1,721,027 research outputs found
Thermodynamic analysis of (bi)sulphate adsorption on a Pt(111) electrode as a function of pH
No full textA complete thermodynamic study of (bi)sulphate adsorption on Pt(1 1 1) electrodes from solutions at four different pHs (pH 0.43, 2.1, 3.1 and 4.1) is reported. The effect of pH on the sum of the Gibbs excesses of sulphate and bisulphate species, standard Gibbs energies of adsorption and formal partial charge numbers is analyzed. The results provide relevant information on the nature of species involved in the different voltammetric features. The experiments at pH 0.43 were performed in a higher base electrolyte concentration (0.5 M), that allows the study of (bi)sulphate adsorption in a broader range of concentrations. Under these conditions, two adsorption steps are clearly defined, associated to two different voltammetric features, between 0.30 and 0.60 V and between 0.65 and 0.90 V (standard hydrogen scale, SHE). Once the pH is increased, a marked decrease in absolute value of the (bi)sulphate adsorption Gibbs energy is observed, concomitant with an increasing amount of OH co-adsorptio
Layer-by-layer PMIRRAS characterization of DMPC bilayers deposited on a Au(111) electrode surface
No full textA combination of Langmuir-Blodgett and Langmuir-Schaefer techniques was employed to deposit 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers at a gold electrode surface. One leaflet consisted of hydrogen-substituted acyl chains, and the second leaflet was composed of molecules with deuterium-substituted acyl chains. This architecture allowed for layer-by-layer analysis of the structure of the bilayer. Photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to determine the conformation and orientation of the acyl chains of DMPC molecules in the individual leaflets as a function of the potential applied to the gold electrode. The bilayer is adsorbed onto the metal surface when the field applied to the membrane does not exceed 108 V/m. When adsorbed, the bottom leaflet is in contact with a hydrophobic metal surface, and the top leaflet is interacting with the aqueous solution. The asymmetry of the environment has an effect on the orientation of the DMPC molecules in each leaflet. The tilt angle of the acyl chains of the DMPC molecules in the bottom leaflet that is in contact with the gold is ~10° smaller than that observed for the top leaflet that is exposed to the solution. These studies provide direct evidence that the structure of a phospholipid bilayer deposited at an electrode surface is affected by interaction with the metal
Elucidation of the chemical nature of adsorbed species for Pt(111) in H2SO4 solutions by thermodynamic analysis
No full textThe nature of the adsorbed species for Pt(111) in sulfuric acid solutions has been elucidated by a careful thermodynamic analysis of the effect of pH on charge density data. This analysis takes advantage of the fact that, for solutions of constant total sulfate + bisulfate concentration, an increase of pH would increase the sulfate concentration, at the expense of decreasing the bisulfate concentration. As a result, sulfate adsorption would be shifted toward lower potentials, while bisulfate adsorption would follow the opposite trend. In the present work, coulostatic data for Pt(111) in (0.2 ? x) M Me2SO4 + x M H2SO4 (Me: Li, Na; x: 10?4 ? 0.2) and (0.1 ? x) M KClO4 + x M HClO4 + 10?3 M K2SO4 (x: 10?4 ? 0.1) solutions are carefully analyzed. It is concluded that sulfate rather than bisulfate adsorption takes place at potentials higher than the potential of zero charge. This result agrees with the fact that similar FTIRRAS bands for adsorbed sulfate species are observed for pH 0.8?3.5 in (0.2 ? x) M K2SO4 + x M H2SO4 solution
Thermodynamic evidence for K+–SO42? ion pair formation on Pt(111). New insight into cation specific adsorption
No full textThis work contributes to the understanding of cation specific effects on platinum electrochemistry by means of a thorough thermodynamic analysis of potassium adsorption on Pt(111) in sulfuric acid solutions. It is concluded that potassium specific adsorption is better described as the adsorption of the K+–SO2?4 ion pair. From the evaluation of the potassium sulfate concentration, it is found that potassium specific adsorption only takes place in the presence of coadsorbed sulfate species. Within the main sulfate adsorption state, for [similar]0.3 V < E < [similar]0.4 V (vs. SHE), the extent of potassium specific adsorption is small, reaching [similar]0.1 × 1014 species per cm2 for cK+ > 0.1 M. Then, at higher potentials, E > 0.55 V (vs. SHE), a second potassium adsorption process takes place, concomitant with the second sulfate adsorption process (associated to the small voltammetric feature called “the hump”). This last process involves the adsorption of an equal amount of potassium and sulfate species, leading to the adsorption of [similar]0.5 × 1014 ion pair species per cm2 ([similar]0.03 ion pair species per platinum surface atom). Furthermore, the results of the formal partial charge numbers corroborate that potassium adsorption involves sulfate cooperative coadsorption, in such a way that the effective adsorbing species is anionic, rather than cationic. In conclusion, this work evidences that cation specific effects may originate from the formation of surface ion pairs, which is probably related to the presence of ion pairs in solutio
Effect of the surface structure of gold electrodes on the coadsorption of water and anions
No full textThe potential-dependent water adsorption on gold surfaces in perchloric and sulfuric acid solutions has been studied by surface-enhanced infrared absorption spectroscopy (SEIRAS). It is found that the surface structure of the gold electrodes has a major impact on the SEIRAS spectra. When the gold films are composed of nanoparticles of 47 ± 11 nm, the SEIRAS spectra are in agreement with previous reports. However, when the size of the gold nanoparticles is decreased to 27 ± 8 nm, by depositing the gold at 1 Å/s instead of 0.1 Å/s, it is found that the SEIRAS bands associated with water molecules coordinated to coadsorbed anions are absent. The combination of both types of gold electrodes allows a detailed study of the properties of the adsorbed water molecules. It is found that water molecules coadsorbed with sulfate and perchlorate anions appear to belong to the hydration shell of the anions because (i) the intensity of the SEIRAS bands of these water molecules increase with potential in the same way as the SEIRAS bands of the adsorbed anions and (ii) the frequencies of the O–H stretch resemble those of the water molecules in the hydration shell of the anions in solutio
Self-promotion mechanism for CO electrooxidation on gold
No full textCO electrooxidation on Au(111), Au(100) and Au(110) electrodes in 0.1 M HClO4 and 0.1 M NaOH solutions has been studied by means of voltammetric measurements with hanging meniscus rotating bead-type electrodes . It is found that the reaction order in CO concentration is close to unity at potentials close to the onset of CO oxidation , and then it becomes higher than unity at higher potentials for all studied surfaces except for Au(111) in 0.1 M HClO4. This behavior indicates that CO oxidation on gold takes place through a self-promotion mechanism, in which the presence of CO on the gold surface enhances the adsorption of its own oxidant. Specifically, this mechanism offers a plausible explanation for the higher catalytic activity in alkaline solutions, since CO adsorption is stronger under these conditions, and thus it can lead to a higher enhancement of the adsorption of the oxidant specie
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Design of surface-modified electrodes for the electrochemical adsorption of Platinum-based anticancer drugs
Full text linkThe design of a hemofiltration method that minimizes the side effects produced by the accumulation of Pt-based drugs such as cisplatin represents an important technology for cancer treatment. This work establishes the foundation for the development of an efficient platform for the electrochemical hemofiltration and simultaneous sensing of antitumor drugs, such as cisplatin and carboplatin, from physiological medium. The electrochemical filtration system is based on a gold surface-modified electrode using tris(carboxyethyl)phosphine ligands as scavengers of the Pt-based drug complexes. The platinum-phosphine interaction is shown to be more time efficient, as compared to the naturally occurring form of a glutathione-platinum adduct, and displayed a highly cooperative nature of binding. Altogether, we demonstrate that the presence of ligands capable of binding to Pt results in a differential spectroscopic and electrochemical response and the potential to adsorb and monitor the concentration of Pt salts in complex aqueous media.</p
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