Schultz Mary Jane, Baldelli Steve, Schnitzer Cheryl, Simonelli Danielle
Department of Chemistry, Pearson Lab., Tufts University, Medford, Massachusetts 02155.
J Phys Chem B. 2002 May 30;106(21):5313-5324. doi: 10.1021/jp014466v.
An important issue for developing a molecular-level mechanism of heterogeneous interactions at the aqueous interface is determining changes in the interface with changes in the bulk composition. Development of the nonlinear spectroscopy, sum frequency generation (SFG) provides a technique to probe these changes. Several molecular and ionic solutes have been used to investigate changes in the structure of the aqueous interface. Molecular solutes include glycerol and ammonia. Ionic and associated ion complexes include sulfuric acid as well as alkali sulfate and bisulfate salts. Molecular solutes and associated ion complexes penetrate to the top monolayer of the aqueous-air interface displacing water from the interface. Specifically, the conjectured ammonia-water complex is observed with ammonia tilted, on average, 25-38° from the normal. Ionic solutes generate a double layer in the interfacial region due to the differential distribution of anions and cations near the interface. The strength of the double layer is dependent on ion size and charge. Due to the extreme size of the proton, the strongest field is generated by acidic solutes. As the ionic solute concentration increases, associated ion pairs form and these penetrate to the top monolayer. These results have wide implications because the aqueous interface is ubiquitous in atmospheric and biological systems.
对于在水界面处开发异质相互作用的分子水平机制而言，一个重要问题是确定界面随本体组成变化的情况。非线性光谱学——和频产生（SFG）的发展提供了一种探测这些变化的技术。几种分子和离子溶质已被用于研究水界面结构的变化。分子溶质包括甘油和氨。离子及相关离子络合物包括硫酸以及碱金属硫酸盐和硫酸氢盐。分子溶质和相关离子络合物渗透到水 - 空气界面的顶层单分子层，将水从界面置换出来。具体而言，推测的氨 - 水络合物被观察到，氨平均相对于法线倾斜25 - 38°。离子溶质由于界面附近阴离子和阳离子的差异分布在界面区域产生一个双层。双层的强度取决于离子大小和电荷。由于质子的尺寸极小，酸性溶质会产生最强的场。随着离子溶质浓度增加，相关离子对形成并渗透到顶层单分子层。这些结果具有广泛的意义，因为水界面在大气和生物系统中无处不在。
