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Time-Resolved Studies of Organic Monolayer Reactivity

 

Natural air-water interfaces have an impact on the global climate. Processes such as air-sea gas exchange are mediated by the marine surface microlayer or other surfactant layers. These thin films of organic surface-active substances alter the physical and chemical properties of aqueous aerosols and the ocean surface itself. Moreover, in natural systems all surfaces are subject to chemical aging (e.g., atmospheric oxidation by ozone or OH radicals, photochemistry). Hence, the surface composition, the surfactant characteristics, and the physical properties of natural water-air interfaces are constantly changing.

 

langmuirtroughVibrational sum-frequency generation (VSFG) spectroscopy is a surface sensitive method, which has been widely used to study the static structure of organic monolayers on a submonolayer scale. To establish VSFG spectroscopy as a quantitative and time-resolved detection tool for surface reaction kinetics at liquid interfaces, we combine VSFG spectroscopy with a Langmuir trough setup. In this configuration, changes of the surface concentration of the surfactants can be followed both by VSFG spectral intensity and surface pressure. Moreover, the 2D phase behavior of the monolayer can be directly related to molecular orientation effects.

 

 

sfg_kinetics2_webAs a next step, we investigate the ozonolysis of monolayers of unsaturated fatty acids using a small hemispherical reactor. Fatty acids serve as model system for natural degradation processes of surfactant layers. Here, we try to answer the overarching research question "What determines the reactivity of organic monolayers at the water-air interface?". Recently, the reactor was equipped with a fiber-coupled solar simulator lamp allowing us to investigate the photochemistry of surfactants in future experiments. 

 

In order to perform quantitative surface concentration measurements, careful calibration of the VSFG signal from the surfactant monolayer has to be performed at different surface concentrations. Relying on this calibration experiments, changes of the surface concentration upon ozone induced surface oxidation or photolysis is measured and the resulting concentration-time profiles are evaluated to extract rate constants for the heterogeneous surface reaction rate constant.

 

oavstimeFor example, oleic acid (OA) ozonolysis has been studied as a proxy for unsaturated surfactants in atmospheric aerosols. Our kinetic results are consistent with a reactive ozone uptake coefficient of γ ~ 10−6. This is in agreement with recent monolayer studies based on other surface sensitive techniques, but contradicts older OA droplet experiments reporting γ ~ 10−4. Obviously, many older studies have been biased by bulk reactivity and hence did not measure the surface process alone.

Currently, in order to investigate structure-reactivity relationships for heterogeneous O3 oxidation, we perform systematic studies on several fatty acids with shifted double-bond position in the alkyl chain. Interestingly, a distinct reactivity trend has been found. Further experiments assisted with molecular dynamics simulations will be performed to better asses the roles of surface accommodation (resulting in ozone adsorption layer)s, ozone solubilities, and different ozone permeabilities through monolayers with variable surface densities.

 

Contributing researchers: A. Dabrowski, F. Lange, G. Friedrichs and (formerly) J. Kleber, K. Laß

 


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Last Updated on Monday, 09 January 2017 09:55
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