Cowen Scott, Al-Abadleh Hind A
Department of Chemistry, University of Guelph, Guelph, Ontario, Canada.
Phys Chem Chem Phys. 2009 Sep 28;11(36):7838-47. doi: 10.1039/b905236d. Epub 2009 May 27.
Humic like substances (HULIS) are important components of atmospheric aerosols, yet little is known about their photochemical transformation and the role of adsorbed water in this photochemistry. We report herein in situ and surface-sensitive spectroscopic studies on (1) the photodegradation of solid tannic acid, (2) structure of adsorbed water before and after photodegradation, and (3) the change in the hydrophilicity of tannic acid as a result of this photochemistry. Tannic acid (TA) was chosen as a synthetic proxy for HULIS because it has a defined molecular structure. Photochemical studies were conducted using diffuse reflectance infrared spectroscopy (DRIFTS) as a function of time (3 h), relative humidity (5-30%) and total irradiance (7, 20, 290 W m(-2) at 555 nm). Water adsorption isotherm measurements were recorded before and after photodegradation, which provided information on the structure of interfacial water and the thermodynamics of adsorption. The structure of water adsorbed on TA resembles that of water at the interface with polar organic solvents. Difference spectral data collected during irradiation shows loss features in the 1700-1000 cm(-1) range and growth in carbonyl features that are blue shifted relative to the starting material, suggesting oxidative photodegradation of TA and formation of aryl aldehydes. Under our experimental conditions, we observed no enhancement in water uptake after photodegradation relative to that on unirradiated samples. The implications of our results to the understanding of heterogeneous photochemistry of HULIS and the role of adsorbed water in these reactions are discussed.
类腐殖质（HULIS）是大气气溶胶的重要组成部分，但人们对其光化学转化以及吸附水在这种光化学过程中的作用知之甚少。我们在此报告了关于以下方面的原位和表面敏感光谱研究：（1）固体单宁酸的光降解；（2）光降解前后吸附水的结构；（3）这种光化学作用导致的单宁酸亲水性变化。选择单宁酸（TA）作为HULIS的合成替代物，因为它具有明确的分子结构。使用漫反射红外光谱（DRIFTS）作为时间（3小时）、相对湿度（5 - 30%）和总辐照度（555 nm处7、20、290 W m(-2)）的函数进行光化学研究。在光降解前后记录水吸附等温线测量结果，这提供了关于界面水结构和吸附热力学的信息。吸附在TA上的水的结构类似于与极性有机溶剂界面处的水的结构。辐照期间收集的差分光谱数据显示，在1700 - 1000 cm(-1)范围内有损失特征，并且羰基特征相对于起始材料发生蓝移而增加，表明TA发生了氧化光降解并形成了芳醛。在我们的实验条件下，我们观察到光降解后相对于未辐照样品，水的吸收没有增强。讨论了我们的结果对理解HULIS的非均相光化学以及吸附水在这些反应中的作用的影响。
