Electrosorption of L-α-dipalmitoylphosphatidylcholine at the mercury solution interface

Electrocapillary curves were measured for L-α-dipalmitoylphosphatidylcholine in 97% (υ/υ) methanol-water solutions containing 0·01 M NH₄NO₃. The electrosorption isotherms are similar to those of simpler aliphatic compounds. Saturated coverage of the electrode surface with this lipid in 97% (υ/υ) methanol-water was not possible owing to the formation of micelles in the bulk solution. It is shown that it is possible from the electrocapillary curves to estimate the critical micelle concentration of lipids in non-aqueous electrolyte solutions.     

Specific adsorption of formate ions at the mercury—aqueous solution interface

Adsorption on polarized Hg electrode of formate ions from aqueous solutions of pure HCOONa using differential capacity measurements was studied. Anion specific adsorption was indicated by the cathodic shift of potential of zero charge with increase in bulk-electrolyte activity. Variation of charge due to surface excess of sodium ions (Γ ₊ Z₊ F) and of charge due to specifically adsorbed formate ions (q¹), with electrode charge (q^M) indicated super-equivalent adsorption of formate ions at all positive charges and for all concentrations studied. Esin—Markov coefficient was found to be unreliable criterion of the occurence of specific adsorption. Logarithmic form of constant charge adsorption isotherms were found to be followed. The plots of φ^{M ? 2}vs q¹ at constant q^M were not linear and resembled those for F^?, BF₄^? and CH₃COO^? ions; which is attributed to relatively weak specific adsorption of anions.     

Adsorption of triethanolamine at the solution/mercury and the solution/air interface

Comparison of adsorption of triethanolamine at the Hg/solution and the air/solution interface has been carried out by means of interfacial and surface tension and differential capacity measurements. Adsorption is stronger at the metal/solution interface than at the free surface of the solution. This is explained in terms of some metal—adsorbate specific interaction. Adsorption parameters for the Hg/solution interface indicate that the adsorbate molecules interact with neighbouring water molecules. While the flat orientation of the large organic molecule does not change with electrode charge, the functional OH groups, slightly turned to the solution, are very likely to rotate under the action of the electrode potential thus giving rise to a field dependent inner layer capacity, C₁.     

Adsorption of phloroglucinol at the mercury/aqueous solution interface

The adsorption of phloroglucinol on mercury electrodes has been studied from 0.5 M NaClO₄ aqueous solution by means of capacitance and electrocapillary methods. Capacitance curves show a condensation type behaviour. Congruence Tests indicate simultaneous congruence at negative surface charges. Frumkin isotherm is obeyed both at constant charge and contant potential. The interaction parameter obtained through Frumkin fit corresponds to a repulsive interaction. A new isotherm based on Free Volume Theory was tried. A model for the structure of the double layer in presence of phloroglucinol has been advanced, based on these results and the available crystallographic data of phloroglucinol and its dihydrate.     

Adsorption and association of xanthine in absence and presence of some divalent metal ions at the mercury/solution interface

The adsorption and interfacial orientations of xanthine were studied by out-of-phase ac voltammetry at a hanging mercury drop electrode. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential, temperature, adsorption time, the nature of anions of the supporting electrolyte and the bulk concentration of xanthine. The changes in the stacking interactions of xanthine molecules in the presence of Cd(II), Cu(II), Co(II), Ni(II), Ca(II) and Mg(II) were studied by ac voltammetry as changes in “pits” or “wells” on double layer capacitance curves due to adsorption and association of M(II)-xanthine complexes on the charged interface. The results indicate that the complexation of xanthine molecules enhances the stacking interactions and hence would be expected to facilitate the formation of perpendicularly stacked layer of M(II)-xanthine complex on the electrode surface. The time dependence of the electrode impedance indicates that the formation of a compact film in absence and presence of these metal ions controlled by a fixed number of nuclei and the data were analysed according to the Avrami equation. The adsorption parameters of xanthine have been computed at different pH values in the absence and the increased presence of metal ions.     

Faradaic rectification studies of some reactions at the mercury/solution interface

The kinetic parameters of Hg₂²⁺, 1·0 M HClO₄/Hg and Fe(Ox)₃³⁻, Fe(Ox)₃⁴⁻, 0·5 M K₂ (Ox), 0·7 N H₂SO₄/Hg reactions have been obtained by faradaic rectification at radio frequencies. The value of I_a°, the exchange current density for discharge of 1·0 mM and 0·5 mM of mercurous ions on mercury in 1·0 M perchloric acid is 3·9 A/cm² and 1·1 A/cm² respectively. The rate constant k_a° for the reaction is of the order of 0·9 × 10⁻² cm/s at 35°C and the transfer coefficient is 0·56. The values of kinetic parameters obtained for the discharge of mercurous ions have been compared with data obtained by others using different methods. The value of α for Fe(Ox)₃³⁻/Fe(Ox)₂⁴⁻, 0·5 M K₂(Ox)/Hg reaction is 1·0.     

Adsorption of Cd(II) ions at the hydroxyapatite/electrolyte solution interface

The study of the Cd(II) ions adsorption at the hydroxyapatite/electrolyte solution interface and the changes of the electrical double layer (EDL) structure in this system are presented. The adsorption of Cd(II) ions was calculated from the loss of their concentration from the solution using the radioisotope method. The adsorption was studied in the range of the initial concentration from 0.000001 to 0.001mol/dm³ and as the function of pH. The results of measurements of Cd(II) ions adsorption kinetics on hydroxyapatite were fitted using the pseudo-first, pseudo-second, and multiexponential modes to check which model describes this process in the best way. In addition, the main properties of the EDL, i.e., surface charge density and zeta potential were determined by the potentiometeric titration and electrophoresis measurements, respectively. The adsorption of Cd(II) ions at hydroxyapatite contributed to the increase of the zeta potential and also to the change of the double electrical layer structure.     

The adsorption of strychnine at a mercury—solution interface

The adsorption of strychnine in an aqueous acetate buffer has been studied in order to explain the optimum conditions of an electrochemical asymmetric synthesis in which it is used as inductor. In dilute solutions (m < 10^?4) the adsorption is a slow kinetic process while at more concentrated values (m > 10^?4) it is affected by an association in solution. This explains the fact that the optimum conditions for the synthesis are known to correspond to a concentration 10^?4 M in strychnine.Further more while it is not adsorbed at very positive charges, this molecule saturates the surface of the electrode for negative charges between ?2 and ?7 μCcm^?2ie between ?500 and ?900 mV (Calomel electrode KCl 0.1 M). At still more negative charges (?1300 mV) hydrogen is catalytically discharged.     

An effect of hydration entropy on adsorption of various alkylammonium ions at the mercury/water interface

The adsorption of various alkylammonium cations was investigated by measurement of the interfacial tension. Tetraethanolammonium ions (HOC₂H₄)₄N⁺ showed weaker adsorbability than that expected from its ionic radius. The adsorbabilities (relative surface excess of cation FΓ₊ and charge of specifically adsorbed cation q¹₊) of the ethyl series (C₂H₅)_nH_4?nN⁺(n = 0 ≈ 4), and of the methyl series (CH₃)_nH_4?nN⁺n = 0 ≈ 3) were directly proportional to the hydration enthalpies of these cations - ΔH⁰_h, but the proportionalities were different between the methyl and the ethyl series. A linear relation was, however, found between FΓ₊ and the hydration entropies - ΔS⁰_h without regard to the kinds of cations. A similar relation was also found between q¹₊? - ΔS⁰_h. In conclusion, the adsorbabilities of these cations were governed by the hydration entropy, not by the hydration enthalpy.     

On the kinetics of adsorption and desorption of perchlorate ions in the presence of 3-methyl-isoquinoline at the mercury—aqueous solution interface

Coadsorption of perchlorate ions and 3-methyl-isoquinoline molecules at the mercury—aqueous solution interface results in the formation of a condensed film. The process of ClO^?₄ incorporation (or removal) is very slow. Characteristic relaxation times (about 10^?2 s) are several orders of magnitude larger than values typical of ion adsorption at a mercury—solution interface which is free from organic surfactant. The relationships between the rate of the process, the electrode potential and the amount of adsorbed ClO^?₄ ions are determined and discussed. The results indicate that the rate of the overall process is controlled by a slow adsorption (or desorption) step whose kinetics are consonant with an isotherm of the Temkin type.     

Stochastic phenomena during phase transition of lecithine films at the mercury solution interface

1,2-Dipalmitoyllecithine forms adsorbed films at electrodes in two distinct potential regions. At temperatures below the phase transition temperature the film at negative potentials is highly ordered and blocks the surface for electrode reactions. Potential steps between both regions enable us to monitor the random nucleation during the film transition by its influence upon the charge transfer process. The time distribution of current pulses obeys the Poisson distribution. The current power spectrum is given.     

Specific adsorption of tetra-alkyl ammonium iodide at the mercury—solution interface and the influence of the double-layer structure on the kinetics of zinc ions

The effect of adsorption of tetra-alkyl ammonium iodide at the dme has been examined with Faradaic impedance measurements. The differential capacitance of mercury in the salt solutions of the type R₄NI, where R is CH₃, C₂H₅ and n-C₃H₇ at 25°C as a function of polarization is measured. The parameters of the electrocapillary maximum, E_max and σ_max, are evaluated. By comparing the differential capacitance curves with those observed in other aqueous salt solutions of the same concentration and assumed to exhibit specific adsorption, it is shown that the cation is specifically adsorbed and that adsorption increases from tetra-methyl to tetra-ethyl to tetra-propyl salts. On the anodic limb of the capacity curve it is assumed that there is simultaneous specific adsorption of cations and anions. The anions thus form bridges between the cations and the mercury surface. The apparent standard rate constant K_s for the discharge of Zn(II)/Zn(Hg) in 1 M NaClO₄ was found to decrease in the presence of the quaternary cations and accelerated by iodide only. This phenomena is similar to that observed with the hydrogen evolution reaction at the mercury electrode and is opposite of those observed with anions of the persulphate type.     

Specific adsorption of acetate ions at the mercury—lithium acetate (AQ) interface

Specific adsorption of acetate ions at the mercury—lithium acetate (aq) interface has been investigated using both differential capacity and electrocapillary measurements. Variation of charge due to surface excess of lithium ions (q⁺) and of charge due to specifically adsorbed acetate ions (q^?1 with electrode charge (q^M) have been discussed. Such discussion inferred superequivalent adsorption of acetate ions at all positive charges for concentrations of lithium acetate ≥0.20 M whereas Esin-Markov plots predicted the absence of specific adsorption; the prediction of Esin-Markov plots has been shown to be faulty under certain conditions. The plots of φ^M-2vsq^?1 at constant q^M for acetate ion resembled the corresponding plots for F^? and BF^?₄ ions; the slope of these plots is negative up to coverages corresponding to 0.20 M lithium acetate. The identification of the isotherm parameter, (?φ^M-2/?q^?1)_q^M with the reciprocal of inner layer capacity at constant charge (_^?1Cⁱ) has been questioned.     

The adsorption of trifluoromethanesulfonate anions at the mercury/electrolyte interface

Recently, there has been much interest in the use of trifluoromethanesulfonic acid as an electrolyte in advanced fuel cell systems because of the enhanced rate of oxygen reduction in this medium. In order to elucidate this characteristic from a double layter point of view, the present work was undertaken to study the adsorption of trifluoromethanesulfonate ions on mercury electrodes. Differential capacities and potentials of zero charge were measured in constant ionic strength solutions of composition X M CF₃SO₃ K + (1-X) M KF at 25°C. Analysis of the results shows that the extent of specific adsorption of CF₃SO₃^? anions is small, and the anions even desorb from the surface at sufficiently positive electrode charges. The structure of the inner layer is elucidated, and an isotherm for the adsorption of CF₃SO₃^? based on electrostatic models of the double layer is derived. The results indicate that at the potentials for the electroreduction of oxygen CF₃SO₃^? anions will not interfere with the adsorption of the reactant and of intermediates, allowing faster rates to be achieved.     

Adsorption of the No−3 ions at the polycrystalline silver electrode-aqueous electrolyte solution interface

Differential capacity of polycrystalline silver electrode-aqueous LiNO₃ solution interface was measured for 0.01, 0.03, 0.06, 0.1 and 0.2 M concentrations. Adsorption of the nitrate ion has been analyzed in terms of the Damaskin's method [Elektrokhimiya12, 646 (1976)]. Some adsorption parameters have been determined and compared with the results obtained for mercury and gold in order to determine the role of metal in the double-layer structure.     

Adsorption of chloride ion at the mercury/ solution interface in LiCl—methanol and HCl—ethanol systems—III

The distance ratio of the inner and outer Helmholtz planes, estimated previously from the capacitance ratio method, has been additionally determined as λ-values,¹ from the reduced isotherm (virial and Langmuir types) with the Frumkin-isotherm results of individual q^sat_Cl^? and q′_Cl^? data. Similarly, the so-called chemical term of adsorption energy (Stern's adsorption potential) was evaluated in the investigated systems, using the isotherm parameters.     

Adsorption stages and interfacial orientations of cytidine sulphate at the mercury/electrolyte interface

The adsorption and interfacial orientations of cytidine sulphate were studied by out-of-phase ac voltammetry. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential, adsorption time, temperature and the bulk concentration of cytidine sulphate. The time dependence of the electrode impedance indicates that the formation of a compact film controlled by nucleation/growth mechanism and the data were analysed according to the Avrami equation. Moreover, the role of the anions of the indifferent supporting electrolyte in the formation of the condensed film of the adsorbed molecules has been elucidated. The stacking interactions between adsorbed molecules are affected by the nature of the anions of the supporting electrolyte and are hindered by their specific adsorption. The adsorption parameters of cytidine sulphate has been computed at various pHs and for various anions in the supporting electrolyte.     

Two-dimensional surface properties of 2-methoxy ethyl oleate at the air/water interface

Methoxy ethyl oleate, a nonionic surfactant, has been investigated at the air/water interface for various surface properties by employing the Langmuir film balance technique. The ester forms an expanded isotherm at the air/water interface. The minimum area of packing (A ₀), initial area of increase of surface pressure (A _i ), collapse pressure (π _c ), and area/molecule at collapse pressure (A _c ) have been estimated from the isotherm curve.The higher compressibility coefficient (K) suggests that the ester forms a more expanded liquid film than the parent oleic acid. Besides, the ester film is fairly stable as suggested by only about 30% loss in area over a period of 20 min. The relaxation rates of the ester film at different surface pressures of 10, 15, and 20 mN·m^?1 have been estimated from changes in the surface area/molecule with time. Interestingly, the surface area (54.2 Ų/molecule) that corresponds to a minimized structure projected for the ester, calculated theoretically, agrees reasonably well with the experimental value (57.2 Ų/molecule).     

Adsorption of N-tosylglycine at the Hg/aqueous solution interface at various pH values

The adsorption of N-tosylglycine (CH₃-Ø-SO₂-NH-CH₂COOH) has been studied at the mercury/aqueous solution interface (0.1 M NaClO₄ as base electrolyte) with the aim of comparing the interfacial properties of the molecular forms of the amino acid present in solution at different pH values. The analysis of the interfacial behaviour was performed by tensammetric measurements using an ac phase selective polarographic method of pH = 4, 6 and 12.It has been shown that the adsorption isotherm. (Frumkin type) is congruent with respect to the potential, with a slight attractive interaction factor at pH = 4, but higher and repulsive at pH = 6 and 12. At pH = 4 the species adsorbed seems to be the neutral one, white at pH = 6 and 12 the monoanionic species seems to be involved in the electrode interaction. Moreover, there is evidence that the molecule, in its neutral or ionic form, is always adsorbed with the tosyl group flat on the surface and the glycine moiety turned toward the solution.