RESUMO
Vapor pressure and aqueous solubility are important parameters used to estimate the potential for transport of chemical substances in the atmosphere. For fluorotelomer alcohols (FTOHs), currently under scrutiny by environmental scientists as potential precursors of persistent perfluorocarboxylates (PFCAs), vapor pressure is the more significant property since these compounds are only very sparingly soluble in water. We have measured the vapor pressures of a homologous series of fluorotelomer alcohols, F(CF2CF2)nCH2CH2OH (n = 2-5), in the temperature range 21-250 degrees C by three independent methods: (a) a method suitable for very low vapor pressures at ambient temperatures (gas-saturation method), (b) an improved boiling point method at controlled pressures (Scott method), and (c) a novel method, requiring milligram quantities of substance, based on gas-phase NMR, a technique largely unfamiliar to chemists and holding promise for studies of relevance to environmental chemistry. The concordant values obtained indicate that recently published vapor pressure data overestimate the vapor pressure at ambient temperature, and therefore the volatility, of this series of fluorinated compounds. It was suggested that substantial intramolecular -O-H...F- hydrogen bonding between the hydroxylic proton and the two fluorines next to the ethanol moiety was responsible for their putative high volatility. Therefore, we have used gas-phase NMR, gas-phase FTIR, 2D NMR heteronuclear Overhauser effect measurements, and high-level ab initio computations to investigate the intramolecular hydrogen bonding in fluorotelomer alcohols. Our studies unequivocally show that hydrogen bonding of this type is not significant and cannot contribute to and cause unusual volatility. The substantially lower vapor pressure at ambient temperatures than previously reported resulting from our work is important in developing a valid understanding of the environmental transport behavior of this class of compounds.
RESUMO
Carbon nanotubes are man-made one-dimensional carbon crystals with different diameters and chiralities. Owing to their superb mechanical and electrical properties, many potential applications have been proposed for them. However, polydispersity and poor solubility in both aqueous and non-aqueous solution impose a considerable challenge for their separation and assembly, which is required for many applications. Here we report our finding of DNA-assisted dispersion and separation of carbon nanotubes. Bundled single-walled carbon nanotubes are effectively dispersed in water by their sonication in the presence of single-stranded DNA (ssDNA). Optical absorption and fluorescence spectroscopy and atomic force microscopy measurements provide evidence for individually dispersed carbon nanotubes. Molecular modelling suggests that ssDNA can bind to carbon nanotubes through pi-stacking, resulting in helical wrapping to the surface. The binding free energy of ssDNA to carbon nanotubes rivals that of two nanotubes for each other. We also demonstrate that DNA-coated carbon nanotubes can be separated into fractions with different electronic structures by ion-exchange chromatography. This finding links one of the central molecules in biology to a technologically very important nanomaterial, and opens the door to carbon-nanotube-based applications in biotechnology.