Effect of tetraalkylammonium cations on gas coalescence at a hydrogen-evolving microelectrode.

Hydrogen gas evolution at the surface of a microelectrode may result in periodic release of single bubbles larger than the electrode diameter. Bubbles often grow by incorporating smaller bubbles that coalesce with them. To explore the coalescence, we investigate how a series of six tetralkylammonium cations (TXA(+)), where the number of carbons on the alkyl chain varies from 1 to 6, affects the oscillatory behavior of the gas-evolving microcathode. Different concentrations of TXA(+) bromide salts ranging from a few micromolar up to 1 M were added in the acid electrolyte. The frequency of bubble release and the transition from periodic to aperiodic release are related to the inhibition of bubble coalescence and gas streaming. The concentration range where this transition occurs depends strongly on the cation hydrophobicity and it ranges from very small values for the hydrophobic cations to over 1 M for the most hydrophilic one. For some of the TXA(+) cations, the transition shows a smooth increase in release frequency before switching completely to bubble-stream behavior, while for others the transition is abrupt. A smooth increase in the gas oscillator frequency with concentration indicates that the adsorption of TXA(+) cations on the bubble surface is mass transport-limited. The inhibition of bubble coalescence by the smallest cations is electrochemically driven, facilitated by specific interactions established between the ions and the electrode surface.

Influence of growth conditions on magnetite nanoparticles electro-crystallized in the presence of organic molecules.

Magnetite nanoparticles were synthesized by electrocrystallization in the presence of thiourea or sodium butanoate as an organic stabilizer. The synthesis was performed in a thermostatic electrochemical cell containing two iron electrodes with an aqueous solution of sodium sulfate as electrolyte. The effects of organic concentration, applied potential and growth temperature on particle size, morphology, structure and magnetic properties were investigated. The magnetite nanoparticles were characterized by X-ray diffraction, electron microscopy, magnetometry and Mössbauer spectrometry. When the synthesis is performed in the presence of sodium butanoate at 60 °C, a paramagnetic ferric salt is obtained as a second phase; it is possible to avoid formation of this phase, increase the specific magnetization and improve the structure of the oxide particles by tuning the growth conditions. Room-temperature magnetization values range from 45 to 90 Am2kg-1, depending on the particle size, type of surfactant and synthesis conditions. Mössbauer spectra, which were recorded at 290 K for all the samples, are typical of nonstoichiometric Fe3-δO4, with a small excess of Fe3+, 0.05 ≤ δ ≤ 0.15.

Supramolecular ion-pair interactions to control monolayer assembly.

We demonstrate that noncovalent ion-pair interactions in solution can be employed to control the molecular spacing of thiols in a self-assembled monolayer (SAM) on gold. Ion-pairs formed between the carboxylate tail-group of 16-mercaptohexadecanoic acid (MHA) and tetraalkylammonium (TAA+) hydroxide salts of various alkyl side-chain lengths remain intact during chemisorption of the thiol on gold. The resulting ion-pair SAMs exhibit a 1:1 molar ratio of MHA:TAA+ on the surface and are covalently bound to the gold surface through the thiol headgroup of MHA. We hypothesize that the incorporation of the bulky TAA+ group competes with the strong tendency of the thiols to organize into an ordered monolayer, which highlights the strength of the ion-pair complexes. The ion-pair films can be converted into a loosely packed MHA monolayer by rinsing the SAM with a solution of potassium perchlorate, which releases the TAA+ from the surface. Contact angle measurements and X-ray spectroscopy (XPS) confirm the stoichiometry and covalent attachment of the monolayers. XPS analysis and contact angle measurements indicate that the surface density of bound MHA decreases with increasing size of the TAA+ cation. These results suggest that steric hindrance created by the bulky side-chains of the TAA+ cation dictates the lateral spacing of MHA chains on the surface.

Electrochemical study of a dendritic family at the water/1,2-dichloroethane interface.

The transfer of six dendritric molecules, DMs, across the water/1,2-dichloroethane interface was investigated using cyclic voltammetry. From the variation of peak potential with pH, two different mechanisms of transfer were postulated depending on the nature of the molecules. Voltammetric parameters were employed to evaluate the hydrophilic/hydrophobic character and calculate the acid dissociation constant of these molecules. The results were explained taking into account the nature and multiplicity of functional surface groups.