Amphoteric Surface Research Articles

Dynamic stability ratio of a colloidal dispersion

The stability ratio of a dispersed system containing colloidal particles having amphoteric surfaces is estimated theoretically. We consider the case where the degree of dissociation of the functional groups on particle surface as a response to the variation in the conditions of the surrounding liquid phase is a function of time. The dynamic nature of the distributions of ions in the electrical double layer near a particle is also taken into account. The result of numerical simulation reveals that the dynamic nature of the system under consideration has the effect of increasing its stability. Due to the fact that the diffusivity of protons in the aqueous solution is much larger than that of other ion species, the level of increase in the stability ratio is less than ∼40%. However, if the rates of surface reactions are slow, the dynamic stability ratio can exceeds ten times the corresponding equilibrium value. Also, the smaller the particle and/or the thicker the double layer, the more significant the dynamic behavior of the system under consideration.

Exact Poisson-Boltzmann solution for the interaction of dissimilar charge-regulating surfaces.

An efficient method is proposed to calculate the electric double layer force between two flat surfaces of dissimilar composition and ionization properties. The approach is based on explicit expressions for the solution of the (nonlinear) Poisson-Boltzmann equation and allows for boundary conditions of charge regulation, i.e., chemical equilibrium of both surfaces with a bulk electrolyte at all surface separations. As an illustration, we discuss in some detail the interaction between a weakly acidic and a strongly acidic latex surface, and between an acidic (silica) surface and an amphoteric (rutile) surface.

Pore-surface characterization of poly(acrylonitrile) membrane having amphoteric charge groups by means of zeta potential measurement

Pore-surface characteristics of poly(acrylonitrile)-based porous membranes having amphoteric charge groups were investigated by zeta potential obtained from streaming potential measurements. The amphoteric-charged pore surface was theoretically examined by a single-acid, single-base site dissociation model, whose model is based on the assumption that the surface charge arises only from the protonation and deprotonation of the charged surface groups. From the theoretical approach, some characteristic parameters of the amphoteric pore surface such as the isoelectric point, dissociation constant, apparent site density of each acidic and basic group, and acid-to-base ratio were successfully determined. These parameters were changed in response to the variation of the feed monomer ratio. This approach can be expected to provide useful information concerning the pore-surface charge of an amphoteric-charged membrane.

Surface Evaluation of Plasma-Modified Polysulfone (Udel P-1700) Films

The paper presents a new method for the evaluation of surface properties. Polysulfone films modified by plasma treatment in carbon dioxide, nitrogen, and vapors of n-butylamine were taken as test samples. Surface concentrations of acidic or basic functionalities were estimated by contact angle measurements. Aqueous solutions of HCl and NaOH with various pH values were used. The method of surface evaluation is based on fitting the γLcosϑ versus pH (or pOH) relationship with a polynomial and finding its inflection point. The pH (or pOH) values at this point are recalculated into apparent surface concentrations of basic (or acidic) functionalities by means of Gibbs' and Langmuir's equations. It was found that surfaces contain acidic functionalities (0.15 μmol/m2) when polysulfone was modified in carbon dioxide and basic functionalities (2.50 μmol/m2) for butylamine plasma. Modification with N2 resulted in the creation of an amphoteric surface.

The Structure of U6+ Sorption Complexes on Vermiculite and Hydrobiotite

AbstractThe sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ionexchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ionexchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Stability of Colloidal Dispersions: Charge Regulation/Adsorption Model

The modified stability ratio, W‘, and the critical coagulation concentration, CCC, of a colloidal dispersion containing spherical particles are analyzed theoretically. We consider the case where the surface of a particle is capable of regulating its charged condition through the dissociation of functional groups, the exchange of ions with the liquid phase, or both. An amphoteric surface is assumed for the former. The analysis leads to an approximate analytic expression for the electrical interaction energy between two particles. We show that the smaller the (pK- − pK+), K- and K+ being the dissociation constants, the greater the interaction energy. It is found that a system may have two different CCC's and two different electrolyte concentrations at which W‘ = 1. However, the greater the (pK- + pK+), or the greater the (pK- − pK+), the less the significance of the second CCC and the electrolyte concentration at which W‘ = 1. The number of CCC and that of electrolyte concentration at which W‘ = 1 may not b...

Influence of additives on the protoplasts electrofusion

Various neutral or charged surface active substances were used for testing the influence of additives on the electrofusion of barley protoplasts. It was found that neutral surface active agents DX, TAGB, Span-80 and AEO-9 could promote the electrofusion. The positively charged surface active agents Bardac 2080, Bardac 2280 and amphoteric surface active agents dodecyl-propyl betaine and CAB betaine also promote the electrofusion, but at high concentration the electrofusion efficiency will reduce. The negatively charged polymer agents Cibacron blue DX, Fluoresceinylthiocarbamoyl DX, and active surface substances K 12 and Carsonol TLS − presented negative effect. These phenomena were discussed from the view of adsorption of additives on the membrane and the interactions between protoplasts.

Electrokinetic and contact angle measurements of grafted carbon fibers

The grafting method that has successfully been applied to methacrylic acid and liquid crystalline monomers was expanded to prepare amphoteric carbon fiber surfaces using 2-(N,N-dimethylamino)ethyl methacrylate as monomer. The obtained carbon fiber surfaces were characterized by contact angle and ζ-potential measurements. The expected basic behavior was not observed, instead an amphoteric character of the modified carbon fiber surface was found. The fiber surfaces display a basic character in the acidic pH-range, while they are acidic in the alkaline part of the pH-scale. An important influence is derived from the amount of initiator used to graft the monomers onto the fibers. The smaller the initiator concen-tration used during polymerization, the larger the amount of amino functionalities introduced to the carbon fiber surface. The wetting behavior versus water depends on the overall conformation of the immobilized polymer. During immersion into water the polymer acts hydrophobic, while during emersion, a hydrophilic character is observed, probably derived from conformational changes and swelling during the contact angle measure-ments in water.

Characterization of an Amphoteric-Charged Layer Grafted to the Pore Surface of a Porous Membrane

Porous membranes having amphoteric charge groups were prepared by heterogeneous graft polymerization from aqueous solution containing acrylic acid and (N,N-dimethylamino)propyl acrylamide. The charging properties of the amphoteric charge groups grafted to the pore surface were investigated by ζ potentials obtained from streaming potential measurements. Theoretical development in terms of the amphoteric pore surface was attempted by introducing a site dissociation model of pH-dependent ζ potentials. The theoretical model is based on the assumption that the surface charge arises only from the protonation and deprotonation of the charged surface groups exposed to electrolyte solution. Good fits between experimental results and theoretical equations were attained, and thereby the isoelectric point, dissociation constant, apparent surface site density, and acid-to-base ratio of the amphoteric pore surface were determined. Depending on the variation in the feed monomer ratio for graft polymerization, these para...

Effect of surface active additives on partitioning of proteins and enzymes in poly(ethylene glycol)/dextran aqueous two-phase systems

The effect of anionic (sodium butylbenzene sulfonate, sodium butylmonoglycol sulfate), cationic (tetrabutyl ammonium bromide), nonionic(Tween 20) and amphoteric (proline) surface active additives on the partitioning of proteins and enzymes, such as BSA, lysozyme, glucose oxidase and β-lactoglobulin, in a bipolymeric aqueous two-phase system of polyethylene glycol and dextran has been studied. The partitioning of proteins and enzymes in the aqueous two-phase system is influenced by surface active additives depending upon their structure and charge. The amphiphiles themselves partition unevenly between the two phases. Their effect on protein partitioning can be explained on the basis of electrostatic and hydrophobic interactions. In the presence of ionic amphiphiles, proteins have an affinity for the other phase if an amphiphile carrying a charge of the same sign partitions to that phase. The hydrophobic effect contributes to protein partitioning if the proteins have significant members of surface hydrophobic amino acid residues. © 1998 Society of Chemical Industry

An XPS study of the adsorption of lead on goethite (α-FeOOH)

The adsorption behavior of divalent metal ions on amphoteric surface hydroxyl groups of metal oxides is an important factor in the transport of subsurface environmental pollutants. We have studied the adsorption of lead ions from aqueous solution on the surface of the mineral goethite (α-FeOOH) as a function of pH and adsorbate concentration in 0.1 M NaNO 3 solution. Results obtained from the dried surface by X-ray photoelectron spectroscopy (XPS) are in good agreement with data from spectrophotometric analysis of lead ions remaining in the supernatant liquid. Lead adsorption as a function of pH shows an adsorption edge near pH 6 and maximizes by pH 7.5. The surface atomic Pb/Fe ratio at saturation as measured by XPS is 0.93 leading to an apparent high adsorption site density of 14.2 sites/nm 2. A characteristic low binding energy (BE) XPS feature is present in the Pb 4f XPS signal in the pH range 6–8. Comparison with literature predictions of the speciation of lead at a goethite surface suggests that this XPS feature is perhaps associated with a FeOPb + complex. At high Pb(II) concentrations some polynuclear complexes probably form. They may be responsible for the high site density value which, while crystallographically possible, is much higher than values reported by others working at lower Pb concentrations.

Complex Formation between Bovine Serum Albumin and Strong Polyelectrolytes: Effect of Polymer Charge Density

Light scattering and pH titration were used to examine the binding of bovine serum albumin (BSA) to poly(diallyldimethylammonium chloride) (PDADMAC), poly(acrylamidomethylpropyl sulfonate) (PAMPS), poly(methacrylamidopropyltrimethylammonium chloride) (PMAPTAC), and an AMPS−acrylamide random copolymer (PAMPS80AAm20). The critical protein charge required to induce protein−polyelectrolyte complexation, (Zpr)c, was found to vary linearly with the square root of the ionic strength (I1/2), i.e., with the Debye−Hückel parameter (κ), the proportionality constant being a function of polyelectrolyte chain parameters such as intrinsic stiffness and charge density. This linearity was remarkably continuous through Zpr = 0, with (Zpr)c occurring predominantly “on the wrong side” of the isoionic point; i.e., the onset of binding was typically observed when the global protein charge was of the same sign as the polyelectrolyte. Binding of BSA to the lower charge density polyanion (PAMPS80AAm20) unexpectedly occurred under conditions where binding to the more highly charged homopolyanion (PAMPS) did not. The theoretical treatment of Muthukumar was used to interpret the linearity of (Zpr)c vs I1/2 and the observed influence of polyelectrolyte structural parameters. The apparent applicability of this model to the heterogeneous amphoteric protein surface suggests that binding of polyelectrolytes takes place at “charge patches” whose effective charge densities are different from, but nevertheless linearly dependent on, the global charge density.

Interaction theory for double electric layers of dissimilar particles for equilibrium regime of surface ionization Low surface potentials

The solution to the problem of the interaction of double electric layers formed by dissimilar particles is obtained on the basis of the linearized Poisson-Boltzmann equation assuming the equilibrium regime of particle surface charging. Three types of particle surface ionization by the ions of the electrolyte whose concentration corresponds to the Henry law region are considered: (a) cations (anions) adsorption on an initially uncharged surface which does not possess any non-compensated charge before the adsorption of ions takes place; (b) ions adsorption (ion exchange) on an initially charged particle surface that possesses this charge; (c) ions adsorption (ion exchange) on amphoteric surfaces. The three cases can be reduced to the case (c) with unequal Nernst potentials of the surfaces. The dependencies of potentials, densities of plate charges, and electrostatic component of the Gibbs free energy of the particle interactions on the nearest distance between the particles are studied qualitatively, and the corresponding analytical expressions are derived.

Thermodynamic Modelling of Heterogeneous Sorption of Trace Metals Onto Synthetic and Natural Non-Stoichiometric Mn Oxides

Recent research highlighted the intricate relationships between solubility of Mn oxides and trace metal sorption in stratified marine and lacustrine water columns (see abstract by Kersten et al.). Existing speciation, solubility and surface complexation models taken separately are insufficient in rationalising such processes. This paper presents a new multi-site multi-surface thermodynamic model combining metal adsorption/ ion exchange with solubility of non-stoichiometric Mn oxide (8MnOx). The model can account for both particleand solutionconcentration effects on trace metal partitioning in suboxic natural environments. Gibbs energy minimisation (GEM) approach (see abstract by Kulik) implemented in Selektor-A 3.2 code was used for all calculations. The code can solve aqueous equilibria involving 'sorption nanophases' composed of surface complexes arranged on up to six surface types on a solid-solution carrier of non-zero specific surface area A. The built-in thermodynamic dataset was extended for surface species using REACDC format of Selektor-A database. TLM (Hayes) was used for amphoteric surface types; non-electrostatic model (NEM) for ion exchange surface. 8-MnOx was represented by MnO2-MnOOH ideal solid solution (see abstract by Kersten et al.). Single-site TLM fits of 8-MnO~ surface acidity yield pHpz C 2 F.m -2, pKa2 < 6.0 and outer-sphere pKcat around 3.0 (Carts and Langmuir, 1986). However, Tamura et al. (1996) found pHpzc between 3.76 and 4.72 by titration of synthetic ~{MnOx samples. Healy et al. (1966) reported pHpzc < 2.0 for 8-MnOx, but pHpzc = 5.5 _+ 0.2 for y-MnO2. Microscopic studies reveal energetically different sites for cation binding on surfaces of Mn oxides, probably responsible for solution-concentration effects in adsorption data for 8-MnO• causing large scatter of fitted pKads. Moreover, the ion exchange is potentially significant on synthetic and natural birnessites/buserites with formal CEC ~< 0.3 (Silvester et al., 1997). All this suggests that MnOx Institute for Baltic Sea Research, University of Rostock, 18119 Wamemuende, Germany

A mechanistic description of Ni and Zn sorption on Na-montmorillonite Part I: Titration and sorption measurements

In this paper experimental investigations into the acid/base titration characteristics of Na-montmorillonite and the sorption behaviour of Ni and Zn under a wide variety of conditions are presented. From these measurements the dominant sorption mechanisms could be deduced. In the following paper (Bradbury and Baeyens, 1997) the titration and sorption data are analysed to determine the parameters in cation exchange and surface complexation based models which together provide a quantitative description of the titration and sorption data. A conditioning procedure was applied to the SWy-1 Na-montmorillonite starting material in order to remove background metal impurities, soluble salts and sparingly soluble minerals which could influence titration and sorption measurements. The purified clay, in the homo-ionic Na form, was thoroughly physico-chemically characterised before carrying out batch titration measurements on suspensions in 0.1 and 0.5 M NaClO 4. The influence of background impurities, not removed by the conditioning, and cation exchange processes on the form of the titration curves is discussed. Titration data can be analysed to yield site capacities and protonation/deprotonation constants for the amphoteric surface hydroxyl groups (≡SOH). The acid endpoint in the titration data was used to estimate an ≡SOH site capacity of 0.08 mol kg −1. The sorption of Ni and Zn on conditioned Na-montmorillonite was studied at trace concentrations as a function of pH over a range from ∼ 3 to ∼ 10 to produce so-called “sorption edges”. In the case of Ni, such measurements were carried out as a function of the NaClO 4 background electrolyte concentration. In addition, sorption isotherms were determined for both nuclides at several fixed pH values in 0.1 M NaClO 4. From the form of the “edges” it was deduced that two main sorption mechanisms were controlling the uptake of Ni and Zn onto the clay mineral; a pH-independent component, identified as cation exchange on the permanent charge sites, and a pH-dependent one, interpreted as surface complexation on the amphoteric surface hydroxyl groups. The non-linearity of the sorption isotherms indicated that at least two different ≡SOH type sites were contributing to the overall sorption on Na-montmorillonite.

Using chemical models for evaluating the geochemical confinement capacity of a geological barrier application to the Centre de Stockage de l'Aube (France)

The feasibility of using a chemical reaction-based approach for evaluating and modelling the role of adsorption reactions in determining the geochernical confinement capacity of natural geological barriers is being studied as part of an on-going R & D programme. The confined superficial aquifer underlying the Centre de Stockage de l'Aube facility, a geological barrier for this site, has been used as a case study with the following aims. First, development of a site characterisation protocol and demonstration of its use to determine the principal geochemical characteristics of aquifer materials using batch experiments and to represent the information obtained in terms of a chemical model. The experimental results obtained for Ni 2+ partitioning as a function of total Ni, pH, total Ca and total solid can be satisfactorily represented in terms of reactions with an ion exchange site and a single amphoteric surface hydroxyl site with ferrihydrite reaction constants. A second objective is the incorporation of the reactions in a coupled geochemistry/transport code, and to verify the applicability of the coupled code predictions for Ni 2+ mass transfer by comparison with the results obtained during column tracer experiments. The breakthrough curve and equilibrium solid phase Ni loading, predicted by a one-dimensional coupled model for a column tracer experiment, agree closely with observed data. Additional studies are underway to reduce model conditionality, to extend the adsorption model to other analogue cations and anions, to incorporate the effect of natural organic matter and to take into consideration precipitation/dissolution of amorphous Fe surface phases.

Theory of ionic-surface electrical conduction in porous media

We present a model describing ionic electrical conduction in porous media, with particular emphasis given to surface conduction. The porous medium is assumed to consist of an insulating matrix and an interconnected pore volume that is saturated with an electrolyte. When in contact with an electrolyte, mineral surfaces get an excess of charge that is balanced by mobile ions in an electrical diffuse layer above the surface. Electrical conduction in this diffuse layer can contribute substantially to the effective electrical conductivity of the porous medium. Our surface conduction model is based on a description of surface chemical reactions and electrical diffuse layer processes. For this purpose, we consider an amphoteric mineral surface described by a five-site-type model. We derive the fractional occupancies of positive, negative, and neutral sites on the surface, and the fractional ionic diffuse layer densities, as a function of the salinity and the pH. Finally, the specific surface conductance used to describe the surface electrical conduction is related to the previously mentioned properties, via the electrical surface potential, and is found to be dependent on the electrolyte concentration and pH.

Electrical conductivity, spontaneous potential and ionic diffusion in porous media

Abstract A consistent set of equations describing electrical conduction, spontaneous potential and ionic diffusion phenomena in porous media, with a particular emphasis given to surface electrical phenomena, is presented. The porous medium is assumed to consist of an insulating matrix and an inter-connected pore volume that is saturated with an electrolyte. When in contact with an electrolyte solution, mineral surfaces have an excess of charge that is balanced by mobile ions in an electrical diffuse layer (EDL). Electrical conduction and ionic diffusion in this diffuse layer can contribute substantially to the effective electrical conductivity of the porous medium, and can play an important role in the strength of several spontaneous electrical potentials (i.e., membrane potential, streaming potential, and thermo-electric potential) and ionic diffusion phenomena. Our surface electrical properties model is based on a thermodynamic description of surface chemical reactions and electrical diffuse layer processes. As an example, we consider an amphoteric mineral surface described by a three-site-type model. We derive the fractional occupancies of positive, negative and neutral sites, and the fractional ionic diffuse layer densities on the surface as a function of the salinity and the pH. The surface charge density at a given pH, is found to be dependent on the electrolyte concentration. Finally, parameters of interest in describing macroscopic effects (effective electrical conductivity, spontaneous potential coupling coefficients and effective diffusion coefficient), are related to the previously mentioned properties, via the mineral surface electrical potential (called the Stern potential).

Modeling the interfacial properties of an amorphous aluminosilicate dispersed in aqueous NaCl solutions

The structural and interfacial properties of an amorphous aluminosilicate prepared from aluminum chloride and sodium silicate solutions are reported. IR and 27Al NMR spectroscopy revealed that the structure of the studied sample was similar to that of (hydrous) feldspathoids and amorphous aluminosilicate catalysts, which are characterized by the presence of negatively charged SiOAl groups (SiAlO −) and hydroxylated surface groups. The particles were non-porous and dissolved appreciably at pH < 5.2. H +, Cl − and Na + adsorption data ( Γ H, Γ Cl and Γ Na), together with electrophoretic measurements, revealed, however, that the point of zero saline effect (PZSE) was different from the isoelectric point (IEP) and from the pH where Γ Cl − Γ Na = 0. This behavior differs from that of (hydr)oxides that contain only variable charge (amphoteric) surface sites. A simple model of the solidNaCl solution interface is proposed to account for the cation exchange properties of SiAlO − sites and for the acid-base character of the hydroxylated ones. The lack of coincidence between the PZSE and the IEP and the invariability of the IEP with the electrolyte concentration is attributed to the presence, and cation exchange properties, of SiAlO − sites. However, the model suggests that a high proportion of structural charge sites are neutralized by sodium ions in the particle bulk and cannot be assessed for cation exchange. The ability of the model to fit experimental adsorption data suggests that it can be used to improve the understanding of the amorphous aluminosilicate-electrolyte solution interface.

Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaCl), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCl 2, Na 2SO 4, and MgSO 4) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.