Solvation structure of hydroxyl radical by Car-Parrinello molecular dynamics

Car-Parrinello molecular dynamics simulations of a hydroxyl radical in liquid water have been performed. Structural and dynamical properties of the solvated structure have been studied in details. The partial atom-atom radial distribution functions for the hydrated hydroxyl do not show drastic differences with the radial distribution functions for liquid water. The OH is found to be a more active hydrogen bond donor and acceptor than the water molecule, but the accepted hydrogen bonds are much weaker than for the hydroxide OH- ion. The first solvation shell of the OH is less structured than the water's one and contains a considerable fraction of water molecules that are not hydrogen bonded to the hydroxyl. Part of them are found to come closer to the solvated radical than the hydrogen bonded molecules do. The lifetime of the hydrogen bonds accepted by the hydroxyl is found to be shorter than the hydrogen bond lifetime in water. A hydrogen transfer between a water molecule and the OH radical has been observed, though it is a much rarer event than a proton transfer between water and an OH- ion. The velocity autocorrelation power spectrum of the hydroxyl hydrogen shows the properties both of the OH radical in clusters and of the OH- ion in liquid.     

A Titration Microcalorimetric Study of the Effects of Halide Counterions on Vesicle-Forming Aggregation in Aqueous Solution of Branched-Chain Alkylpyridinium Surfactants

Titration microcalorimetry is used to study the influences of iodide, bromide, and chloride counterions on the aggregation of vesicle-forming 1-methyl-4-(2-pentylheptyl)pyridinium halide surfactants. Formation of vesicles by these surfactants was characterised using transmission electron microscopy. When the counterion is changed at 303 K through the series iodide, bromide, to chloride, the critical vesicular concentration (cvc) increases and the enthalpy of vesicle formation changes from exo- to endothermic. With increase in temperature to 333 K, vesicle formation becomes strongly exothermic. Increasing the temperature leads to a decrease in enthalpy and entropy of vesicle formation for all three surfactants. However the standard Gibbs energy for vesicle formation is, perhaps surprisingly, largely unaffected by an increase in temperature, as a consequence of a compensating change in both standard entropy and standard enthalpy of vesicle formation. Interestingly, standard isobaric heat capacities of vesicle formation are negative, large in magnitude but not strikingly dependent on the counterion. We conclude that the driving force for vesicle formation can be understood in terms of overlap of the thermally labile hydrophobic hydration shells of the alkyl chains. Copyright 2000 Academic Press.     

Effects of Adsorbed Polyaniline on Redox Process on MoO3 Surface

The effects exhibited by adsorbed conducting polyaniline on the redox process on a molybdenum oxide surface were studied. Thermogravimetric results indicate a 4% polyaniline deposition. Cyclic voltammograms of the adsorbed polymer on MoO3 show that polyaniline exerts remarkable effects on the molybdenum blue oxidation-reduction process, with oxidation and reduction potentials of 0.33 and 0.18 V, respectively. This effect strongly enhances the electrode response, and can be used as an important tool in qualitative and/or quantitative determinations of molybdenum in solution as well as in any substrate. Copyright 1999 Academic Press.