RESUMO
Platinum phases of general formula [Pt(n-), M+, MX] can be electrogenerated from cathodic polarization in dry dimethylformamide containing a supporting electrolyte, MX. The reaction of these electrogenerated Pt phases as reducing agent with aryldiazonium salts was investigated for preparing controlled metal-organic interfaces and characterizing the reactivity of the "reduced platinum phases". In a two-step process, the "reduced platinum phase" locally reacts with aryldiazonium salts, leading to the attachment of aryl groups onto the metal surface in the previously modified areas. Detailed experiments using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and in situ electrochemical atomic force microscopy (EC-AFM) were carried out to follow the reaction in solution with the example of NaI as supporting electrolyte (MX = NaI). These studies demonstrate the irreversible attachment of aryl groups onto the platinum electrode. Comparison between the direct electroreduction of aryldiazonium compounds (4-nitrophenyl- and 4-bromophenyldiazonium) on a platinum electrode and their reaction with [Pt2-, Na+, NaI] suggests that a similar general mechanism is responsible for the grafting. However in the second case, no applied potential is required to stimulate the binding thanks to the reductive properties of [Pt2-, Na+, NaI]. Competitive reduction of the organic layer and growth of the layer were observed and analyzed as a function of the injected charge used to initially produce [Pt2-, Na+, NaI]. Similar reactions are highly probable with other MX salts owing to the redox properties observed for this type of platinum phase ([Pt(n-), M+, MX]).
RESUMO
Three polypyrrole-silica nanocomposites were characterized by inverse gas chromatography (IGC) at 60 and 80degreesC using both linear and branched alkane probes. The IGC data for these materials were compared with those obtained for the ultrafine silica sol and polypyrrole (PPy) bulk powder reference materials. The London components of the surface energies (gammasd) of the nanocomposites were in the range 115-225 mJ m-2 at 60degreesC. These values are much higher than those obtained for either the polypyrrole powders or the colloidal silica sol, which suggests that the nanocomposites have significant microporosity. The gammasd values for the nanocomposites decrease in the order chloride-doped PPy > sulfate-doped PPy > tosylate-doped PPy, which is the same trend as that found for the heat of adsorption (DeltaHa) of a given n-alkane. DeltaHa values of the linear and branched alkanes are up to three times higher than the heats of vaporization, which confirms that the nanocomposites (and, to a lesser extent, the polypyrrole powders) have high adsorption capacities for these solutes. For a series of alkane isomers (e.g., C7H16 or C8H18), the difference between the DeltaHa values for the nanocomposites and the corresponding polypyrrole powders decreased as the degree of branching for these probes was increased. These observations suggest that the more sterically hindered probes are excluded from the microporous interior of the nanocomposites. Copyright 1997 Academic Press. Copyright 1997Academic Press
