RESUMEN
The selectivity of adsorption of chiral surfactants to a chiral monolayer at the solid-liquid interface was studied using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). One enantiomer of the chiral surfactant was deuterated, which causes a change in the IR absorption frequency, and allows independent measurement of the adsorption of each molecule. Both the surfactant, N-lauroyl phenylalanine (NLP), and the chiral monolayer, N-L-phenylalaninoyl, 11-undecyl-silicon, were amino acid derivatives. An enantiomeric excess of 56 +/- 22% of the L over D was observed for adsorption to the interface between a carbon tetrachloride solution containing a quasi-racemate of N-lauroyl phenylalanine and the N-L-phenylalaninoyl, 11-undecyl monolayer film on silicon. In contrast, equimolar adsorption occurred from an equimolar mixture of hydrogenated and deuterated forms of the L surfactant. The measured enantiomeric excess strongly depended on the density of chiral surface groups: the higher the density of chiral groups on the surface, the better the enantiodiscrimination, even though the total adsorption was roughly constant. This nonlinear behavior indicates that more than one chiral surface group is required for significant selectivity.
Asunto(s)
Aminoácidos/química , Tensoactivos/química , Adsorción , Membranas Artificiales , Silicio/química , Espectroscopía Infrarroja por Transformada de Fourier , Estereoisomerismo , Propiedades de SuperficieRESUMEN
Several recent papers have described the existence of stable nanobubbles in bulk, which is surprising given that the high curvature of these bubbles is expected to place such bubbles under a high pressure and therefore lead to rapid dissolution. Here, we investigate the possible existence of nanobubbles in mixtures of water plus an organic solvent using both static and dynamic light scattering and infrared spectroscopy. The mixing of solvents was designed to introduce nanobubbles into bulk solution via supersaturation of the solution. The solutions scatter light for a long period (days) after mixing, which is consistent with the formation of nanoscale objects, but we show that these scattering objects originate from water-insoluble impurities in the organic solvents. Our results are inconsistent with the presence of gas nanobubbles in bulk solution: Degassing the solutions, either before or after mixing, has a minimal effect on the scattering, and purification of the organic solvent before mixing reduces the scattering after mixing. Therefore, previous reports of nanobubbles based on scattering experiments should be reconsidered with the hypothesis that the scattering objects are not actually gaseous.
RESUMEN
The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.