Walz M-M, Werner J, Ekholm V, Prisle N L, Öhrwall G, Björneholm O
Uppsala University, Department of Physics and Astronomy, Box 516, 75120 Uppsala, Sweden.
Phys Chem Chem Phys. 2016 Mar 7;18(9):6648-56. doi: 10.1039/c5cp06463e. Epub 2016 Feb 12.
Surface-active organic molecules at the liquid-vapor interface are of great importance in atmospheric science. Therefore, we studied the surface behavior of alcohol isomers with different chain lengths (C4-C6) in aqueous solution with surface- and chemically sensitive X-ray photoelectron spectroscopy (XPS), which reveals information about the surface structure on a molecular level. Gibbs free energies of adsorption and surface concentrations are determined from the XPS results using a standard Langmuir adsorption isotherm model. The free energies of adsorption, ranging from around -15 to -19 kJ mol(-1) (C4-C6), scale linearly with the number of carbon atoms within the alcohols with ΔGAds per -CH2-≈-2 kJ mol(-1). While for the linear alcohols, surface concentrations lie around 2.4 × 10(14) molecules per cm(2) at the bulk concentrations where monolayers are formed, the studied branched alcohols show lower surface concentrations of around 1.6 × 10(14) molecules per cm(2), both of which are in line with the molecular structure and their orientation at the interface. Interestingly, we find that there is a maximum in the surface enrichment factor for linear alcohols at low concentrations, which is not observed for the shorter branched alcohols. This is interpreted in terms of a cooperative effect, which we suggest to be the result of more effective van der Waals interactions between the linear alcohol alkyl chains at the aqueous surface, making it energetically even more favorable to reside at the liquid-vapor interface.
液-气界面处的表面活性有机分子在大气科学中具有重要意义。因此，我们利用对表面和化学敏感的X射线光电子能谱（XPS）研究了不同链长（C4 - C6）的醇异构体在水溶液中的表面行为，该技术能在分子水平上揭示表面结构信息。使用标准的朗缪尔吸附等温线模型，从XPS结果中确定吸附的吉布斯自由能和表面浓度。吸附自由能范围约为 -15至 -19 kJ mol⁻¹（C4 - C6），与醇中碳原子数呈线性关系，每-CH₂-的ΔGAds≈ -2 kJ mol⁻¹。对于直链醇，在形成单分子层的本体浓度下，表面浓度约为每平方厘米2.4×10¹⁴个分子，而所研究的支链醇表面浓度较低，约为每平方厘米1.6×10¹⁴个分子，这两者均与分子结构及其在界面处的取向一致。有趣的是，我们发现直链醇在低浓度时表面富集因子存在最大值，而较短的支链醇未观察到这一现象。这可以用协同效应来解释，我们认为这是由于直链醇烷基链在水表面之间更有效的范德华相互作用导致的，使得它们在液-气界面处的存在在能量上更有利。
