Department of Chemistry and Biochemistry, University of Maryland, College Park, College Park, MD 20742, USA.
Anal Bioanal Chem. 2009 Oct;395(4):1063-73. doi: 10.1007/s00216-009-2957-8. Epub 2009 Jul 28.
Resonance-enhanced, second harmonic generation (SHG) is used to measure the electronic structure of solutes sensitive to specific solvation adsorbed to liquid/liquid and liquid/solid interfaces. Here, specific solvation refers to solvent-solute interactions that are directional and localized. N-methyl-p-methoxyaniline (NMMA) is a solute whose first allowed electronic transition wavelength remains almost constant (approximately 315 nm) in non-hydrogen-bonding solvents regardless of solvent polarity. However, in hydrogen-bond-accepting solvents such as dimethylsulfoxide, NMMA's absorbance shifts to longer wavelengths (320 nm), whereas in hydrogen-bond-donating solvents (e.g., water), the absorbance shifts to shorter wavelengths (approximately 300 nm). SHG experiments show that at alkane/silica interfaces, surface silanol groups serve as moderately strong hydrogen-bond donors as evidenced by NMMA's absorbance of 307 nm. At the carbon tetrachloride/water interface, NMMA absorbance also shifts to slightly shorter wavelengths (298 nm) implying that water molecules at this liquid/liquid interface are donating strong hydrogen bonds to the adsorbed NMMA solutes. In contrast, experiments using newly developed molecular ruler surfactants with NMMA as a model hydrophobic solute and a hydrophilic, cationic headgroup imply that, as NMMA migrates across an aqueous/alkane interface, it carries with it water that functions as a hydrogen-bond-accepting partner.
共振增强的二次谐波产生（SHG）用于测量对吸附在液/液和液/固界面上的特定溶剂化的溶质的电子结构敏感的溶质。这里，特定溶剂化是指具有方向性和局部性的溶剂-溶质相互作用。N-甲基-p-甲氧基苯胺（NMMA）是一种溶质，其第一允许的电子跃迁波长在非氢键溶剂中几乎保持不变（约 315nm），无论溶剂极性如何。然而，在氢键接受溶剂（如二甲亚砜）中，NMMA 的吸收峰移向较长波长（320nm），而在氢键供体溶剂（如水）中，吸收峰移向较短波长（约 300nm）。SHG 实验表明，在烷烃/二氧化硅界面上，表面硅醇基团作为中等强度的氢键供体，这可以从 NMMA 的吸收峰为 307nm 中得到证明。在四氯化碳/水界面上，NMMA 的吸收峰也移向较短的波长（298nm），这意味着在该液/液界面上的水分子向吸附的 NMMA 溶质提供强氢键。相比之下，使用新开发的分子标尺表面活性剂进行的实验表明，当 NMMA 作为疏水溶质和亲水阳离子头基的模型时，当 NMMA 穿过水/烷烃界面时，它会携带作为氢键接受体的水。
