pH-dependent Surface Charge Research Articles

PH-dependent surface charging and points of zero charge II. Update

Recently published PZC (points of zero charge) of metal oxides and related materials are compiled to update the previous compilations (M. Kosmulski, Chemical Properties of Material Surfaces, Dekker, New York, 2001; J. Colloid Interface Sci. 253 (2002) 77). The electroacoustic method has been widely used; it has become a standard tool, and it has proved to produce IEP (isoelectric points) comparable with those obtained by means of classical electrokinetic methods. The recently published numerical values of PZC/IEP of various materials corroborate the old results, with one exception: the PZC of magnetite found at pH 8 is substantially higher than the values reported in the old literature. New approaches to the electrokinetics of sparingly soluble salts have recently been proposed; e.g., the hysteresis in electrokinetic curves of (nominally) BaTiO 3 has been interpreted in terms of changes in the surface stoichiometry caused by leaching.

The pH-Dependent Surface Charging and the Points of Zero Charge

The recently published points of zero charge of metal oxides and related materials are compiled to update the previous compilation (Kosmulski, M., “Chemical properties of materials surfaces.” Dekker, New York, 2001). The purity of materials is the most important factor responsible for discrepancies in the literature data. In contrast to the success of new spectroscopic methods in the studies of specific adsorption, the progress in studies of primary surface charging of oxides over recent years is less spectacular.

Synthesis and pH Dependent Optical Properties of Gold Nanoparticles Capped with Mercaptopropionic Acid

A simple method is reported for preparing gold nanoparticles capped with 3-mercaptopropionic acid(MPA). The capped Au nanoparticles can be isolated as a dark brown solid which can be re-dispersed in sodium hydrate solution. The average size of nanoparticles is 4.9 ± 0.7 nm, determined by HRTEM. The surface plasmon band of Au nanoparticles around 520nm was found red-shifted together with a slight damping and broadening, as the pH of the colloid is adjusted from 11 to 3.5. Such a shift is attributed to the pH dependent surface charge of the nanoparticles.

Processes at the magnesium-bearing carbonates/solution interface. I. a surface speciation model for magnesite

The surface charge of magnesite (MgCO 3) was measured at 25°C as a function of pH (4.6 to 11) and ionic strength (0.01, 0.1 and 0.5 M NaCl) under pCO 2 from 10 −3.5 to 0.96 atm. The acid-base titrations were performed in a limited residence time reactor following the approach developed by Charlet et al. (1990) for rhodochrosite and siderite. Magnesite zeta potential (ζ) was determined as a function of pH (1.5 to 12), ionic strength (0.001 to 0.1 M) and Mg 2+ and HCO 3 − concentrations. Based on these results and spectoscopic data on the calcite and dolomite/water inerfaces, a surface complexation model is presented which postulates the formation of the primary hydration species >CO 3H° and >MgOH°. Within this scheme, magnesite surface speciation is governed by the following species: >CO 3H°, >CO 3 −, >CO 3Mg +, >MgOH°, >MgO −, >MgOH 2 +, >MgHCO ° 3, and >MgCO 3 −. The intrinsic stability constants of these species were determined by fitting the pH dependent surface charge over a wide range of solution composition. For the conditions investigated in this study, the measured ζ-potential values of magnesite are in good agreement with the sign of the surface potential and its dependence on solution composition calculated using the surface complexation model generated in this study.

Particle aggregation in complex aquatic systems

The influence of humate, gallate and salicylate on the surface charge character and particle aggregation of permanently and/or conditionally charged clay and oxide minerals (montmorillonite, kaolinite and aluminium-oxide) was investigated at pH 7–8 in the presence of an indifferent electrolyte. pH-dependent surface charging of minerals were determined by acid–base titration. The pH- and ionic strength dependent particle aggregation was measured in both the absence and presence of organic anions by means of dynamic light scattering (DLS). Relatively small amount of organic anions, less than one species per nm 2, adsorbed on the Al-OH surface sites of minerals induces a significant increase in the colloidal stability of dilute suspensions. Bound organic anions smear out the heterogeneity of surface charge and the differences between the clay and oxide minerals.

Infrared Spectroscopy of the TiO2/Aqueous Solution Interface

An innovative approach has been used to probe the molecular nature of the metal oxide/aqueous solution interface. Internal reflection spectroscopy of thin colloidal TiO2 films, under aqueous solutions of pH 11.7−2.3, has been used to obtain differential in situ infrared spectra related to interfacial species. An alkaline solution wash of the TiO2 films was shown to remove contaminants arising from the sol evaporation−film deposition process. Specific infrared absorptions have been assigned to terminal Ti−OH and Ti−OH2+, adsorbed water Ti−OH2, and bridging Ti−OH+−Ti species from the pH dependence of spectra and from deuteration experiments. These surface species determine the pH-dependent surface charge and the enhanced interfacial ionic concentrations observed in our previously published STIRS results. The enhanced interfacial ionic concentrations were also observed to have spectral effects related to interfacial water structure which are similar to those observed from concentrated aqueous solutions conta...

Modeling of the Impact of Ionic Strength on the Electroosmotic Flow in Capillary Electrophoresis with Uniform and Discontinuous Buffer Systems

A new dynamic computer model permitting the combined simulation of the temporal behavior of electroosmosis and electrophoresis under constant voltage or current conditions and in a capillary which exhibits a pH-dependent surface charge has been constructed and applied to the description of capillary zone electrophoresis, isotachophoresis, and isoelectric focusing with electroosmotic zone displacement. Electroosmosis is calculated via use of a normalized wall titration curve (mobility vs pH). Two approaches employed for normalization of the experimentally determined wall titration data are discussed, one that considers the electroosmotic mobility to be inversely proportional to the square root of the ionic strength (method based on the Gouy-Chapman theory with the counterion layer thickness being equal to the Debye-Hückel length) and one that assumes the double-layer thickness to be the sum of a compact layer of fixed charges and the Debye-Hückel thickness and the existence of a wall adsorption equilibrium of the buffer cation other than the proton (method described by Salomon, K.; et al. J. Chromatogr. 1991, 559, 69). The first approach is shown to overestimate the magnitude of electroosmosis, whereas, with the more complex dependence between the electroosmotic mobility and ionic strength, qualitative agreement between experimental and simulation data is obtained. Using one set of electroosmosis input data, the new model is shown to provide detailed insight into the dynamics of electroosmosis in typical discontinuous buffer systems employed in capillary zone electrophoresis (in which the sample matrix provides the discontinuity), in capillary isotachophoresis, and in capillary isoelectric focusing.

Controls on polyacrylamide adsorption to quartz, kaolinite, and feldspar

Potentiometric titrations of quartz, kaolinite, feldspar, and partially hydrolysed polyacrylamide (HPAM), and sorption measurements of HPAM on the minerals, allows identification of the general mechanisms of polyacrylamide adsorption to aluminosilicates and quartz. Adsorption was monitored at the mineral solution interface by way of X-ray photoelectron spectroscopy (XPS). XPS spectra of the unreacted minerals show bands in the Ols, Si2p, Al2p, and Cls regions. Additional peaks are observed in the C1s and N1s regions after treatment with polyacrylamide and the latter is used in this study to monitor polymer adsorption. N1s peak intensities increase with polymer concentration to a maximum corresponding to surface site saturation. At a fixed polymer concentration, adsorption varies with pH-dependent surface charge. The adsorption mechanism changes with pH, reflecting variation in the pH-dependent concentrations of ionizable groups on polyacrylamide and at aluminosilicate surfaces, and the extent of hydrogen-bonding between uncharged mineral surface sites and polymer amide groups.

PH Dependent changes in the optical properties of carboxylic acid derivatized silver colloidal particles

The optical properties of silver colloidal particles derivatized using an aromatic bifunctional molecule, 4-carboxythiophenol, are presented in this communication. The capping molecule forms a thiolate bond with the silver colloidal particle yielding a carboxylic acid terminal functionality which may then be charged to varying degrees by controlling the silver hydrosol pH. A progressive red shift together with a damping and broadening of the surface plasmon feature ( λ max) of the silver particles is observed as the pH is reduced from 10 to 3. Ag colloidal particles with high surface coverage of the bifunctional molecule showed negligible flocculation with time at high solution pH indicating good stabilization due to coulombic repulsive interactions. At low pH, considerable flocculation was observed even for high surface coverage due to lack of coulombic stabilization. A tentative explanation is put forward to explain changes in the optical properties of the colloidal particles due to variation in the pH dependent surface charge of the particles.

Characterization and impact of the temporal behavior of the electroosmotic flow in capillary isoelectric focusing with electroosmotic zone displacement

A computer model permitting the combined simulation of the temporal behavior of electroosmosis and electrophoresis under constant voltage or current conditions and in a capillary which exhibits a pH dependent surface charge has been constructed and applied to the description of capillary isoelectric focusing with electroosmotic zone displacement (cIEF). Electroosmosis for each column segment is calculated based upon the voltage gradient and a wall titration curve (mobility vs. pH) which takes into account the dissociation (p K) of surface groups of the inner capillary wall, a wall mobility at full dissociation (high pH) and the non-vanishing surface charge at low pH. Then, the net electroosmotic flow is taken to be the average of all of the segment flows and considered to represent a plug flow. For a titration curve with p K around 6, a wall mobility around 7·10 −8 m 2/Vs and a residual mobility of about 1.2·10 −8 m 2/Vs at low pH, qualitative agreement between simulation data and experimental data obtained with an untreated fused-silica capillary is obtained. With the sample initially occupying 25% of the column length, electroosmosis is predicted to decrease four to six-fold during the course of the run. Simulation data further reveal (i) that the gradually displaced equilibrium gradient defined by the carrier ampholytes represents the main contributor (pump) to electroosmosis and (ii) that the entire cIEF process is characterized by the asymptotic formation of a stationary steady state zone configuration in which anionic electrophoretic transport and cathode electroosmotic zone displacement are opposite and of equal magnitude. The ampholyte pattern is predicted not to completely leave the capillary at its cathodic end. The position of the stationary boundary between the anolyte and the most acidic ampholyte is dependent on the residual electroosmotic flow at low pH and is thus defined by the composition of the anolyte and the most acidic carrier ampholytes. Having an untreated fused-silica capillary, 10 m M H 3PO 4 as anolyte and a sample occupying 25% of the initial column length the boundary is predicted to be immobilized at about 92% of the column length.

Hydrophobic and electrostatic cell surface properties of Cryptosporidium parvum.

Microbial adhesion to hydrocarbons and microelectrophoresis were investigated in order to characterize the surface properties of Cryptosporidium parvum. Oocysts exhibited low removal rates by octane (only 20% on average), suggesting that the Cryptosporidium sp. does not demonstrate marked hydrophobic properties. A zeta potential close to -25 mV at pH 6 to 6.5 in deionized water was observed for the parasite. Measurements of hydrophobicity and zeta potential were performed as a function of pH and ionic strength or conductivity. Hydrophobicity maxima were observed at extreme pH values, with 40% of adhesion of oocysts to octane. It also appeared that ionic strength (estimated by conductivity) could influence the hydrophobic properties of oocysts. Cryptosporidium oocysts showed a pH-dependent surface charge, with zeta potentials becoming less negative as pH was reduced, starting at -35 mV for alkaline pH and reaching 0 at isoelectric points for pH 2.5. On the other hand, variation of surface charge with respect to conductivity of the suspension tested in this work was quite small. The knowledge of hydrophobic properties and surface charge of the parasite provides information useful in, for example, the choice of various flocculation treatments, membrane filters, and cleaning agents in connection with oocyst recovery.

Influence of amorphous silica and iron hydroxide on interparticle action and soil surface properties

The study of the physicochemical properties of pure amorphous materials (complexes) consisting of Fe2O3 and SiO2 in various proportions indicates that the amorphous complexes will exhibit different properties and characteristics depending on the proportions of Fe2O3 and SiO2. Addition of the amorphous complexes with illitic clay studied shows that the properties of the clay admixture will also vary according to the properties of the amorphous complex, albeit to a lesser degree. The properties and behaviour observed for the amorphous complexes and the clay admixtures can be linked directly to the large specific surface area and high surface charge of the amorphous complexes. The contribution of amorphous complexes to the clay – amorphous complex mixtures (clay admixtures) is twofold: firstly, by the amount of amorphous complex in the clay admixture, and secondly by the composition of the amorphous complex used. The contribution from the amorphous complex is in two forms: water-holding capacity and bonding action. The presence of pH-dependent surface charges associated with the amorphous complexes makes the physicochemical properties and behaviour of the clay admixtures (e.g., liquid limits and zeta potential) sensitive to the pH environment. Coating of amorphous colloids onto clay particle surfaces, shown by scanning electron microscopy, appears to, be enhanced by a decrease in pH of the system, indicating that the enhancement is likely due to the increased electrostatic attraction resulting from the increased amounts of positive charges on the amorphous colloids. Key words : amorphous materials, mass ratio, zeta potential, Bingham yield stress, clay admixtures, hydrogen bonding, specific surface area, cation exchange capacity, anion exchange capacity, fabric and soil structure.

The nature of laponite and its aqueous dispersions

A study has been made of the physical properties and aqueous dispersion characteristics of the synthetic magnesium silicate Laponite XLG. Results obtained from electron microscopy, electron diffraction, ultracentrifuge analysis, and small angle X-ray scattering studies indicate that the primary particles of this material are thin plate-like crystals of reasonably uniform size, having (plate) diameters of ca. 20 nm. In dilute, colloidally stable aqueous dispersions the average thickness of discrete crystals is within the range 2–4 nm. Significant dissolution of this Laponite material occurs in aqueous solutions of pH < 9 with solution concentrations of magnesium exceeding 10 −3 mol dm −3 at pH ⩽ 7. In the region of pH 7–8 precipitation of colloidal silica results in a marked increase in the cation-exchange capacity (CEC) of Laponite dispersions. CEC values obtained for dispersions having pH values within the range 8–10 indicate that the Laponite particles develop no significant pH-dependent surface charge in aqueous electrolyte (NaCl) solutions. The results of this investigation indicate that with regard to particle morphology and high pH surface charge characteristics Laponite XLG constitutes a reasonable model material for studies of the behavior of natural smectitie clay minerals. It is clear however that the use of Laponite for such studies requires careful attention to dispersion preparation and “purification” methods and to the choice and control of experimental pH ranges.

Effect of calcination temperature of alumina on the adsorption/impregnation of Pd(II) compounds

A series of alumina supports derived from a common source material was subjected to a range of calcination temperatures and the results of this treatment on the adsorption of Pd(II) compounds were studied. Increasing calcination temperature lead to a decrease in BET surface area and produced a series of alumina samples carrying different inventories of hydroxyl groups. The surface chemistry of these hydroxyl groups was studied using mass and potentiometric titration techniques and from these data the protonation/deprotonation equilibrium constants for the surface hydroxyl groups were obtained. This allowed us to construct the pH-dependent surface charge distribution for each alumina sample. Cationic and anionic Pd(II) complexes were used to titrate the negative and positive surface charge sites. We find that the adsorption mechanism is primarily controlled by electrostatic effects in that saturation capacities of adsorbed complex showed a correlation with the surface charge density evaluated at the pH used for the adsorption/impregnation experiments.

Relationship between metal ion adsorption and catalytic properties of carbon-supported nickel catalysts

During the preparation of supported metal catalysts, the adsorption of aqueous metal ions is influenced by the intrinsic properties of the support. Any attempt to model this adsorption phenomenon requires at least three elements: the pH-dependent surface charge speciation of the support, the pH-dependent aqueous phase speciation of the precursor ion, and a suitable model to describe the equilibrium exchange reaction. A series of activated carbons derived from a low ash content carbon source was prepared by controlled oxidation using nitric acid. The surface ionization constants for the protonation/de proton ation equilibria of the hydroxyl groups on each carbon were determined assuming a classical Gouy-Chapman diffuse layer. They were found to vary systematically with extent of oxidation. Nickel adsorption data were obtained over a wide range of pH and Ni concentration. Under the conditions of this study, Ni 2+ was the dominant ion. A surface complexation model was employed to test the hypothesis that the model's parameters could be correlated to the results of characterization studies of these Ni/carbon catalysts. Specifically, it was found that the surface stability constant (≈Δ G ads) of the precursor and the point of zero charge of the carbon varied in accordance with results from temperature-programmed reduction and temperature-programmed oxidation. It is proposed that the point of zero charge of the carbon support can serve as a convenient index for the design of carbon-supported metal catalysts with desired catalytic properties.

Characterization of surface change properties of synthetic microfiltration membranes by means of a streaming current method

By use of an improved streaming current method we characterized the pH dependent surface charge properties of six synthetic microfiltration membranes at the constant electrolyte concentration of 10 −4 mol/l. The observed pH dependence of the surface charge properties of the cellulose acetate and the nylon 66 membranes could be explained by the pH influence on the dissociation and protonation of the surface end groups of membrane materials, while the reaction of the surface modification agents, i.e., the wetting agents, with the electrolytes and the ion adsorption on the membrane surfaces, were the main causes for the surface charge of nonionic polymer membranes (polyvinylidenfluoride and polysulfone membranes) in the solutions which had different pH values.

Estimation of surface ionization constants for amphoteric solids

Modeling of aqueous ion adsorption onto amphoteric solids requires the determination of the equilibrium constants for the surface ionization reactions. From these data, the pH dependent surface charge development can be determined on the basis of any number of models for the electrical double layer. Experimentally, one can obtain the sum of these ionization constants, but an additional relationship is required to determine each uniquely. Numerous schemes have been proposed to accomplish this based on certain assumptions. Unfortunately, data analysis is often confined to a limited pH range, and contradicts the assumptions inherent to each scheme. It is proposed that a reasonable estimate of the ionization constants can be obtained by combination of a graphical method and numerical regression. The methodology was applied to existing data from simple oxide systems and recent data from activated carbons. The results show that the values of the ionization constants are strongly dependent on the pH range used. Consequently, it is suggested that a uniform range be established for the purpose of comparing inter-laboratory results: data within 3 pH units from the point of zero charge toward pH 7 should be taken as the data base for consistent regression results.

Relationship of Surface Area of pH-Dependent Suspensions and Water Retention

AbstractThe equilibrium water retention, MW/MS, of suspensions having a pH-dependent surface charge (carbonate-containing aluminum hydroxide and fumed silica) appears to be due to water adsorbed by the surface as well as water retained in pores. The water retained in pores has a lower limit determined by the closest packing porosity. Maximal water retention occurred when the pH was equal to the point of zero charge and attractive particle interactions predominated. The contribution of pore water to MW/MS can be minimized by selecting a pH significantly different from the point of zero charge. The water retention properties of the suspension under these conditions are believed to be due to adsorption and the closest packing porosity. Preliminary results indicate that this MW/MS can be related to the surface area of the suspended solid.

DISPERSION OF KAOLINITE BY DISSOLVED ORGANIC MATTER FROM DOUGLAS-FIR ROOTS

The organic constituents of water extracts from Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) roots that cause kaolinite dispersion were investigated. The dissolved organic matter was fractionated according to molecular size and chemical characteristics into acids, neutrals, and bases of the hydrophilic and hydrophobic groups. The dominant fraction causing dispersion included the hydrophobic acids and organics with molecular sizes greater than 104 nominal molecular weight. Partial oxidation of the dissolved organic matter increased its carboxylic acid content and dispersion potential. Organic acids promoting kaolinite dispersion included aliphatic and aromatic carboxylic acids. The dispersing ability of a carboxylic acid was related to its charge and charge density. Kaolinite has a pH-dependent surface charge; specific adsorption of carboxyl groups makes the clay more negative and promotes dispersion. Polycarboxylic acids appear to play the major role in kaolinite dispersion by dissolved organic matter in forests. Key words: Fulvic acid, specific adsorption, carboxylic acid, fractionation

Adsorption and Availability of Phosphorous Following the Application of Limestone to an Acid, Aluminous Soil

AbstractChanges in the adsorption and availability of P in an acid, aluminous soil (surface and subsoil) were evaluated following liming to neutralize 1N KCl‐exchangeable Al (minimal liming) or to pH 6.8 as determined by the SMP (Shoemaker‐McLean‐Pratt) buffer method. Langmuir adsorption isotherms were utilized to ascertain changes in P adsorption. A greenhouse bioassay was conducted to evaluate differences in P availability to plants (Oats, Avena sativa L. cv. Korwood) due to liming and incubation treatments. Results from these studies confirm that minimal liming is the most effective approach, from both agronomic and economic standpoints. Minimal liming optimized P uptake and minimized the P adsorption maxima (Langmuir b values). Liming by the SMP recommendation did not improve P uptake despite the application of three times as much lime, and also markedly reduced exchangeable Mg. Aging of adsorbate surfaces in the soil reduced their bonding intensity for P (Langmuir K), as did increasing soil pH. Development of ordered, less soluble Al‐OH polymers with fewer nonstructural OH groups capable of ligand exchange with P is postulated as the mechanism underlying these observations. Increases in pH‐dependent negative surface charge and P adsorption by free CaCO3 may be involved.