CSIRO Minerals, Bayview Avenue, Clayton South, Victoria 3169, Australia.
Phys Chem Chem Phys. 2009 Dec 28;11(48):11507-14. doi: 10.1039/b916046a. Epub 2009 Oct 21.
The effect of water on the surface structure of 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonylimide [C(4)mpyr][NTf(2)] ionic liquid was investigated using X-ray reflectometry. The measured reflectivity data suggests a significant amount of water is adsorbed at the surface, with the first layer from the gas (nitrogen)-liquid phase boundary mainly occupied by a mixture of cations and water. Beyond the cation + water layer, the scattering length density increases towards the bulk value, indicating a decreasing amount of water and cations, and/or an increasing amount of anions. The orientation of the butyl chain of cation at the phase boundary and the population of water at the surface were described based on results from an independent molecular dynamics (MD) simulation. We show that the presence of water in the ionic liquid has a non-monotonic effect on the overall thickness of the surface. At low water content, the addition of water does not change the surface thickness since water is mainly present in the bulk. As the water content increases, the surface swells before eventually shrinking down close to the solubility limit of water. The non-monotonic surface thickness is used to explain the anomalous trend of surface tension in ionic liquid-water mixtures reported in the literature.
采用 X 射线反射法研究了水对 1-丁基-1-甲基吡咯烷六氟磷酸离子液体[C(4)mpyr][NTf(2)]表面结构的影响。测量得到的反射率数据表明，大量的水被吸附在表面，从气相（氮气）-液相边界的第一层主要由阳离子和水的混合物组成。在阳离子+水层之外，散射长度密度向体相值增加，表明水和阳离子的数量减少，和/或阴离子的数量增加。基于独立分子动力学（MD）模拟的结果，描述了相边界处阳离子的丁基链的取向和表面处水的分布。我们表明，水在离子液体中的存在对表面的整体厚度具有非单调的影响。在低含水量下，由于水主要存在于本体相中，因此添加水不会改变表面厚度。随着含水量的增加，表面先膨胀，然后最终在接近水的溶解度极限时收缩。这种非单调的表面厚度可以用来解释文献中报道的离子液体-水混合物表面张力的异常趋势。
