Steel William H, Walker Robert A
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
Nature. 2003 Jul 17;424(6946):296-9. doi: 10.1038/nature01791.
Molecular dynamics simulations have previously described how the physical properties across immiscible liquid-liquid interfaces should converge from aqueous to organic limits, but these predictions have largely gone untested, owing to difficulties associated with probing buried interfaces. X-ray and neutron scattering experiments have created detailed pictures of molecular structure at these boundaries, but such scattering studies cannot probe how surface-altered solvent structures affect interfacial solvating properties. Given that surface-mediated solvent properties control interfacial solute concentrations and reactivities, identifying the characteristic dimensions of interfacial solvation is essential for formulating predictive models of solution phase surface chemistry. Here we use specially synthesized solvatochromic surfactants that act as 'molecular rulers' and resonance-enhanced second-harmonic generation to measure the dipolar width of weakly and strongly associating liquid-liquid interfaces. Dipolar width describes the distance required for a dielectric environment to change from one phase to another. Our results show that polarity converges to a nonpolar limit on subnanometre length scales across a water-cyclohexane interface. However, polarity across the strongly associating, water-1-octanol interface is dominated by a nonpolar, alkane-like region. These data call into question the use of continuum descriptions of liquids to characterize interfacial solvation, and demonstrate that interfacial environments can vary in a non-additive manner from bulk solution limits.
分子动力学模拟先前已描述了互不相溶的液-液界面的物理性质应如何从水相极限收敛到有机相极限，但由于探测埋藏界面存在困难，这些预测在很大程度上未得到验证。X射线和中子散射实验已绘制出这些边界处分子结构的详细图像，但此类散射研究无法探测表面改变的溶剂结构如何影响界面溶剂化性质。鉴于表面介导的溶剂性质控制着界面溶质浓度和反应活性，确定界面溶剂化的特征尺寸对于构建溶液相表面化学的预测模型至关重要。在此，我们使用专门合成的作为“分子尺”的溶剂致变色表面活性剂以及共振增强二次谐波产生来测量弱缔合和强缔合液-液界面的偶极宽度。偶极宽度描述了介电环境从一个相转变到另一个相所需的距离。我们的结果表明，在水-环己烷界面上，极性在亚纳米长度尺度上收敛到非极性极限。然而，在强缔合的水-1-辛醇界面上，极性主要由非极性的、类似烷烃的区域主导。这些数据对使用液体的连续介质描述来表征界面溶剂化提出了质疑，并表明界面环境与本体溶液极限相比可能以非加和的方式变化。
