Potential Of Maximum Adsorption Research Articles

Adsorption of sodium dodecylsulphate on mercury as an example of micellization within a multilayer interphase

The adsorption of sodium dodecylsulphate from aqueous electrolytic solutions on a polarized mercury electrode was studied by means of differential capacitance measurements over a wide range of concentration and potential, and in the presence of different supporting electrolytes with different ionic strengths. An interpretation of the differential capacitance vs. applied potential curves is given, based on theoretical treatments developed previously. It is shown that at concentrations below the cmc, two-dimensional aggregates are formed on the electrode surface within a polarization region which is bounded by two capacitance peaks at extreme positive and negative polarizations. This film is not particularly stable and is transformed into a compact layer at polarizations close to the potential of maximum adsorption, resulting in a capacitance pit. With increasing bulk concentration the aggregation process extends across the interphase and at least two layers of aggregates—micelles are formed at concentrations around and above the cmc. This three-dimensional aggregation is characterized by the appearance of deformed and/or split capacitance peaks which determine the polarization region wherê this phenomenon occurs.

Adsorption of pyridine at the Au(210)—solution interface

Adsorption of pyridine at an Au(210) single-crystal electrode has been investigated using chronocoulometry, cyclic voltammetry and phase-sensitive ac voltammetry. The adsorption parameters, such as the relative Gibbs surface excesses, the Gibbs energies of adsorption, the electrosorption valencies and the potential drop across the inner layer, have been calculated. These results suggest that pyridine molecules assume a vertical, N-bonded, orientation at the Au(210) surface. The full coverage Gibbs energy of adsorption at the potential of maximum adsorption is approximately equal to −63 kJ mol −1, which is a value typical of weak chemisorption. The adsorption of pyridine at the (210) surface was compared with adsorption at the (311), (110), (100) and (111) planes of gold.

Adsorption of pyridine at the Au(311)—solution interface

The energetics of pyridine adsorption onto a Au(311) single-crystal electrode surface have been investigated using chronocoulometry. Adsorption parameters such as the relative Gibbs surface excesses, the Gibbs energies of adsorption and electrosorption valencies have been determined. The data have been analysed in terms of both the electrode potential and surface charge density. Over the potential region investigated (−0.8 V to +0.6 V), pyridine has been found to adsorb in the vertical orientation with the nitrogen end of the pyridine molecule facing the Au(311) surface. The limiting surface concentration for this orientation was found to be 6.6±0.3×10 −10 mol cm −2. The Gibbs energies of adsorption were calculated using the Henry's law isotherm. At the potential of maximum adsorption, Δ G max 0 was found to be equal to −44 kJ mol −1. The absolute value for Δ G max 0 is fairly large, indicating that pyridine is weakly chemisorbed onto the Au(311) electrode surface.

Adsorption of pyridine at the Au(111)-solution interface

The energetics of pyridine adsorption onto an Au(111) single crystal electrode surface have been characterized using chronocoulometry. The adsorption parameters such as the relative Gibbs surface excesses, the Gibbs energies of adsorption, and electrosorption valencies have been determined as a function of the electrode potential and surface charge density. Over the potential region investigated (−0.8 V to +0.6 V) pyridine has been found to adsorb in two different orientations. The flat orientation is observed on the negatively charged electrode surface while the vertical orientation is present on the positively charged surface. Reorientation between the flat and the vertical orientations takes the form of a two-dimensional phase transition. The limiting surface concentrations have been found to be 1.4 × 10 −10 mol cm −2 and 6.7 × 10 −10 mol cm −2 for flat and vertical orientations of pyridine, respectively. For pyridine adsorbed in the flat orientation the Gibbs energies of adsorption, Δ G°, were determined using the Henry's Law isotherm. At the potential of maximum adsorption, Δ G 0 max is equal to −37 kJ mol −1. The absolute value for Δ G max is fairly large indicating that the pyridine molecules are weakly chemisorbed at the Au(111)-solution interface.

Adsorption of pyridine at the Au( 110)-solution interface

The energetics of pyridine adsorption onto a Au(110) surface have been characterized quantitatively using chronocoulometry. The adsorption parameters such as relative Gibbs surface excesses, Gibbs energies of adsorption, and electrosorption valencies have been determined as a function of the electrode potential and the surface-charge density. The maximum surface excess is equal to 6.1 × 10 −10 mol cm −2, the Gibbs energy at the potential of maximum adsorption is −42 kJ mol −1, and the shift of the potential of zero charge (pzc) due to displacement of a monolayer of water molecules by a monolayer of adsorbed pyridine is −0.65 V. It is concluded that pyridine molecules are adsorbed vertically on a Au(110) surface, attached to the metal by the nonbonding orbital of the nitrogen atom. Adsorption of pyridine at Au(110) and Au(100) surfaces are compared. The strong effect of surface crystallography on the energetics of pyridine adsorption at gold electrodes is shown.

Quantitative investigations of the adsorption of tert-amyl alcohol at the Au (100) / aqueous solution interface

The energetics of adsorption of tert-amyl alcohol (t-AA) onto a gold (100) single crystal electrode surface have been characterized quantitatively using chronocoulometry. The adsorption parameters were obtained from measurements of the charge density at the electrode surface. The precision of these measurements shows a marked improvement over previous results. Film and surface pressures, relative Gibbs surface excesses, Gibbs energies of adsorption, effective dipole moments and electrosorption valencies were determined as functions of the electrode potential and the surface charge density. The maximum surface excess obtained for this system is equal to 4.4 × 10 −10 mol cm −2 and the Gibbs energy is equal to − 17.4 kJ mol −1 at the potential of maximum adsorption (approximately 0.15 V(SCE)). These values are very close to those obtained for the adsorption of t-AA on the mercury electrode. Similarities and differences between the adsorption of t-AA at gold and mercury electrodes are discussed in detail.

On the adsorption of hexanol-1 on polycrystalline gold from kf methanolic solutions

Using the potentiostatic pulse method, the differential capacity on gold for 0.1 M KF methanolic solution and for the solutions containing hexanol-1 of several concentrations has been measured. Surface fractional coverage θ = Γ/Γ max with hexanol-1 molecules for potentials near to the potential of maximum adsorption and energy of adsroption have been calculated according to the Frumkin theory of adsorption.

The electrosorption valency of organic electrosorbates: Part II. Aromatic and heterocyclic compounds

Thermodynamic data (electrosorption valency γ and its potential dependence ∂γ/∂ E, the Gibbs energy of adsorption Δ G ad and others) are collected for the electrosorption of aromatic molecules at mercury. Two sets of data are obtained for the flat (|) and perpendicular (⊥) orientation, respectively. For most substances the flat orientation dominates at low surface concentrations Γ | m < 3 × 10 −10 mol/cm 2 and positive potentials. A charge transfer from the adsorbate to the metal is obtained which contributes 50 kJ/mol per trasferred electron to the Gibbs energy of adsorption. Increasing λ | diminishes the potential dependence ∂γ |/∂ E and increases the potential of maximum adsorption E | m. The increasing charge of the adsorbate causes lateral repulsion between the molecules. In the perpendicular orientation,γ ⊥ N increases with the dipole κ s. Water desorption gives a small contribution but the charge transfer is negligible. Reorientation processes are determined by the potential and surface concentrations. General rules are derived and α-picoline and phenylthiourea are discussed as examples. Ionic adsorbates have γ N values determined mainly by the charge z. Charge transfer depends on the electron density in the ring: compared with the neutral molecules, it is smaller for cations and larger for anions. Heterocyclic compounds behave similarly, but the charge transfer is smaller than for aromatics.

Adsorption of a mixture of sodium dodecyl sulphate and dodecyl alcohol on a mercury electrode and its effect on the electrochemical processes of cadmium(II) in sodium chloride solution

The adsorption behaviour of a mixture of the surfactants sodium dodecyl sulphate (SDS) and dodecyl alcohol (DOH) in 0.5 mol dm −3 NaCl has been studied on a hanging mercury drop electrode (HMDE) on the basis of capacity measurements using phase-sensitive ac voltammetry with prior accumulation of the surfactants at a potential E=−0.6 V (vs. SCE). Investigations were carried out in a wide concentration range of SDS (2.08×10 −7−5.20×10 −3 mol dm −3) and in the presence of trace quantities of DOH (0.5 and 1.4 wt%) in the mixture. The effect of surfactants on the electrode reaction of cadmium was studied by the same technique, based on faradaic current measurements. The standard rate constants of the electrode reaction of cadmium, k s, in the absence and presence of surfactants were estimated from the shape and height of ac voltammograms using the theoretical curves obtained by digital simulation. The results show that the adsorbed film on the mercury surface, which is formed by the mixture of sodium dodecyl sulphate and dodecyl alcohol, has different properties from those of films formed by individual components of the mixture. Two very interesting phenomena were observed: an increase of the electrode double-layer capacity of the mixture at the potential of maximum adsorption in comparison with the values of individual components, and an increased permeability of the mixed adsorbed layer for cadmium ions. Both phenomena were observed in a higher concentration range of surfactants (near the CMC value of SDS). It may be assumed that these phenomena are connected with the solubilization of dodecyl alcohol, which is sparingly soluble in water, and its penetration into the polylayer or micellar film of sodium dodecyl sulphate on the electrode surface.

Electrosorption from pseudobinary surfactant mixtures: II. Binary surfactant mixtures of weakly adsorbable components investigated by electrocapillarity

The electrocapillary curves of the mixtures hexahydrophenol/i-butanol and n-butanol/triethylamine-hydrochloride, respectively, were measured by the drop time technique in aqueous electrolyte solutions (1 M KCl) at different mole fractions and concentrations of the surfactants. The electrocapillary data were evaluated by the method of Hutchinson, and the total surface excess concentrations, the surface excess concentrations of the components in the mixed adsorption layer, and their mole fractions have been determined. In the system hexahydrophenol/i-butanol the adsorbability is different but both components have the same potential of maximum adsorption. It could be shown that the hexahydrophenol is adsorbed preferably above the concentration c > 3 × 10 −3 M; i.e., the mole fraction of hexahydrophenol in the interface is larger than the mole fraction of i-butanol. In the system n-butanol/triethylaminehydrochloride the adsorbabilities at the potentials of maximum adsorption are similar, but the potentials of maximum adsorption are very different ( E = −0.6 V; E = −1.0 V). It could be shown that at very negative potentials, a potential-dependent “demixing” of the surface phase occurs; i.e., at very negative potentials only triethylaminehydrochloride is adsorbed while the n-butanol molecules are displaced from the electrode surface.

Capacitance and surface conductance study of the adsorption of pentan-1-ol on gold

The adsorption of pentan-1-ol on thin gold film electrodes was studied by capacitance and surface conductance, The pentan-1-old adsorption conforms to the Frumkin isotherm. The free energy of adsorption at the potential of maximum adsorption shows that the adsorption of pental-1-ol is weaker on gold than on mercury and very similar to that of the diethyl ether on (110) gold single crystals. The surface resistivity in the presence of the alcohol is lower than that of the base electrolyte, showing a minimum near the minimum of capacitance. The changes of resistance are tentatively explained in terms of the field effect and the scattering of surface electrons produced by the solvent molecules.

Interfacial interactions between aqueous solutions of 4,4'-bipyridyl and mercury

From analysis of electrocapillary curves obtained by drop-time measurements with spindle-type capillary and the method of controlled convection the data on reversible equilibrium adsorption of 4,4'-bipyridyl on mercury from aqueous 0·1M solutions of Na2SO4 and H2SO4 were obtained. In neutral solution 4,4'-bipyridyl is strongly adsorbed: at potential of maximum adsorption the Langmuir coefficient β = 1.4 . 102 m3 mol-1 and Γm = 1.8 . 10-6 mol m-2; in acid solution the adsorption of the doubly protonated 4,4'-bipyridylium cation is weaker and its potential dependence goes through a minimum. The increase of adsorption of the bipyridylium cation and a simultaneous increase of negative charge with increasing negative potential are explained by charge transfer in adsorbed state. The electroreduction of 4,4'-bipyridyl is preceded by formation at the electrode surface of a layer of sparingly soluble salt of the bipyridyl cation radical.

Electrosorption and inhibition properties of p-norborn2-yl phenolate ions at the mercury/solution interface

The electrosorption properties of p-norborn-2-yl phenolate ions in alkaline solutions were investigated by ac polarographic and electrocapillary measurements. Two adsorption regions were found. At low bulk surfactant concentrations the adsorption at the positively charged electrode (−0.2 ⩾ E ⩾ −0.6 V) is predominant while at higher surfactant concentrations the adsorption at the negatively charged electrode (−0.6 ⩾ E ⩾ −1.0 V) is more pronounced. At E = −0.40 V the adsorption parameters were determined ( a ≈ 2; Δ G° A = −32.5 ± 1 kJ mol −1. Between −0.6 ⩾ E ⩾ −1.0 V one potential of maximum adsorption for all concentrations does not exist and therefore the adsorption parameters could not be calculated. At E = −0.40 V progressive two-dimensional nucleation with a nucleation order of 3 was observed which corresponds well with the high attraction constant. The electrode reaction S 2O 2− 8 + 2 e − → 2 SO 2− 4 is inhibited by norborn-2-yl phenolate ions in the potential range −0.2 ⩾ E ⩾ −0.6 V. In the second potential range of capacity decrease the electrode process is much less retarded. At E = −0.40 V, in a similar manner as described for neutral molecules, a linear dependence of the log k s ( k s apparent rate constant) on ln c A and π (π = surface film pressure), respectively, has been found.

Electrocatalytic oxidation of organic molecules in alkaline solutions—II. Electroadsorption and electrooxidation of ethylene glycol at platinum

The electrocatalytic oxidation of ethylene glycol (EG) at polycrystalline platinum in 1 M KOH was studied under potentiodynamic and potentiostatic conditions. EG electroadsorption processes were clearly distinguished from the oxidation of EG electroadsorbed residues by using the flux technique after adsorption. The maximum adsorption potential was between 0.3 and 0.4 V in the H-adatoms potential region. An electroadsorbed residue which oxidizes at more positive potentials was formed. The (eps) ads and (eps) ox values were independently determined by the potentiostatic i/t transients at 0.35 V and the potentiodynamic oxidation profiles at low sweep rates respectively. The experimental values obtained allowed to postulate a plausible reaction pathway sequence. The comparison of EG with electroadsorbed 1,3 dioxolane and adsorbed CO electrooxidation spectra showed similar surface oxidation behavior.

Electrocatalytic oxidation of organic molecules in alkaline solutions—I. Oxidation of 1,3 dioxolane at platinum

The electroadsorption and electrooxidation of 1,3 dioxolane at platinum in 1 M KOH solutions were investigated under a wide range of experimental conditions. The potential of maximum adsorption ( E max ad) is ca. 0.4 V and the degree of surface coverage depends linearly with the logarithm of organic concentration as predicted by a Temkin type adsorption isotherm. The kinetics of the electroadsorption process follows a Roginskii-Zel'dovich equation. From (eps) ad, (eps) ox and analysis of oxidation products a mechanism is postulated which consists of an adsorption with simultaneous dehydrogenation step followed by cycle rupture and electroadsorption and dehydrogenation of further residues.

Flotation and adsorption investigations of the systemn-dodecanoic acid/mercury at different pH values

The systemn-dodecanoic acid/mercury in 0,1 N KCl solutions has been investigated within pH range 4,0–10,5 in order to characterize the adsorption of surfactant and to determine its influence on the flotation of mercury. Flotation tests have been carried out in a model mercury flotation apparatus developed by Pomianowski and Para. Some characteristic parameters of adsorption (attraction constant of Frumkin isotherm, adsorption coefficient at maximum adsorption potential, free standard adsorption enthalpy and area per molecule) have been determined by a. c. polarographic capacity-potential and capacity-time curves. Flotation recovery as well as attraction constant, adsorption coefficient and surface excess reach maximum at pH≈5,1. It is explained by the assumption of an ion-molecule associate formation.

Kinetics and Mechanism of Adsorbed Ethylene Electrooxidation on Platinized Platinum under Potentiodynamic Conditions

The potentiodynamic electrooxidation of the adsorbed ethylene species on platinized platinum is studied at 40°, 60°, and 80°C in saturated with ethylene at 1 atm pressure. The perturbation conditions are chosen to avoid the influence of ethylene readsorption. Experimental conditions to obtain the maximum coverage by the adsorbed species are established. The electrosorption process at the potential of maximum adsorption is related to a transient anodic current assigned to the electrooxidation of hydrogen adatoms. The kinetic parameters reveal that the electrochemical reaction is an activated process. Its interpretation and discussion are based on two possible schemes which are different than those earlier proposed for the ethylene electrooxidation under stationary conditions. The initial step in any of them is rate determining and corresponds either to the deprotonation of the adsorbed radical or to the water molecule discharge on the completely covered electrode surface.

Adsorption of dicarboxylic acids on lithiated nickel oxide electrode

Adsorption of dicarboxylic acids on NiO electrodes was studied by means of the galvanostatic transient method, mainly for oxalic acid. The oxalate or hydrogen oxalate anion or both were concluded to adsorb on the electrode. The adsorption was maximal at a potential less positive than the flat-band potential of the electrode, owing to preferential adsorption of hydroxyl anion in the region above the maximum adsorption potential. The quantity of adsorption was dependent on carrier concentration of the electrode, and high for the electrode of high carrier concentration. This phenomenon is connected to the charged condition of the electrode that when the maximum adsorption takes place, positively charged sites remain on the surface of the electrode which is charged negatively as a whole and the amount of the sites is much more different between the electrodes of different N than their bulk carrier concentrations. Adsorptions of malonic and succinic acid were distinctly low compared with that of oxalic acid. The adsorbability was in the order; oxalic acid>malonic acid>succinic acid. This order was in accord with the stability constants of nickel dicarboxylic acid chelates.

Electrocapillary phenomena in the Hg, Tl, Tl +, H 2O system in the presence of phenol

Summary The mutual effect of thallium and phenol adsorption in the Hg, Tl, Tl+, H2O system at 0.2 M phenol concentration has been studied. It has been shown that phenol and thallium adsorption suppress each other in a wide potential range with the result that at the potential of maximum total thallium adsorption (ΓTl+ΓTl+) phenol adsorption drops nearly to zero. However, at the most negative potentials the mutual suppression changes to mutual increase of adsorption. The picture of the structure of the mercury surface layer in the presence of thallium and NO3− ions, developed in the previous communication, allows a qualitative explanation of the effects observed. The results of ref. 3 obtained without taking into consideration the changes in Tl concentration in amalgam caused by its oxidation by atmospheric oxygen have been partly corrected.

Double layer capacity studies of adsorption at mercury-solution interfaces effects of expanding and receding surfaces stability of oriented langmuir monolayers

Double layer capacity measurements were performed on aqueous solutions of n-octyl alcohol, n-nonanoic acid, and n-butyric acid with a pendant mercury drop electrode capable of expansion or recession. No appreciable differences of double layer capacity were observed whether the mercury drop was formed by expansion or by recession. Temporary differences are probably due to different rates of approach to equilibrium rather than to structural differences. A Langmuir isotherm is applicable for n-nonanoic acid at concentrations below two-thirds of saturation. Beyond that concentration there is evidence of multilayer adsorption. Compressed, oriented Langmuir monolayers of palmitic acid on mercury-coated platinum are found to reach the same equilibrium state as bare mercury-coated platinum. The double layer capacity of an oriented monolayer of palmitic acid is 4.3 μf. /cm 2. at the potential of maximum adsorption, as measured against 0.1 M NaClO 4.