Quantitative time-resolved vibrational sum frequency generation spectroscopy as a tool for thin film kinetic studies: new insights into oleic acid monolayer oxidation.
Quantitative time-resolved vibrational sum frequency generation spectroscopy as a tool for thin film kinetic studies: new insights into oleic acid monolayer oxidation.
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DOI：
10.1021/jp404087s
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发表时间：
2013-08
期刊：
影响因子：
--
通讯作者：
Joscha Kleber;K. Lass;G. Friedrichs
中科院分区：
文献类型：
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作者：
Joscha Kleber;K. Lass;G. Friedrichs
环境中的气-水界面通常被表面活性有机物薄膜所覆盖，这些表面活性有机物在气-海气体交换和气溶胶老化中起着重要作用。表面灵敏振动和频产生（VSFG）光谱已被广泛用于研究静态结构的有机单分子膜作为这种薄膜的简单模型系统。可能是由于SFG信号强度与表面浓度的相关性的困难，相应的时间分辨的表面反应的研究是稀缺的。在这项研究中，定量的时间分辨测量已经进行了油酸单层臭氧分解，这被认为是一个基准系统，调查的反应性和命运的不饱和天然有机物。通过结合压力-面积等温线和VSFG光谱采集获得了表面浓度校准数据，从而可以适当地考虑油酸膜的2D相行为。与文献报道相反，发现表面活性氧化产物可以忽略不计，并且不干扰VSFG测量。对时间分辨数据进行准一级动力学分析，得到双分子速率常数k2（油酸+O3 →产物）=（1.65 ± 0.64）× 10（-16）cm（3）molecules（-1）s（-1），对应的吸收系数γ =（4.7 ± 1.8）× 10（-6）。这一结果是非常好的协议与最近的单层测量的基础上替代方法，并强调了时间分辨VSFG方法的可靠性。
Environmental air-water interfaces are often covered by thin films of surface-active organic substances that play an important role for air-sea gas exchange and aerosol aging. Surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy has been widely used to study the static structure of organic monolayers serving as simple model systems of such films. Probably due to the difficulties to correlate the SFG signal intensity with the surface concentration, corresponding time-resolved studies of surface reactions are scarce. In this study, quantitative time-resolved measurements have been performed on the oleic acid monolayer ozonolysis, which is considered a benchmark system for investigating the reactivity and fate of unsaturated natural organics. Surface concentration calibration data have been obtained by combining the pressure-area isotherm and VSFG spectra acquisition such that the 2D phase behavior of the oleic acid film could be properly taken into account. In contrast to literature reports, surface-active oxidation products were found to be negligible and do not interfere with the VSFG measurements. A pseudo-first-order kinetic analysis of the time-resolved data yielded a bimolecular rate constant of k2(oleic acid + O3 → products) = (1.65 ± 0.64) × 10(-16) cm(3) molecules(-1) s(-1), corresponding to an uptake coefficient of γ = (4.7 ± 1.8) × 10(-6). This result is in very good agreement with most recent monolayer measurements based on alternative methods and underlines the reliability of the time-resolved VSFG approach.