Vibrational Sum Frequency Generation (VSFG) is a surface sensitive nonlinear laser spectroscopic technique, which has been widely used in physics and physical chemistry to investigate interface processes and heterogeneous chemistry. As the technique advances from basic to applied research, we have assessed its suitability for investigations of natural air-water interfaces, which are typically covered with an organic nanolayer. Vibrational spectra of natural sea surface nanolayer samples are presented. Strong signals attributable to the alkyl chains of lipids and indications for more or less soluble carbohydrate material have been found. Furthermore, the use of VSFG spectroscopy as a time-resolved detection tool for monitoring surface layer formation and its reactive decay is demonstrated. Ozone-induced film oxidation served as an example. We successfully applied VSFG to characterize the efficiency of a surface water screen sampler as well. Overall, VSFG spectroscopy turns out to be a versatile tool that can provide valuable information about composition, structure, chemical reactivity, and film formation dynamics of the sea surface nanolayer.*Corresponding author: E-mail: friedrichs@phc.uni-kiel.de
AcknowledgmentsWe are indebted to the people from the Boknis Eck project at IFM-GEOMAR for the opportunity to take samples during their sample cruises, and to captain and crew of the R/V Littorina for their support during the cruises. Thanks to Friedrich Temps and Doug Wallace for helpful discussions. Financial support by the German Science Foundation (DFG -EC 80) in the framework of the cluster of excellence The Future Ocean is gratefully acknowledged.
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.
In situ separation of a chiral target compound is realized for the first time as distillate on top of an integrated biocatalytic batch reactive distillation column. The applied reaction system is the racemic resolution of (R/S)-2-pentanol in a transesterification with propyl butyrate, which is catalyzed by immobilized Candida antarctica lipase B (Novozym435). Biocatalyst integration is realized in baskets of wire gauze catalytic packings as column internals. The equilibrium limited reaction is shifted to the product side by fractional distillation of low boiling propanol and the target compound (S)-2-pentanol. Increased molar fractions up to x (S)-2-PeOH = 65 ± 4% with an enantiomeric excess of 90 ± 4% and 51 ± 3% overall conversion are reached for (S)-2-pentanol. Feasibility of the reaction system is investigated in our preselection tool. This tool is developed to evaluate predefined operation conditions in the column setup for transesterification reactions based on criteria regarding biocatalyst stability and boiling point differences between the reactants. The subsequently selected reaction of (R/S)-2pentanol with propyl butyrate is successfully carried out in a batch reactive distillation column with in situ separation of our target compound (S)-2-pentanol. In contrast to current examples in the literature, this clearly demonstrates the possibility of one-pot target compound isolation within chiral synthesis applying biocatalytic reactive distillation.
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