Jan Langmaier

Charge-transfer processes at the interface between hydrophobic ionic liquid and water

This article provides a brief review of theoretical and methodological concepts in the area of the charge-transfer processes at the interface between a hydrophobic ionic liquid (IL) and an electrolyte solution in water (W). Electrochemical methods of study of the W|IL interfaces are described, current experimental problems are indicated, and the most important experimental results are summarized. The relevance of electrochemistry at the W|IL interfaces to the extraction behavior of ILs is outlined.

Polarization phenomena at the water | o-nitrophenyl octyl ether interface. Part 1. Evaluation of the standard Gibbs energies of ion transfer from the solubility and voltammetric measurements

Standard Gibbs energies of ion transfer from water to o-nitrophenyl octyl ether are evaluated for a series of ions from voltammetric and solubility measurements by using the tetraphenylarsonium tetraphenylborate hypothesis. Thermodynamic scales derived from these measurements are consistent with each other and with analogous scales for ion transfer to nitrobenzene and 1,2-dichloroethane.

Substituent effects in cyclic voltammetry of titanocene dichlorides

Abstract Methyl-substituted titanocene dichlorides (C5H5−nMen)2TiCl2 (n=0–5), [C5Me4(SiMe3)]2TiCl2, (C5Me4Ph)2TiCl2 (Ph=phenyl), [C5Me4(FPh)]2TiCl2 (FPh=para-fluorophenyl), [C5Me4(CH2Ph)]2TiCl2 and ansa-compounds Me2Si(C5H4)2TiCl2 and Me2Si(C5Me4)2TiCl2 were investigated by means of cyclic voltammetry at a mercury electrode in tetrahydrofuran. The standard potential (E°1) of the first electron uptake shifts to more negative values proportionally to the number of methyl groups in the (C5H5−nMen)2TiCl2 (n=0–3) compounds, with an increment of 0.093 V per one methyl group. A decline from this linear dependence is observed for (C5HMe4)2TiCl2 and a positive shift for (C5Me5)2TiCl2. The [C5Me4(R)]2TiCl2 (R=SiMe3, Ph, FPh and CH2Ph) compounds show even larger positive shifts of E°1. These positive shifts can be brought about by a steric strain between the cyclopentadienyl ligands which lowers the dihedral angle between cyclopentadienyl ring planes (φ) and thus decreases energies of bent titanocene 1a1 and b2 LUMO orbitals. This opinion is corroborated by the voltammetry of ansa-compounds Me2Si(C5H4)2TiCl2 and Me2Si(C5Me4)2TiCl2, having a large and fixed angle φ. Their E°1 values are close to those of (C5H5)2TiCl2 and (C5HMe4)2TiCl2, respectively.

Voltammetry of ion transfer across a polarized room-temperature ionic liquid membrane facilitated by valinomycin: theoretical aspects and application.

Cyclic voltammetry is used to investigate the transfer of alkali-metal cations, protons, and ammonium ions facilitated by the complex formation with valinomycin at the interface between an aqueous electrolyte solution and a room-temperature ionic liquid (RTIL) membrane. The membrane is made of a thin (approximately 112 microm) microporous filter impregnated with an RTIL that is composed of tridodecylmethylammonium cations and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions. An extension of the existing theory of voltammetry of ion transfer across polarized liquid membranes makes it possible to evaluate the standard ion-transfer potentials for the hydrophilic cations studied, as well as the stability constants (K(i)) of their 1:1 complexes with valinomycin, as log K(i) = 9.0 (H(+)), 11.1 (Li(+)), 12.8 (Na(+)), 17.2 (K(+)), 15.7 (Rb(+)), 15.1 (Cs(+)), and 14.7 (NH(4)(+)). These data point to the remarkably enhanced stability of the valinomycin complexes within RTIL, and to the enhanced selectivity of valinomycin for K(+) over all other univalent ions studied, compared to the conventional K(+) ion-selective liquid-membrane electrodes. Selective complex formation allows one to resolve voltammetric responses of K(+) and Na(+) in the presence of an excess of Mg(2+) or Ca(2+), which is demonstrated by determination of K(+) and Na(+) in the table and tap water samples.

Diffusion Coefficients of Alkali Metal Cations in Nafion® from Ion‐Exchange Measurements An Advanced Kinetic Model

An advanced kinetic model of the coupled diffusion of two counterions in a fixed-site ion-exchange membrane is developed considering the effect of the varying ionic composition on the membrane water content. The transport problem is solved numerically for a set of ratios of the diffusion coefficients of the two counterions and 1:1 ion-exchange stoichiometry. The model is used to evaluate the diffusion coefficients of alkali metal cations in the as-received and expanded H- and M-form Nafion® (M = Li, Na, K, Rb, Cs) from ion-exchange measurements. Owing to a compensating effect of the electro-osmotic pore fluid flow, the initial rates of ion exchange correspond to a fixed water content which, however, is different in H- and M-form membranes. A strong correlation is revealed between the ratio of the membrane to aqueous ion diffusion coefficients and the polymer-phase volume fraction. It is concluded that the polymer phase presents mainly a steric effect without significantly changing the mechanism of transport of alkali metal cations or protons, which resembles that in bulk water. The different behaviors of the as-received and expanded Nafion® forms are probably associated with the prevailing cluster- or pore-network morphology, respectively

Polarization phenomena at the water|o-nitrophenyl octyl ether interface. Part II. Role of the solvent viscosity in the kinetics of the tetraethylammonium ion transfer

Abstract The kinetics of the tetrahylammonium ion transfer from water to o -nitrophenyl octyl ether has been studied by impedance measurements at the equilibrium cell potential. Enhanced capacity of the interface points to a significant specific adsorption of the tetraethylammonium ion, probably via forming the interfacial ion pairs. In spite of the coupling between the ion transfer and the ion adsorption, a non-linear least squares fitting of impedance data to the Randles-type equivalent circuit or to an impedance model accounting for the specific ion adsorption yields similar values of the kinetic parameters. The most important finding is that the ratio of the rate constants of the tetraethylammonium ion transfer from water to nitrobenzene and to o -nitrophenyl octyl ether correlates with the inverse ratio of the organic solvent viscosities. The low viscosity of 1,2-dichloroethane can then be responsible for rather fast ion transfer kinetics at the water|1,2-dichloroethane interface. The viscosity effect is likely to be associated with the friction exerted by the organic solvent molecules on the hydrated ion penetrating the organic solvent to some depth prior to the ion transfer step.

Amperometric solid-state NO2 sensor based on plasticized PVC matrix containing a hydrophobic electrolyte

Abstract An amperometric solid-state sensor for the detection of nitrogen oxide in air has been constructed using a gold minigrid indicator electrode, Pt/air reference and platinum counter electrodes in contact with a solid polymer electrolyte containing 9.6% PVC, 87.0% 2-nitrophenyl octyl ether and 3.4% tetrabutylammonium hexafluorophosphate. The stationary electric current corresponding to the NO 2 reduction at the indicator electrode at − 0.3 V versus Pt/air electrode exhibited a linear dependence on the NO 2 concentration in the range 0.2 to 5.0 ppm, with a sensitivity of 21.7 nA ppm −1 (at 54% relative humidity), a standard deviation of 3.3 nA at 1.06 ppm NO 2 , and a detection limit of 75 ppb. The time constant or time required to attain 90% of the steady-state response is 3.3 s or 14 s, respectively. The response decreases linearly with increasing relative humidity (RH) with a slope of 0.14 nA ppm −1 per 1% RH. The sensor shows an excellent stability over one month.

Indicator and reference platinum | solid polymer electrolyte electrodes for a simple solid-state amperometric hydrogen sensor

Abstract Factors that influence the open-circuit potential of a platinum | solid polymer electrolyte (SPE) electrode in air containing low concentrations of hydrogen were investigated. The electrodes were prepared from platinum in the form of a wire or a grid mechanically pressed, or a Pt layer chemically deposited, onto a Nafion® 117 membrane. The parameter underlying the behaviour of the Pt | SPE electrodes is the ratio of the rate of reduction of the platinum oxides to the hydrogen transport through a layer adjacent to the electrode surface, which can be controlled by, for example, the roughness factor of the platinum electrode. The lower is this ratio, the more sensitive is the electrode potential to the changes in the hydrogen concentration in the gas phase. Depending on the value of the roughness factor, the Pt | SPE electrode can be employed as either an indicator or a reference electrode. The design of a simple amperometric hydrogen sensor, which combines the Pt wire as an indicator electrode and the chemically deposited Pt as a reference electrode, is outlined.

Fine tuning of the catalytic effect of a metal-free porphyrin on the homogeneous oxygen reduction

The catalytic effect of tetraphenylporphyrin on the oxygen reduction with ferrocene in 1,2-dichloroethane can be finely tuned by varying the molar ratio of the acid to the catalyst present in the solution. The mechanism involves binding of molecular oxygen to the protonated free porphyrin base, in competition with ion pairing between the protonated base and the acid anion present.

Cyclic voltammetry of methyl- and trimethylsilyl-substituted zirconocene dichlorides

Abstract Redox properties of the substituted zirconocene dichlorides: (C 5 H 5− n Me n ) 2 ZrCl 2 ( n =0–5), [C 5 H 5− n (SiMe 3 ) n ] 2 ZrCl 2 ( n =0–3), [C 5 Me 4 (SiMe 3 )](C 5 HMe 4 )ZrCl 2 and two ansa -analogues Me 2 Si(C 5 H 4 ) 2 ZrCl 2 and Me 2 Si[C 5 H 2 (SiMe 3 ) 2 ] 2 ZrCl 2 were investigated by cyclic voltammetry on a mercury electrode in tetrahydrofuran. In the (C 5 H 5− n Me n ) 2 ZrCl 2 ( n =0–4) compounds, standard electrode potential ( E °) of the one-electron uptake shifts to more negative values by 0.071 V per one methyl group. A deviation from this linear dependence to a less negative E ° is observed for (C 5 Me 5 ) 2 ZrCl 2 . This effect is attributed to the steric hindrance between rotating C 5 Me 5 ligands which tends to decrease the angle between the cyclopentadienyl ring planes ( φ ) and consequently, the energy difference between MO frontier orbitals. In the trimethylsilylated compounds, the net effect of SiMe 3 is negligible, giving virtually the same value of E ° for n =0–3. In μ-SiMe 2 -bridged ansa -compounds the difference in E ° of 147 mV corresponds to the negative shift of 37 mV per one SiMe 3 group. Owing to the rigid angle φ , this shift can be tentatively accounted for the electronic effect of the SiMe 3 groups. In the non- ansa -compounds, the negative shift due to the electronic effect of SiMe 3 groups is assumed to be roughly compensated by a positive shift resulting from a sterically controlled diminution of φ .