Kinetics of phosphate adsorption on goethite: comparing batch adsorption and ATR-IR measurements

The adsorption kinetics of phosphate on goethite has been studied by batch adsorption experiments and by in situ ATR-IR spectroscopy at different pH, initial phosphate concentrations and stirring rates. Batch adsorption results are very similar to those reported by several authors, and show a rather fast initial adsorption taking place in a few minutes followed by a slower process taking place in days or weeks. The adsorption kinetics could be also monitored by integrating the phosphate signals obtained in ATR-IR experiments, and a very good agreement between both techniques was found. At pH 4.5 two surface complexes, the bidentate nonprotonated (FeO)(2)PO(2) and the bidentate protonated (FeO)(2)(OH)PO complexes, are formed at the surface. There are small changes in the relative concentrations of these species as the reaction proceeds, and they seem to evolve in time rather independently. At pH 7.5 and 9 the dominating surface species is (FeO)(2)PO(2), which is accompanied by an extra unidentified species at low concentration. They also seem to evolve independently as the reaction proceeds. The results are consistent with a mechanism that involve a fast adsorption followed by a slow diffusion into pores, and are not consistent with surface precipitation of iron phosphate.     

Mechanisms of fibrinogen adsorption on latex particles determined by zeta potential and AFM measurements

The adsorption of fibrinogen on polystyrene latex particles was studied using the concentration depletion method combined with the AFM detection of residual protein after adsorption. Measurements were carried out for a pH range of 3.5-11 and an ionic strength range of 10(-3)-0.15 M NaCl. First, the bulk physicochemical properties of fibrinogen and the latex particle suspension were characterized for this range of pH and ionic strength. The zeta potential and the number of uncompensated (electrokinetic) charges on the protein were determined from microelectrophoretic measurements. It was revealed that fibrinogen molecules exhibited amphoteric characteristics, being on average positively charged for pH <5.8 (isolectric point) and negative otherwise. However, the latex particles did not show any isoelectric point, remaining strongly negative for this pH range. Afterward, systematic measurements of the electrophoretic mobility of fibrinogen-covered latex were carried out as a function of the amount of adsorbed protein, expressed as the surface concentration. A monotonic increase in the electrophoretic mobility (zeta potential) of the latex was observed in all cases, indicating a significant adsorption of fibrinogen on latex for pH below 11. It was also proven that fibrinogen adsorption was irreversible, with the maximum surface concentration varying between 2.5 and 5 × 10(3) μm(-2) (weight concentration of a bare molecule was 1.4 to 2.8 mg m(-2)). These measurements revealed two main adsorption mechanisms of fibrinogen: (i) the unoriented (random) mechanism prevailing for lower ionic strength, where adsorbing molecules significantly penetrate the fuzzy polymeric layer on the latex core and (ii) the side-on adsorption mechanism prevailing for pH > 5.8 and a higher ionic strength of 0.15 M. It was also shown that in the latter case, variations in the zeta potential with the protein coverage could be adequately described in terms of the electrokinetic model, previously formulated for planar substrate adsorption. On the basis of these experimental data, an efficient procedure of preparing fibrinogen-covered latex particles of controlled monolayer structure and coverage was envisaged.     

Improvement of the spinning electrophorometer for zeta potential measurements

In this study, bubbles are held by centripetal force at the center of a rotating cylinder filled with an aqueous solution. Their velocities along the axe of rotation, after application of an electrophoretic force, are used for the calculation of the so-called electrokinetic potential. But this process necessitates the elimination of the electro-osmosis which occurs on the interior sides of the glass cylinder by superposing a concurrent force on the bubble. Efficiency of DEAE-Dextran reticulated with 1,4 Butanediol Diglycidyl Ether can be tested by the observation of a cloud of latex microspheres injected in the interior of the tube and allowed to move in respect with the application of an electric field. The experimental control of these velocity profiles proves the adequacy of the polymer for many cases such as surfactant solutions, presence of electrolytes, utilization with moderate pH.The dynamic interpretation of the electrophoretic motion of bubbles is possible by considering that small ones behave like rigid spheres moving in a rotating fluid. In the second part of this paper and in a previous publication, we have experimentally proved that the use of the theoretical expressions of the forces involved for rigid spheres is justified for small bubbles. So, the electrokinetic potential can be expressed versus the velocity, leading to possible interpretations of the adsorption on gas-water interfaces.     

The adsorption of cationic and amphoteric copolymers on glass surfaces: zeta potential measurements,adsorption isotherm determination,and FT Raman characterization

The adsorption of cationic and amphoteric copolymers onto controlled pore glass (CPG) powders has been studied by measurement of the powder particle zeta (zeta) potential, by determination of the adsorption isotherm, and by FT Raman measurements of the polymer-coated powder. The cationic polymers consisted chiefly of homopolymers of dimethyldiallylammonium chloride (DMDAAC) or copolymers of DMDAAC and acrylamide. The amphoteric polymers studied included copolymers of DMDAAC and acrylic acid. The comonomer ratio was varied to explore the dependence of cationic charge density on the extent and effect of adsorption. Both types of polymers adsorb onto the anionic glass surface via an ion-exchange mechanism. Consequently, a correspondingly higher mass of a low-charge-density copolymer adsorbs than of a cationic homopolymer. The presence of the anionic portion in the amphoteric polymers does not significantly alter this picture. The zeta potential, however, reflects the overall nature of the polymer. Cationic polymers effectively neutralize the glass surface, while amphoteric polymers leave the zeta potential net negative. Adsorption isotherms, determined via the depletion technique using colloidal titration, were used to "calibrate" a FT Raman method. The latter was used to determined the amount of adsorbed polymer under solution conditions in which colloidal titration could not be performed.     

Influence of roughness and capillary size on the zeta potential values obtained by streaming potential measurements

Streaming potential measurements are performed to determine the zeta potential of flat surfaces, particles, or fibers. Although the zeta potential is a well-defined property of solid surfaces in a liquid, there are indications that the absolute values of the zeta potential calculated using the Helmholtz-Smoluchowski equation are affected by surface roughness and—in case of particle or fiber assemblies—their packing density. The study at hand investigates these influences using flat polymer surfaces with different roughness and topography and assemblies of basalt spheres. It was found that increasing roughness of the flat surface and larger size or smaller number of particles in particle assemblies result in flatter slopes of the streaming potential versus pressure and thus lower apparent absolute values of the zeta potential. The interpretation of streaming potential measurements should therefore not focus on absolute zeta potential values but on trends in pH- and concentration-dependent measurements.     

Bovine serum albumin adsorption onto colloidal Al2O3 particles: a new model based on zeta potential and UV-vis measurements

We investigated the adsorption of bovine serum albumin (BSA) on colloidal Al2O3 particles in an aqueous environment. Changes in the zeta potential of the Al2O3 particles upon the adsorption of BSA were measured using an electro-acoustic technique. The mass of protein adsorbed was determined by using UV-vis spectroscopy. The change of the isoelectric point of the Al2O3 powder-protein suspension was found to be a function of adsorbed protein mass. It was shown that approximately one monolayer of BSA was needed to fully mask the surface and to compromise the charge of Al2O3. From titration experiments it follows that about 30-36% of the negatively charged groups of the protein form bonds with the protonated and charged Al2O3 surface. On the basis of our observations we introduced a new adsorption model for BSA on Al2O3 particles.     

Optimal MEMS device for mobility and zeta potential measurements using DC electrophoresis

We have developed a novel microchannel geometry that allows us to perform simple DC electrophoresis to measure the electrophoretic mobility and zeta potential of analytes and particles. In standard capillary geometries, mobility measurements using DC fields are difficult to perform. Specifically, measurements in open capillaries require knowledge of the hard to measure and often dynamic wall surface potential. Although measurements in closed capillaries eliminate this requirement, the measurements must be performed at infinitesimally small regions of zero flow where the pressure driven‐flow completely cancels the electroosmotic flow (Komagata Planes). Furthermore, applied DC fields lead to electrode polarization, further questioning the reliability and accuracy of the measurement. In contrast, our geometry expands and moves the Komagata planes to where velocity gradients are at a minimum, and thus knowledge of the precise location of a Komagata plane is not necessary. Additionally, our microfluidic device prevents electrode polarization because of fluid recirculation around the electrodes. We fabricated our device using standard MEMS fabrication techniques and performed electrophoretic mobility measurements on 500 nm fluorescently tagged polystyrene particles at various buffer concentrations. Results are comparable to two different commercial dynamic light scattering based particle sizing instruments. We conclude with guidelines to further develop this robust electrophoretic tool that allows for facile and efficient particle characterization.     

Probing of polyelectrolyte monolayers by zeta potential and wettability measurements

Detection of the very first step of polyelectrolyte adsorption onto a solid support is of great importance for understanding mechanisms of solid surface modification. It was shown that streaming potential and contact angle measurements can be successfully used for polyelectrolyte (PE) adsorption characterization in a broad range of surface coverage. Cationic polyallylamine hydrochloride (PAH) was used for the formation of the layer. The electrokinetic characteristics of the substrate covered by the PAH layer were compared with contact angles measured under wet (captive air bubble/substrate in water) and dry (sessile water droplet/dried substrate) conditions. It has been demonstrated that contact angle values determined under both conditions are in good agreement. The observed rapid increase in the contact angle from zero for the bare mica surface to the value close to one characteristic of the PAH monolayer appears in the same PAH coverage range as zeta potential value changes due to adsorption. These results show that wettability can be as sensitive to the presence of small amounts of adsorbed species as electrokinetic measurements.     

Phosphate adsorption on synthetic goethite and akaganeite

Low crystalline iron hydroxides such as goethite (alpha-FeOOH) and akaganeite (beta-FeOOH) were synthesized, and the selective adsorption of phosphate ions from phosphate-enriched seawater was examined. The results of the distribution coefficients (K(d)) of oxoanions in mixed anion solutions at pH 8 follow the selectivity order Cl-, NO3-, SO4(2-) < CO3(2-), HPO4(2-) for goethite, and Cl-, CO3(2-) < NO3- < SO4(2) < HPO4(2-) for akaganeite. In seawater, both adsorbents show high selectivity for phosphate ions despite the presence of large amounts of major cations and anions in seawater. The adsorption isotherms fitted better with the Freundlich equation and the maximum uptake of phosphate from phosphate-enriched seawater was 10 mg P/g at an equilibrium phosphate concentration of 0.3 mg P/L on both adsorbents. The phosphate adsorption/desorption cycles show that akaganeite is an excellent adsorbent even after 10 cycles and its chemical stability is good.     

Influence of surface conductivity on the apparent zeta potential of TiO2 nanoparticles

Zeta potential is a physico-chemical parameter of particular importance in describing ion adsorption and electrostatic interactions between charged particles. Nevertheless, this fundamental parameter is ill-constrained, because its experimental interpretation is complex, particularly for very small and charged TiO(2) nanoparticles. The excess of electrical charge at the interface is responsible for surface conductance, which can significantly lower the electrophoretic measurements, and hence the apparent zeta potential. Consequently, the intrinsic zeta potential can have a larger amplitude, even in the case of simple 1:1 electrolytes like NaCl and KCl. Surface conductance of TiO(2) nanoparticles immersed in a NaCl solution is estimated using a surface complexation model, and this parameter and particle size are incorporated into Henry's model in order to determine a constrained value of the zeta potential from electrophoresis. Interior conductivity of the agglomerates is calculated using a differential self-consistent model. The amplitude of estimated zeta potential is greater than that derived from the von Smoluchowski equation and corresponds to the electric potential at the outer Helmholtz plane calculated by our surface complexation model. Consequently, the shear plane may be located close to the OHP, contradicting the assumption of the presence of a stagnant diffuse layer at the TiO(2)/water interface.     

Dilatometric study of the adsorption of heavy-metal cations on goethite

The specific volumes of adsorption of Cd, Co, Cu, Ni, Pb, and Zn on goethite determined by means of the dilatometric method are 21, 32, 32, 31, 31, and 42 cm3/mol, respectively, and are independent of pH. The effect of NaCl (up to 0.5 mol dm(-3)) on the specific volume of adsorption is rather insignificant. The specific volume of precipitation of corresponding hydroxides (determined experimentally and calculated) is about 60 cm3/mol. Apparently, the adsorbed heavy-metal cations lose half of their hydration water. The adsorption constant decreases as the pressure increases, and the effect becomes significant at pressures of > 10(7) Pa, i.e., more than 1 km of water column.     

Estimation of the zeta potential and the dielectric constant using velocity measurements in the electroosmotic flows

In this paper we develop a method for the determination of the zeta potential zeta and the dielectric constant epsilon by exploiting velocity measurements of the electroosmotic flow in microchannels. The inverse problem is solved through the minimization of a performance function utilizing the conjugate gradient method. The present method is found to estimate zeta and epsilon with reasonable accuracy even with noisy velocity measurements.     

Hydrodynamic effects on the oscillations of supported bubbles: implications for accurate measurements of surface properties

Oscillating bubble techniques are commonly used to infer dynamic surface tensions (DST) from the measurements of the dynamic pressure differences across an interface. In inferring DST from such measurements, it is assumed that hydrodynamic efects either are negligible or can be approximated. In virtually all previous studies, the dynamic bubble shapes, which are set by the requisite balance of forces at the bubble surface, are taken to be nearly spherical and assumed to be governed by the static Young–Laplace (Y–L) equation alone. To examine these assumptions, the Navier–Stokes and continuity equations governing the flow of a liquid outside an axisymmetric bubble supported by a narrow capillary are solved simultaneously by a rigorous finite element method. Cases of constant surface tension (pure liquids or solutions with very fast adsorption) are tested to focus on understanding the effects of fluid motion on surface tension measurements. To test the capability of the computational algorithm on describing the hydrodynamics, computed and experimental velocity profiles are compared and found to be consistent, as are the apparent surface tensions. Parameters such as forcing frequency, forcing amplitude, chamber dimensions, and surface tension and viscosity of the liquid are varied to find the limits where pulsating bubbles depart from spherical and where hydrodynamic effects impact the determination of surface tension. In the commercial pulsating bubble surfactometer (PBS), the bubble shapes remain nearly spherical for low pulsation or oscillation rates (≤ 100 Hz) with moderate volume amplitudes. Under these conditions, however, two major hydrodynamic effects, due to the inertia of the bulk liquid, or to the liquid viscosity and the viscous forces acting on the chamber walls, are found to be important and can cause large errors in the surface tension measurements. For measurements of low surface tensions (≤ 5 mN/m) in a PBS, oscillating bubble methods that do not take into account shape deformations from the spherical are found to be inaccurate, and the results from dynamic bubble shape analysis (solving the Y–L equation for a nonspherical axisymmetric surface) are shown to provide another approach to obtain accurate results provided that hydrodynamic effects can be neglected. At higher frequencies (≥ 200 Hz), because of strong convection around the surface of the bubble, the bubble shapes become highly deformed. Further extensions of this algorithm are needed, to include the surfactant diffusion and adsorption effects by which the surface tension may change with time.     

Determination of the surface potential for adsorption layers of polyelectrolytes by the streaming potential method

A physicochemical model is proposed to describe electrokinetic phenomena in capillaries and pores the surface of which is coated with a charged porous adsorption layer. The use of this model makes it possible to explain experimental data on the surface potentials of polyelectrolyte adsorption layers upon their deformation resulting from solution flow. The commonly used Smoluchowski equation is shown to lead to large errors in the determination of the potential and charge of the surface of an adsorption layer.     

Tangential flow streaming potential measurements: Hydrodynamic cell characterization and zeta potentials of carboxylated polysulfone membranes

Computational fluid dynamics calculations were carried out to ensure that a self-made tangential flow mode streaming potential measurement cell meets the hydrodynamic stipulations of laminar, steady and established electrolyte flow necessary for reproducible electrokinetic measurements. The calculations show that the cell design meets all of these conditions.Six carboxylated polysulfones with a range of different degrees of substitution (DS) from 0.26 to 1.74 carboxyl groups per polymer repeat unit were synthesized in a two-stage process of lithiation and carboxylation. Ultrafiltration membranes were made from both the unmodified polysulfone and these hydrophilic materials. The zeta potentials of these membrane surfaces were determined in 0.001 M KCl solution as a function of pH. The curves show the theoretically expected profiles for non-ionic and weakly acidic materials. The growing influence of the COOH dissociation on the surface charge formation is indicated by the flattening of the curves at low pH values. The magnitude of the negative zeta potentials plateau values ranged from −52 to −20 mV. While unmodified PSU has a plateau value of −52 mV this value decreases continuously with increasing DS to −20 mV for the PSU-COOH 1.74 material. It is suggested that this arises from a shift of the electrokinetic shear plane into the bulk electrolyte solution due to an extended swelling layer reflecting the enhanced hydrophilicity of these membrane surfaces.     

A new insight into the adsorption of bovine serum albumin onto porous polyethylene membrane by zeta potential measurements, FTIR analyses, and AFM observations

The mode of adsorption of bovine serum albumin (BSA) on porous polyethylene (PE) membrane was studied as a function of time and concentration, which may contribute to the surface coverage. An improved physical model for adsorption is initiated based on the results of the adsorptional and desorptional measurements, FTIR analysis, and AFM observations as well as streaming potential measurements. The results obtained indicate that the adsorptional mode depend on both time and concentration. It is shown that a critical concentration (about 1000 ppm here) exists in the adsorptional process. Below this concentration, the adsorption seems to be conducted in a normal side-on way but time elapse gives rise to greater conformational change than concentration increase; above this concentration, the aggregation of protein molecular plays a decisive role and the adsorption is in an aggregation way, which is similar to end-on, but a relative large gap between the adsorbed molecules exists due to aggregation. This conclusion is general and can be expected to apply in other globular protein-hydrophobic porous surface systems.     

Characterization of an effective cleaning procedure for aluminum alloys: surface enhanced Raman spectroscopy and zeta potential analysis

We have developed a cleaning procedure for aluminum alloys for effective minimization of surface-adsorbed sub-micrometer particles and nonvolatile residue. The procedure consists of a phosphoric acid etch followed by an alkaline detergent wash. To better understand the mechanism whereby this procedure reduces surface contaminants, we characterized the aluminum surface as a function of cleaning step using surface enhanced Raman spectroscopy (SERS). SERS indicates that phosphoric acid etching re-establishes a surface oxide of different characteristics, including deposition of phosphate and increased hydration, while the subsequent alkaline detergent wash appears to remove the phosphate and modify the new surface oxide, possibly leading to a more compact surface oxide. We also studied the zeta potential of <5 microm pure aluminum and aluminum alloy 6061-T6 particles to determine how surface electrostatics may be affected during the cleaning process. The particles show a decrease in the magnitude of their zeta potential in the presence of detergent, and this effect is most pronounced for particles that have been etched with phosphoric acid.