Investigation of Sea Surface Nanolayer Properties and Photochemistry Using Modern Laser Spectroscopic and Surface Analytical Techniques
At the interface between natural waters and the sea, an enrichment of surfaceactive substances can be observed. This has a strong impact on the physical and chemical properties of this system, including the gas exchange. Because of the high local concentration of different organic substances and, in comparison with the bulk, the high intensity of solar radiation, a large number of photochemically induced reactions take place within this interfacial zone. The scope of this thesis covers both the investigation of natural sea water surface samples and the development synthetic model systems to study such photochemical processes using surface-selective analytical techniques. Out of the different techniques used, Vibrational Sum Frequency Spectroscopy (VSFG) was employed as the primary experimental method. The first part of the thesis covers the investigation of natural water surface samples by means of VSFG spectroscopy for the semi-quantitative determination of surface-active matter abundance in the so-called Sea Surface Nanolayer (SNL), the direct molecular interface between air and sea. A set of software tools was implemented to allow for an automatized and systematic analysis of large amount of spectral data. These were used to re-examine the data obtained by monthly sampling of natural surface water collected at Boknis Eck time series station over ten years with respect to seasonal variation of the surfactant abundance as well as to correlate surfactant abundance with other oceanographic parameters. A seasonal variation of this SNL abundance through the year could be confirmed. The second part of the thesis describes the synthesis and characterization of a novel class of surface-active photosensitizer molecules based on benzoylbenzoic acid. It was possible to obtain three new substances not yet described in the literature by esterification of the acid with three symmetric diols (hexane, nonane and dodecane diol). Those compounds turned out to be very surface-active and yielded VSFG spectra of remarkable quality and showing pronounced carbonyl peaks. Upon irradiation with artificial sunlight from a xenon lamp, efficient autophotolysis was observed for those molecules, clearly proving their photochemical reactivity. To better mimic the surface of natural waters, in which typically only a small fraction of the interfacial molecules is susceptible to photochemistry, mixed layers of the novel sensitizers and naturally occurring fatty acids (myristic acid, palmitic acid and stearic acid) have been prepared and investigated by means of VSFG spectroscopy, LANGMUIR compression isotherms and simultaneous application of both methods. The mixture of the sensitizer BX12 and stearic acid was identified as the most promising candidate for a model system for future studies
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