The double layer and ion adsorption at the interface between two non miscible solutions: Part I. Interfacial tension measurements for the water-nitrobenzene tetraalkylammonium bromide systems

The classical treatment of the double layer has been extended to the interface between two immiscible solutions. The model presented is composed of an inner compact layer, characterized by a dipolar potential drop, between two diffuse type layers. The systems studied are composed of C₂ to C₅ quaternary ammonium bromides at partition equilibrium between water and nitrobenzene for which the inner potential difference, for a given electrolyte, is independent, at least in the lower concentration range, of the concentration. Drop weight interfacial measurements and the use of the Gouy-Chapman approach show that the tetraethyl-, tetrapropyl- and tetrabutylammonium ions are not adsorbed within the inner compact layer, and the dipolar potential drop of this layer can then be determined. Tetrapentylammonium ions on the contrary are specifically adsorbed but the amount of adsorbed ions within the compact inner layer cannot be evaluated because of the impossibility, in this case, of determining the dipolar potential drop.     

Adsorption of sugar surfactants at the air/water interface

The adsorption isotherms of n-decyl-β-D-glucoside (β-C(10)G(1)) as well as various n-alkyl-β-D-maltosides (β-C(n)G(2)) with n=8, 10, 12 and 14 were determined from surface tension measurements. Based on the analysis of the adsorption isotherms, the total free energy change of adsorption was determined and a novel method was proposed to determine the maximum adsorbed amount of surfactant. It can be concluded that the driving force for adsorption first increases with increasing adsorbed amount of the sugar surfactants and then levels off in a plateau. This peculiar behaviour is interpreted as formation of a thin liquid-like alkane film of overlapping alkyl chains at the air/water interface once a certain adsorbed amount is exceeded. The driving force of adsorption depends on the alkyl chain length only and is not affected by the type of the head group. The hydrophobic contribution to the standard free energy change of adsorption was compared with the values of sodium alkylsulfate and alkyltrimethylammonium bromide surfactants. This comparison reveals that the hydrophobic driving force of adsorption is the largest for the sodium alkylsulfates, whereas it is the same for the sugar surfactants and the alkyltrimethylammonium bromides.     

Electrical Double-Layer Structure at the Rutile-Water Interface as Observed in Situ with Small-Period X-Ray Standing Waves

X-Ray standing wave (XSW) measurements were made of Rb and Sr adsorbed from aqueous solutions at the rutile (110)-water interface. These experiments were performed to address the extent to which direct measurements of electrical double-layer structure are possible. The experimental results show that the Bragg XSW technique, using small-period standing waves generated by Bragg diffraction from the substrate, can precisely measure ion locations within the condensed layer and the in situ partitioning of ions between the condensed and diffuse layers. Differences in condensed layer ion positions were observed for Sr ions (measured in situ) as compared with Rb ions (in situ) and also for Sr ions (ex situ). An additional constraint on the ex situ Sr site geometry was provided by polarization-dependent surface EXAFS measurements. Such measurements can provide important constraints for the development and verification of electrical double-layer theory especially as applied to ion adsorption at the solid-water interface. Copyright 2000 Academic Press.     

Coadsorption of Cd(II) and oxalate ions at the TiO2/electrolyte solution interface

The study of the adsorptions of cadmium and oxalate ions at the titania/electrolyte interface and the changes of the electrical double layer (edl) structure in this system are presented. The adsorption of cadmium or oxalate ions was calculated from an uptake of their concentration from the solution. The concentration of Cd(II) or oxalate ions in the solution was determined by radiotracer method. For labeling the solution 14C and 115Cd isotopes were used. Coadsorption of Cd(II) and oxalic ions was determined simultaneously. Besides, the main properties of the edl, i.e., surface charge density and zeta potential were determined by potentiometer titration and electrophoresis measurements, respectively. The adsorption of cadmium ions increases with pH increase and shifts with an increase of the initial concentration of Cd(II) ions towards higher pH values. The adsorption process causes an increase of negatively charged sites on anatase and a decrease of the zeta potential with an increase of initial concentration of these ions. The adsorption of oxalate anions at the titania/electrolyte interface proceeds through the exchange with hydroxyl groups. A decrease of pH produces an increase of adsorption of oxalate ions. The processes of anion adsorption lead to increase the number of the positively charged sites at the titania surface. However, specific adsorption of bidenate ligand as oxalate on one surface hydroxyl group may form inner sphere complexes on the metal oxide surface and may overcharge the compact part of the edl. The presence of oxalate ions in the system affects the adsorption of Cd(II) ions on TiO2, increasing the adsorption at low pH range and decreasing the adsorption at high pH range. Using adsorption as a function of pH data, some characteristic parameters of adsorption envelope were calculated.     

On the theory of polyelectrolyte adsorption : The effect on adsorption behavior of the electrostatic contribution to the adsorption free energy

A theory has been developed for the adsorption of polyelectrolytes on charged interfaces from an aqueous salt solution. This adsorption is determined by the electrical charge density of the polyelectrolyte, the adsorption energy, the salt concentration, the molecular weight, solubility, flexibility, and concentration of polymer. The theory relates these parameters to the properties of the adsorbed polymer layer, i.e., the amount of polymer adsorbed, the fraction of the adsorbent interface covered, the fraction of the segments actually adsorbed on the interface versus the fraction of the segments in the dangling loops, the final surface charge density, and the thickness of the adsorbed layer. As polyelectrolyte adsorption should resemble nonionic polymer adsorption at high ionic strength of the solution or low charge density on the polymer, this work is an extension of the nonionic polymer adsorption theory to polyelectrolyte adsorption. The following effects are taken into account: (a) the conformational change upon adsorption of a coil in solution into a sequence of adsorbed trains interconnected by loops dangling in solution; (b) the interactions of the adsorbed trains with the interface and with each other; (c) the interaction of the dangling loops with the solvent; (d) the change in surface charge density of the adsorbent due to adsorption of charged trains and the accompanying changes in the electrical double layer which contains “small” ions as well as charged loops; (e) the (induced) dipole interaction of the adsorbed trains with the charged adsorbent interface. The theory is worked out for low potentials (Debye—Hückel approximation); in Appendix B an outline of a more complete treatment is given. The predicted adsorption isotherms have the experimentally observed high-affinity character. A relation between the adsorption energy, the surface charge density on the adsorbent, the degree of dissociation of the polymer, and the salt concentration predicts the conditions under which no adsorption will occur. For adsorbent and polymer carrying the same type of charge (both positive or both negative) the adsorption is predicted to decrease with increased charge density on polymer or adsorbent and to increase with salt concentration. If adsorbent and polymer carry different type charges, the adsorption as a function of the degree of dissociation, α, goes through a maximum at a relatively low value of α and, depending on the adsorption energy, an increase in the salt concentration can then increase or decrease the adsorption. At finite polymer concentration in solution the number of adsorbed segments and the fraction of the interface covered practically do not change with an increase in polymer concentration, whereas the total number of polymer molecules adsorbed increases slightly, as does the average fraction of segments in loops. The experimental results for polyelectrolyte adsorption have been reviewed in general and, as far as data are available, the predictions of the theory seem to follow the experimentally observed trends quite closely, except for the thickness of the adsorbed layer. This thickness is systematically overestimated by the theory and two reasons for this are given. The theoretical model implies a not too low ionic strength of the solution. Extrapolation of results to solutions of very low ionic strength is not warranted.     

Adsorption monitoring of phospho-l-serine on hydroxyapatite

The adsorption of the aminoacid phospho-l-serine (PLS) on the surface of hydroxyapatite {Ca₅(PO₄)₃OH, HAP}, was investigated using streaming potential measurements. Solutions saturated with respect to HAP, containing different concentrations of PLS, were brought in contact under carefully controlled flow conditions through plugs made of well dispersed HAP powder. The measurement of PLS adsorption during the equilibration of the solute with the HAP substrate, showed a plateau regime corresponding, according to geometrical considerations, to monolayer surface coverage. The PLS uptake measurements on HAP suggested that it is possible to monitor in situ adsorption during the monolayer surface coverage measuring the streaming potential of HAP. Analysis of the surface potential measurements suggested that during the monolayer surface coverage step, the negatively charged (HL^2?) PLS species were adsorbed. The adsorbed HL^2? was located at the inner Helmholtz plane of the electrical double layer, forming surface complexes with the positively charged ≡CaOH ₂ ⁺ sites on the surface of HAP. The rate constants of adsorption and desorption were calculated from the kinetics of adsorption of PLS on HAP.     

Varying the counterions at a charged interface

The influence of different counterions on the adsorption behavior of the ionic soluble surfactant dodecyl-dimethylammonium-pyridimium bromide is investigated. The addition of potassium halogenides to aqueous solutions of the surfactant modifies the surface activity of the amphiphile and has a profound influence on the surface tension isotherms. The measured critical micelle concentration follows the order of the periodic table of elements which is in strong contrast to the surface excess. The number density of the adsorbed surfactants at the cmc does not depend on the amount of counterions in the solution but on the nature of the counterion. Furthermore, evidence is provided that the surface region is depleted on fluoride ions. Surface second harmonic generation and ellipsometry have been used to gain direct structural information which complement the thermodynamic considerations. The combination of both optical techniques yields the number density of the condensed counterions within the compact layer. A strategy to retrieve selected parameters of the ion binding model of Radke et al. is presented. The analysis of the optical data reveal the existence of a phase transition towards a surface condensed state with increasing salt condensation.     

Hydrodynamic thickness of petroleum oil adsorbed layers in the pores of reservoir rocks

The hydrodynamic thickness delta of adsorbed petroleum (crude) oil layers into the pores of sandstone rocks, through which the liquid flows, has been studied by Poiseuille's flow law and the evolution of (electrical) streaming current. The adsorption of petroleum oil is accompanied by a numerical reduction in the (negative) surface potential of the pore walls, eventually stabilizing at a small positive potential, attributed to the oil macromolecules themselves. After increasing to around 30% of the pore radius, the adsorbed layer thickness delta stopped growing either with time or with concentrations of asphaltene in the flowing liquid. The adsorption thickness is confirmed with the blockage value of the rock pores' area determined by the combination of streaming current and streaming potential measurements. This behavior is attributed to the effect on the disjoining pressure across the adsorbed layer, as described by Derjaguin and Churaev, of which the polymolecular adsorption films lose their stability long before their thickness has approached the radius of the rock pore.     

Effect of Inorganic Ions on Oleate Adsorption at a Nigerian Hematite–Water Interface

Using “pure” natural hematite selected from a high silica Nigerian hematitic ore, oleate adsorption densities at the hematite–water interface were determined in the presence of various inorganic ions (anions and cations) of different charges and at varying concentrations. Adsorption density was determined using electrical conductivity measurements. The specific surface area of the hematite particles was determined using the method of adsorption of paranitrophenol in aqueous solution. Inorganic ions in solution depressed oleate adsorption at the aqueous hematite surface. The charge of the ion proved to be the dominant factor determining the depression of oleate adsorption. Ionic strength also was an influence, up to a limiting value at which monolayer oleate coverage of the hematite surface occurred. The inorganic ions in solution are considered to function through nonspecific adsorption in the diffuse region of the electric double layer.     

Monte carlo study of the adsorption and aggregation of alkyltrimethylammonium chloride on the montmorillonite-water interface

Organically modified clays exhibit adsorption capacities for cations, anions, and nonpolar organic compounds, which make them valuable for various environmental technical applications. To improve the understanding of the adsorption processes, the molecular-scale characterization of the structures of organic aggregates assembled on the external basal surfaces of clay particles is essential. The focus of this Monte Carlo simulation study was on the effects of the surface coverage and the alkyl chain length n on the structures of alkyltrimethylammonium chloride ((C(n)TMA)Cl) aggregates assembled on the montmorillonite-water interface. We found that the amount of adsorbed C(n)TMA(+) ions is independent of the alkyl chain length and increases with the C(n)TMA(+) surface coverage. The C(n)TMA(+) ions predominantly adsorb as inner-sphere complexes; the fraction of outer-sphere adsorbed ions equals only about 10%. The conformational order of the C(n)TMA(+) alkyl chains substantially decreases with decreasing alkyl chain length. In agreement with previous experiments, the amount of C(n)TMA(+) ions that are aggregated at the mineral surface increases with increasing chain length. The maximum value of 0.66 C(n)TMA(+) adsorption complex per unit cell area of the clay surface considerably exceeds the amount of cations required to compensate the negative charge of the montmorillonite surface. Furthermore, in most of the studied systems, fractions of Na(+) surface cations remain adsorbed on montmorillonite. The resulting interfacial positive charge excess is counterbalanced by coadsorbed chloride ions forming ion pairs with both C(n)TMA(+) and Na(+).     

Structure of electrical double layer at gold electrode in cyanide solution

The potential distribution in electrical double layer is calculated, basing on the data on the electrode charge and cyanide-ion adsorption at the gold electrode. It is shown that the integral capacitances of regions in the dense layer are not unambiguous functions of the electrode potential or charge per se, but depend also on the amount of specifically adsorbed ions Γ. A function is proposed for the describing of the Γ dependence of the dense layer integral capacitances.     

Studies of the Influence of Polyelectrolyte Adsorption on Some Properties of the Electrical Double Layer of ZrO2- Electrolyte Solution Interface

An influence of the molecular weight of the polymer and inorganic contaminations of zirconia on the adsorption and electrokinetic properties on ZrO₂-electrolyte solution interface was studied. Two polymers were used; polyacrylic acid (PAA) and polyacrylamide (PAM). On the basis of the obtained dependencies, main factors responsible for observed changes of zeta potential and surface charge of washed and contaminated ZrO₂ were determined. It was showed, that the change of ionic structure in the Stem layer depends on the number and arrangements of-COOH groups in PAA and PAM macromolecules. These groups are responsible for the conformation of polymer chains near the surface and have direct influence on the amount of the adsorbed polyelectrolyte. The inorganic ions, present on the surface of the oxide, blocking some part of active sites, making them inaccessible for adsorbing by carboxylic groups polymer chains. That makes the adsorption on the contaminated oxide lower than on the washed one. From the comparison of the determined values of the diffuse layer charge and surface charge, the main factor influencing the zeta potential changes at different pH, molecular weight and polymer concentration was determined. Also was demonstrated, that the contaminations of the solid are reason for considerable shift of pH_pzc in relation to pH_icpof the zirconia. Thickness of the adsorption layers and free energies of the adsorption of polyacrylic acid and polyacrylamide on the surface of ZrO₂ were calculated.     

Mechanism of metals’ electrodeposition from complex electrolytes allowing for the anions’ adsorption: Effect of the position of ligands and complex ions in the compact layer on the reaction orders

Effect of the relative position in the compact layer of specifically adsorbed electroactive complex ion and complexing ligand on the magnitude of current is considered. Effect of parameters that characterize the adsorption of such ions and the electrical double layer structure on the dependence of current on the solution composition and effective reaction orders is analyzed.     

Comparison of surface properties between kaolin and metakaolin in concentrated lime solutions

The surface adsorption of calcium hydroxide onto kaolin and metakaolin was investigated by monitoring with atomic emission spectroscopy and pH measurements the amounts of ions left in solution after exposing clays to calcium hydroxide solutions of various concentrations. Both clays adsorb calcium and hydroxyl ions but differently. Kaolin adsorbs calcium hydroxide not only at the edges of the clay particles but also onto the basal faces. The adsorbed hydrated calcium ions form a layer on the clay particle surfaces, preventing further dissolution of the clay mineral platelet. Metakaolin shows high pozzolanic activity, which provides the quick formation of hydrated phases at the interfaces between metakaolin and lime solutions. The nature of the hydration products has been investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA). The most important hydrated phases like CSH (hydrated calcium silicate) and C₂ASH₈ (gehlenite) have been identified.     

Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO₂ anatase (001) surface is studied by first‐principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O₂ molecule is high enough to break the O?O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O?O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.     

Inner- and outer-sphere complexation of ions at the goethite-solution interface

Formation of inner- and outer-sphere complexes of environmentally important divalent ions on the goethite surface was examined by applying the charge distribution CD model for inner- and outer-sphere complexation. The model assumes spatial charge distribution between the surface (0-plane) and the next electrostatic plane (1-plane) for innersphere complexation and between the 1-plane and the head end of the diffuse double layer (2-plane) for the outersphere complexation. The latter approach has been used because the distance of closest approach to a charged surface may differ for different ions. The surface structural approach implies the use of a Three-Plane model for the compact part (Stern layer) of solid-solution interface, which is divided into two layers. The thickness of each layer depends on the capacitance and the local dielectric constant. The new approach has been applied to describe the adsorption of magnesium, calcium, strontium, and sulfate ions. It is shown that the concept can successfully describe the development of surface charge in the presence of Ca(+2), Mg(+2), Sr(+2), and SO4(-2) as a function of loading, pH, and salt level, and also the shift in the isoelectric point (IEP) of goethite. The CD modeling revealed that, for the conditions studied, magnesium is mainly adsorbed as a bidentate innersphere complex, calcium can be a combination of bidentate innersphere and a monodentate inner- or outer-sphere complexes, and strontium is probably adsorbed as an outersphere complex. Sulfate is present as a mixture of inner- and outer-sphere monodentate complexes. Outersphere complexation is less pH dependent than innersphere complexation. The CD model predicts that the outersphere complexation of divalent cations and anions is relatively favorable at respectively low and high pH. Increase of ion loading favors the formation of innersphere complexes.     

Adsorption of randomly annealed polyampholytes at the silica-water interface

We have investigated the adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) with various molar compositions. The adsorption was performed from dilute aqueous solutions onto silicon substrates. The adsorbed layers were characterized by reflectivity techniques such as reflectometry, ellipsometry, and neutron specular reflection. As expected for annealed polyampholytes, the adsorption was found to depend strongly on the pH, with a maximum within the isoelectric domain of the polyampholyte. The monomer volume fraction profiles of the adsorbed layers were determined from neutron specular reflection measurements. In the isoelectric domain, the polyampholyte chains adopt a compact conformation, with a layer thickness of about 60 A. The polyampholyte layer is as dense as the adsorbed layer of fully charged polyelectrolyte but much thicker. Finally, we found that changing the ratio of neutral units along the polyampholyte chain in the isoelectric domain had no significant effect on the concentration profile of the adsorbed layer.     

Electrical double layer interactions between dissimilar oxide surfaces with charge regulation and Stern-Grahame layers

Models of surfaces with intrinsic ionisable amphoteric surface sites governed by the dissociation of acid-base potential determining ion species together with the capacity for the adsorption of anion and cations of the supporting electrolyte are required to describe both the results of electrokinetic and titration measurements of inorganic oxides. The Gouy-Chapman-Stern-Grahame (CGSG) model is one such model that has been widely used in the literature. The electrical double layer interaction between two dissimilar CGSG surfaces has been studied by Usui recently [S. Usui, J. Colloid Interface Sci. 280 (2004) 113] where erroneous discontinuities in the slope of the pressure-separation relation were observed. We revisit this calculation and provide a simple general methodology to analyse the electrical double layer interaction between dissimilar ionisable surfaces with ion adsorption.     

Adsorption of poly(vinyl formamide-co-vinyl amine) onto silica particle surfaces and stability of the formed hybrid materials

In order to produce silica/polyelectrolyte hybrid materials the adsorption of the polyelectrolyte poly(vinyl formamide-co-vinyl amine), P(VFA-co-VAm) was investigated. The adsorption of the P(VFA-co-VAm) from an aqueous solution onto silica surface is strongly influenced by the pH value and ionic strength of the aqueous solution, as well as the concentration of polyelectrolyte. The adsorption of the positively charged P(VFA-co-VAm) molecules on the negatively charged silica particles offers a way to control the surface charge properties of the formed hybrid material. Changes in surface charges during the polyelectrolyte adsorption were studied by potentiometric titration and electrokinetic measurements. X-ray photoelectron spectroscopy (XPS) was employed to obtain information about the amount of the adsorbed polyelectrolyte and its chemical structure. The stability of the adsorbed P(VFA-co-VAm) was investigated by extraction experiments and streaming potential measurements. It was shown, that polyelectrolyte layer is instable in an acidic environment. At a low pH value a high number of amino groups are protonated that increases the solubility of the polyelectrolyte chains. The solvatation process is able to overcompensate the attractive electrostatic forces fixing the polyelectrolyte molecules on the substrate material surface. Hence, the polyelectrolyte layer partially undergoes dissolving process.     

Dissociative adsorption of molecular hydrogen onto Pt<Subscript>6</Subscript> and Pt<Subscript>19</Subscript> platinum clusters located on the tin dioxide surface: Quantum-chemical modeling

It is suggested that, for the operation of platinum catalysts based on tin dioxide in air hydrogen fuel cells, hydrogen spillover (migration) leading to a change in the electron and proton contributions of the catalyst conductivity is of crucial importance. The hydrogen adsorption, dissociation, and migration in the platinum-tin dioxide-hydrogen system surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that the adsorption energy of a hydrogen molecule onto a platinum cluster increases from 1.6 to 2.4 eV as the distance to the SnO₂ substrate decreases. The calculated Pt-H bond length for adsorbed structures is 1.58–1.78 Å. The computer modeling has demonstrated that: (1) the hydrogen adsorption energy on clusters is higher than on the perfect platinum surface; (2) dissociative chemisorption onto Pt _n clusters can occur without a barrier and depends on the adsorption site and the cluster structure; (3) the adsorption energy of hydrogen onto the SnO₂ surface is higher than the adsorption energy onto the platinum cluster surface: (4) multiple H₂ dissociation on the tin dioxide surface occurs with a barrier; (5) the dissociation adsorption of hydrogen molecules onto the platinum cluster surface followed by atom migration (spillover) is energetically favorable.