Helmholtz Plane Research Articles

Cation Effects on Infrared Reflection Absorption Spectra of Cyanide Adsorbed on Pt(111) Electrode in Electrolyte Solutions

Abstract In-situ infrared reflection absorption spectroscopy was applied to the investigation of cation effects on the C–N stretching frequency of cyanide ions on Pt(111) electrode in electrolyte solutions. Only a single absorption peak was observed at ca. 2100 cm−1 in solutions containing H3O+, Li+, Na+, K+, or Cs+. In a KClO4 solution (pH = 9.5), a change in slope was observed in the peak frequency–potential curve at ca. −0.5 V vs. the saturated calomel electrode. It was concluded that in the cathodic region in the KClO4 solution, H3O+ dominates the outer Helmholtz plane as a result of cation exchange. In alkaline solutions, the slope for the frequency shift is dependent on the cations in the order K+ > Na+ > Li+ ≈ Cs+. In HClO4, a change in slope was also observed in the peak frequency-potential curve. In a Mg(ClO4)2 solution, three peaks were observed for CN on Pt(111), indicating that factors other than the simple electrochemical Stark effect must be considered for interpreting the influence of this strongly adsorbed cation.

Relationship between Heterogeneous and Homogeneous Kinetics of Electron Transfer between Transition Metal Complexes in Aqueous Solution: Volumes of Activation

Electrochemical rate constants kel and volumes of activation ΔVel⧧ for self-exchange at an electrode of the aqueous couples Co(phen)33+/2+, Co(en)33+/2+, Fe(H2O)63+/2+, Co(diamsar)3+/2+, Co(diamsarH2)3+/2+, Co(sep)3+/2+, Co(ttcn)23+/2+, Fe(phen)33+/2+, Mo(CN)83-/4-, and Fe(CN)63-/4- have been measured by high-pressure AC voltammetry over the range 0.1−200 MPa at 25 °C; the respective values of ΔVel⧧ are −9.1, −8.3, −5.5, −3.5, −3.8, −3.0, −2.8, −1.6, +7.3, and +11 cm3 mol-1. Although the theory of Marcus (Electrochim. Acta 1968, 13, 1005) suggests that ln kel should be linearly related to 1/2 ln kex, where kex is the rate constant of the corresponding homogeneous (bimolecular) self-exchange reaction, ln kel is often sensitive to the nature of the working electrode and the supporting electrolyte and is only weakly correlated with ln kex, with slope ≈0.1. In contrast, ΔVel⧧ = (0.50 ± 0.02)ΔVex⧧, in precise agreement with an extension of Marcus' theory, regardless of the nature of the electrode and the supporting electrolyte. This result implies that electron transfer in these couples occurs adiabatically on direct ion−ion and ion−electrode contact (i.e., within the outer Helmholtz plane), and also that ΔVel⧧ values are predictable in the manner described elsewhere (Can. J. Chem. 1996, 74, 631) for ΔVex⧧. Conversely, where ΔVex⧧ cannot be measured for technical reasons (e.g., where paramagnetism of both reactants precludes NMR measurements of kex), it can be reliably estimated as 2ΔVel⧧.

The behaviour of cadmium monocrystals and polycrystals in acid chloride solution

The polarisation characteristics of Cd polycrystals and monocrystals, with (0001), (112̄0), and (101̄0) surfaces, were studied in 0.1 M NaCl + 0.04 M HCl (pH 1.3) and compared with thermodynamic equilibria. Anodic dissolution rates were independent of surface orientation, exhibiting Tafel slopes ( b a ) near 27 mV that were reasonably consistent with a rate controlled process based on mass transport of CdCl + from the Outer Helmholtz Plane (OHP). Other processes are likely to have played a contributory role in the anodic behaviour, because crystallographic faceting accompanied dissolution. Cathodic evolution of hydrogen exhibited mixed rate control, with a cathodic Tafel slope ( b c ) near −81 mV. Hydrogen evolution appeared to be sensitive to surface orientation, with the (101̄0) surface showing the largest deviations in behaviour.

Advances in the Mechanism of Deposition of the CrO42-, HCrO4-, and Cr2O72- Species on the Surface of Titania Consisting of Anatase and Rutile

Following a methodology based on the “2pK/one site” and “triple layer” models, we further investigated the mechanism of deposition of the CrO42-, HCrO4-, and Cr2O72- species on the titania/electrolyte solution interface. The surface of the titania used is consisted of anatase and rutile patches. This methodology involved the following steps: (i) The assumption of tentative deposition equilibria. (ii) The derivation of the corresponding equations. (iii) The calculation of the concentration of each of the deposited CrVI−oxo species as well as the total concentration of the deposited CrVI at various pH's and bulk solution concentrations. (iv) The calculation of the variation with pH of the difference in the hydrogen ions consumption by the titania surface in the presence and absence of the CrVI−oxo species in the impregnating solution. (v) The calculation of the variation with pH of the ζ-potentials developed above the anatase and rutile patches. (vi) The comparison of the calculated parameters and variations mentioned above with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis. It was found that the deposition takes place following the equilibria: TiOH2+ + CrO42- ↔ TiOH2+···CrO42-; TiOH2+ + HCrO4- ↔ TiOH2+···HCrO4-; TiOH2+ + Cr2O72- ↔ TiOH2+···Cr2O72-; TiOH + CrO42- ↔ Ti−O−(CrO3-) + OH-; TiOH + HCrO4- ↔ Ti−O−(CrO2)−OH + OH-; TiOH + Cr2O72- ↔ Ti−O−(Cr2O6-) + OH-. At pH's 7.5 and 8.0 the deposition occurs exclusively by surface reaction (fourth and fifth equilibria). In the pH range 6.0−4.0 all the above equilibria contribute to the deposition. The only exception is that the fourth equilibria does not practically occur at pH ≤ 4.5. However, in the pH range 6.0−4.0 the deposition by adsorption of the CrVI species, mainly of the HCrO4- species, is the predominant process. It should be noted that the extent of the deposition in the anatase region is much larger than that on the rutile region. Lateral, attractive, interactions are exerted between the deposited CrVI−oxo species through water molecules being in the inner Helmholtz plane (IHP). The intensity of these interactions is proportional to the charge of the deposited CrVI−oxo species.

The influence of an acid treatment on the surface charge density of silica gel

The influence of an acid treatment on the pore structure as well as on the surface charge density of porous silica was investigated. It is shown that this treatment causes only small changes of the pore structure. Positive values of the surface charge density at pH>4 are interpreted in terms of surface impurities consisting of Na+ ions resulting from the synthesis of the gel from sodium silicate solution. This effect is strongly influenced by the acid treatment. The surface charge density parameters were evaluated on the basis of the triple-layer model for the electrical double layer. Here two different mechanisms of counterion attachment in the inner Helmholtz plane are discussed.

Study of pH effect on the relaxation phenomenon of polyaniline by electrochemistry and XPS

In this work, we report the pH effect on the relaxation phenomenon of polyaniline for the first redox system. The redox peaks of polyaniline are pH dependent over a certain pH range (up to pH approximately 1 for the first redox system). However, we observed that the relaxation phenomenon is affected by pH only at pH 2. This can be accounted for by a change in the value of the potential at the outer Helmholtz plane ( ψ 1 ), which conditions the magnitude of the local pH. Since it is known that ψ 1 depends on the ionic concentration, we measured the ionic concentration inside the polyaniline film by XPS and showed that it is indeed a function of pH.

Tungsten–Oxo-Species Deposited on Alumina. I. Investigation of the Nature of the Tungstates Deposited on the Interface of the γ-Alumina/Electrolyte Solutions at Various pH's

The mechanism of the deposition of the W(vi)species from aqueous solutions on the γ-alumina surface is refined in this work by investigating critical mechanistic points. A methodology recently developed to investigate the deposition of the Co(II), Ni(II), Cr(VI), and Mo(VI)on γ-alumina has been applied. This methodology is based on the “2pK/one site” and “triple-layer” models and involves the writing down of various deposition equilibria, the derivation of the corresponding equations, the calculation of the amount of the deposited W(vi)(through the calculated concentrations of the W(vi)species formed on γ-alumina) at various values of the impregnating parameters, the calculation of the variation, with pH, of the ζ-potential and of the difference in the H+ions consumption by the support surface in the presence and absence of the W(vi)species in the impregnating solution. The comparison of the calculated values of the aforementioned parameters with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to establish the mechanism of deposition of W(vi)species on γ-alumina. It was found that although eight different W(vi)species are present in the impregnating solution under the conditions of deposition, the mechanism of deposition was proved to be quite simple:[formula]According to this mechanism only two W(vi)species contribute to the whole deposition. These species move from the bulk solution to the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support particles and the impregnating solution and then are adsorbed on sites created in the IHP by protonated surface hydroxyl groups of the support. Moreover, the monomeric WO42−species, being in the IHP, may react with a single or a pair of adjacent neutral surface hydroxyl groups of the support resulting from the formation of a charged or uncharged W(vi)species, respectively. Lateral intractions are exerted between W(vi)species formed through water molecules also located in the IHP. The study of the variation of the saturation surface concentration of the species illustrated in the rhs of the above equilibria showed that in the pH range 10–6 the deposition occurs practically via reaction (equilibria [c] and [d]). The concentration of Al–O–(WO2)–O–Al and Al–O–(WO3−) species is maximized at pH's 7 and 6, respectively. In the pH range 6–5 both surface reaction and adsorption (equilibria [c] and [a]) contribute to the whole deposition process. The concentration of the AlOH2+…WO42−is maximized at pH 5. Finally in the pH range 5–3.5 the deposition takes place exclusively by adsorption (equilibria [a] and [b]). The concentration of the W(vi)species illustrated in the rhs of the second equilibrium is maximized at pH 3.5. The selective deposition of the WO42−species with respect to the various polymeric species present in the impregnating suspension was attributed to the relatively low negative charge of these species and the negative potential developed at the IHP.

An ion-binding model for ionic surfactant adsorption at aqueous-fluid interfaces

A simple ion-binding model is presented to quantify the equilibrium adsorption of ionic surfactants at aqueous-fluid interfaces. The proposed model adopts a triple layer structure for the interface: a plane of adsorbed surfactants (interface plane), a plane of partially dehydrated, contact-bound counterions (inner Helmholtz plane), and a plane of hydrated counterions (outer Helmholtz plane). An analytic expression for the surface tension is obtained as a function of the physicochemical parameters of the system. It generalizes the classical results of J.T. Davies and E.K. Rideal (Interfacial Phenomena, Academic Press, New York, 1963) as well as those, more recent, of R.P. Borwankar and D.T. Wasan (Chem. Eng. Sci., 1 (1986) 199). In the ion-binding model, the surface tension depends on the electrocapacitance in the layers closest to the interface and the distances between them, in addition to the surface charges on the planes. For the limiting case of a moderate concentration of surfactant, asymptotic formulae for the surface tension are derived. On a semilogarithmic graph of surface tension versus surfactant concentration in the presence of background electrolyte, the asymptotic slope approaches - kT(M t), where k is Boltzmann's constant, T is temperature, and (M t) is the surface concentration of total sites, M t, available for surfactant headgroups in the interface, the parentheses indicating concentration. In the case of no salt added, the asymptotic slope is −2 kT(M t). The asymptotic formulae also establish the influence on the surface tension of the equilibrium constants and the lateral interaction parameter, ω, in Frumkin's isotherm. The ion-binding model results are in good agreement with the surface and interfacial tension data for sodium dodecyl sulfate (SDS). Agreement with measured ξ-potentials is also found for SDS at the air-water boundary.

Ionic surface diffusion coefficients using an electrostatic patchy charge model

A three-dimensional electrostatic patchy charge model is developed to compute surface diffusion coefficients as a function of background ionic strength for sodium counter-ions adsorbed at the inner Helmholtz plane (IHP) of a charged montmorillonite | aqueous interface. The model preserves the discrete nature of the montmorillonite lattice charge by arranging discrete patches of different charge densities at the surface. The salinity dependence of the surface diffusion coefficients arises from an electrical energy barrier to translation of counter-ions along the IHP which is due to the oscillating electrostatic potential profile at the surface. This energy barrier is computed numerically using a three-dimensional finite-element program. We discuss the assumptions implicit in the patchy charge model, and we point out the advantages of performing the three-dimensional calculation over other widely used approaches to modeling heterogeneously charged surfaces. The influence of the critical model parameters on the behavior of the surface diffusion coefficients with changing electrolyte strength is explored. The predicted dependence of the surface diffusion coefficients on ionic strength from the patchy charge calculation yields excellent agreement with experimental data on the conductivity of water-saturated shaly sands.

Effects of electric potentials on surface forces in electrolyte solutions

Atomic force microscopy was employed to analyze the electric double layer on an Au film electrode deposited on mica. An Au-covered or a bare Si3N4 tip was used as a probe. An electrolyte solution was aqueous NaOH of pH 10.9. The electrode and the tip potentials were controlled electrochemically by a potentiostat. Repulsive forces were always observed regardless of the polarity of the potential, i.e., the positive and the negative side of the point of zero charge. The result indicates that the strongly adsorbed OH− anions overcompensated the positive charge at the Au surface. The potential of the outer Helmholtz plane at an electrode potential was estimated from the force–distance curves.

Specific adsorption of potassium penicillin G on mercury electrode

The specific adsorption of penicillin G (salt K) on the mercury in aqueous solution has been determined from electrocapillary and capacity measurements. The adsorption of penicillinium anions can be fitted by a Frumkin isotherm with a value of −3.5 for the interaction parameter. The standard Gibbs energy of adsorption, ΔG 0, is a linear function of the surface charge and the ΔG 0 value at zero charge is −38.6 kJ mol −1 . The inner layer capacity is analysed into its components and these are related to the distances from the inner and outer Helmholtz planes to the metal surface. The possible orientation of the antibiotic on the electrode surface is discussed.

Estimation of the Hamaker constant from flocculation in the secondary minimum and its experimental verification in particle adhesion

The critical flocculation concentration, CFC, of a polystyrene latex (PSL) dispersion was determined from the change in transmittance after adding an electrolyte solution. The apparent values of the CFC obtained from transmittance vs. electrolyte concentration curves decreased with increasing stand time. From the CFC value obtained at the shortest stand time and the experimentally determined outer Helmholtz plane potential, the Hamaker constant between PSL particles in water was calculated by considering particle aggregation in the secondary minimum of the potential energy curve. The Hamaker constant was verified by adhesion experiments: the adhesion of PSL particles to a quartz plate was explained in terms of the total potential energies of interaction calculated from the obtained Hamaker constant on the basis of the heterocoagulation theory.

An experimental trial for the determination of the critical coagulation concentration using the sedimentation method

A method for determining the critical coagulation concentration (Cc) from the change in the transmittance of the sol with stand time after adding a coagulating agent is discussed. Potassium nitrate was used as the coagulating agent because the specific adsorption of electrolyte ions on the particle and the hydrolysis of electrolyte ions are negligible. Apparent critical coagulation concentrations,Cca, of iron (III) hydroxide and silver iodide sols were obtained from the transmittance vs. potassium nitrate concentration curves for various stand times. The values ofCca decreased with increasing stand time. TheCca value obtained for the shortest stand time was closer toCc obtained from the initial turbidity change of the sol by applying Rayleigh's law. The Hamaker constant for the particle in water was calculated from theCca value obtained at the shortest time and the experimentally determined outer Helmholtz plane potential. The calculated Hamaker constants were comparable to the theoretical values for iron (III) hydroxide and silver iodide.

Codeposition of Mo (VI) Species and Ni 2+ Ions on the γ-Alumina Surface: Mechanistic Model

The codeposition of Mo (VI) species and Ni 2+ ions on the γ-alumina surface, using the "equilibrium deposition followed by filtration" (EDF), has been studied in the pH range 4.1-6.2. An increase in the adsorptivity of the γ-alumina for both the Mo (VI) species and Ni 2+ ions was observed at all pH values studied. This was attributed to the stronger lateral interactions exerted between the codeposited Mo (VI) and Ni 2+ ions as they compared with those exerted between the deposited Mo (VI) species in the absence of the Ni 2+ ions and between the deposited Ni 2+ ions in the absence of the Mo (VI) species. A codeposition model was developed and tested on the basis of the results obtained from deposition experiments, potentiometric titrations, and microelectrophoresis. According to this model the deposition of the Mo (VI) species on the γ-alumina surface in the presence of the Ni 2+ ions takes place mainly via adsorption of the MoO 2- 4 and Mo 7O 6- 24 ions on sites created by the protonated surface hydroxyls. Each adsorption site is created by one protonated surface hydroxyl. These sites are located in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support and the impregnating solution. The extent of deposition through reaction of one MoO 2- 4 ion with two neutral surface hydroxyls was proved to be negligible. Moreover, it was found that the extent of adsorption of MoOn 2- 4 (Mo 7O 6- 24) decreased (increased) as pH decreased. Finally, it was shown that the total deposition of Mo (VI) increased as pH decreased. Concerning the Ni deposition, according to the established model, it takes place through adsorption of one Ni 2+ ion on a site created by one deprotonated surface hydroxyl in the IHP. As pH increased, the concentration of the deprotonated surface hydroxyls and thus the amount of the deposited Ni increased.

Mechanistic aspects of the deposition of the Cr(VI) species on the surface of TiO 2 and SiO 2

The deposition of the negatively charged Cr(VI) species on TiO 2 and SiO 2 surfaces suspended in aqueous electrolyte media has been studied in the pH range 3.0–8.0. The investigation included deposition isotherm and microelectro-phoretic mobility measurements. From the experimental results and a theoretical analysis of the deposition isotherms, it was concluded that the Cr x O y z− ions taken up were located in energetically equivalent sites at the inner Helmholtz plane (IHP) of the electric double layer that had developed between the support particles and the impregnating solution. The deposition sites were mainly created by protonated surface hydroxyl groups in the case of SiO 2 and by protonated and undissociated surface hydroxyl groups in the case of TiO 2. The lateral interactions operative among the deposited Cr(VI) species were, in general, strong, being stronger in SiO 2 supports in comparison with those of TiO 2. Moreover, it was found that upon a pH decrease from 6.0 to 3.0, the extent of deposition of the Cr(VI) species on silica increased from 0.01 to 0.13 μmol Cr(VI) m −2. The extent of deposition of the Cr(VI) species on titania increased also from 0.23 to 1.68 μmol Cr(VI) m −2 with a pH decrease from 8.0 to 4.8. Further decrease in the pH in the case of titania resulted in a considerable decrease in the extent of deposition.

In‐situ‐Monitoring of Electrochemical Double Layer Structure Changes at Gold with a Phase‐Controlled Quartz Microbalance

AbstractContinuum hydrodynamics and no‐slip boundary conditions apply to electrochemical quartz microbalances of the thickness shear mode type with gold in aqueous contact. Electrochemical phase‐stabilized quartz micro‐balance measurements can give insight into ion adsorption, solvation, hydrogen bonding, and water clustering at charged surfaces. Species in the outer and inner Helmholtz layer can be treated as rigidly coupled masses. Frequency changes on polycrystalline gold electrodes in alkaline aqueous contact, and in the potential range from the hydrogen evolution up to the bulk oxide formation, are primarily caused by ion solvation and ion pair formation. Specifically adsorbed anions, like sulfate and hydroxide, are stripped of nearly half of their original solvation shell, and function as counter charge carriers analogously to completely solvated anions in the outer Helmholtz plane. Their saturation coverage is limited by lateral electrostatic repulsion and steric crowding by their solvation shells. Specifically adsorbed anions which form neutral ion pairs with alkali metal cations practically do not exhibit electrostatic repulsion and solvation shell crowding. They contribute much stronger to the double layer loading than specifically adsorbed partially hydrated ions and non‐specifically attracted species. In the oxide monolayer potential region, electrosorbed hydroxyl functions are deprotonated, and become neutralized by alkali metal ions at high pH. In a first order approximation, viscous fluid coupling and roughness changes do not have to be invoked to explain the observed frequency data.

Calculation of outer Helmholtz potential at mercury as a function of temperature and concentration of aqueous sodium fluoride solutions

Abstract Potentials of the outer Helmholtz plane, o 2 , at mercury as function of electrode potential and temperature are calculated on the basis of the Gouy-Chapman theory using Grahame's data for surface charge and capacitance in aqueous NaF solutions. An empirical equation for calculating o 2 at negative rational potential is presented. Grahame's original data of surface charge and capacitance are summarized in ASCII files on floppy disk.

Electrophoretic Deposition of Phosphors

As part of a fundamental study of the electrophoretic deposition of phosphor particles, a suspension composed of Zn2SiO4; Mn (P-1) or ZnS:Ag (P-11) phosphor particles in isopropyl alcohol (IPA) containing various nitrate salts was characterized by conductivity and zeta-potential measurements. Dissociation constants and limiting ionic mobilities of NaNO3, Mg(NO3)2, and Y(NO3)3 in IPA were determined from conductivity measurements. It was found that the dissociation constants are very low and decrease as the metal ion valency increases. The zeta potential of the phosphors was measured in IPA under a wide range of salt concentrations and pH values. In all cases, the zeta potential was approximately -40 mV at salt concentrations less than 10-6M. As the concentration of Mg(NO3)2, Y(NO3)3, or Al(NO3)3 increased, the zeta potential increased and became positive, reaching a maximum of approximately 40 mV at 10-5 M. In IPA or IPA containing 10-5M Mg(NO3)2, the zeta potential was positive at pH < 6, but became negative for pH > 6. The zeta potential remained positive at all pH values for Mg(NO3)2 concentrations of 10-4 and 10-3 M. The dependence of zeta potential on salt concentration was modeled using the Gouy-Chapman-Stern-Grahame model of the double layer. Using low values of capacitance for the inner and outer Helmholtz planes and a large adsorption potential, the model results agreed well with the experimental data.

Determination of the high temperature zeta potential and pH of zero charge of some transition metal oxides

The zeta potential of a particle (i.e. the potential difference between the outer Helmholtz plane of the electrical double layer formed at the particle/fluid interface and the bulk fluid) is a measure of its surface charge. Typically, measurements of the zeta potential have been made at ambient temperature. This paper provides a brief description of new equipment used to measure streaming potentials of oxides at elevated (235°C) temperature from which zeta potentials were calculated. Measurements are reported of the zeta potential of simulated colloids (i.e. corrosion products such as magnetite and hematite) typically generated in the operation of nuclear and fossil-fired power plants. Because zeta potentials provide a framework for interpreting the transport and deposition of colloids, measurements also are reported of oxides that could possibly serve as coatings to prevent deposition of corrosion products on selected surfaces in power plants.

Molybdenum-oxo Species Deposited on Alumina by Adsorption: III. Advances in the Mechanism of Mo(VI) Deposition

The refinement of the mechanism of Mo(VI) deposition (proposed previously) from aqueous solutions on the γ-alumina surface by investigating critical mechanistic points is the subject of the present work. A mechanistic model involving the adsorption of Mo7O6−24 and MoO2−4 ions on sites created by the protonated surface hydroxyls of γ-alumina in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the γ-alumina particles and the molybdate solutions, as well as the deposition of the MoO2−4 ions through surface reaction with the neutral surface hydroxyls has been developed; ft has been tested over a wide range of impregnation parameters (pH = 8.5-4.1 at 25°C, temperature 25-50°C at pH = 5 and doping of the carrier with various amounts of Na+ and Li+ ions). The testing of the model included the derivation of various equations corresponding to the model deposition equilibria, the calculation of the amount of the deposited Mo, of the difference in the isotherms of the hydrogen adsorption in presence and absence of molybdates, and of the ζ-potential of the γ-alumina particles in the molybdate solution (using an interactive code for the calculation of chemical equilibria in aqueous systems called SURFEQL), and the comparison of these parameters with the corresponding ones determined experimentally. The agreement between the calculated and experimentally determined parameters over a wide range of impregnating conditions allowed us to shed more light on the mechanism of the Mo deposition. It was established that both the adsorption of Mo7O6−24 and MoO2−4 ions and the deposition of MoO2−4 ions by surface reaction with two neutral hydroxyls of the support take place, but the contribution of each of these processes depends on the impregnating conditions. Specifically, in the pH range 8.5-6. 1 the deposition practically occurs through the reaction of MoO2−4 ions with neutral surface hydroxyls of the support resulting in the formation of the surface complex [formula] whereas at pH = 5 the adsorption of Mo7O6−24 and MoO2−4 ions is predominant and at PH = 4.1 the adsorption of Mo7O6−24 ions prevails. Increasing the impregnating temperature, at pH = 5, from 25 to 50°C increases considerably the adsorption of Mo7O6−24 ions and decreases further the deposition of MoO2−4 ions by surface reaction, whereas it does not change considerably the extent of adsorption of MoO2−4 ions. Finally, the doping with Li+ and Na+ ions increases both the extent of adsorption of Mo7O6−24 ions and the extent of MoO2−4 deposition by surface reaction, whereas it does not change considerably the extent of adsorption of MoO2−4 ions. Moreover, it was demonstrated that each protonated surface hydroxyl creates one adsorption site and that strong lateral interactions are exerted between the deposited species, mainly between the adsorbed MoO2−4 and Mo7O6−24 ions, through water molecules located at the IHP. On the other hand, comparison of the adsorption constant with the reaction constant demonstrated that the ionic sorptive band is much stronger than the covalent Al-O-Mo bonds on the above-mentioned surface complex. Finally, the fact that the ratio [MoO2−4]b/[Mo7O6−24]b calculated in the bulk solution was always lower than the ratio [MoO2−4]adsorb+react/[Mo7O6−24]adsorb calculated in the deposited state corroborated the finding reported previously that, concerning deposition, the selectivity of the support surface for the MoO2−4 ions is higher than the selectivity for the Mo7O6−24 ions.