Parsons-Zobel plots: an independent way to determine surface complexation parameters?

Parsons-Zobel plots can in principle be used to estimate inner Helmholtz-layer capacitance values and electrochemical surface areas for mineral particles. Their application to aqueous suspensions of various minerals has been documented in the literature. For the experimental data used so far, the expected linear relationship between the overall and the diffuse-layer capacitances has been reported. The extracted values either have not been used at all subsequently in a surface complexation model to describe the mineral surface charge versus pH curves, or were found not to be suitable entirely for such purposes. In the latter case, the reported failure was not explained. In one part of the present paper, the Parsons-Zobel plot concept is tested with data generated from a surface complexation model, for which the interfacial structure closely corresponds to that assumed in the application of the Parsons-Zobel plot. From the analysis of the results it turns out that electrolyte binding and non-Nernstian surface potential-pH curves more or less strongly affect the outcome of Parsons-Zobel plots. Despite the fact that the analysis in this paper is restricted to iron(III) minerals only, it is concluded, in general, that the use of Parsons-Zobel plots with aqueous mineral suspensions to determine inner Helmholtz-layer capacitances for subsequent application to surface complexation models cannot be recommended, since the reasons for failure can be traced very nicely with applications to model-generated data. Such application requires the determination of further parameters, and it was found that low electrolyte binding and Nernstian slopes should be imposed. Of these two issues, the more important is electrolyte binding. For the surface complexation models, an inner Helmholtz-layer capacitance and weak electrolyte binding were required for a good fit to experimental data. The values of the electrolyte binding constants required to achieve this end are in conflict with the assumptions of the Parsons-Zobel plot (absence of specific adsorption). However, these parameters would not necessarily cause specific adsorption in terms of a classical colloid chemistry definition (i.e., would not shift isoelectric points). The electrochemical surface areas were found to be in good agreement with the value used to generate the data. Based on this, there is a potential for using the approach to determine surface areas in situ from titration curves. Consequently, in a second part of the paper, Parsons-Zobel plots are applied to experimental data with the objective of determining electrochemical surface areas in situ. Application to various sets of published experimental titration data for hydrous ferric oxide yielded consistently very large electrochemical surface areas for fresh samples. This can be explained by very small particles and/or inclusion of substantial amounts of water in the suspended particles. As would be expected, the electrochemical surface area for aged ferrihydrite was found to be substantially lower.     

Benzenecarboxylate Surface Complexation at the Goethite (alpha-FeOOH)/Water Interface

A surface complexation model describing the adsorption of three benzenecarboxylates (phthalate, trimellitate, and pyromellitate) on goethite (alpha-FeOOH) was calibrated on data using goethite particles of 37 and 43 m(2)/g surface area. The models predict potentiometric titration and batch adsorption data with the multisite complexation model coupled with the three-plane model to account for surface electrostatics. The modeling parameters were found to be similar to those calibrated on benzenecarboxylate adsorption data on goethite particles of 90 m(2)/g (Boily et al. Geochim. Cosmochim. Acta, in press). The significance of the benzenecarboxylate-dependent values of the modeling parameters is also discussed. The values of the capacitances of the inner- and outer-Helmholtz planes were shown to be important modeling parameters to model the benzenecarboxylate-dependent slopes of the adsorption edges. It was shown that the larger the charge of the ligand, the larger the capacitance of the outer-Helmholtz plane. Copyright 2000 Academic Press.     

On derivatives of surface charge curves of minerals

Surface titrations of minerals in aqueous electrolyte solutions are used as building blocks for surface complexation modelling. However, these potentiometric data may contain less model relevant information than previously and presently assumed. In the literature, derivative analyses have been applied to experimental surface charge versus pH curves and four or more pK values were extracted for goethite or aluminium oxide. Derivative analysis of specific surface charge versus pH curves calculated for various published model variants for goethite shows that not more than the net-zero proton surface charge condition can be extracted from computer generated data. Generated data can be produced in density and precision superior to experimental data, but yield only relatively little output from such derivative analysis compared to what has previously been extracted from derivatives of experimental data. For the generated goethite data and for all model variants only the point of zero could be extracted. For the various goethite model variants tested a nearly symmetrical peak appeared at the point of zero charge in the derivative curve. A different pattern could be obtained for generic models, for which two sites with unequal sites densities and different pK values were assumed. Variation of these parameters could result in derivatives of the charging curves with two maxima or one maximum and a broad tailing. In the literature, curves with features nearly identical to these generated curves have been interpreted by up to four pK values (i.e., four different sites within a 1-pK model). It is concluded that the interpretation of the generated data is in all cases hampered by the overwhelming electrostatic contributions to the free energy of proton ad/desorption. In no case except for the one-site 1-pK model was it possible to extract the input pK value(s) from the derivatives. Plausible explanations for the discrepancy between generated data and published experimental data are discussed.     

Determination of Surface Silanols of Silica Gels and HPLC Bonded Phases

Abstract

A rapid method determining the surface silanols of silica gels and HPLC bonded phases involves a titration of silica gel with sodium hydroxide, in a medium of any of the following 10% aqueous salts: sodium chloride, sodium nitrate, sodium sulfate, potassium chloride, potassium nitrate and potassium sulfate. The silanols are quantified as m eg/g. It is possible to determine cation exchange capacity of a cation-exchanger due to an acid as an end group and surface silanols separately.     

Evaluation of DLVO theory with disjoining-pressure and film-conductance measurements of common-black films stabilized with sodium dodecyl sulfate

We develop a unique film holder combining a thin-film balance with AC impedance spectroscopy to measure disjoining pressure, film conductance, and film thickness simultaneously. Foam films stabilized by sodium dodecyl sulfate (SDS) are investigated with and without added sodium chloride (NaCl) electrolyte. Classical colloidal theory, Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, is tested rigorously over a wide range of solution conditions by comparing the surface charge densities fit to disjoining-pressure isotherms with those estimated independently from film-conductance and surface-tension data. Film-conductance measurements strongly suggest that the adsorbed anionic surfactant is partially complexed with counterions. Therefore, to reconcile the different values of charge densities calculated from surface tension and film conductance with those from disjoining pressure, we propose a simple ion-binding electrostatic model. The ion-complexation framework predicts increased ion complexing with increasing solution ionic strength, in agreement with surface-tension and film-conductance data. Unfortunately, it is not possible to describe similarly the trends of the measured disjoining-pressure isotherms because the diffuse-layer charge density increases, or equivalently, the ion complexation decreases with increasing ionic strength. Accordingly, the ion-binding extension of classical DLVO theory does not permit agreement between theory and independent experimental data from surface tension, disjoining pressure, and film conductance.     

Sorption of selenite ions on hematite

The sorption of selenite from aqueous solutions onto hematite was investigated as a function of pH (2-12), ionic strength (0.01-0.1 M), and concentration of selenium (10(-7)-10(-2) M). The sorption may proceed according to two processes: surface complexation, followed by the precipitation of ferric selenite starting at approximate [Se] = 4 x 10(-4) M (surface coverage>ca. 2 at nm(-2)). The sorption isotherms have been fitted by a Tempkin equation. A surface complexation model (2-pK/Constant Capacitance Model) was used to fit the sorption data. The nature of the surface species of selenite cannot be determined by modeling since monodentate >FeOSe(O)O- or >FeOSe(O)OH and bidentate (>FeO)2SeO surface complexes are both able to fit the experimental data. The reversibility and kinetics of sorption were also studied. The affinity of selenite ions toward hematite, expressed as the distribution coefficient with respect to the surface area (K(D) in L m(-2)), was compared with results published for other ferric oxides (goethite and amorphous ferric oxide). It was found that the reactivity toward selenite is similar, contrary to acid-base properties which depend on the nature of the oxide and its level of purity.     

Amperometric method for determining the degree of complexation of polyelectrolytes with cationic surfactants

The complexation of sodium polystyrene sulfonate with monovalent cationic surfactants at a microsized liquid/liquid interface has been studied using electrochemistry. The method is based on measurement of surfactant ion transfer across the interface between two immiscible electrolyte solutions (ITIES). The complexation of various cationic surfactants (alkylpyridinium- and trimethylammonium-) with oligosized polystyrene sulfonate was measured. Binding isotherms were used to determine the degree of binding as a function of the surfactant chain length and type of head group. It was found that the hydrophobicity of the surfactant was the predominant factor. The effect of the polyelectrolyte chain length on the binding mechanism was studied using cetylpyridinium chloride as a complexing agent. It was found that binding affinity, as well as cooperativity of the binding process, decreases with decreasing polyelectrolyte chain length. Thermodynamics of surfactant binding was measured using titration microcalorimetry. The thermodynamic data obtained show that the enthalpy of surfactant binding is not dependent on polymer chain length, but an increase in chain length makes the binding process entropically more favorable.     

Adsorption of alpha amino acids at the water/goethite interface

The adsorption of amino acids onto mineral surfaces plays an important role in a wide range of areas, e.g., low-temperature aqueous geochemistry, bone formation and protein-bone interactions. In this work, the adsorption of three alpha aminoacids (sarcosine, MIDA and EDDA) onto goethite (alpha-FeOOH) was studied as a function of pH and background electrolyte concentration at 25.0 degrees C, and the molecular structures of the surface complexes formed were analyzed by means of ATR-FTIR spectroscopy. The results showed that adsorption of alpha amino acids were strongly dependent on the functionality and structure of the ligands. No adsorption was detected for the zwitterionic sarcosine indicating that simple alpha amino acids without other ionizable and/or functional groups display insignificant affinity for mineral surfaces such as goethite. With respect to the more complex amino acids, which are surface reactive, the number and relative positions of carboxylate and amine groups determine the types of surface interactions. These interactions range from non-specific outer-sphere to specific inner-sphere interactions as shown by the MIDA and EDDA results, respectively. The results presented herein suggest that isomerically-selective adsorption might only occur for amino acids that are capable of specific surface interactions, either through site-specific hydrogen bonding or inner-sphere complexation.     

Measurement of surface potential at silver chloride aqueous interface with single-crystal AgCl electrode

The surface potential at the silver chloride aqueous interface was measured by means of a single-crystal silver chloride electrode (SCr-AgCl). The measurements were conducted by titration of the KCl solution with AgNO3, and vice versa. The SCr-AgCl electrode potentials were converted to surface potentials psi(0) by setting zero at the point of zero charge at pCl = 5.2. The psi(0)(pCl) function was linear, with a slope 12% lower with respect to the Nernst equation. It was demonstrated that the surface potential at the silver halide aqueous interface could be interpreted by means of the surface complexation model, originally developed for metal oxides.     

Ionic strength dependence of formation constants, complexation of nitrilotriacetic acid with tungsten(VI) ion

The dependence on ionic strength of protonation of nitrilotriacetic acid and its complexation with W(VI) is reported in sodium perchlorate, sodium nitrate and sodium chloride solutions as background salts. The measurements have been performed at 25°C and various ionic strengths in the range 0.1–1.0 mol dm⁻³, using a combination of potentiometric and spectrophotometric techniques. The overall analysis of the present and the previous data dealing with the determination of stability constants at different ionic strengths allowed us to obtain a general equation, by which a formation constant determined at a fixed ionic strength can be calculated, with a good approximation, at another ionic strength, if 0.1 ≤ ionic strength ≤ 1.0 mol dm⁻³ sodium perchlorate, sodium nitrate or sodium chloride.     

Studies on liquid–liquid extraction of La(III) from acidic nitrate medium using TBP in petrofin

The present study aims at investigating the capability of tri-n-butyl phosphate (TBP) in petrofin as extracting agent for La(III) from acidic nitrate medium. The concentrations of TBP and sodium nitrate have positive influence while temperature shows a negative impact on the extraction. The values of standard enthalpy change and entropy change confirm the extraction process as exothermic with a decrease in randomness due to complexation. Regression analysis results fit well with the experimental data in the cases of TBP and nitrate concentration variation.     

Surface complexation modeling of uranium(VI) sorbed onto lanthanum monophosphate

Sorption/desorption are basic processes in the field of contaminant transport. In order to develop mechanistically accurate thermodynamic sorption models, the simulation of retention data has to take into account molecular scale informations provided by structural investigations. In this way, the uranyl sorption constants onto lanthanum monophosphate (LaPO(4)) were determined on the basis of a previously published structural investigation. The surface complexation modeling of U(VI) retention onto LaPO(4) has been performed using the constant capacitance model included in the FITEQLv3.2 program. The electrical behavior of the solid surface was investigated using electrophoretic measurements and potentiometric titration experiments. The point of zero charge was found to be 3.5 and surface complexation modeling has made it possible to calculate the surface acidity constants. The fitting procedure was done with respect to the spectroscopic results, which have shown that LaPO(4) presents two kinds of reactive surface sites (lanthanum atoms and phosphate groups). The uranyl sorption edges were determined for two surface coverages: 40 and 20% of the surface sites that are occupied, assuming complete sorption. The modeling of these experimental data was realized by considering two uranyl species ("free" uranyl and uranyl nitrate complex) sorbed only onto phosphate surface groups according to the previously published structural investigation. The obtained sorption constants present similar values for both surface complexes and make it possible to fit both sorption edges: logK(U)=9.4 for z.tbnd;P(OH)(2)+UO(2)(2+)<-->z.tbnd;P(OH)(2)UO(2)(2+) and logK(UN)=9.7 for z.tbnd;P(OH)(2)+UO(2)NO(3)(+)<-->z.tbnd;P(OH)(2)UO(2)NO(3)(+).     

Surface charge and adsorption of Na+ and Cl− ions at the zirconium dioxide/water interface

The surface charge and adsorption densities of Na⁺ and Cl^– ions at the zirconium dioxide/electrolyte interface have been determined as a function of pH for 0.1, 0.01 and 0.001 M solutions of NaCl. Using potentiometric titration of the surface hydroxy groups, it was found that the point of zero charge occurred at pH 4.3±0.15. The results are discussed in terms of site binding model of the electric double layer. The ionization and complexation constants have also been determined.     

Separation of superparamagnetic magnetite nanoparticles by capillary zone electrophoresis using non‐complexing and complexing electrolyte anions and tetramethylammonium as dispersing additive

Separations of bare superparamagnetic magnetite nanoparticles (BSPMNPs, approx. 11 nm diameter) was performed using non‐complexing (nitrate) and complexing (chloride, citrate and phosphate) electrolyte ions with additions of tetramethylammonium hydroxide (TMAOH), which is commonly applied to control the synthesis of stable iron oxides. The use of TMAOH as a background electrolyte (BGE) additive for capillary electrophoresis (CE) separations provided for the first time electropherograms of BSPMNPs exhibiting symmetrical and highly reproducible peaks, free of spurious spikes characteristic of nanoparticle clusters. Consequently, accurate determination of the electrophoretic effective mobility of BSPMNPs was possible, yielding a value of −3.345E‐08 m² V⁻¹ s⁻¹ (relative standard deviation (RSD) of 0.500%). The obtained mobilities of BSPMNPs in the presence of various electrolyte ions show that the degree of complexation with the surface of BSPMNPs follows the order chloride < citrate < phosphate, correlating with the stabilities of Fe(III) complexes with the respective anions. Finally, bare and carboxylated iron oxide nanoparticles were successfully separated in only 10 min using 10 mM Tris‐nitrate containing 20 mM of TMAOH as electrolyte. Our findings show that simple and rapid CE experiments are an excellent tool to characterise and monitor properties and interactions of iron oxide nanoparticles with other molecules for surface modification purposes.     

Effects of Energetic Surface Heterogeneity on Ion Adsorption at the Electrolyte/Oxide Interfaces: Non-symmetrical Binding-to-Surface Energy Dispersions

The equations developed by us for the surface complexation models, taking into account energetic heterogeneity of surface oxygens, are applied here to study the effects of the shape of adsorption energy distribution on the ion adsorption at the oxide/electrolyte interface. The paper presents comparison of two models: one assuming that the energetical heterogeneity of oxide surface is described by a symmetrical and next a non-symmetrical adsorption energy dispersion for the formation of various surface complexes. The comparison of these two models was based on the obvious assumption that the variances of both the symmetrical and the non-symmetrical distributions are equal. The potentiometric titration data are not sensitive enough to choose the right adsorption model. So, in addition the individual isotherms of adsorption of cation and anion, of the inert electrolyte measured radiometrically, have been taken into consideration. The symmetrical adsorption energy distribution seems to represent the features of these adsorption systems better.     

Electrochemical Synthesis and Characterization of Electroactive Conducting Polypyrrole Polymers

Conducting/electroactive polypyrrole polymers are synthesized electrochemically on glassy carbon in various electrolytes (counterions). The polymers' electroactivity is measured using cyclic voltammetry. The electrolytes are chloride, nitrate, p-toluene sulfonate, dodecyl sulfate, and dodecylbenzenesulfonate of sodium and potassium ferrocyanide. It is found that the electrolyte (dopant) markedly affects the redox behavior of the polypyrrole films.