Frumkin Adsorption Isotherm Research Articles

Adsorption of 1,10-Phenanthroline in HClO4Solutions on Columnar-Structured Gold Electrodes. The Excluded Volume Effect

The electroadsorption of 1,10-phenanthroline on columnar Au electrodes from x M 1,10-phenanthroline + 0.1 M HClO 4 aqueous solutions (5 x 10 -4 ≤ x ≤ 5 x 10 -3 ), at constant potential and 298 K, was studied using voltammetry, chronocoulometry, and ac impedance combined with scanning tunneling microscopy to characterize the substrate topography. The roughness factor(R) was defined as the ratio of the O-adatom monolayer electrodesorption charge at the columnar Au electrode to that at the initial smooth Au electrode. When both R and the molar 1,10-phenanthroline concentration (c) were kept constant, the degree of surface coverage by 1,10-phenanthroline (θ) diminished as R was increased. The θ vs c plots, irrespective of R, were interpreted by a Frumkin adsorption isotherm including a constant standard adsorption free energy. The analysis of parameters derived from adsorption isotherms reflects the existence of an excluded volume effect, as should be expected from the 1,10-phenanthroline molecule adsorption cross section, and the size of the smallest voids and crevices at the substrate determined by scanning tunneling microscopy.

Time-Resolved Luminescence Investigation of the Adsorption of Ru(bpy)32+ onto Antimony-Doped SnO2 Colloidal Particles

The adsorption of Ru(bpy)32+ (bpy = 2,2‘-bipyridine) onto negatively charged antimony-doped SnO2 colloidal particles (ca. 4-nm diameter) was monitored by time-resolved luminescence. Electron injection from photoexcited Ru(bpy)32+ into the conduction band of the semiconductor particle results in luminescence quenching predominantly within the laser pulse when the complex is bound to the particle, whereas the luminescence of the unbound complex decays with a lifetime of several hundred nanoseconds. The relative concentrations of the bound and free species are thus readily obtained by determining the relative contributions of the quenched and unquenched components in the luminescence decay curve. Binding of Ru(bpy)32+ to the particles conforms to the Frumkin adsorption isotherm and suggests that the adsorbed Ru(bpy)32+ molecules self-associate with a free energy of interaction equal to −1.9 ± 0.7 kcal mol-1. The average maximal capacity of each particle for adsorbed Ru(bpy)32+ determined by the binding data, 29 ± 10, is in agreement with the value obtained by ion exchange, 30 ± 4, but is smaller than predicted for a close-packed monolayer.

Gibbs adsorption and the compressibility equation

A new approach for deriving the equation of state is developed. It is shown that the integral in the compressibility equation is identical to the isotherm for Gibbs adsorption in radial coordinates. The Henry, Langmuir, and Frumkin adsorption isotherms are converted into equations of state. It is shown that using Henry’s law gives an expression for the second virial coefficient that is identical to the result from statistical mechanics. Using the Langmuir isotherm leads to a new analytic expression for the hard-sphere equation of state which can be explicit in either pressure or density. The Frumkin isotherm results in a new equation of state for the square-well potential fluid. Conversely, new adsorption isotherms can be derived from equations of state using the compressibility equation. It is shown that the van der Waals equation gives an adsorption isotherm equation that describes both polymolecular adsorption and the unusual adsorption behavior observed for supercritical fluids.

Study of the enhancement of the boron-beryllon III system by anion surfactant using oscillopolarography

A sensitive oscillopolarographic method for the determination of trace boron, based on the complex adsorptive current of the boron-beryllon III system with the anion surfactant sodium dodecyl sulfate (SDS), which increased the current response about 16 times compared to that obtained without SDS, was reported. Under optimum experimental conditions the limit of detection was 0.5 ng/ml boron. The coordination ratio and the conditional stability constant (K) of the boron-beryllon III complex were estimated to be 1:1 and 3.8 × 10 5, respectively. The existing form of beryllon III and boron-beryllon III at pH 3.7 has been confirmed. The electrochemical behaviour of beryllon III and its complex with boron(III), both in the absence and in the presence of SDS in acetic acid-ammonium acetate (pH 3.7) was investigated. On the surface of a dropping mercury electrode, the adsorption of the B(III)-beryllon III complex in the presence of SDS obeyed the Frumkin adsorption isotherm. The adsorption coefficient ( \\ ̄ gb = 6.4 × 10 4 ), the attraction factor ( v ̄ = 1.41 ) and the amount of saturated adsorption ( Γ s = 5.8 × 10 −10 mol/cm 2) were evaluated. The mechanism of the electrode reaction and the enhancement mechanism of SDS have been suggested.

Numerical simulation of kinetically controlled electrosorption processes under cyclic voltammetric conditions

The computations described in the paper are based on the Frumkin adsorption isotherm. Variation of the electrosorption valency with potential is taken into account. The rate-determining step can be either diffusion or interfacial kinetics. The kinetics of charge transfer associated with the formation of a chemical bond between an adsorbate and an electrode is described in terms of the Butler-Volmer equation. The effect of lateral interactions between adsorbed molecules and the activated complex is also considered. The presented dependences of the peak width and peak potential on the electrode coverage and the sweep rate allow the determination of various kinetic and thermodynamic parameters of the adsorption process. Computations were carried out for both linear and spherical symmetry of diffusion.

Kinetics and thermodynamics of thioglycol adsorption on mercury ultramicroelectrodes

A method for determination of the thermodynamic and kinetic parameters of electrosorption from fast sweep rate cyclic voltammograms was developed and tested using a simulation program. The method was then used for determination of the thioglycol adsorption kinetics on Hg in KOH and H 2SO 4 aqueous solutions. Cyclic voltammetric experiments were carried out with a hemispherical Hg ultramicroelectrode (5 μm radius) at sweep rates ranging from 1000 to 100 000 V s −1. The experimental data fit relatively well to a theoretical model based on the Frumkin adsorption isotherm, despite the fact that the current model assumes independence of the electrosorption valency and the interaction parameter of the potential and the electrode coverage, and that it neglects possible reorientation of adsorbed molecules on the surface with a change in electrode coverage and/or potential. It was found that the electrosorption valency for thioglycol in both solutions studied is similar and close to unity; the standard energies of adsorption are 2.93 and −46.6 kJ mol −1 in 1 M H 2SO 4 and 1 M KOH respectively; the interaction parameters are −3670 J mol −1 (in 1 M H 2SO 4) and 2450 J mol −1 (in 1 M KOH); the rates of adsorption and desorption steps extrapolated to the zero charge potential are k a 0 = 4.7 × 1O 5 s −1, k d 0 = 9.8 × 10 6 s −1 (in 1 M H 2SO 4) and k a 0 = 1.7 × 10 12 s −1, k d 0 = 2.12 × 10 4 s −1 (in 1 M KOH). The interaction parameter for the activated complex in both studied solutions is 3000 J mol −1 larger than the corresponding interaction parameter of the product. The results are discussed in terms of participation of protons in the adsorption-desorption process and the effect of potential on the orientation of adsorbed molecules.

Inhibition studies in sweet corrosion systems by a quaternary ammonium compound

Benzyl dimethyl-n-hexadecylammonium chloride (BHDC) was investigated for its effectiveness as a corrosion inhibitor of pure iron in carbon dioxide saturated 3% sodium chloride solutions. Both a.c. impedance and d.c. electrochemical techniques were used. The compound inhibited the anodic reaction and was effective at low temperatures. Data obtained fitted the Frumkin adsorption isotherm. The inhibition characteristics are interpreted in terms of thermodynamics and the structure of the compound.

Thiourea and substituted thioureas as corrosion inhibitors for aluminium in sodium nitrite solution

The effect of sulphur-containing organic compounds such as thiourea (TU), phenyl thiourea (PTU), diphenyl thiourea (DPTU), allyl thiourea (ATU), p-tollyl thiourea (p-TTU) and N, N-dimethyl thiourea (DMTU) on the corrosion of aluminium in sodium nitrite solution at 303 K was investigated from weight loss and polarization techniques. A significant decrease in the corrosion rate of aluminium was observed by the presence of traces of these compounds. The corrosion rate was found to be a function of the nature, concentration of the inhibitor and temperature of the medium. The degree of surface coverage calculated from the results were used to evaluate the free energy of adsorption ( ΔG a o ) of the inhibitors. All the inhibitors follow the Frumkin adsorption isotherm. The mechanism of inhibitor action has been discussed on the basis of adsorption and molecular polarizability of the inhibitor. The observed experimental data indicate that these compounds act as cathodic inhibitors. The negative values of free energy of adsorption and high degree of protection also confirm the strong interaction of these compounds on the corroding aluminium surface.

The adsorption behaviour of long-chain N-alkyl betaines and their hydrobromides

Homologous N- n-alkyl- N, N-dimethylammonio acetic acid bromides with alkyl chain lengths of C 8C 26 were synthesized to characterize their adsorption behaviour at the air/solution interface at pH 7 (betaine form) and a: pH 0.5 (cationic form). The p K a of these components was 2.4 and was independent of the alkyl chain length. The adsorption behaviour was described using the Frumkin adsorption isotherm. A strong influence of the electrolyte content was observed only for the cationic form of the surfactant, on addition of KBr to the solution. The surface tension vs concentration (σ-log C) curves were independent of the addition of KBr at pH 7 up to a concentration of 0.1 M. The critical micelle concentrations of the cationic hydrobromides were higher than those of the betaines at pH 7, whereas the interfacial activities, b F, of the betaines were higher. The interfacial activities increased linearly in the b F vs n plot as the alkyl chain length n was increased. The saturation adsorption was almost independent of the alkyl chain length at pH 7. The interfacial interaction parameters were equal to zero and the adsorption parameters agreed with those of the Langmuir isotherm for samples containing up to 13 carbon atoms at pH 7 and up to 11 carbon atoms at pH 1.5. For higher alkyl chain lengths, the interaction parameters depended linearly on n and were in general higher for the cationic form than for the corresponding betaine form surfactant. Two betaines were synthesized using the corresponding hydrobromides as starting materials to compare the properties of the hydrobromides at pH 7 with those of the betaines. The similarity of the σ-log C curves and the corresponding adsorption parameters was demonstrated.

New aspects of the chemistry and mechanism of electrochemical reduction of kynurenine at mercury electrodes in aqueous media

New features of the chemistry of kynurenine in aqueous media over the pH range of 0 to 12 have been obtained using a combination of spectroscopic and electrochemical techniques at mercury electrodes. While 13C nuclear magnetic resonance (NMR) spectroscopic data show exchange averaged spectra over the pH range examined, polarographic techniques reveal the presence of three kynurenine reduction processes which are predominant at close to the 100% level at pH 2, 7 and 10 respectively. Thus, the various forms of kynurenine exchange rapidly on the NMR time scale, but not at a diffusion controlled rate, so that the species present in solution can be studied individually by electrochemical methods. The protonated form(s) of kynurenine present at low pH is adsorbed at mercury electrodes in an orientation which is parallel to the electrode surface and obeys a Frumkin adsorption isotherm. At higher concentrations of the acidic form and at all concentrations of the neutral and basic forms, the reduction processes are diffusion controlled in the limiting current region of polarograms and involve an overall irreversible two electron-two proton reduction process at the carbonyl group, with the chemical steps rather than the charge transfer process being rate determining. NMR, infra-red and mass spectral studies of the products of bulk electrolysis at a mercury pool electrode are consistent with the formation of diastereoisomers of γ( o-aminophenyl)homoserine and in the analytical context, pH 2 offers the most sensitive response towards kynurenine with limits of detection being 3.1×10 −8 M with conventional differential pulse polarography and 1.7×10 −8 M with a modified pulse method which discriminates against the overlapping hydrogen ion reduction wave.

Hydrazine and Substituted Hydrazines as Corrosion Inhibitors for Mild Steel in Alkaline Mine Water

The corrosion of mild steel in alkaline mine water containing various concentrations of hydrazine, phenyl hydrazine and 2,4,Dinitro phenyl hydrazine has been studied by weight loss and polarization techniques at 303 K. The hydrazine and substituted hydrazines inhibit corrosion effectively even at low concentrations. The degree of corrosion inhibition is a function of temperature, concentration and nature of the inhibitor. The corrosion inhibition is explained by considering the adsorption and/or complex formation by the inhibitors on the corroding mild steel surface. Thermodynamic parameters for adsorption are calculated by using the Frumkin adsorption isotherm. These organic compounds act as anodic inhibitors.

Adsorption of poly(oxyethylene)dodecyl ethers at the water—nitrobenzene interface

The adsorption phenomena of non-ionic surfactants poly(oxyethylene)dodecyl ethers C 12E x at the water—nitrobenzene interface were studied by means of ac impedance measurement. The interfacial capacitance C was measured as a function of potential difference E at various concentrations of C 12E x in nitrobenzene. The inner layer capacitance C i and the inner layer potential difference Δ w nφ i decreased when the non-ionic surfactant C 12E x was added to the nitrobenzene phase. The surface coverage was evaluated from impedance data fitted to the Frumkin adsorption isotherm at the lower concentrations. The experimental results indicate that all the three C 12E x studied are strongly adsorbed at the water-nitrobenzene interface. The surface activities of these adsorbates are in the order C 12E 6 > C 12E 4 ⩾C 12E 8.

Studies on the polarographic behaviour of estazolam

In 0.1 M NH 3-NH 4Cl buffer (pH 9.2) a sensitive 2-electron reduction wave of estazolam is obtained by single-sweep oscillopolarography. The peak potential is −1.08 V ( vs. SCE). The peak height is proportional to the concentration of estazolam over the range 1.0 × 10 −7–9.0 × 10 ∂ M. The detection limit is 5.0 × 10 −8 M. The behaviour of the reduction wave has been studied and applied to the determination of estazolam. The reduction process is irreversible and the wave shows adsorptive characteristics, the behaviour obeying the Frumkin adsorption isotherm. The adsorption coefficient β is 1.16 × 10 6 1./mole and the interaction factor α is −1.06. The mechanism of the electrode reaction is discussed.

The inhibitive action of dimethyltin dichloride towards the corrosion of aluminium in hydrochloric acid and sodium hydroxide solutions

AbstractThe dissolution of aluminium in hydrochloric acid and sodium hydroxide solutions in the presence of dimethyltin dichloride as corrosion inhibitor has been studied using hydrogen evolution and thermometric methods. The two methods gave consistent results. The higher inhibition efficiency of this compound in acidic than in alkaline media may be due to the less negative potential of aluminium in hydrochloric acid solution, favouring adsorption of the additive. The adsorption of this compound was found to obey Frumkin adsorption isotherm.

Electrosorptive detection of simple organic compounds in liguid chromatography

The detection of a range of neutral organic compounds in liquid chromatography based on decreases in differential double-layer capacitance, ΔCd, at a mercury—aqueous interface caused by analyte specific adsorption is described. The arrangement employs alternating current phase-selective measurements using a large-volume wall-jet configuration. The low-molecular-weight organic solutes examined include aliphatic alcohols, diols, mono- and dicarboxylic acids, amines and alkanolamines. The differential capacitance measurements were conducted close to the potential of zero charge, where adsorption of such species is most extensive. Plots of ΔCdversus analyte concentration were generally sigmoidal, in accordance with expectations from the Frumkin adsorption isotherm. A potential-step coulometric method, where variations in the non-faradaic charge, Δ(Δq), are measured, was found to be a useful alternative detection scheme. Methods based on measuring transient capacitive currents associated with tensammetric adsorption—desorption behavior were also briefly investigated. A virtue of these double-layer capacitance detection schemes is that the magnitude of the ΔCd or Δ(Δq) response exhibits a clear sensitivity to the organic molecular structure, as anticipated in view of the known dependence of the adsorption thermodynamic upon solute hydrophobicity.

Standard states and the frumkin adsorption isotherm for substances at electrode interphases

Using the saturation condition of the interphase, a relationship between the surface tension and the chemical potential of both the solvent and the adsorbate is obtained. Starting with the adsorption reaction, the bulk and surface chemical potentials of one component are functions of the activity terms only. The classical experimental isotherm cannot be used in the case of a Frumkin isotherm, nor in order to obtain an invariant standard state. Reported values of Δ G o ads modified and then, some qualitative conclusions obtained by comparing different adsorption systems are given.

Adsorption of phospholipids at the interface between two immiscible electrolyte solutions: Part I. Equilibrium adsorption of phosphatidylcholines at the water/nitrobenzene interface

An impedance technique was used to study the adsorption of 1,2-dilauroyl- (DLPC), 1,2-dimyristoyl-(DMPC) and 1,2-dipalmitoylphosphatidylcholine (DPPC) at the interface between 0.05 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene. The surface coverage evaluated from impedance data fits the Frumkin adsorption isotherm with the adsorption Gibbs energy decreasing in the sequence DLPC > DMPC > DPPC from −35.7 to −37.9 kJ mol −, which points to a strong adsorption of all three phosphatidylcholines at the water/nitrobenzene interface, and with the lateral interaction parameter equal to ∮.4, which indicates weak repulsive interactions in the adsorbed layer. The observed increase of the inner-layer potential difference as well as a positive shift of the zero-charge potential difference are probably connected with the preferential orientation of the adsorbed phosphati-dylcholine molecules at the interface with their polar heads or hydrocarbon chains directed towards the aqueous or organic solvent phase, respectively.

Adsorption of thiourea at the polycrystalline silver electrode from aqueous KF solutions—II

The Frumkin adsorption isotherm parameters and the virial isotherm parameters are determined from the earlier published experimental data on double layer differential capacity measurements of polycrystalline silver electrode in aqueous KF solutions containing thiourea [M. Mi l ̵ kowska, Electrochim. Acta 32, 159 (1987)] using the Parsons analysis. Thiourea is also used as the probe dipole to evaluate the electrostatic parameters of the inner layer.

Monolayer Formation of Dilauroylphosphatidylcholine at the Polarized Nitrobenzene–Water Interface

Abstract Double layer capacitance of the polarized nitrobenzene–water interface has been measured in the presence of the adsorption of l-α-dilauroylphosphatidylcholine (DLPC). The double layer capacitance was lowered with the increase of the DLPC concentration in the negative branch of the potential range where the aqueous phase was in negative potential with respect to the nitrobenzene phase. A saturated monolayer of DLPC was formed at a concentration of DLPC above 20 μmol dm−3. The area occupied by a DLPC molecule at the saturated monolayer was 0.73 nm2 which was estimated by comparing the capacitance data with those obtained from a spread monolayer of DLPC at the interface. In the negative branch the monolayer was stable over 100 mV of the applied potential across the monolayer. The adsorption was described by the Frumkin adsorption isotherm. The interaction between the adsorbed DLPC molecule was weak, corresponding to the penetration of nitrobenzene molecules and, possibly, ions between the hydrocarbon chains of DLPC. This penetration also explains the high capacitance value of 11 μF cm−2 for the saturation monolayer. In the potential range where the aqueous phase is positive, the capacitance increased with the DLPC concentration.

Inhibitive action of some schiff bases and amines towards the corrosion of copper in an aqueous alcoholic medium

Some Schiff bases and relative amines were tested as copper corrosion inhibitors in aqueous 25 vol.% ethanolic 0.1 M NaCl solution at 30 °C. Polarization curves and weight loss measurements allowed the inhibitor efficiencies of the additives to be screened. The data showed a remarkable decrease in the corrosion rate only in the presence of N-2-mercaptophenyl-salicylideneimine (BSS) or 2-mercaptoaniline (AS). Nevertheless, better results were always obtained with the Schiff bases than with the amines. In order to study the nature of the solid products formed in free corrosion conditions or, in the case of BSS, in anodic galvanostatic conditions, their IR spectra were compared with those of synthetic complexes obtained with these organic substances and copper(I) or copper(II). The surface analysis of the specimens at the end of the immersion tests was carried out using scanning electron microscopy and photopotential measurements. Further investigation showed that the inhibition of copper corrosion by BSS and AS followed the Langmuir and the Frumkin adsorption isotherms, respectively, over a wide range of concentrations. The computed adsorption energies together with some tests on the pre-filming action of the two substances indicated chemisorption as the inhibition mechanism.