Adsorption of poly(styrene sulfonate) of different molecular weights on alpha-alumina: effect of added sodium dodecyl sulfate

The adsorption of poly(styrene sulfonate), PSS, of different molecular weights (70,000, 500,000, and 1,000,000 mol/kg), from aqueous solutions on alpha-alumina has been investigated. PSS of the lower molecular weight adsorbs less than the others whose adsorption isotherms overlap. The adsorption is found to increase with increasing ionic strength of the solutions indicating that both electrostatic and non-electrostatic contributions are involved in the adsorption process. Upon addition of the anionic surfactant, sodium dodecyl sulfate, SDS, PSS is found to adsorb less the more SDS added. SDS is found to be preferentially adsorbed as shown both from the simultaneous adsorption of the components and also from the sequential adsorption process where SDS in all cases displaces preadsorbed PSS from the solid surface. The displacement of preadsorbed polyelectrolyte by surfactant is a very slow process and the displacement is less pronounced as the molecular mass of the polyelectrolyte increases indicating the fewer number of contact points to the surface. This is further underlined by the effect on the displacement of PSS by SDS upon increasing the ionic strength of the solutions.     

Adsorption of branched-linear polyethyleneimine-ethylene oxide conjugate on hydrophilic silica investigated by ellipsometry and Monte Carlo simulations

The adsorption of and conformation adopted by a branched-linear polymer conjugate to the hydrophilic silica-aqueous solution interface have been studied by in situ null ellipsometry and Monte Carlo simulations. The conjugate is a highly branched polyethyleneimine structure with ethyleneoxide chains grafted to its primary and secondary amino groups. In situ null ellipsometry demonstrated that the polymer conjugate adsorbs to the silica surface from water and aqueous solution of 1 mM asymmetric divalent salt (calcium and magnesium chloride to emulate hard water) over a large pH range. The adsorbed amount is hardly affected by pH and large charge reversal on the negatively charged silica surface occurred at pH = 4.0, due to the adsorption of the cationic polyelectrolyte. The Monte Carlo simulations using an appropriate coarse-grained model of the polymer in solution predicted a core-shell structure with no sharp boundary between the ethyleneimine and ethyleneoxide moieties. The structure at the interface is similar to that in solution when the polymer degree of protonation is low or moderate while at high degree of protonation the strong electrostatic attraction between the ethyleneimine core and oppositely charged silica surface distorts the ethyleneoxide shell so that an "anemone"-like configuration is adopted. The adsorption of alkyl benzene sulfonic acid (LAS) to a preadsorbed polymer layer was also investigated by null ellipsometry. The adsorption data brought additional support for the existence of a strong polymer adsorption and showed the presence of a binding which was further enhanced by the decreased solvency of the surfactant in the salt solution and confirmed the surface charge reversal by the polymer adsorption at pH = 4.0.     

Impact of environmental conditions on the adsorption behavior of radionuclide Ni(II) onto hematite

In this work, adsorption of Ni(II) from aqueous solution onto hematite under various solution chemistry and temperature was investigated. The results indicated that the pseudo-second-order rate equation fitted the kinetic adsorption well. The adsorption of Ni(II) onto hematite was strongly dependent on pH and ionic strength. At low pH, the adsorption was dominated by outer-sphere surface complexation or ion exchange, whereas inner-sphere surface complexation was the main adsorption mechanism at high pH. A positive effect of FA on Ni(II) adsorption was found at pH < 8.0, whereas a negative effect was observed at pH > 8.0. The Langmuir, Freundlich, and D–R models were applied to simulate the adsorption isotherms at three different temperatures of 293.15, 313.15 and 333.15 K. The thermodynamic parameters were calculated from the temperature dependent adsorption, and the results indicated that the adsorption was endothermic and spontaneous.     

The influence of functionality on the adsorption of p-hydroxy benzoate and phthalate at the hematite-electrolyte interface

Kinetics of adsorption of p-hydroxy benzoate and phthalate on hematite-electrolyte interface were investigated at a constant ionic strength, I = 5 x 10(-4) mol dm(-3), pH 5 and at three different temperatures. The state of equilibrium for the adsorption of p-hydroxy benzoate onto hematite surfaces was attained at 70 h, whereas it was 30 h for phthalate-hematite system. None of the three kinetics models (Bajpai, pseudo first order and pseudo second order) is applicable in the entire experimental time period; however, the pseudo second order kinetics model is considered to be better than the pseudo first order kinetics model in estimating the equilibrium concentration both the p-hydroxy benzoate-hematite and phthalate-hematite systems. The variation of adsorption density of p-hydroxy benzoate and phthalate onto hematite surfaces as a function of concentration of adsorbate was studied over pH range 5-9 at a constant ionic strength, I = 5 x 10(-4) mol dm(-3) and at constant temperature. The adsorption isotherms for both the systems were Langmuir in nature and the maximum adsorption density (Gamma(max)) of p-hydroxy benzoate is approximately 1.5 times more than that of phthalate on hematite at pH 5 and 30 degrees C in spite of an additional carboxylic group at ortho position in phthalate. This is due to the more surface area coverage by phthalate than that of p-hydroxy benzoate on hematite surface. The activation energy was calculated using Arrhenius equation and the activation energy for adsorption of p-hydroxy benzoate at hematite-electrolyte interface is approximately 1.8 times more than that of phthalate-hematite system. The negative Gibbs free energy indicates that the adsorption of p-hydroxy benzoate and phthalate on hematite surfaces is favourable. The FTIR spectra of p-hydroxy benzoate and phthalate after adsorption on hematite surfaces were recorded for obtaining the bonding properties of adsorbates. The phenolic nu(CO) appears at approximately 1271 cm(-1) after adsorption of p-hydroxy benzoate on hematite surfaces, which shifted by 10 cm(-1) to higher frequency region. The phenolic group is not deprotonated and is not participating in the surface complexation. The shifting of the nu(as)(COO-) and nu(s)(COO-) bands and non-dissolution of hematite suggest that the p-hydroxy benzoate and phthalate form outer-sphere surface complex with hematite surfaces in the pH range of 5-7.     

Mixed adsorption of poly(vinylpyrrolidone) and sodium dodecylbenzenesulfonate on kaolinite

The mixed adsorption of the nonionic polymer poly(vinylpyrrolidone) (PVP) and the anionic surfactant sodium dodecylbenzenesulfonate (SDBS) on kaolinite has been studied. Both components adsorb from their mixture onto the clay mineral. The overall adsorption process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. Interpretation of the experimental data addresses also the patchwise heterogeneous nature of the clay surface. In the absence of PVP, SDBS adsorbs on kaolinite by electrostatic and hydrophobic interactions. However, when PVP is present, surfactant adsorption at 10(-2) M NaCl is mainly driven by charge compensation of the edges. The adsorption of PVP from the mixture shows similar behavior under different conditions. Three regions can be distinguished based on the changing charge of polymer-surfactant complexes in solutions with increasing SDBS concentration. At low surfactant content, PVP adsorbs by hydrogen bonding and hydrophobic interactions, whereas electrostatic interactions dominate at higher surfactant concentrations. Over the entire surfactant concentration range, polymer-surfactant aggregates are present at the edges. The composition of these surface complexes differs from that in solution and is controlled by the surface charge.     

Control on shape, porosity and surface hydrophilicity of hematite particles by using polymers

The shape, porosity, and surface hydrophilicity of hematite particles formed from a forced hydrolysis reaction of acidic FeCl₃ solution were controlled by using a trace of polymers (0.001 and 0.003 wt%). The spherical particles were produced on the systems with polyvinyl alcohol (PVA) and polyaspartic acid (PAS). In the case of polyacryl amide (PAAm), slightly small spherical particles were precipitated at 0.003 wt%. However, polyacrylic acid (PAAc) and poly-γ-glutamic acid (PGA) gave ellipsoidal particles. This morphological change on hematite particles depended on the order of functional groups of polymers as –OH<–CONH₂<–COOH<–COOH and ⟩C=O, corresponding to the order in extent of polymer molecules for complexation to Fe³⁺ ions and adsorption onto particle surface. Accompanying this order, the hematite particles produced were changed from less porous to microporous. On the other hand, only the system with 0.003 wt% of PAAm produced mesoporous hematite particles. Choosing the kinds of polymers also controlled the ultramicroporosity and surface hydrophilicity of the particles.     

pH dependence of adsorption of n-dodecyl-beta-D-maltoside on solids

Adsorption of surfactants on solids plays an important role in industrial operations such as separation, lubrication, flotation, dispersion, chemical mechanical polishing, and enhanced oil recovery. In this work, adsorption of a typical biodegradable nonionic surfactant, n-dodecyl-beta-d-maltoside, on solids was studied to explore its potential applications. Even though it is a nonionic surfactant, significant pH-dependence was revealed for the adsorption on alumina in the range from pH 4 to 7. The adsorption density was found to be proportional to the concentration of surface AlOH group among Al(OH(2))(+) and AlO(-) groups. The equilibriums among the surface species are governed by pH through surface ionization reactions. The surface AlOH group evidently determines the formation of hydrogen bonding between the surfactant molecules and the solid surface and thus the adsorption. Similar correlation was also found in the case of hematite. The results help to understand the mechanism of adsorption of sugar-based surfactant on solids.     

Molecular mobility in micellar complexes of a nonionic surfactant and the poly(acrylic acid)-based hydrogel

The interaction between the polyelectrolyte gel of crosslinked poly(acrylic acid) (PAA) and nonionic surfactant Brij 58 based on poly(ethylene glycol) (C₁₆H₃₃(CH₂CH₂O)₂₀OH) is studied. It is established that poly(acrylic acid)-surfactant complexes are formed. Nondissociated carboxyl groups of poly(acrylic acid) and oxygen atoms of the surfactant are involved in the complexation. Surfactant micelles are a kind of bridge that connects polymer chains. The presence of the surfactant decreases the equilibrium swelling of the hydrogel. The spin probe method is employed to determine the local mobility of the hydrocarbon core of a micelle in the complex. It was shown that the local mobility is independent of the hydrogel crosslink density and is much lower in acidic than in alkaline media. In acidic media, much more surfactant molecules of micelles are involved in the complexation than in alkaline media. However, even in alkaline media, surfactant micelles cannot leave the hydrogel, while the spin probes located in micelles are at equilibrium with the spin probes present in the external aqueous medium. The prospects for applying the considered systems as carriers for controlled release drugs are discussed.     

Equilibrium and Dynamic Surface Tension of Carboxymethylchitosan and Alkyl Trimethylammonium Bromide Mixtures

A water-soluble derivative of chitosan, carboxymethylchitosan (CMCH), was mixed with alkyltrimethylammoniumbromides (C_mTAB) and was studied on the adsorption at air/water interface using equilibrium and dynamic surface tension method. The effects of surfactant and polymer concentrations, surfactant chain length, as well as pH of solution were investigated. Addition of the surfactants remarkably promotes the polymer adsorption. Increasing any one of surfactant concentration, surfactant chain length, and pH will facilitate the adsorption of the mixture whereas little effects of polymer concentration were observed. The results are explained in terms of the interaction between CMCH and C_mTAB under different conditions.     

Radiotracer analysis of the physico-chemical state of trace elements in aqueous solutions III. Lead

The physico-chemical state of trace lead in aqueous nitrate solutions (I=0.01) was studied using equilibrium calculation, free-liquid electrophoresis, dialysis and centrifugation methods and^{210, 212}Pb. It has been found that lead is present exclusively as nonhydrolyzed cations in solutions of pH 2–3.5. Colloidal or larger particulate forms of lead exist in solutions of lower acidity (pH>4). They are of pseudocolloidal nature, i.e. they are formed by adsorption of lead on solid impurities in the studied solutions. The pseudocolloids are negatively charged at pH>5 and reversible at pH 4–7. Reversibility of the adsorption of lead on impurities decreases at pH>7. Experiments have indicated that a significant hydrolysis of bivalent lead takes place already at pH>4. Anionic hydroxo-complexes of lead predominate in alkaline solutions of pH>12.     

The effect of chloride concentration and pH on pitting corrosion of AA7075 aluminum alloy coated with phenyltrimethoxysilane

The effect of chloride ion concentration and pH of solution on the corrosion behavior of aluminum alloy AA7075 coated with phenyltrimethoxysilane (PTMS) immersed in aqueous solutions of NaCl is reported. Potentiodynamic polarization, linear polarization, open circuit potential, and weight loss measurements were performed. The surface of samples was examined using SEM and optical microscopy. Elemental characterization of the coating by secondary ion mass spectrometry indicates an intermediate layer between coating and aluminum alloy surface. The corrosion behavior of the aluminum alloy AA7075 depends on chloride concentration and pH of solution. In acidic or neutral solutions, general and pitting corrosion occur simultaneously. On the contrary, exposure to alkaline solutions results in general corrosion only. Results further reveal that aluminum alloy AA7075 is susceptible to pitting corrosion in all chloride solutions with concentrations between 0.05 M and 2 M NaCl; an increase in the chloride concentration slightly shifted both the pitting and corrosion potentials to more active values. Linear polarization resistance measurements show a substantially improved corrosion resistance value in case of samples coated with PTMS as compared to uncoated samples in both neutral (pH = 7), acidic (pH = 0.85 and 3), and alkaline chloride solutions (pH = 10 and 12.85). The higher corrosion resistance of the aluminum alloy coated with PTMS can be attributed to the hydrophobic coating which acts as a barrier and prevents chloride ion penetration and subsequent reaction with the aluminum alloy.     

Adsorption of l-histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

The adsorption of l-histidine on a copper electrode from H₂O- and D₂O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO²⁻₄ and Cl⁻ ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO²⁻₄ and Cl⁻ ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH₂ group and the neutral imidazole ring. The Cl⁻ ions cause the protonation and detachment of the α-NH₂ group from the surface and the formation of the ion pair NH⁺₃ … Cl⁻ can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO⁻ group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH₂ group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.     

Contact angle and adsorption behavior of carboxylic acids on α-Al2O3 surfaces

There is good correlation of contact angle measurements and contact angles calculated from surfactant adsorption density data for an electrically neutral surface, as reported in a previous paper for the system hematite-aqueous solution-ketone, with surfactant hexadecyl sulfonic acid. The same method is not sufficient when the hematite surface is electrically charged. Data was collected to develop the appropriate form of an electrostatic term for the analysis. Acid-base titration was used to evaluate surface electrical properties versus pH for the hematite used in the study. Surfactant adsorption isotherms were measured at pH of 4.5, 5.5, 6, and 7 to use in developing an equation for effect of surface potential on contact angle. After adding a term for the contribution of the electric field, the contact angles calculated from adsorption data follow the measured contact angles well.     

Adsorption of neutral polymers on negatively charged liposomes. A novel quantitative method to measure the rate of polymer adsorption on the liposomal surface

We studied the adsorption of two neutral polymers [poly(vinyl pyrrolidone) and poly(vinyl alcohol) (PVA)] on negatively charged liposomes composed of 25:2:3 (molar ratio) soy lecithin/dicetyl phosphate/cholesterol.The liposomes were prepared in buffered solution at pH 7.4 and were mixed with the solution of the polymers in the desired polymer/lipid ratios. Adsorption was measured by determination of the equilibrium bulk concentration of the polymer. Protamine hydrochloride was used to aggregate the liposomes with polymers adsorbed on their surface and to facilitate their separation from the equilibrium bulk solution. In the case of PVA, quantitative adsorption measurements with a specific reagent were possible. Adsorption isotherms were recorded at 25 ± 0.2 °C. It was concluded that adsorbed and nonadsorbed PVA molecules are in equilibrium even at low polymer/lipide ratios. The results were confirmed by dynamic laser light scattering, X-ray diffraction and thermal activity monitoring experiments. Received: 13 October 2000 Accepted: 8 March 2001     

Protein co-adsorption on different polystyrene latexes: Electrokinetic characterization and colloidal stability

 The latex agglutination immunoassay technique uses polymer colloids as a carrier for the adsorbed proteins to enhance the antigen–antibody reaction. Competitive co-adsorption of IgGaCRP and m-BSA proteins on polystyrene latexes with different functionality (sulfate and sulfonate groups) was carried out looking for the increase in the immunoreactivity and colloid stability of latex–protein complexes. The preferential adsorption of a protein is also studied, comparing both surface types. Regarding the application in the development of a diagnostic test system, it is necessary to study the latex–protein complexes from an electrokinetic and colloid stability point of view. The presence of protein on the surface latex shifts the iso-electric point (i.e.p.) of the latex–protein complexes to pH values near the i.e.p. of the protein which is the majority. Thus by the adsorption of m-BSA we can obtain complexes with the i.e.p. near pH 5 and, therefore, with a significant electrostatic repulsion at neutral pH. Due to the higher surface charge density of the sulfonate latexes there is a higher adsorption of both proteins, which can provide a better colloidal stability (by the adsorption of m-BSA) and a better immunoreactivity (by the adsorption of IgG). Received: 27 March 1996 Accepted: 1 September 1996     

Adsorption of -histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

The adsorption of -histidine on a copper electrode from H₂O- and D₂O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO²⁻₄ and Cl⁻ ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO²⁻₄ and Cl⁻ ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH₂ group and the neutral imidazole ring. The Cl⁻ ions cause the protonation and detachment of the α-NH₂ group from the surface and the formation of the ion pair NH⁺₃ … Cl⁻ can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO⁻ group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH₂ group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.     

Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Studies on interaction of colloidal silver nanoparticles (SNPs) with five different bacterial species

Silver nanoparticles (SNPs) are being increasingly used in many consumer products like textile fabrics, cosmetics, washing machines, food and drug products owing to its excellent antimicrobial properties. Here we have studied the adsorption and toxicity of SNPs on bacterial species such as Pseudomonas aeruginosa, Micrococcus luteus, Bacillus subtilis, Bacillus barbaricus and Klebsiella pneumoniae. The influence of zeta potential on the adsorption of SNPs on bacterial cell surface was investigated at acidic, neutral and alkaline pH and with varying salt (NaCl) concentrations (0.05, 0.1, 0.5, 1 and 1.5 M). The survival rate of bacterial species decreased with increase in adsorption of SNPs. Maximum adsorption and toxicity was observed at pH 5, and NaCl concentration of <0.5 M. A very less adsorption was observed at pH 9 and NaCl concentration >0.5 M, there by resulting in less toxicity. The zeta potential study suggests that, the adsorption of SNPs on the cell surface was related to electrostatic force of attraction. The equilibrium and kinetics of the adsorption process were also studied. The adsorption equilibrium isotherms fitted well to the Langmuir model. The kinetics of adsorption fitted best to pseudo-first-order. These findings form a basis for interpreting the interaction of nanoparticles with environmental bacterial species.     

Study of the surface properties of titanium hydroxide using a radioactive tracer technique

The surface properties of titanium hydroxide precipitates, formed under different conditions were studied in the presence of carrier-free radioactive⁸⁹Sr. It was found that titanium with concentrations equivalent to 24 mg TiO₂/litre, was enough to remove by adsorption more than 95% of the carrier-free strontium activity from an alkaline solution (pH≈13) in less than 6 hrs, regardless of whether the precipitate was freshly prepared or aged for a few days. If the precipitate was formed in situ, at the above pH value, the percentage removal of the strontium activity from the solution was even greater, and this was attributed to an internal adsorption process. At a lower pH value (∼2.5), however, the adsorption of strontium activity by titanium hydroxide was very low (<5%), whether the precipitate was preformed or formed in situ. The increase in the adsorption ability of titanium hydroxide precipitate on going from low pH (2. 5) to higher values was attributed to the inversion of the charge of titanium hydroxide sols from cationic particles at pH 2. 5 to negatively-charged particles at pH>3. 5, these latter particles adsorbing any positive strontium ions in solution.     

The role of electrolyte and polyelectrolyte on the adsorption of the anionic surfactant, sodium dodecylbenzenesulfonate, at the air-water interface

The role of the polyelectrolyte, poly(ethyleneimine), PEI, and the electrolytes NaCl and CaCl(2), on the adsorption of the anionic surfactant, sodium dodecylbenzenesulfonate, LAS, at the air-water interface have been investigated by neutron reflectivity and surface tension. The surface tension data for the PEI/LAS mixtures are substantially affected by pH and the addition of electrolyte, and are consistent with a strong adsorption of surface polymer/surfactant complexes down to relatively low surfactant concentrations. The effects are most pronounced at high pH, and this is confirmed by the adsorption data obtained directly from neutron reflectivity. However, the effects of the addition of PEI and electrolyte on the LAS adsorption are not as pronounced as previously reported for PEI/SDS mixtures. This is attributed primarily to the steric hindrance of the LAS phenyl group resulting in a reduction in the ion-dipole attraction between the LAS sulfonate and amine groups that dominates the interaction at high pH.