Daniel Homolka

Slovak Academy of Sciences, Presburg, Bratislavský, Slovakia

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Publications (21)48.42 Total impact

  • Daniel Homolka · Hartmut Wendt ·
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    ABSTRACT: Transfer from water to nitrobenzene of Iron(II) and (III), Ni(II) and Zn(II) complexed in the aqueous phase by one, two and three molecules of o-phenanthroline and o,o'-bipyridine was investigated using cyclic voltammetry. The kinetically inert complexes of Fe(II), Fe(III) and Ni(II) are keeping their ligand-metal bonds intact and are transferred reversibly from water into nitrobenzene. The uncomplexed metal ions are not transferred within the voltage range limited by transfer of the ions of the aqueous and nonaqueous supporting electrolyte. Ease of phase-transfer is greatly enhanced by complexation with the organic ligand and successive addition of the first, second and third bidentate nitrogen base changes the Gibb's transfer enthalpy of the Ni(II) by approximately equal amounts (ΔΔG° ≈ − 37 kJ/Mol;). The molecular structure of the complex ligand bears only a minor but clearly discernible influence on the Gibb's transfer enthalpies as demonstrated for Ni(II)-phenanthroline and bipyridine complexes. Comparative investigation of phase transfer of Fe(II)-and Fe(III)-trisphenanthroline complexes reveals that due to higher charge of the Fe(III)-complexes they are more strongly solvated in aqueous solution and hence less easily extracted into nitrobenzene than Fe(II)- complexes. Voltammograms for transfer of complexed zinc ions which form kinetically (fast) labile complexes in water as well as in nitrobenzene are to a great deal determined by fast homogeneous kinetics. - In particular mono- and bicomplexed zinc ions (ZnL2+ and ZnL2+2) undergo rapid disproportionation with preferential formation of triscomplexed zinc (ZnL2+3) in the nonaqueous phase. Diffusion coefficients of differently complexed species in water are determined and show a shift to lower values with increasing degree of complexation.
    Berichte der Bunsengesellschaft/Physical Chemistry Chemical Physics 10/1985; 89(10):1075 - 1082. DOI:10.1002/bbpc.19850891012
  • Zdenèk Samec · Vladimír Marec̄ek · Daniel Homolka ·
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    ABSTRACT: From fast galvanostatic pulse measurements at 25°C the capacitance of the water/nitrobenzene interface was evaluated as a function of the interfacial potential difference Δowϕ for systems consisting of NaBr, LiCl or MgSO4 in water and tetrabutylammonium tetraphenylborate, tetraphenylarsonium tetraphenylborate or tetraphenylarsonium dicarbollylcobaltate in nitrobenzene. The modified Verwey—Niessen model, in which an inner layer of solvent molecules separates two space-charge regions (the diffuse double layer), describes the structure of the water/nitrobenzene interface well at electrolyte concentrations above ca. 0.02 mol dm−3, provided that the ions are allowed to penetrate into the inner layer over some distance. For all the systems studied the zero-charge potential difference was found at Δwoϕpzc ≈ 0 on the basis of the standard potential difference Δwoϕ0TMA + = 0.035 V for tetramethylammonium cation which was used as a reference ion. At zero surface charge a comparison was made with the theoretical capacitance calculated using the mean spherical approximation for a model consisting of two ion and dipole mixtures facing each other. The effect of ion penetration on the interfacial capacitance was estimated from the solution of the linearized Poisson-Boltzmann equation for a triple dielectric model with a continuous distribution of the point ions. The concentration-independent inner layer potential difference and capacitance can only be inferred from the capacitance data if the ion size effect is taken into account. A non-iterative procedure based on the hypernetted-chain equation was used for the evaluation of the potential drop across the diffuse double layer. The extend of the penetration into the inner layer appears to be a function of ion solvation, e.g. the more hydrated ion the less extensive ion penetration is likely.
    Journal of Electroanalytical Chemistry 05/1985; 187(1):31-51. DOI:10.1016/0368-1874(85)85573-8 · 2.87 Impact Factor
  • Z. Samec · V. Mareček · D. Homolka ·

    Journal of Electroanalytical Chemistry 07/1984; 170(1-2):383-386. DOI:10.1016/0022-0728(84)80062-5 · 2.87 Impact Factor
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    ABSTRACT: The partition of a series of protonated amines (aniline, benzylamine, 2-phenylethylamine and related compounds) between water and nitrobenzene was investigated using electrochemical approaches (cyclic voltammetry and differential pulse stripping voltammetry) which made it possible to infer the transport and thermodynamic parameters of the partition process. Micromolar concentrations of the protonated amines in the aqueous phase can be determined by DPSV at the electrolyte hanging drop electrode. The function of an amine as the proton acceptor in the facilitated proton transfer across the water/organic solvent phase was discussed.
    Journal of Electroanalytical Chemistry 03/1984; 163(1-2-163):159-170. DOI:10.1016/S0022-0728(84)80049-2 · 2.87 Impact Factor
  • Zdeněk Samec · Vladimír Mareček · Daniel Homolka ·
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    ABSTRACT: The electrical double layer at the interface between two immiscible electrolyte solutions (ITIES) has been studied by the fast-galvanostatic-pulse method for the system consisting of aqueous NaBr and a solution of tetrabutylammonium tetraphenylborate in nitrobenzene. The double-layer capacity has been evaluated as a function of the potential difference across the interface. The modified Verwey–Niessen model, in which a layer of oriented solvent molecules (the inner layer) separates two space-charge regions (the diffuse double layer), seems to provide a reasonable framework to interpret the experimental data, assuming (i) that the approximations to the Poisson–Boltzmann equation by Gouy and Chapman are removed and (ii) that the boundary between the space-charge region and the inner layer is considered to be diffuse rather than sharp. The use of the tetrabutylammonium cation as the reference ion in voltammetric studies of the water/nitrobenzene interface is discussed.
    Faraday Discussions of the Chemical Society 01/1984; 77:197-208. DOI:10.1039/DC9847700197

    Journal of electroanalytical chemistry 12/1983; 159(1):233-238. DOI:10.1016/S0022-0728(83)80329-5 · 2.73 Impact Factor
  • Zdenék Samec · Vladimír Mareček · Daniel Homolka ·
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    ABSTRACT: Kinetic and thermodynamic parameters of the transfer of choline and acetylcholine cations across the water/nitrobenzen interface were evaluated using convolution potential sweep voltammetry. In order to trace the factors which control the ion-transfer kinetics the semi-phenomenological theory was used, assuming that: (1) the temperature dependence of the rate constant has the form of the Arrhenius equation; (2) the reaction site is located in the outer Helmholtz plane (oHp); (3) there exists a Brønsted-type relationship between the true activation Gibbs energy and the reaction Gibbs energy for the ion transfer from the oHp in water to that in the organic solvent phase. The apparent rate constants were corrected for the double-layer effect using the capacity data and the Gouy-Chapman theory. It is concluded that the observed potential dependence of the apparent rate constants arises largely from the effect of the potential on the concentration of the transferred ions at the reaction planes. The correlation of the true (corrected) rate constant with the reaction Gibbs energy for a series of the ions with similar structure indicates that the true charge-transfer coefficient α1≅0.3, which would correspond to the asymmetric potential energy barrier for the ion-transfer step.
    Journal of electroanalytical chemistry 11/1983; 158(1-158):25-36. DOI:10.1016/S0022-0728(83)80335-0 · 2.73 Impact Factor

    Journal of Electroanalytical Chemistry 08/1983; 151(1-2):277-282. DOI:10.1016/S0022-0728(83)80441-0 · 2.87 Impact Factor

    Journal of Electroanalytical Chemistry 03/1983; 145(1):213-218. DOI:10.1016/S0022-0728(83)80307-6 · 2.87 Impact Factor
  • Daniel Homolka · Karel Holub · Vladimir Mareček ·
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    ABSTRACT: The transfer of the metal cation across the interface between two immiscible electrolyte solutions facilitated by complex formation with a ligand at the interface was investigated both theoretically and experimentally. The theory of single-scan voltammetry was derived which enables the complex stoichiometry (1:1, 1:2 or 1:3. cation to ligand) to be determined as well as the thermodynamic and transport parameters of the facilitated charge transfer controlled by the diffusion of the ligand. Application of the theoretical results was illustrated for the transfer of Li+ and Cd2+ ions across the water/nitrobenzene interface facilitated by complexation with the neutral macrocyclic polyether diamine.
    Journal of Electroanalytical Chemistry 08/1982; 138(1):29-36. DOI:10.1016/0022-0728(82)87125-8 · 2.87 Impact Factor
  • Zdeněk Samec · Daniel Homolka · Vladimir Mareček ·
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    ABSTRACT: Facilitated transfer of proton, alkali and alkaline earth-metal cations across the water/nitrobenzene interface was observed in the presence of either N, N′-di[(11′-ethoxycarbonyl)undecyl]-N, N′,4,5-tetramethyl-3,6-dioxaoctanediamide (DODA) or 7,19-dibenzyl-2,3-dimethyl-7,19-diaza-1,4,10,13,16-pentaoxacycloheneicosane-6,20-dione (PEDA) in the nitrobenzene phase. These neutral synthetic compounds act as the ion carriers which, through the complex formation, mediate the translocation of the ion across the water/nitrobenzene interface. Using the convolution potential sweep voltammetry, the stoichiometry (r:s) and the thermodynamic, transport as well as the kinetic parameters were evaluated for the single-step charge-transfer model: rM2+ (w) + sL(n)=MrLsr2+(n), where M2+(w) is the metal cation with the charge number z in water and L(n) is the neutral ligand in nitrobenzene. The formation of 1 : 1 or 1 : 2 (cation to ligand) complexes is involved in the case of the monovalent or divalent cations respectively. The highest stability constant K in nitrobenzene was found for calcium ion: K= 5.8 × 1021M−2 or 2.0 × 1018M−2 for the acyclic (DODA) or the cyclic (PEDA) ligands respectively. The kinetic parameters were evaluated only for the transfer of the divalent metal cations, since the transfer of the monovalent cations is too fast. While the apparent charge-transfer coefficient is invariably very close to 0.5, the apparent rate constants at the formal potentials of the charge-transfer reactions differ considerably from each other and are sensitive to the nature of the ligand. For the acyclic ligand (DODA) the following sequence of the apparent rate constants was observed: Ca2+⪢Ba2+∼Sr2+⪢Mg2+, whereas for the cyclic ligand (PEDA) it was: Ba2+⪢Sr2+∼Mg2+⪢Ca2+. Among the factors underlying the kinetic behaviour there seem to be the double-layer effects, the internal as well as the solvent contributions to the activation energy.
    Journal of Electroanalytical Chemistry 05/1982; 135(2):265-283. DOI:10.1016/0368-1874(82)85125-3 · 2.87 Impact Factor
  • Z. Samec · V. Mareček · J. Weber · D. Homolka ·
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    ABSTRACT: Convolution analysis was used in the evaluation of the thermodynamic and kinetic parameters of two charge-transfer systems at the water/nitrobenzene interface: Cs+ ion transfer and the electron transfer between ferrocene in nitrobenzene and hexacyanoferrate(III) in water. Attention was focused in particular on the potential dependence of the rate constant of the ion or electron transfer. The apparent rate constant was corrected for the double-layer effect using the capacity data and the Gouy-Chapman theory. It is concluded that the observed potential dependence of the apparent rate constant of Cs+ ion transfer arises from the effect of the total potential difference on the concentration of reactants at the reaction planes. In the electron transfer the analysis is considerably complicated by the possibility of ion-pairing, the bridge mechanism of electron transfer and the existence of the different planes of the closest approach for the reactant and the base electrolyte ions. Nevertheless, an attempt at analysis indicates that an intrinsic potential dependence of the rate constant is involved.
    Journal of Electroanalytical Chemistry 09/1981; 126(s 1–3):105–119. DOI:10.1016/S0022-0728(81)80422-6 · 2.87 Impact Factor
  • Z. Samec · V. Mareček · D. Homolka ·
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    ABSTRACT: From ac impedance measurements the capacity of the water/nitrobenzene interface was evaluated as a function of the potential difference between two phases in contact. In each phase an electrolyte was dissolved: LiCl in water and tetrabutylammonium tetraphenylborate in nitrobenzene. The experimental results were interpreted in terms of the compound double-layer model in which the layer of the oriented solvent molecules (the inner or compact layer) separates two space-charge regions (diffuse double layer). The capacity of the diffuse double layer calculated using the Gouy-Chapman theory was found to fit well for the capacity of the interface. It was concluded that the potential drop across the inner compact layer remains constant and close to zero when the total potential drop across the interface is varied.
    Journal of Electroanalytical Chemistry 09/1981; 126(1-3):121-129. DOI:10.1016/S0022-0728(81)80423-8 · 2.87 Impact Factor

    Journal of Electroanalytical Chemistry 08/1981; 125(1):243-247. DOI:10.1016/S0022-0728(81)80341-5 · 2.87 Impact Factor
  • Daniel Homolka · Vladimír Mareček ·
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    ABSTRACT: The transfer of the picrate ion across the interface between two immiscible electrolyte solutions, 0.05 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene was investigated by electrolysis with the electrolyte dropping electrode and by cyclic voltammetry. Under the conditions of the experiments the charge-transfer process is controlled solely by diffusion. The maximum which appears on the polarogram of the picrate ion close to the limiting current can be suppressed by the addition of a surface-active substance (gelatine). The diffusion coefficients of the picrate ion in the aqueous and nitrobenzene phase were determined from the limiting polarographic current and from the peak current on the cyclic voltammogram. The value of the formal potential of the charge-transfer reaction, which was calculated from the half-wave potential or from the peak potential, is in good agreement with that inferred from the extraction data.
    Journal of Electroanalytical Chemistry 09/1980; 112(1):91-96. DOI:10.1016/S0022-0728(80)80010-6 · 2.87 Impact Factor
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    ABSTRACT: The interface of an aqueous and an organic electrolyte can be electrochemically polarized from an external voltage or current source. The faradaic ion transfer taking place as an effect of the polarization has been studied by means of chronopotentiometry and cyclic voltammetry. These methods are suitable for determination of semihydrophobic ions present in one of the phases. Cation transfer is facilitated by macrocyclic complex formers (ionophores) present in the organic phase. This can be used for determination of the complexing agents and for stability constants of the complexes formed in the organic phase.
    Analytical Chemistry 09/1980; 52(11). DOI:10.1021/ac50061a017 · 5.64 Impact Factor
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    ABSTRACT: A simple membrane model is the interface between water and an organic liquid immiscible with water, with a strongly hydrophilic electrolyte dissolved in the aqueous phase and a strongly hydrophobic electrolyte in the organic phase. This interface can be electrochemically polarized in the same way as the interface electrode electrolyte solution using various modes of voltammetry or the galvanostatic method. A fourelectrode potentiostatic system is required for such studies. An electrolyte dropping electrode, analogous to Heyrovský's DME, was also constructed. The voltammograms fully resemble those obtained with metallic electrodes.The faradaic processes studied so far are mainly connected with the transfer of hydrophobic ions across the interface. These processes are quite rapid and the half-wave potential of a particular ion is related to its standard Gibbs transfer energy. Observed electron-transfer effects model redox processes at membranes.Macrocyclic ionophores facilitate transfer of alkali metal ions across this interface. Very fast ion transfer as well as complex formation was observed in the systems under investigation so that, generally, the diffusion of the ionophore toward the interface and of the complex into the organic phase is the rate-controlling step, no surface reaction retarding the overall process.Apart from the investigation of membrane processes, this approach can be used for elucidation of processes in ion-selective electrodes and in phase-transfer catalysis.
    Bioelectrochemistry and Bioenergetics 03/1980; 116(1):61-68. DOI:10.1016/S0022-0728(80)80221-X
  • Z. Samec · V. Marecek · J. Weber · D. Homolka ·

    Journal of Electroanalytical Chemistry 06/1979; 99(3):385–389. DOI:10.1016/S0022-0728(79)80106-0 · 2.87 Impact Factor
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    ABSTRACT: 6 and 7-substituted 2-amino-4-hydroxy-5,6,7,8-tetrahydropteridines are oxidized at a platinum electrode in several waves in the potential range between hydrogen and oxygen evolution potentials. The electrode process in the first (main) anodic wave was investigated in more detail using a stationary and rotating disc platinum electrode. The process in this wave is “semireversible” with an exchange of two electrons. The standard rate constants of this reaction were determined. The oxidation product is deactivated by an irreversible chemical reaction. Some of the products block the surface of the electrode.
    Journal of Electroanalytical Chemistry 12/1976; 74(2):205-214. DOI:10.1016/S0022-0728(76)80236-7 · 2.87 Impact Factor
  • Jan Weber · Karel Holub · Daniel Homolka · J PRADAC ·
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    ABSTRACT: The theory of double-step chronocoulometry has been verified for the e.c. electrode reaction type, a reversible electron transfer being followed by a chemical reaction in solution leading to inactivation of the primary product. In contrast to methods proposed previously, the inactivation reaction rate constants were calculated using the charge values obtained by integration up to current decay after the second potential step. The anodic oxidation of tetrahydropterin derivatives on a Pt electrode was employed as a model system. It has been found that the rate of inactivation of the anodic product decreases with increasing pH of the solution and is different for various derivatives studied. A special case of zero inactivation rate was verified using the Fe3+/Fe2+ redox system in an acidic medium.