Effect of the diffuse double layer on the interpretation of EQCM results in dilute NaClO4 solutions

The response of the electrochemical quartz crystal microbalance (EQCM) in dilute NaClO₄ solutions was studied with gold and iron electrodes during a stepwise increase of the perchlorate concentration. In the range from 10⁻⁴ M to 7.8×10⁻² M, the quartz resonant frequency of the 10 MHz AT cut crystals increased by about 700 Hz, indicating a mass loss on the electrode. A model was developed in which the diffuse double layer and the oscillating bulk electrolyte layer, characterised by the velocity decay length of the damped shear wave in solution, are treated as two independent, superimposed sheets. By assuming a characteristic thickness of the diffuse double layer according to the Gouy–Chapman theory and by treating the diffuse double layer as a rigid sheet, the measured mass loss could be simulated qualitatively. The viscosity changes in the diffuse double layer as well as in the sensed electrolyte bulk layer were found to be negligible in the concentration range investigated. In dilute solutions, the frequency shift following a concentration change is entirely due to thinning of the diffuse double layer with increasing concentration. The results demonstrate the importance of diffuse double layer effects for EQCM measurements in dilute electrolytes.     

Der Einfluß der Kationen alkalischer Elektrolyte auf die Geschwindigkeit der kathodischen Sauerstoffreduktion an Platin

The dependence of the rate of the oxygen reduction on the nature of cations [Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, (CH₃)₄N⁺] is studied at smooth platinum and porous carbon loaded with a platinum catalyst in alkaline solutions.The rest potentials are shifted to more negative values from Li⁺ to (CH₃)₄N⁺, likewise the cathodic polarization is increased with the size of the cations. A change of the potential drop within the diffuse double layer caused by increasing cation concentration with growing size of ions is of minor importance in 0.5N alkaline solutions. Specific adsorption of (Rb⁺), Cs⁺ and (CH₃)₄N⁺ has to be considered, which would give rise to a decrease of the rate of the electrochemical reaction.Secondly the observed effect can be attributed to ion pairing of charged species (O₂ ^–) involved in the overall reaction and the cations. The stability of the hyperoxide ion is increased from (CH₃)₄N⁺ to Li⁺ by interaction with the cations. Consequently the velocity of the rate determining charge transfer step is accelerated in this direction.The experimental findings are in favour of the second interpretation, because the effect is not enhanced in more dilute solutions.

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Mit 7 Abbildungen     

Transient natural convection induced by electrodeposition of Li+ ions onto a lithium metal vertical cathode in propylene carbonate

Transient natural convection caused by Li⁺ electrodeposition at constant current along a vertical Li metal cathode immersed in a 0.5 M LiClO₄–PC (propylene carbonate) electrolyte was compared with that by Cu²⁺ ion electrodeposition in aqueous CuSO₄ solution. The concentration profile of the Li⁺ ions was measured in situ by holographic interferometry. The interference fringes start to shift with time at a higher current density. The concentration boundary layer thickness for Li⁺ ions was successfully determined. With the progress of electrodeposition, the density difference between the electrolyte at the cathode surface and the bulk electrolyte increased to induce upward natural convection of the electrolyte. The electrolyte velocity was measured by monitoring the movement of tracer particles. The measured transient behavior of the ionic mass and momentum transfer rates normalized with respect to the steady-state value was numerically analyzed. Transient natural convection along a vertical cathode due to Li metal electrodeposition can be reasonably explained by boundary layer theory, similar to the case of Cu electrodeposition in aqueous CuSO₄ solution.     

Increase of ionic conductivity in the microporous lithosilicate RUB-29 by Na-ion exchange processes

The ionic conductivity in the zeolite-like lithosilicate RUB-29 (Cs₁₄Li₂₄[Li₁₈Si₇₂O₁₇₂]·14H₂O [S.-H. Park, J.B. Parise, H. Gies, H. Liu, C.P. Grey, B.H. Toby, J. Am. Chem. Soc. 122 (2000) 11023–11024]) increases via simple ion-exchange processes, in particular when Na cations replace a part of Cs⁺ and Li⁺ of the material. The resulting ionic conductivity value of 3.2×10⁻³ S cm⁻¹ at 885 K is about two orders higher than that for the original material [S.-H. Park, J.B. Parise, M.E. Franke, T. Seydel, C. Paulmann, Micropor. Mesopor. Mater., in print (doi:10.1016/j.micromeso.2007.03.040 available online since April 19, 2007)]. The structural basis of a Na⁺-exchanged RUB-29 sample (Na-RUB-29) at 673 K could be elucidated by means of neutron powder diffraction. Rietveld refinements confirmed the replacement of Na⁺ for both parts of Cs and Li cations, agreeing with idealized cell content, Na₈Cs₈Li₄₀Si₇₂O₁₇₂. As a result of the incorporation of Na⁺ in large pores, the number of Li⁺ vacancies in dense Li₂O-layers of the structure could increase. This can be one of the main reasons for the improved conductivity in Na-RUB-29. In addition, mobile Na cations may also contribute to the conductivity in Na-RUB-29 as continuous scattering length densities were found around the sites for Na in difference Fourier map.     

Surface charge and adsorption of Na+ and Cl− ions at the zirconium dioxide/water interface

The surface charge and adsorption densities of Na⁺ and Cl^– ions at the zirconium dioxide/electrolyte interface have been determined as a function of pH for 0.1, 0.01 and 0.001 M solutions of NaCl. Using potentiometric titration of the surface hydroxy groups, it was found that the point of zero charge occurred at pH 4.3±0.15. The results are discussed in terms of site binding model of the electric double layer. The ionization and complexation constants have also been determined.     

Lithium transport within the solid electrolyte interphase

A LiClO₄ SEI film grown on copper was examined with time-of-flight secondary ion mass spectrometry. The SEI porosity profile and Li⁺ transport processes within the SEI were studied with isotopically labeled ⁶LiBF₄ electrolyte. An ~ 5 nm porous region, into which electrolytes can easily diffuse, was observed at the electrolyte/SEI interface. Below the porous region, a densely packed layer of Li₂O and/or Li₂CO₃ prevents electrolyte diffusion, but Li⁺ transports through this region via ion exchange.     

Effect of salt on the inter- and intramolecular hydrophobic interactions of macromolecules

The effect of salt on the structure of a low density lipoprotein (LDL) and on the reversible polymerization of bovine serum albumin (BSA) reduced with 2-mercaptoethanol was investigated by means of ultracentrifugal analysis. The chaotropic anion, e.g., SCN^– and I^–, at 5M completely disrupted the LDL structure and effectively dissociated BSA oligomers at lower concentrations. The parallelism between the anion order of these effects and that of the chaotropic effect suggested that the observed salt effects are primarily based on the disruption of hydrophobic interactions. The cation effectiveness disrupting the LDL structure followed the order of their promoting effect on the water structure, i.e., Li⁺>Na⁺>K⁺>Cs⁺. However, Cs⁺ was most effective in dissociating BSA oligomers, and this was attributed to the -complex formation with the aromatic amino acid side chains which otherwise contribute to the promotion of the intermolecular hydrophobic association.     

Partial molal volumes of ions in organic solvents from ultrasonic vibration potential and density measurements. III. Dimethylsulfoxide

The partial molal volumes of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Cl^–, Br^–, I^–, and NO ₃ ^- in DMSO at 25°C have been determined from ultrasonic vibration potential data and density data for solutions of uni-univalent electrolytes. Hepler's semiemprirical equation has been used to split ionic partial molal volumes into geometric and electrostrictive contributions. The results obtained in this work confirm the conclusion of our previous studies, namely, that the contribution of electrostriction is essentially determined by the properties of that layer of atoms, 0.3 to 0.4 nm thick, in contact with the ion and by the degree of steric hindrance of the poles of the dipole of the solvent molecule. On the other hand, the geometric contribution depends on the size of the solvent molecule and also on the arrangement of the solvent molecules about the ions. It is shown that the geometric contribution to the partial molal volume of ions is largely increased when ions cannot come close enough to the poles of the solvent-molecule dipole, owing to steric hindrance.     

Ab initio pair potentials for the ionic lithium-formate system

The potential energy surfaces for the interatomic interaction in the Li⁺HCOO^– system have been investigated byab initio methods within the rigid-molecule approximation. Analytical potential expressions were fitted to 133 calculated SCF energies for the Li⁺-HCOO^– interaction, 42 SCF energies for the Li⁺-Li⁺ interaction, and 332 SCF energies for the HCOO^–-HCOO^– interaction. The global minimum on the Li⁺-HCOO^– surface is –170 kcal/mol and corresponds to the lithium ion lying on the C₂ axis of the formate ion at 2.2 Å from the carbon atom on the oxygen side. The cation-cation and anion-anion interactions are repulsive everywhere, although the potential surface is markedly anisotropic for the HCOO^–-HCOO^– interaction.     

The alkali metal cation effect on the surface-enhanced Raman spectra of phosphate anions adsorbed at silver electrodes

The interaction of adsorbed phosphate anions with alkali metal cations at the Ag|aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver–oxygen (Ag---O′), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, δ_as(PO) and ν_as(PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential (0.00 V vs. Ag | AgCl) the spectral parameters for the Cs⁺ and K⁺ cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na⁺ and Li⁺ cations. An increase in ν_as(PO) frequency by ca. 10 cm⁻¹ was the characteristic spectral signature for the interaction of phosphates with Li⁺. The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the ν_as(PO) mode and the previously observed frequency sensitivity of this band to solvent H₂O substitution by D₂O. At negative potential (−0.80 V), a stabilization effect of Cs⁺ on the phosphate adlayer was detected based on the twofold increase in intensity of the ν(Ag---O′) mode compared with Li⁺. Splitting of the ν_as(PO) mode suggested the contact interaction of anions with specifically adsorbed Cs⁺ cations.     

An Application of Functional Analysis Method to the Potential of Electrical Double Layer for Spherical Micelles

With the help of the iterative method in functional analysis theory based on the Gouy–Chapman model in the colloid and interface chemistry an analytic solution of the potential of electrical double layer of spherical micelles has been obtained. This method has eliminated the restriction that the Poisson–Boltzmann equation, which represents the distribution of the potential in the double layer, can be solved only under the condition of zekT so far. The connections between the present results and those from Verwey and Overbeek's previous work have also been discussed. Our approach provides a simple but effective method for the calculation of the potential of electrical double layer under general potential condition.     

Non-ideality in Born-free energy of solvation in alcohol-water and dimethylsulfoxide-acetonitrile mixtures: Solvent size ratio and ion size dependence

Recent extension of mean spherical approximation (MSA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol-water) and non-associating (dimethylsulfoxide-acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MSA theory for the first time. The solvent-solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. Smaller ions such as Li⁺ and Na⁺ show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs⁺ and Bu₄N⁺). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA)-water mixture is found to be very different from that in other alcohol-water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaker in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities.     

ESR investigation on the phase transitions of DPPC vesicles in presence of high concentrations of Li+, Na+, K+ and Cs+

The partition of the spin label TEMPO in the hydrophobic region of di-palmitoyl-phosphatidylcholine unilamellar vesicles has been used to investigate the influence of high concentration (up 3M) of Li⁺, Na⁺, K⁺, and Cs⁺ on the phase transitions at 20–60°C. All of the above salts increase the permeation of TEMPO. The efficiency of monovalent cations in inducing the partition of the spin label in the hydrophobic environment of the bilayer increases in the order: Cs⁺+++. The disappearing of the pretransition and the downward shift of the main phase transition temperature from 37°C to 33.5°C is related to the increased permeation of TEMPO into the bilayer. The presence of salts in the bulk solution disturbs the hydration of the zwitterionic polar head of the DPPC molecules and changes the electrical interaction between the polar groups of the bilayer. This reduces the packing density of the lipid molecules and promotes the permeation of TEMPO.     

Double Layer at the Pt(111)–Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy–Chapman Screening

We report, for the first time, the observation of a Gouy–Chapman capacitance minimum at the potential of zero charge of the Pt(111)‐aqueous perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concentrations (<10^?3 m ). By comparison to gold and mercury double‐layer data, we conclude that the diffuse double layer structure at the Pt(111)‐electrolyte interface deviates significantly from the Gouy–Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean‐field Poisson–Boltzmann theory.     

Selectivity ratios of cation-sensitive glass electrodes in ethanol-water mixtures

Summary The response of cation-sensitive glass electrodes to univalent cations in ethanol-water mixtures shows increasing e.m.f. values with increasing fractions of ethanol for Na⁺, K⁺, Cs⁺, Rb⁺, Li⁺, and NH₄ ⁺, but anomalous behavior for H⁺. The calculated selectivity of the glass electrode for metal ions over the hydrogen ion thus increases sharply over the range of 10–70 weight % ethanol. It is suggested that sodium rather than hydrogen ion may be adopted as the reference ion for the definition of selectivity ratios in mixed solvents.

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Zusammenfassung Die Ansprechbarkeit von kation-empfindlichen Glaselektroden auf einwertige Kationen (Na⁺, K⁺, Cs⁺, Rb⁺, Li⁺, NH₄ ⁺) in Äthanol-Wassergemischen weist mit steigendem Äthanolgehalt steigende E.M.K.-Werte auf; H⁺ zeigt ein abweichendes Verhalten. Die berechnete SelektivitÄt der Glaselektrode für Metallionen nimmt daher im Bereich von 10–70% Äthanol stark zu. Es wird empfohlen, für die Definition von SelektivitÄtsverhÄltnissen in Lösungsmittelgemischen Na⁺ an Stelle von H⁺ als Bezugsion zu verwenden.

     

Acid–Base Zeta-Metric Titration of Quartz Dispersions in Ethanol Solutions of Electrolytes

The conductivity of dilute quartz suspensions and electrophoretic mobility of quartz particles in solutions with the concentration C = 10^–5–10^–2 M XBr (X = H, Cs, Na, and Li) and NaOH, as well as in mixed solutions of 10^–4 M XBr (X = Cs, Na, and Li) + 10^–4–10^–2 M HBr and 10^–4 M XBr + 10^–4–10^–2 M XOH (X = Cs, Na, and Li) in ethanol containing 6 vol % of water were measured using conductometry and microelectrophoresis. The values of surface conductivity of quartz were calculated by the Wagner formula and used to calculate zeta potential by the Henry–Booth formula. The resultant dependences (logC) suggest that the value and sign of zeta potential are determined not only by the adsorption of potential-determining ions ⁺ and ^–, but also by the competitive specific adsorption of all ions of the aforementioned electrolytes, the adsorption values increasing in a cation series Li⁺ < Na⁺ < Cs⁺ < H⁺ and an anion series Br^– < OH^–. In particular, it is found that the titration of the above suspensions with XOH bases results in the reversal of zeta potential sign from negative to positive at a concentration depending on the adsorption capacity of alkali cation.     

Fourier transform SERS studies of the chemisorption of ferricyanide on gold electrode surfaces

Surface enhanced Raman spectroscopy (SERS) studies of ferricyanide and ferrocyanide ions at gold electrodes show that in solutions containing 0.1 M Cs⁺, ferricyanide ions are adsorbed, but not ferrocyanide. The strength of the interaction with the surface is such that the electrode can be removed from the solution without loss of the SERS signal. This chemisorbed layer is also observed from solutions containing Li⁺ ions. The implications for the mechanism of the heterogeneous electron transfer reaction are discussed.     

Free energies of transfer of alkali metal halides in isodielectric acetonitrile and ethylene glycol mixtures at 25°C

Standard free energies of transfer (G _t ^o ) of alkali metal chlorides MCl (M=Li, Na, K, Rb, Cs) and of potassium bromide and iodide from ethylene glycol to its isodielectric mixtures containing 20, 40, and 60 wt. % acetonitrile have been determined from emf measurements at 25°C. The standard potentials of the M/M⁺, the Ag–AgBr, and the Ag–AgI electrodes have been calculated using auxiliary emf measurements. All electrolytes were found to be increasingly destabilized in mixed solvents of increasing acetonitrile content; the relative order of cation destabilization is Li⁺>Na⁺>K⁺>Rb⁺>Cs⁺ and that of anion destabilization is Cl^–>Br^–>I^–, both following (a) the order of increasing softness of ions and increasing interaction with the soft base, acetonitrile,and (b) the order of decreasing hydrogen-bonding propensity of ions and decreasing interactions with the hard base, ethylene glycol.