Department of Chemistry, University of California, Irvine, California 92697-2025, USA.
J Phys Chem A. 2009 Mar 12;113(10):2060-9. doi: 10.1021/jp808710n.
A combination of experiments and molecular dynamic (MD) simulations has been applied to elucidate the nature of water on organic self-assembled monolayers (SAMs) before and after oxidation. SAMs mimic organics adsorbed on environmental urban surfaces. Water on clean or SAM-coated borosilicate glass surfaces was measured at equilibrium as a function of relative humidity (RH), using transmission Fourier transform infrared (FTIR) spectroscopy at 1 atm and 22 +/- 1 degrees C. The SAMs included C18 and C8 alkanes, as well as the C8 terminal alkene. Oxidation of the terminal alkene SAM was carried out with either KMnO(4) solution or gaseous O(3). The FTIR data showed at least two distinct peaks due to water on these surfaces, one at approximately 3200 cm(-1), which dominates at low RH (20%), and one at approximately 3400 cm(-1) at high RH (80%), which is similar to that in bulk liquid water. Temperature-programmed desorption (TPD) experiments showed that oxidation leads to more strongly adsorbed water. However, the amount of water in equilibrium with water vapor on the oxidized alkene was not significantly different from that on the unoxidized SAM, although there was a change in the relative intensities of the two contributing infrared peaks at 80% RH. MD simulations with hydrogen bond analysis suggest that molecules on the surface of small water clusters that dominate on SAM surfaces at low RH have fewer hydrogen bonds, while those in the interior of the clusters have three and four hydrogen bonds similar to bulk liquid water. Taken together, the experimental infrared data and MD simulations suggest a correlation between the relative intensities of the 3200 cm(-1)/3400 cm(-1) bands and the hydrogen-bonding patterns of the water on the surface and in the interior of clusters on the SAM surfaces. These studies suggest that water clusters will be present even on hydrophobic surfaces in the atmosphere and hence are available to participate in heterogeneous chemistry. In addition, oxidation of organic coatings on atmospheric particles or surfaces in the boundary layer may not lead to enhanced water uptake as is often assumed.
已经应用实验和分子动力学（MD）模拟来阐明有机自组装单层（SAM）氧化前后水的性质。SAM 模拟环境城市表面上吸附的有机物。在平衡状态下，使用传输傅里叶变换红外（FTIR）光谱法在 1 个大气压和 22 ± 1°C 下，测量清洁或 SAM 涂覆的硼硅酸盐玻璃表面上水作为相对湿度（RH）的函数。SAM 包括 C18 和 C8 烷烃，以及 C8 末端烯烃。使用 KMnO(4)溶液或气态 O(3)氧化末端烯烃 SAM。FTIR 数据显示，由于这些表面上水的存在，至少有两个不同的峰，一个在大约 3200 cm(-1)处，在低 RH（20%）下占主导地位，一个在大约 3400 cm(-1)处在高 RH（80%）下，类似于在块状液态水中的情况。程序升温脱附（TPD）实验表明氧化导致更强烈吸附的水。然而，与氧化烯 SAM 上的水蒸气平衡的水量与未氧化 SAM 上的水量没有显著差异，尽管在 80%RH 时两个贡献红外峰的相对强度发生了变化。氢键分析的 MD 模拟表明，在低 RH 下主导 SAM 表面的小水分子簇表面上的分子氢键较少，而在簇内部的分子具有三个和四个氢键，类似于块状液态水。综合起来，实验红外数据和 MD 模拟表明，相对强度 3200 cm(-1)/3400 cm(-1)带与 SAM 表面上水的氢键模式之间存在相关性。这些研究表明，即使在大气中的疏水性表面上也会存在水分子簇，因此可以参与非均相化学。此外，边界层中大气粒子或表面上有机涂层的氧化可能不会像通常假设的那样导致水的吸收增加。
