Brandes Eva, Stage Christiane, Motschmann Hubert, Rieder Julian, Buchner Richard
Department of Physical and Theoretical Chemistry, University of Regensburg, Regensburg, Germany.
J Chem Phys. 2014 Nov 14;141(18):18C509. doi: 10.1063/1.4895969.
This contribution aims to elucidate the connection between ion-ion-solvent interactions in the bulk of aqueous electrolyte solutions and the properties of their liquid-air interface. In particular, we were interested in the conditions under which ion pairs form at the surface and whether this is linked to ion pairing in the bulk. For this reason different combinations of hard (Cl(-), Li(+)) and soft ions (I(-), Cs(+)) were investigated. Ion hydration and possible ion association in the bulk was probed with dielectric relaxation spectroscopy. This technique monitors the cooperative reorientation of the dipolar solvent molecules and detects all ion-pair species possibly present in the solution. At the interface, the formation of contact ion pairs was investigated by infrared-visible-sum frequency spectroscopy (SFG). This nonlinear optical technique possesses an inherent surface specificity and can be used for the characterization of interfacial water. The intensity of the SFG-active vibrational stretching modes depends on the number of oriented water molecules. The electric field at the surface of a charged aqueous interface aligns the water dipoles, which in turn increases the SFG response. Hence, the enhancement of the oscillator strengths of the water vibrational modes can be used to draw some conclusions on the strengths and geometrical extension of the electric field. The formation of ion pairs at the interface reduces the intensity of the band associated with hydrogen-bonded water. The underlying theory is presented. The combined data show that there are no contact ion pairs in the bulk of the fluid and--at best--only small amounts of solvent shared ion pairs. On the other hand, the combination of hard/hard or soft/soft ions leads to the formation of ion pairs at the liquid-air interface.
本论文旨在阐明水性电解质溶液主体中的离子-离子-溶剂相互作用与其液-气界面性质之间的联系。特别地，我们关注表面形成离子对的条件，以及这是否与主体中的离子配对有关。因此，我们研究了硬离子（Cl(-)、Li(+)）和软离子（I(-)、Cs(+)）的不同组合。通过介电弛豫光谱探测主体中的离子水合作用和可能的离子缔合。该技术监测偶极溶剂分子的协同重排，并检测溶液中可能存在的所有离子对物种。在界面处，通过红外-可见和频光谱（SFG）研究接触离子对的形成。这种非线性光学技术具有固有的表面特异性，可用于表征界面水。SFG活性振动拉伸模式的强度取决于取向水分子的数量。带电水界面表面的电场使水偶极取向，进而增加SFG响应。因此，水振动模式振子强度的增强可用于对电场强度和几何扩展得出一些结论。界面处离子对的形成降低了与氢键水相关的谱带强度。文中介绍了相关基础理论。综合数据表明，流体主体中不存在接触离子对，充其量只有少量溶剂共享离子对。另一方面，硬/硬或软/软离子的组合会导致在液-气界面形成离子对。
