对由两种常见离子液体形成的离子液体混合物的体相和表面性质进行了全面研究：1-乙基-3-甲基咪唑鎓和1-十二烷基-3-甲基咪唑鎓双（三氟甲基磺酰）亚胺（[Cmim][TfN]和[C12mim][TfN]）。物理性质测量（粘度、电导率和密度）表明，这些离子液体混合物不能用简单的混合定律很好地描述，这意味着它们的结构和动力学强烈依赖于组成。小角X射线和中子散射测量以及分子动力学（MD）模拟表明，在[Cmim][TfN]的低摩尔分数下，离子液体的主体由[Cmim]离子在[Cmim][TfN]基质中的小聚集体组成，这是由长烷基链和离子液体的极性部分的纳米分离驱动的。随着混合物中[Cmim][TfN]比例的增加，聚集体的尺寸和数量增加，直到C12烷基链渗透过系统并形成极性和非极性域的双连续网络。反应原子散射-激光诱导荧光实验也得到MD模拟的支持，已被用于探测这些混合物的表面结构。发现即使在长链成分含量低的混合物中，真空-离子液体界面中C12烷基链也显著富集。这些数据表明，与先前的观点相反，[Cmim]离子在这种二元离子液体混合物中具有表面活性。然而，随着其在混合物中的比例增加，表面在C12链中不会饱和，并且在纯[Cmim][TfN]中仍不饱和。
