Isoelectric Point pHiep Research Articles

Adsorption of Ag (I) Ions at the Zirconium Phosphate/KNO3 Aqueous Solution Interface.

The paper presented the mechanical (MChT), microwave (MWT), and hydrothermal (HTT) methods of zirconium phosphate samples modification in order to improve its adsorption affinity for the Ag (I) ions. The FTIR studies proved that the modification of both gel and xerogel samples with the ultrasonic microwaves causes an increase in the concentration of phosphate groups on the surface of MWT-modified zirconium phosphate: the isoelectric point pHiep = 2.2–2.9 for these samples against 3.9 for the initial sample and pKa2 values were 4.7–5.6 and 6.3, respectively. As resulting from the Ag+ ion adsorption studies, the MWT treatment of zirconium phosphate samples caused the greatest affinity of Ag+ ions for the surface of MWT zirconium phosphate. Compared with the initial ZrP sample, the shift of the Ag (I) ion adsorption edge towards lower pH values was observed, e.g., with adsorption of Ag (I) ions from the solution with the initial concentration of 1 µmol/dm3 for the initial ZrP sample pH50% = 3.2, while for the sample MWT ZrPxero pH50% = 2.6.

Electokinetic and adsorption properties of different titanium dioxides at the solid/solution interface

Abstract Six samples of titanium dioxide of different phase compositions and specific surface areas have been characterized by XRD, Raman-and FTIR spectroscopy, adsorption of nitrogen, electrophoresis. Adsorption of Zn(II) ions at the TiO2/NaCl aqueous solution interface as well as the effect of adsorption on the structure of electrical double layer have been studied. The influence of ionic strength, pH and presence of ions on the adsorption of Zn(II) ions at the TiO2/NaCl solution interface have also been investigated. The zeta potential, surface charge density, parameters of adsorption edge pH50% and ΔpH10–90% for different concentrations of basic electrolyte have been determined. Studied unpurified samples showed lower values of isoelectric point pHiep compared with literature data due to the presence of anion impurities. The antibate dependence between pHiep values and particle size has been established. Adsorption of Zn(II) ions using monophase samples is completed at a lower pH than for the biphase TiO2. Appearance of the point CR3 is associated with the charge turnover from positive to negative at high values of pH and formation of Zn(OH)2.

Collapse of acrylamide‐based polyampholyte hydrogels in water

AbstractThe swelling behavior of balanced acrylamide (AAm)‐based polyampholyte hydrogels in water and in aqueous salt (NaCl) solutions was investigated. Equimolar ratio of the ionic comonomers 4‐vinylpyridine (cationic monomer) and acrylic acid (anionic monomer) were used together with the nonionic monomer AAm in the hydrogel preparation. The variations of the hydrogel volume in response to changes in pH were measured. It was found that the hydrogels are in a collapsed state not only at the pH of the isoelectric point pHIEP but also over a wide range of pH including pHIEP. The width of the collapsed plateau increased and the hydrogels assumed a more compact state as the ionic group content is increased. The antipolyelectrolyte behavior was observed along the collapsed plateau region, where the gel occupies a larger volume in salt solution. The experimental swelling data were compared with the predictions of the Flory‐Rehner theory of swelling equilibrium including the ideal Donnan equilibria. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Surface chemistry and reactivity of plant phytoliths in aqueous solutions

In order to better understand the reactivity of plant phytoliths in soil solutions, we determined the solubility, surface properties (electrophoretic mobilities and surface charge) and dissolution kinetics of phytoliths extracted from fresh biomass of representative plant species (larch tree and elm, horsetail, fern, and four grasses) containing significant amount of biogenic silica. The solubility product of larch, horsetail, elm and fern phytoliths is close to that of amorphous silica and soil bamboo phytoliths. Electrophoretic measurements yield isoelectric point pH IEP = 0.9, 1.1, 2.0 and 2.2 for four grasses, elm, larch and horsetail phytoliths respectively, which is very close to that of quartz or amorphous silica. Surface acid–base titrations allowed generation of a 2-pK surface complexation model (SCM) for larch, elm and horsetail phytoliths. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 1 ≤ pH ≤ 8, were found to be very similar among the species, and close to those of soil bamboo phytoliths. Mechanistic treatment of all plant phytoliths dissolution rates provided three-parameters equation sufficient to describe phytoliths reactivity in aqueous solutions: R ( mol / cm 2 / s ) = 6 ⋅ 10 − 16 ⋅ a H+ + 5.0 ⋅ 10 − 18 + 3.5 ⋅ 10 − 13 ⋅ a OH − 0.33 Alternatively, the dissolution rate dependence on pH can be modeled within the concept of surface coordination theory assuming the rate proportional to concentration of > SiOH 2 +, > SiOH 0 and > SiO − species. In the range of Al concentration from 20 to 5000 ppm in the phytoliths, we have not observed any correlation between their Al content and solubility, surface acid–base properties and dissolution kinetics. It follows from the results of this study that phytoliths dissolution rates exhibit a minimum at pH ∼ 3. Mass-normalized dissolution rates are similar among all four types of plant species studied and these rates are an order of magnitude higher than those of typical soil clay minerals. The minimal half life time of larch and horsetail phytoliths in the interstitial soil solution ranges from 10–12 years at pH = 2–3 to < 1 year at pH above 6, comparable with mean residence time of phytoliths in soil from natural observations.

Determination of surface potentials from the electrode potentials of a single-crystal electrode

Determination of surface potentials Ψ 0 at the metal oxide/aqueous solution interface from measured electrode potentials of a metal oxide single-crystal electrode (SCrE) is described. The proposed method is based on the surface complexation model and evaluates the surface potential at the isoelectric point, i.e., at pH iep. This value is used for calculation of Ψ 0 values from the measured electrode potentials. Both 1-p K and 2-p K models produced the same result so that the procedure does not depend on the assumed mechanism of the surface charging. It is proposed to determine the pristine point of zero charge pH eln and the isoelectric point pH iep, and use these data to set the scale of surface potentials. The value of pH eln can be obtained at a sufficiently low ionic strength where pH pzc coincides with pH iep. The method is demonstrated on the example of the anatase single-crystal electrode. From the shifts of pH iep and pH pzc with respect to the pristine point of zero charge pH eln it was concluded that Cl − ions exhibit higher affinity for association with positively charged surface groups than ClO − 4 ions. Also, preferential surface association of Na + cations compared to both anions was detected. The slopes of the Ψ 0 ( pH ) functions were found to be significantly lower in magnitude with respect to the Nernst equation, which is due to the high degree of counterion association at the surface caused by their relatively high concentration.

Surface properties, solubility and dissolution kinetics of bamboo phytoliths

Although phytoliths, constituted mainly by micrometric opal, exhibit an important control on silicon cycle in superficial continental environments, their thermodynamic properties and reactivity in aqueous solution are still poorly known. In this work, we determined the solubility and dissolution rates of bamboo phytoliths collected in the Réunion Island and characterized their surface properties via electrophoretic measurements and potentiometric titrations in a wide range of pH. The solubility product of “soil” phytoliths ( p K sp 0 = 2.74 at 25 °C) is equal to that of vitreous silica and is 17 times higher than that of quartz. Similarly, the enthalpy of phytoliths dissolution reaction ( Δ H r 25 - 80 ° C = 10.85 kJ / mol ) is close to that of amorphous silica but is significantly lower than the enthalpy of quartz dissolution. Electrophoretic measurements yield isoelectric point pH IEP = 1.2 ± 0.1 and 2.5 ± 0.2 for “soil” (native) and “heated” (450 °C heating to remove organic matter) phytoliths, respectively. Surface acid–base titrations allowed generation of a 2-p K surface complexation model. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 2 ⩽ pH ⩽ 12, were found to be intermediate between those of quartz and vitreous silica. The dissolution rate dependence on pH was modeled within the concept of surface coordination theory using the equation: R = k 1 · { > SiOH 2 + } n + k 2 · { > SiOH 0 } + k 3 · { > SiO - } m , where {> i} stands for the concentration of the surface species present at the SiO 2–H 2O interface, k i are the rate constants of the three parallel reactions and n and m represent the order of the proton- and hydroxy-promoted reactions, respectively. It follows from the results of this study that phytoliths dissolution rates exhibit a minimum at pH ∼ 3. This can explain their good preservation in the acidic soil horizons of Réunion Island. In terms of silicon biogeochemical cycle, phytoliths represent a large buffering reservoir, which can play an important role in the regulation of silica fluxes in terrestrial aquatic environments.

Determination of electric double layer parameters for spherical silica particles under application of the triple layer model using surface charge density data and results of electrokinetic sonic amplitude measurements

The surface charge density, σ 0, of AEROSIL OX 50 (Degussa) was determined by titration experiments as well as the electrophoretic mobility, μ, using the electrokinetic sonic amplitude method to characterize the electric double layer of spherical SiO 2-particles in dependence on pH and electrolyte concentration. The electrokinetic experiments result in an isoelectric point pH iep 2.4. At the lowest electrolyte concentration investigated a minimum in the μ–pH-plots emerges. Three relationships between μ and the ζ-potential were examined for the evaluation of the experimental data: (i) the Smoluchowski formula, (ii) an equation from O'Brien et al. (J. Colloid Interf. Sci. 173 (1995) 406) for the Dukhin number which includes the contribution of the mobile layer to surface conductivity only, (iii) Dukhin numbers were determined from the results of conductivity measurements under application of the theory of O'Brien and Perrins (J. Colloid Interf. Sci. 99 (1984) 20) with numerical solutions of Kang and Sangani (J. Colloid Interf. Sci. 165 (1994) 195). Here, the contribution of the ions in the stagnant layer is included too. The Smoluchowski formula is not suitable to describe the μ–pH plots in a quantitative manner, specially at low electrolyte concentration. The other two methods taking into account the surface conductivity give a comparable goodness of fit as a result of the multiparameter regression under application of the triple layer model. A significant contribution of the ions in the stagnant layer to surface conductivity follows from the conductivity measurements. This effect results in an increase of the magnitude of the ζ-potential calculated from the electrophoretic mobility in comparison to the Smoluchowski formula.