Standard Gibbs Transfer Energies Research Articles

Standard Electrode Potentials for Electrochemical Hydrogen Production, Carbon Dioxide Reduction, and Oxygen Reduction Reactions in N,N-Dimethylacetamide

Four standard electrode potentials versus the standard hydrogen electrode (SHE) are reported in a polar aprotic solvent, N,N-dimethylacetamide (DMA) at 298 K: (1) −0.319 ± 0.033 V for a H2 production; (2) −0.415 ± 0.033 V and −1.195 ± 0.033 V for CO2 reductions to produce carbon monoxide (CO) in the absence and presence of water, respectively; and (3) +0.918 ± 0.033 V for an O2 reduction (ORR) or O2 evolution (OER) reactions. These potentials were estimated based on measurements of absolute (and conventional) acidity constants for several Brønsted acids in DMA including CO2 with water ([H2O] = 1.1 M) of which absolute pKa is 18.6 ± 0.2 in reference to aqueous proton at standard states. The standard Gibbs transfer energy of proton was calculated to be −31 ± 3 kJ/mol from water to DMA at 298 K.

Is the Oil | Water Interface the Simplest and Best Suited Model for Understanding Biomembranes?

Many studies have been conducted by using the oil (O) | water (W) interface as a simple model for understanding ion transfer (IT) or electron transfer (ET) across biomembranes. In this review, we revisit the usability of the O | W interface as a biomembrane model. For understanding biomembrane IT, the O | W interface is the simplest and best suited model. For example, the standard Gibbs transfer energy of drug ions at the O | W interface is a useful measure for evaluating their membrane permeability in a conventional in vitro assay, called PAMPA. However, the O | W interface is not necessarily a good model for understanding biomembrane ET. This is because no net current can be observed with the O | W interface, owing to the ET-coupled proton transfer. In such a case, the self-assembled monolayer (SAM) formed on a metal electrode serves as a better model for understanding biomembrane ET.

Charge transfer across the water/nitrobenzene interface by three-electrode system

A droplet of aqueous solution containing a certain molar ratio of redox couple is first attached onto a platinum electrode surface, then the resulting drop electrode is immersed into the organic solution containing very hydrophobic electrolyte. Combined with reference and counter electrodes, a classical three-electrode system has been constructed. Ion transfer (IT) and electron transfer (ET) are investigated systematically using three-electrode voltammetry. Potassium ion transfer and electron transfer between potassium ferricyanide in the aqueous phase and ferrocene in nitrobenzene are observed with potassium ferricyanide/potassium ferrocyanide as the redox couple. Meanwhile, the transfer reactions of lithium, sodium, potassium, proton and ammonium ions are obtained with ferric sulfate/ferrous sulfate as the redox couple. The formal transfer potentials and the standard Gibbs transfer energy of these ions are evaluated and consistent with the results obtained by a four-electrode system and other methods.

Gibbs Transfer Energies of Macrocyclic Ligands in Methanol+N-methyl-2-pyrrolidinone Mixtures by Solubility and Partition Measurements

The standard Gibbs transfer energies of 18-crown-6,dibenzo-18-crown-6, cryptands 21, 22 and 222 frommethanol to methanol + N-methyl-2-pyrrolidinone(NMP) mixtures were determined from solubility andpartition measurements at 30 °C. While the Gibbs energy of transfer of 18-C-6 is positive,increases up to XNMP = 0.7 and thereafter decreases, the transfer energy ofdibenzo-18-C-6 is negative and decreases with the addition of NMP. However,the transfer energy of cryptand 21becomes increasingly negative with theaddition of NMP while that of cryptand 22 ispositive and increases under the sameconditions. For cryptand 222, the transferenergy is slightly negative up toXNMP = 0.5 butdecreases markedly at higher compositions of NMP. These results have been explainedin terms of the various types of interaction between the ligand molecules, solventcomponents and the effect of solvent–solvent interactions on them.

SOLVENT EFFECTS ON CRYPTAND (222) COMPLEXATION

The formation of TI+,Na+ and K+ cryptates with the ligand (222) in 10 nonaqueous solvents has been investigated by cyclic voltammetry at 25°C. Based on the results obtained the dependencies of stability constants of resulting 1 : 1 complexes on Gutmann donor numbers (DN) of the solvents and on standard Gibbs transfer energies of the metal ions from water to given solvent have been analyzed. It was found that DN describe the medium effect far better than the corresponding ΔGt o values. It can be deduced, therefore, that the stability constant changes are essentially limited by the cation desolvation free energies, but the residual solvation of the cation encapsulated in the ligand cavity is also important.

STABILITY OF 15-CROWN-5 AND BENZO-15-CROWN-5 COMPLEXES WITH ALKALI METAL CATIONS IN NONAQUEOUS MEDIA

The formation of alkali metal cation complexes with 15-crown-5 and benzo-15-crown-5 in ten organic solvents has been investigated by cyclic voltammetry at 25°C. Stability constants of the resulting 1 : 1 complexes have been determined by monitoring the shift in reduction potentials of the cations against ligand concentration. Based on the results obtained the dependences of stabilities of the complexes on Gutmann donicity of the solvents and on standard Gibbs transfer energies of the metal ion from water to a given solvent have been analyzed and discussed.

Ion-Solvation Behavior of Some Copper(II) Salts in Isodielectric Solvent Mixtures of Methanol and N-Methyl-2-pyrrolidinone at 30 °C

The selective solvation behavior of copper(II) iodate, formate, and benzoate in the isodielectric solvent mixtures of methanol and N-methyl-2-pyrrolidinone has been studied using the standard Gibbs transfer energies and solvent transport number measurements. The standard Gibbs transfer energies of various ions were determined on the basis of the tetraphenylarsonium tetraphenylborate (TATB) assumption. The results were interpreted in terms of heteroselective solvation of all the salts with copper(II) ion being selectively solvated by N-methyl-2-pyrrolidinone and anions by methanol.

Gibbs Transfer Energies of Some Uni-Univalent Salts in Water—N-Methyl-2-Pyrrolidinone Solvent Mixtures at 30°C

Abstract The standard Gibbs transfer energies of some uni-univalent electrolytes like potassium picrate, tetraphenylarsonium picrate, potassium and silver tetraphenyl borate were determined in water + N-Methyl-2-Pyrrolidinone mixtures from solubility measurements at 30 °C. They were split into respective ionic values on the basis of the reference electrolyte method using tetraphenylarsonium tetraphenylborate. The variation of transfer energies of the ions with composition of solvent are examined and discussed in terms of the possible ion-solvent interactions.

Ion-solvent interactions of silver(I)-18-crown-6 perchlorate complex in methanol-acetonitrile mixtures

The standard Gibbs transfer energies of the silver(I)-18-crown-6 perchlorate complex salt from methanol to various compositions of methanol-acetonitrile mixtures were determined from solubility measurements at 30°C and these data were separated into the corresponding ionic contributions by employing the negligible liquid junction potential method of Parkeret al. The solvent transport numbers ΔAN, for the salt were also determined at various solvent compositions using a concentration cell with transference. The Gibbs transfer energy of the silver(I)-18-crown-6 complex cation is negative and decreases with the addition of acetonitrile but the transfer energy of the anion is positive and increases under the same conditions. The solvent transport number, ΔAN, increases and passes through a maximum value of 5.48 at ΔAN=0.55. These results indicate that the complex salt is heteroselectively solvated in these mixtures with the cation being preferentially solvated by acetonitrile and the anion by methanol molecules.

Voltammetric study of the transfer of Dawson-type heteropolyanions across the nitrobenzene—water interface

A voltammetric study has been made for the transfer of four 18-heteropolymolybdate anions including Dawson anions, i.e. [P 2Mo 18O 62] 6−, [As 2Mo 18O 62] 6−, [S 2Mo 18O 62] 4− and [P 2 Mo 18O 61] 4−, and of a related anion [S 2VMo 17O 62] 5− at the nitrobenzene(NB)—water(W) interface. With regard to the hexavalent and pentavalent anions, well-developed voltammetric waves due to their phase transfer could be obtained under certain conditions. The tetravalent anions, however, gave no wave in the polarizable potential range but showed an increase in the final current rise owing to their transfer from NB to W. From the voltammetric data, the standard ion transfer potential (Δ NB wφ j θ) and standard Gibbs transfer energy (Δ G tr,j θ,NB→W) have been determined (or estimated) for the heteropolyanions. It has been found that Δ NB Wφ j θ and Δ G tr,j θ,NB→W depend primarily on the ionic charge. This is in line with our previous result obtained for Keggin anions in 1991. A possible mechanism has also been proposed for the transfer of the hexavalent Dawson anions.

Ion-Solvent Interactions of Zinc(II) Salts in Water- Pyridine Mixtures

Abstract The ion-solvent interactions involving zinc(II) salts viz. iodate, fluoride and benzoate in waterpyridine mixtures have been investigated by solubility and solvent transport number measurements. The standard Gibbs transfer energies of the salts (Δ Gtº), determined from their saturation solubilities in the solvent mixtures, decrease up to xpy = 0.1 for zinc(II) iodate, but up to Xpy = 0.4 for zinc(II) benzoate, and subsequently increase with the addition of pyridine. For zinc(II) fluoride, however, a continuous increase of ΔGtº with the addition of pyridine is observed. The transfer energy of Zn2 + is negative and that of anions is positive with the former decreasing and the latter increasing continuously with the addition of pyridine. The solvent transport number of pyridine (<Δpy) is positive and passes through a maximum for all the salts in the range xpy = 0.25 to 0.55. These results indicate a heteroselective solvation of the salts with Zn2 + being preferentially solvated by pyridine and the anions by water

Effect of temperature on ion transfer at the aqueous/organic solution interface studied by current-scan polarography with the electrolyte solution dropping electrode

The transfer of I −, BF4 − 4, ClO − 4, sulphonate anions (RSO − 3) and alkylammonium cations (R 4N + or RNH + 3) was investigated at the aqueous/nitrobenzene or 1,2-dichloroethane interface in the temperature range from 4 to 80° C. The ion transfers were polarographically reversible for all ions at any temperature investigated. The standard Gibbs transfer energies from the aqueous to the organic solution, Δ G o,w ⇌ org tr, were calculated from the half-wave potentials of ion transfer polarograms. The effect of temperature on Δ G o tr for ions of small volume was explained by the electrostatic interaction between the ion and solvents taking into account the change in the dielectric constants of the solvents and the structure of water. The effect for such voluminous ions as RSO − 3 and RNH + 3 was elucidated by considering both the electrostatic and non-electrostatic solvation energies of ions. The non-electrostatic contribution to the change of Δ G o tr with temperature is discussed from the viewpoint of the structural change of water.

Electrochemical properties of the interface between two immiscible electrolyte solutions Part I. Equilibrium situation and galvani potential difference

The Galvani potential difference at the interface of two immiscible electrolyte solutions (ITIES), Δ α βϕ , can be determined, when concentrations, activity coefficients, standard Gibbs transfer energies of all ions, which are present in the system, volumes of each phase and temperature are known. The general equations for calculation of this potential difference are for-mulated. In certain cases methods of their solution are also presented.

Electrochemical properties of the interface between two immiscible electrolyte solutions: Part I. Equilibrium situation and galvani potential difference

The Galvani potential difference at the interface of two immiscible electrolyte solutions (ITIES), Δ α βϕ, can be determined, when concentrations, activity coefficients, standard Gibbs transfer energies of all ions, which are present in the system, volumes of each phase and temperature are known. The general equations for calculation of this potential difference are for-mulated. In certain cases methods of their solution are also presented.

311 - A new model of membrane transport: electrolysis at the interface of two immiscible electrolyte solutions

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 four-electrode potentiostatic system is required for such studies. An electrolyte dropping electrode, analogous to Heyrovsky's DME, was also constructed. The voltammograms fully resemble those obtained with metallic electrodes. The faradaic proceses 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.

A new model of membrane transport: Electrolysis at the interface of two immiscible electrolyte solutions

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.

Solute-solvent interactions and acid-base dissociation in mixed solvent systems

Summary Data for the p K of cationic weak acids BH + in binary mixed solvents have been collected and critically analyzed. This dissociation reaction is isoelectric, and electrostatic charging effects should have a minimal influence on the change of p K with solvent composition. Thus conditions are favorable for an examination of the effects of solute-solvent interactions on the dissociation reaction with little interference from electrostatic contributions. Standard Gibbs transfer energies derived from the change in p K were xamind, with the aid of solubility data, in terms of the contributions of the individual species. Comparisons were made with the corresponding quantities for the alkali chlorides and the results correlated with the aid of the activities of the two components of the solvent mixture. In solvents of which water was one component, it was possible to relate the transfer energy for hydrochloric acid to the activity of water alone, to a good approximation. At both extremes of the composition range, there are indications that preferential solvation can account for the “observed” differences in transfer energies of the ions BH + and H + . The results therefore confirm the earlier conclusion that solute-solvent interactions, rather than electrostatic effects associated with the change in dielectric constant, are largely responsible for the changes of p K with composition of the solvent mixtures.

Solute-solvent interactions and acid-base dissociation in mixed solvent systems

Data for the pK of cationic weak acids BH+ in binary mixed solvents have been collected and critically analyzed. This dissociation reaction is isoelectric, and electrostatic charging effects should have a minimal influence on the change of pK with solvent composition. Thus conditions are favorable for an examination of the effects of solute-solvent interactions on the dissociation reaction with little interference from electrostatic contributions. Standard Gibbs transfer energies derived from the change in pK were xamind, with the aid of solubility data, in terms of the contributions of the individual species. Comparisons were made with the corresponding quantities for the alkali chlorides and the results correlated with the aid of the activities of the two components of the solvent mixture. In solvents of which water was one component, it was possible to relate the transfer energy for hydrochloric acid to the activity of water alone, to a good approximation. At both extremes of the composition range, there are indications that preferential solvation can account for the “observed” differences in transfer energies of the ions BH+ and H+. The results therefore confirm the earlier conclusion that solute-solvent interactions, rather than electrostatic effects associated with the change in dielectric constant, are largely responsible for the changes of pK with composition of the solvent mixtures.