Specific Adsorption Of Ions Research Articles

Test of the Gouy-Chapman theory at a (111) silver single-crystal electrode

At single-crystal electrodes and in the absence of ionic specific adsorption, plots of the reciprocal of the experimental differential capacity against the reciprocal of the Gouy-Chapman (GC) diffuse layer capacity (the so-called Parsons-Zobel (PZ) plots) exhibit slopes which are often somewhat less than unity at the charge density σ = 0. This apparently anomalous behaviour has usually been explained by surface roughness. On using a (111) silver single-crystal electrode yielding a PZ plot of average slope 0.65 at σ = 0, PZ plots of slopes 1.05 ± 0.05 and linear correlation coefficients better than 0.996 were obtained for − 2 ≤ σ ≤ −1 μC cm -2. Moreover, plots of the applied potential E against the GC potential ø d of slopes 1 ± 0.05 and linear correlation coefficients better than 0.993 were obtained for σ ≤ −2 μC cm -2. A critical analysis of these data was carried out in order to test the validity of the GC theory.

A model for the effect of roughness of single-crystal electrodes on Parsons-Zobel plots

At single-crystal electrodes and in the absence of ionic specific adsorption, Parsons-Zobel (PZ) plots exhibit slopes which are often less than unity at zero charge density, and increase to become equal to, or slightly greater than, unity as the absolute value of the charge density is increased slightly. This behaviour is herein explained on the basis of a model which accounts for the curvature of the asperities on the electrode surface. These asperities are simulated either by hemispheres of a given average radius a or by hemicylinders of a given average cross-sectional radius a′. By a suitable choice of either a or a′, the curvature and slope of experimental PZ plots at the various charge densities are satisfactorily accounted for. It is shown that surface roughnesses as low as 1.05 may give rise to PZ plots at zero charge with slopes as low as 0.6, provided that the radius a (or a′) is sufficiently small.

Testing the film-induced cleavage model of stress-corrosion cracking

The effects of coarsening of porosity within dealloyed layers have been investigated from the viewpoint of the film-induced cleavage model of SCC. Tensile tests and pore coarsening measurements of Ag-20 at%Au foils were carried out in 1 M HClO 4 at room temperature. Upon ageing at potentials above 550 mV(SCE), the ability of a brittle crack to penetrate into the substrate was destroyed quickly due to rapid pore coarsening. At low potentials brittle behaviour persisted for up to 2 h. Chloride ions accelerated the coarsening and pyridine retarded it. The observed time dependence of the coarsening indicates that it occurs by the surface diffusion of Au atoms and is affected by the specific adsorption of ions or molecules.

Single-ion activities based on the electrical double-layer model: an indirect test of the Gouy–Chapman theory

Single-ion activity coefficients for NaOH aqueous solutions in the concentration range m= 0.01–1.0 mol kg–1 have been determined by measuring the potential difference for the cell Hg(σ= const.)|NaOH (m)|HgO|Hg (σ is the Hg surface charge density). The approach rests on the assumption that the electric potential drop across the Hg/solution interface, provided specific adsorption of ions is absent, is independent of the electrolyte concentration once the diffuse-layer contribution is accounted for by the Gouy–Chapman theory. Determinations have been extended from σ= 0 up to σ=–14 µC cm–2 by measuring the potential of zero charge and integrating capacitance data. The results have shown that at m > 0.3 mol kg–1 the model breaks down owing to the presumable attack of Hg by the electrolyte. At m < 0.3 mol kg–1 the activity coefficient of OH– turns out to be lower than the mean ionic coefficient. Calculations by means of the ‘hydration theory’ of Robinson et al. agree with the data of this work only qualitatively.

The role of ξ-potential in the colloid stability of calcium oxalate dihydrate dispersions. 2. Effect of several additives

The ξ-potential and the colloidal stability of dilute dispersions of calcium oxalate dihydrate (COD) in the presence of several additives have been studied. The experimental ξ-potential values vs. the pCa and pH of the dispersions were compared with the theoretical data obtained by using a simple electric double-layer (e.d.l.) model. The effect of magnesium sulfate, urea, creatinine, tartaric acid, sodium citrate, chondroitin sulfate and heparin on the ξ-potential of COD dispersions was explained in terms of pCa changes or because of the specific adsorption of the additives. In the case of sodium citrate, the experimental data are in good agreement with the theoretical results calculated by using the Nernst—Gouy—Stern—Grahame model for the e.d.l., which takes into account the specific adsorption of ions. The colloid stability of a COD sol has been studied using the semiempirical relationship provided by Eilers and Korff(EK). This EK index shows that at acidic pH the COD dispersions are stable in the colloidal sense, and magnesium sulfate and sodium citrate have a dual character, acting like inhibitors of the aggregation at concentrations of 10 −3 M. Urea and creatinine, however, behave like inert agents in the concentration range studied, while chondroitin sulfate and heparin show a high EK index at concentrations greater than 10 −4 gl −1 and can be considered inhibitors of the COD particle aggregation.

The effect of specific adsorption of ions and underpotential deposition of copper on the electro-oxidation of methanol on platinum single-crystal surfaces

The specific adsorption of anions in acid electrolytes containing sulfuric and hydrochloric acids was found to have a strongly inhibiting effect on the rate of methanol electro-oxidation on both the (111) and (100) surfaces of platinum. The effect was much stronger in hydrochloric acid, requiring approximately three orders of magnitude higher concentration of sulfuric acid to achieve the same inhibiting effect (and surface coverage by adsorbed anion). Cl − adsorption isotherms were measured using the emersion technique and ex situ Auger electron spectroscopy analysis. The mechanism of inhibition appeared to be similar for both anions. On the (111) surface, anion adsorption was found to proceed in two stages, one coupled to hydrogen desorption producing a co-adsorbed state, and a second stage of adsorption at more positive potentials into the anion adlattice from which hydrogen has been desorbed. The second stage process on Pt(100) is different, occurring at more positive potentials than on Pt(111) and concurrent with OH formation. Completion of the second stage of adsorption appears to produce the inhibition that causes a current peak to be observed in the voltammetry of methanol in acids containing these anions. Underpotential deposition (UPD) of copper in acids containing these electrolytes caused strong additional inhibition of the methanol oxidation rate. The effect was attributed to induced anion adsorption on platinum atoms near to the UPD copper atoms due to a lowering of the local pzc by the copper adatoms.

アミン浮選におけるグリーンカーボランダムの抑制について 研削屑からの機能性複合酸化物の浮選分離に関する基礎的研究

In order to recover functional double oxides, such as gadolinium gallium garnet (GGG) from its slicing dust by flotation method, the depression of green carborundum (GC) which is an abrasive has been investigated by using various organic colloids.Corn starch was the most effective depressant for GC. The adsorption of corn starch on GC surface was low in the alkaline pH range and increased as pH decreased. It is considered that the adsorption of corn starch on GC surface through hydrogen bonding between hydrogen atoms in corn starch molecule and oxygen atoms on GC surface becomes favorable as pH decreases, because the electrostatic repulsion between corn starch and GC decreases owing to the decrease of negative charge on GC surface. On the other hand, the adsorption of corn starch on GGG surface was high in the alkaline pH range and low in the acidic pH range. The decrease in the adsorption of corn starch on GGG in the acidic pH range may be explained by the reason that hydrogen ions are strongly adsorbed on the oxygen sites on GGG surface so that corn starch can not be adsorbed. Also, the adsorption of corn starch in the acidic pH range was further decreased by the specific adsorption of sulfate ions. Accordingly, corn starch may be adsorbed on GC surface in preference to GGG surface in the acidic pH range, particularly, when sulfuric acid is used as a pH regulator.

Potentials of zero charge and capacitance minima of polycrystalline gold in sodium fluoride solutions

The potential of zero charge ( pzc) of polycrystalline gold in sodium fluoride solutions was determined using the piezoelectric technique and compared with the potential of the capacitance minimum. E c, measured simultaneously. In general the two do not coincide which is in keeping with the surface heterogeneity model of polycrystalline metals. A series of oxidation/reduction cycles causes the pzc to shift negatively and eventually become coincident with E c. This is interpreted as due to the roughening of the surface. The results show no evidence of the specific adsorption of fluoride ions. An improved cell for the piezoelectric method is described.

The electrical double-layer structure in KPF 6 methanol + acetone mixtures

The influence of binary organic solvents on the double-layer structure has been analysed for a mercury-solution interface without detectable specific ionic adsorption. Electrocapillary curves and the differential capacity were measured for 0.01 M and 0.1 M KPF 6 solutions in a number of acetone + methanol mixtures covering the complete range from pure methanol to acetone. The electrocapillary maximum (ECM) in pure acetone is shifted in the positive direction compared with that in methanol. Similarly, the interfacial tension at the ECM in acetone is lower by 5.5 mN/m than in the 0.1 M KPF 6 methanolic solution. Acetone is selectively adsorbed more strongly on mercury, particularly on the cathodic branch of the capacity or the electrocapillary curves. As previously observed in the literature for the KPF 6 ion in acetone, the capacity peak appears very close to the limit of positive polarization in methanol + acetone mixtures, as well as in pure methanolic solutions.

The specific adsorption of halides on mercury in non-aqueous acetonitrile

The adsorption behaviour of various tetraalkylammonium halides R 4NX (R = Et, Bu; X = I −, Br −) and sodium iodide at the mercury/acetonitrile interphase has been investigated employing capacity measurements and measurements of the potential of zero charge at variable ionic strength. The results indicate strong specific adsorption of halide ions in the range between negative and positive electrode charges. For NaI, a more detailed study of the adsorption process based on the virial isotherm was performed including the evaluation of the standard Gibbs energy of adsorption and the interaction parameter. Moreover, the distance ratio ( x 2 − x 1/ x 2 and the Esin-Markov coefficients could be estimated. From differences between the Esin-Markov coefficients determined for all salts under investigation, a counter-ion effect with respect to the halide adsorption could be deduced. The specific adsorption of halide ions in acetonitrile will be discussed, particularly with regard to the influence of solvent and ion association.

Electro-optic investigation of oxide suspensions: Surface charge sign reversal through specific adsorption

Conclusions present in previous papers on the nature of low-frequency electro-optic effects are verified for more drastic variations in the particle double electric layer. The effects are followed during sign reversal of surface charge resulting from specific adsorption of ions. The results support the view that the large electro-optic effects manifested in the kilohertz and the hertz frequency regions are both related to parameters of the diffuse part of double electric layer: the kilohertz effect—to particle surface conductivity, and the negative hertz effect—to the value and profile of electrokinetic potential. The form of the frequency curves follows the changes in the ratio of those effects, hence of diffuse layer parameters.

A new analysis of the differential capacitance of an ideally polarized electrode: Part II. Non-aqueous solvents

A previously presented analysis of the differential capacitance of an ideally polarized electrode without ionic specific adsorption is used for investigating the interface between a mercury electrode and a non-aqueous electrolyte solution. Capacitance data for eight non-aqueous solvents are considered. The solvent contribution C s is obtained by separating from the inner layer capacitance C i deduced from experiment the contribution C m related to the relaxation with charge of the surface dipole of the metal and to the variation with the electrode charge σ of the distance of closest approach of the solvent molecules to the metal surface. The C s(σ) curves are found to be specific to each solvent but all curves have a common simple shape and a single maximum in the range of charges explored. For all solvents but methanol and acetonitrile this maximum is either at a negative charge or close to the pzc. The size of the solvent molecule is found to play an important role in the magnitude of C s near its maximum. The contribution C dip originating from the orientation of the permanent dipoles is analyzed. A clear separation between associated and non-associated solvent is evidenced. The influence of the physical parameters of the solvent and the nature of intermolecular interactions is discussed.

Properties of microfiltration membranes: Computer automated determination of the electrokinetic properties of polycarbonate membranes

The electrokinetic properties of polycarbonate capillary pore membranes have been studied as a function of pore size and pH using a computer automated electro-osmotic technique. The interpretation of the experimental data using existing theories for electro-osmotic flow was complicated by two factors: apparent variations in the surface properties of pore walls for membranes of different pore radii, and the observation of maxima in the profiles of the magnitude of zeta-potential against pH. The former are not inconsistent with the membrane manufacturing conditions. The overall electrokinetic behavior is consistent with a surface protonation reaction and specific adsorption of chloride ions.

The kinetics of electrolyte induced aggregation of Carey Lea Silver colloids

The kinetics of aggregation of colloidal silver as a function of the concentration of NaCl, NaBr, and NaI has been examined. At low concentration (<0.025 mol dm −3) of electrolyte, the electrolyte induced aggregation rate is dependent on the halide ion in the system NaI > NaBr > NaCl. At higher electrolyte concentration, the aggregation rate was found to be independent of the type of halide electrolyte used. Model calculations based on the DLVO theory indicate that specific desorption and adsorption of ions at the colloid surface can explain the observed rate behavior of the different systems.

Interfacial capacitance and ionic association at electrified liquid/liquid interfaces

The capacitance of the interface of immiscible electrolytes has been measured for the Li+, Na+, K+, Rb+ and Cs+ chlorides in 1,2-dichloroethane and nitrobenzene. Evidence for ionic specific adsorption at the interface is presented. The nature of the specifically adsorbed charge is discussed in terms of the formation of interfacial ion pairs between the aqueous and the organic ions. The contribution of specific ionic adsorption to the interfacial capacitance is calculated on the basis of an ionic associated model using the Bjerrum theory of ion-pair formation. A mixed solvent region with varying penetration of the ion pairs into it, dependent on their ionic radii, best represents the structure of these interfaces.

Piezoelectric Response to Specific Adsorption of Chloride Ions on Gold Electrode

The specific adsorption of Cl− ions on gold electrode was investigated by a piezoelectric technique combined with admittance measurement. The specific adsorption contributed greatly to the piezoelectric signal produced by a potential modulation technique. Several parameters of the specific adsorption such as electrosorption valency, γ, and surface excess, were evaluated from the piezoelectric signals. These results indicated that the specific adsorption of Cl− ions took place accompanying a partial charge transfer and induced the structural change of the electric double layer leading to the formation of a surface adsorption compound.

Approximate solutions for cation transport during unsteady, unsaturated soil water flow

Approximate analytical solutions are derived for the transport of reactive solutes during unsteady flow of water in unsaturated porous media. The solutions apply to solutes that undergo adsorption reactions including ion exchange, specific adsorption, or negative adsorption (anion exclusion). No particular form of adsorption or exchange equation is assumed, and solutions are developed for both unfavorable (concave upward) and favorable (convex upward) isotherms, as well as for multisite isotherms. The main approximations that are assumed in deriving the solutions are (1) that local chemical and physical equilibrium is attained rapidly relative to the time scale for transport, (2) that the reaction takes place in a region of constant total charge concentration in both the solution and solid phases, (3) that the effects of dispersion and the shape of the adsorption isotherm may be treated separately and then combined, and (4) that as has been assumed previously for unsteady flow of nonreactive solutes, the water content and pore water velocity may be treated as constant over the region of dispersion. The solutions were tested using experimental data for the transport of exchanging cations during horizontal infiltration in laboratory columns. The solution for unfavorable isotherms was compared with data for sodium invading a calcium‐dominated soil; that for multisite isotherms, which combines the solutions for both favorable and unfavorable isotherms, was compared with data for potassium invading a calcium‐dominated soil. In both cases, good agreement was found between the measured distributions of solution and sorbed cations and those predicted using these solutions together with independently measured cation exchange isotherms.

Another comment on the anomalous behaviour of the inner-layer capacity at low electrolyte concentrations

where C, is the diffuse layer capacity which is generally calculated from the Gouy-Chapman theory. For no specific ion adsorption the Ci is assumed to be independent of the electrolyte concentration so that the behaviour of Ci gives an insight into the double layer structure. One interpretation problem that arises in this theory concerns the behaviour of Ci at very low concentrations. There are systems in which the Ci remain relatively constant for low to medium concentrations but can become infinite or negative at very low concentrations in the neighbourhood of the position of zero charge [l-3]. This anomalous behaviour has been known for some time [l] and is normally interpreted on the basis of the correctness of the Gouy-Chapman theory. In this communication we give further theoretical evidence that the anomalous behaviour in Ci arises through the inadequacy of the double layer model and not of the experimental results. The first fundamental objection to the relation (1) came through the solution of the mean spherical approximation integral equation for a system of charged hard spheres and dipolar hard spheres against a charged hard wall [4,5]. Subsequent improvements have considered a jellium electrode [6,7] and specific ion and dipole adsorption using Baxter’s sticky potential [8]. The present authors [9,10] have treated the ion + dipole mixture for unequal molecular radii and specific dipole adsorption with a mean field closure on that part of the singlet

Wide-range tuning of the titanium dioxide flat-band potential by adsorption of fluoride and hydrofluoric acid

The adsorption of fluoride, (HF){sub n}F{sup {minus}} ions, and hydrofluoric acid at the surface of n-type TiO{sub 2{minus}x}F{sub x} (x {approx} 0.001) electrodes was studied by electrochemical methods. Adsorption of F{sup {minus}} from acetontrile/tetraethylammonium fluoride (TEAF) solution follows a Langmuir isotherm above a fractional coverage of about 0.5, and the equilibrium constant for adsorption is 8,300 M{sup {minus}1}. Specific adsorption of fluoride ions, at millimolar concentration in acetonitrile, results in a large negative shift in the flat-band potential, as manifested by capacitance measurements and cyclic voltammetry of Ru(bpy){sub 3}{sup 2+}. Fluoride ions are strongly complexed by HF in acetonitrile to form (HF){sub n}F{sup {minus}} ions (n = 1,2), which are also specifically adsorbed at the TiO{sub 2{minus}x}F{sub x}(001) surface.

Double-layer structure at an ion-adsorbing surface

The effect of adding a strong, short-range, ion-specific adsorption potential to a fully molecular model of the electrical double layer is investigated within the framework of the reference hypernetted-chain (RHNC) theory. This theory may be solved numerically for potentials strong enough to adsorb anions onto an intrinsically neutral surface so as to produce charge densities equivalent to as much as one electron per 180 A 2