E. V. Stenina

Polarographic maxima of the third kind. III

Abstract The polarographic maxima of the third kind in solutions of 2-oxoadamantane and of pelargic acid (C 8 H 17 COOH) were investigated. The behavior of 2-oxoadamantane is similar to that of other adamantane and camphor derivatives. The surface activity of pelargic acid is markedly enhanced in 2 M MgSO 4 solutions, however a polarographic maximum of the third kind is observed in the neighborhood of the cathodic desorption potential only. The appearance of the polarographic maxima of the third kind seems to be related to the faculty of the surfactant to form under equilibrium conditions two-dimensional condensed layers on the mercury drop surface.

Some properties of two-dimensional condensed layers formed by some terpenoids and adamantane derivatives at the mercury-electrolyte interface

Abstract Adsorption data for two-dimensional condensed layers of some terpenoids and adamantane derivatives are discussed. Capacity-potential curves at the pit edge in the negative potential range were used to investigate the temperature dependence of the two-dimensional condensation of these substances. The experimental data were treated using the Frumkin adsorption model, the quasi-chemical approximation and the lattice gas model (the Ising model). It was found that the adsorption of the organic substances investigated can best be described by the Ising model, i.e. by taking the short-range order of the adsorbed molecules into account. It was also shown that the structure of the spherical organic molecule strongly affects the compactness and the temperature stability of the condensed layer.

Enhancement of the Carbon Nanowall Film Capacitance. Electron Transfer Kinetics on Functionalized Surfaces

The effects of electrochemical oxidation and surfactant adsorption on behavior of vertically oriented carbon-nanowall (CNW)-based electrodes are studied. Electrochemical oxidation is carried out by the electrode polarization in aqueous solutions at high anodic potentials corresponding to water electrolysis, whereas the modification of surface by surfactants is accomplished by the adsorption of molecules characterized by the cage-like structure. Using the methods of cyclic voltammetry and impedancemetry, it is shown that a substantial increase in the capacitance of CNW-based electrodes is observed in both cases (30-50-fold and 3-5-fold, respectively). The as-grown and modified electrodes are characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. A substantial increase in a number of oxygen-containing functional groups is observed on the CNW surface after the electrode polarization at high anodic potentials. The kinetics of redox reactions on the CNW film surface is studied by comparing the behavior of systems [Ru(NH3)6](2+/3+), [Fe(CN)6](4-/3-), Fe(2+/3+), and VO3(-)/VO(2+). It is demonstrated that oxidation of nanowalls makes the electron transfer in the redox reaction VO3(-)/VO(2+) and the redox system Fe(2+/3+) considerably easier due to coordination of discharging ions of these systems with the functional groups; however, no such effect is observed for the redox-systems [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+).

Coadsorption of ions and organic molecules

Abstract Within the framework of the model of a dense part of an electrical double layer as three parallel capacitors coupled with a set of Frumkin adsorption isotherms, equations are derived for simulating coadsorption of two solution components (neutral and ionic species). The adsorption behavior of model systems n-C4H9OH+NaCl (A) and 1-AdOH+NaCl (B) is analyzed and the results of estimation are compared with experimental data. It is concluded that, from the height of anodic capacitance peak (for the system A) and the shape of capacitance curve near the anodic phase transition potential (for the system B), certain conclusions can be drawn regarding the value of lateral interaction between surface-active ions and organic molecules, if the parameters of their individual adsorption are known. A general agreement is found between experimental results and simulated ones.

Adsorption of compounds with the skeleton molecular structure from dimethylsulfoxide solutions on mercury electrode

The adsorption of adamantane (Ad), adamantanol (AdOH), thiocamphor (TC), and a supramolecular complex (cryptate) of sodium ion [Na+ ⊂ 2.2.2.] from DMSO solutions on the mercury electrode is studied by the differential capacitance method. In the considered systems, the surfactants exhibit the high surface activity, which manifests itself in different ways depending on the potential scan direction. For AdOH, TC, and [Na+ ⊂ 2.2.2.] that have either a dipole moment or an electrostatic charge, it is assumed that the important role is played by the adsorbate-solvent interaction at the interface, which can be the key factor determining the formation of a new adsorption layer structure in the positive potential range. The adsorption behavior of the mentioned group of surfactants radically differs from that of Ad hydrocarbon, namely, the adsorption of the latter is not accompanied by the formation of a new adsorption layer structure. The obtained results suggest that for the adsorption of surfactants from nonaqueous solvents (in contrast to aqueous solutions), the interaction between the adsorbate and the solvent molecules, which under certain condition results in the formation of two-dimensional supramolecular structures at the electrode/solution interface, acquires substantial importance.

Electrochemical modification of electrodes based on highly oriented carbon nanowalls

The original and modified vertically oriented carbon nanowalls (CNWs) were applied onto conducting substrates by the plasma-chemical method. Their electrochemical behavior was studied by the methods of cyclic voltammetry and impedance measurements. The modified and original electrodes were characterized by using the methods of scanning and transmitting electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The nanowalls were modified with the functional groups (FG) via the electrolysis of aqueous solutions at the anodic potentials. Their adsorption properties were studied in the solutions of organic surfactants with the skeleton structure. It is shown that, in the first case, the number of oxygen-containing FG on the CNW surface significantly increases and, in both cases, the electrode capacitance considerably increases (by 30–50 and 3–5 times, respectively). A correlation between the rate constants k0 of [Ru(NH3)6]2+/3+, [Fe(CN)6]4–/3–, and Fe2+/3+ redox reactions and a degree of nanowall surface functionalization is revealed. The values of k0 were estimated in the automatic mode using a specially developed program by comparing the potential differences between the peaks of cyclic voltammograms ΔE, which were measured in a wide range of potential scan rate v, and the calculated ΔE (k0, v) dependences, which were obtained by solving the corresponding diffusion equations. It is shown that the functionalization of CNWs leads to a substantial (by ~103 times) increase in k0 for the Fe2+/3+ redox system and has almost no effect on the electron transfer in the [Fe(CN)6]3–/4– and [Ru(NH3)6]2+/3+ systems.

Adsorption characteristics of electrodes containing nanostructured carbon of different morphology

The effect of camphor adsorption on the differential capacitance of electrodes of nanostructured carbon of different morphology (single-walled carbon nanotubes, filiform carbon, and columnar structures) in aqueous electrolyte solutions and also on the electrochemical reactions in these systems is studied. It is shown that irrespective of the ac frequency, the differential capacitance of the nanopaper and columnar electrodes increases 3–5-fold throughout the studied potential range. This experimental fact is explained by the substantial increase in the electrode surface accessible for electrolyte, which is a manifestation of the Rehbinder effect in electrochemistry. The revealed different kinds of effects of camphor adsorption layers formed at the nanostructured carbon/electrolyte interface on the electron transfer processes are as follows: partial inhibition of both the electron injection and the K3[Fe(CN)6] reduction; complete suppression of the reduction of sodium nitrate and nitrite; the absence of effects on the OH radical reduction and solvated electron oxidation.

Electrochemical Behavior of Electrodes Containing Nanostructured Carbon of Various Morphology in the Cathodic Region of Potentials

Voltammograms for electrodes containing nanostructured carbon of various morphology (single-walled carbon nanotubes, filament, columnar structures) are obtained in neutral aqueous electrolytic solutions. Experimental proofs for the existence of injection of solvated electrons into electrolytic solutions at moderate cathodic potentials are presented for all the electrodes. It is established that this effect is connected with the presence of atomically sharp areas on the electrode surfaces. It is assumed that the reason for the appearance of solvated electrons is the autoelectron emission at the interface between the conducting surface of the carbon material and the electrolytic solution. By studying the nitrate anion reduction it is shown that the reduction over-voltage of stable compounds may be lowered by substituting a fast homogeneous reaction of solvated electrons with the initial substance for the hindered heterogeneous stage of the first electron transfer.

Hydrogen Evolution on a Mercury Electrode: The Effect of a Condensed Adsorption Layer

The electroreduction of hydrogen on a dropping mercury electrode with and without condensed adsorption layers (CAL) of organic compounds is studied by methods of classical polarography and laser photoemission. An analysis of effects CAL has on some reactions reveals that the CAL influence on the first-electron transfer and on reactions involving intermediates can retard or even completely block the process. The most profound changes occur in the processes that involve a proton donor. Possible reasons for different effects of CAL on the electron transfer processes are discussed.

Adsorption of surface-active compounds with the skeleton molecular structure from dimethylsulfoxide solutions on carbon nanotubes

The adsorption of adamantane, adamantanol, thiocamphor, and sodium cryptate on electrodes of single-walled carbon nanotubes (SWNT) from dimethylsulfoxide (DMSO) solutions is studied by measuring the differential capacitance (C) vs. potential (E) dependences and cyclic voltammograms. In the tested systems, the high surface activity of these surfactants is observed to result in a noticeable increase in the C of such electrodes in the range of 0.2 ≤ E ≤ −(0.9−1.1) V (SCE). As in the case of aqueous solutions, this experimental fact is explained by the appearance of the so-called Rehbinder effect (the adsorption-induced decrease in the strength), which, in this particular case, consists in a decrease in the surface energy of a solid with the formation of adsorption layers on the side surfaces of SWNTs combined into bundles to afford the partial splitting of these bundles and, as a consequence, the increase in the nanotube surface accessible to the electrolyte. At the same time, the obtained results suggest that for the adsorption of surfactants from nonaqueous solvents (in contrast to aqueous), the interaction between solvent and adsorbate molecules may become important.