| 1. |
- Barišić, Antun, et al.
(författare)
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Experimental Data Contributing to the Elusive Surface Charge of Inert Materials in Contact with Aqueous Media
- 2021
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Ingår i: Colloids and interfaces. - : MDPI. - 2504-5377. ; 5:1
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We studied the charging of inert surfaces (polytetrafluoroethylene, i.e., PTFE; graphite; graphene; and hydrophobic silica) using classical colloid chemistry approaches. Potentiometric titrations showed that these surfaces acquired less charge from proton-related reactions than oxide minerals. The data from batch-type titrations for PTFE powder did not show an effect of ionic strength, which was also in contrast with results for classical colloids. In agreement with classical colloids, the electrokinetic results for inert surfaces showed the typical salt level dependence. In some cases, the point of zero net proton charge as determined from mass and tentatively from acid–base titration differed from isoelectric points, which has also been previously observed, for example by Chibowski and co-workers for ice electrolyte interfaces. Finally, we found no evidence for surface contaminations of our PTFE particles before and after immersion in aqueous solutions. Only in the presence of NaCl-containing solutions did cryo-XPS detect oxygen from water. We believe that our low isoelectric points for PTFE were not due to impurities. Moreover, the measured buffering at pH 3 could not be explained by sub-micromolar concentrations of contaminants. The most comprehensive explanation for the various sets of data is that hydroxide ion accumulation occurred at the interfaces between inert surfaces and aqueous solutions.
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| 2. |
- Boily, Jean-Francois, et al.
(författare)
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Modeling proton binding at the goethite (-FeOOH)-water interface
- 2001
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. ; 179:1, s. 11-27
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Tidskriftsartikel (refereegranskat)abstract
- The basic charging behaviours of goethite particles with different surfaces area (23, 37 and 85 m2 g-1) in 0.003-2.0 M NaNO3 were interpreted using surface complexation theory with the basic Stern model (BSM). The affinity of the goethite surface functional groups for protons was evaluated using the multisite complexation model (MUSIC) framework considering singly, doubly, and triply-coordinated surface oxygens with respect to underlying Fe(III) atoms. The affinity of these functional groups for protons was investigated first by using a calibration curve devised in Hiemstra et al. [J. Colloid Interface Science, 184 (1996) 680]. The calibration curve correlates the proton affinity constants of aqueous metal monomers to the undersaturation of the coordination environment of oxygens by considering the actual bond valences of Fe&unknown;O bonds in goethite, short hydroxyl bonds and hydrogen bonds. The results show that the predictions are sensitive to the range of short hydroxyl bonds/hydrogen bonds found in the literature. The singly- and one type of the triply coordinated sites are, however, most likely responsible for the basic charging behavior of goethite in the pH 2-11. The proton affinity constants of the singly- and triply coordinated sites were also optimized using titration data at different ionic strengths by co-optimizing values for electrolyte ion pairs and the capacitance of the Stern Layer. The optimal proton binding constants were in the range of the predicted values using the aforementioned calibration curve, although the modeling parameters are interdependent. A narrow range of CStern and electrolyte ion pairs was chosen to model the charging behavior of goethite by considering, (i), the range of proton binding constants from the aforementioned calibration curve; and (ii), the success of the models to predict zeta potential measurements assuming that the shear plane coincides with the head of the diffuse layer. Modeling parameters were also produced with the '1pK approximation' whereby the proton affinity constant of the singly- and of the triply-coordinated sites were set to the pH of zero charge and CStern and the electrolyte ion binding constants were co-optimized. In both cases, the values of CStern and of the electrolyte ion binding constants are slightly larger for the 23 and 37 m2 g-1 goethites than for the 85 m2 g-1. This indicates a larger proton uptake capacity of the 23 and 37 m2 g-1 goethites, putatively resulting from the larger surface roughness at the termination of the particles.
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| 3. |
- Lützenkirchen, Johannes, et al.
(författare)
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Comparison of various models to describe the charge−pH dependence of Poly(acrylic acid)
- 2011
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Ingår i: Journal of Chemical and Engineering Data. - : American Chemical Society. - 0021-9568 .- 1520-5134. ; 56:4, s. 1602-1612
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Tidskriftsartikel (refereegranskat)abstract
- The charge of poly(acrylic acid) (PAA) in dilute aqueous solutions depends on pH and ionic strength. We report new experimental data and test various models to describe the deprotonation of PAA in three different NaCl concentrations. A simple surface complexation approach is found to be very successful: the constant capacitance model requires one pKa value and one capacitance for excellent fits to the data, with both parameters depending on ionic strength. The use of a self-consistent set of diffuse double layer parameters with one pKa for flat, spherical, and cylindrical geometry does not result in a satisfactory description of the data, and a number of adjustments to that model were tested to improve the fit. The basic Stern model (BSM) was tested with both plate and cylinder geometry. The cylinder geometry along with strong electrolyte binding was found to be superior to a similar approach involving weak electrolyte binding both in terms of goodness of fit and self-consistency of the parameters. The third approach, the non-ideal competitive consistent adsorption-Donnan (NICCA-Donnan) model, involving one functional group, allows an excellent description of the experimental data. Finally, the polyacid chain was modeled using a mechanistically more realistic self-consistent field (SCF) approach, which allows for radially inhomogeneous distributions of the charges and radial variations in the polymer density and electrostatic potential, while the functional groups can be in protonated, deprotonated, or complexed states. One functional group was insufficient for a satisfactory description of the data. With two segments (one monoprotic, the other diprotic) a reasonable description of the data, including the ionic strength dependence, is achieved, and the tendency of the size of the macro-ion with pH and ionic strength is as expected. This model has the fewest adjustable parameters and is considered the most realistic and comprehensive among the models tested.
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| 4. |
- Lützenkirchen, Johannes, et al.
(författare)
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Limitations of the potentiometric titration technique in determining the proton active site density of goethite surfaces
- 2002
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Ingår i: Geochimica et Cosmochimica Acta: Volume 66, Issue 19, 1 October 2002, Pages. ; 66:19, s. 3389-96
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Tidskriftsartikel (refereegranskat)abstract
- Density of proton active surface sites at mineral surfaces is a property of fundamental importance in equilibrium modeling of surface complexation reactions. In this article, methods for an experimental determination of these sites at the surface of α-FeOOH (goethite) are explored. It is shown that previously obtained saturation data of goethite with respect to protons do not yield a site density that can be considered as an intrinsic sorbent property: the results are below crystallographically expected values and values for different ionic media in terms of composition and concentration yield different numbers—for example, chloride would yield higher values than nitrate at the same concentration, and higher electrolyte concentration would favor higher apparent maxima. Although site saturation might be explained by electrostatic repulsion, which is more efficient at high electrolyte concentration or for certain ions, further independent experimental results show that no saturation occurs on goethite down to ph ≡ −log[H+] = 2.2 and possibly to PH = 1.0 in 0.6 M NaCl. For those very low pH values, the experimental charging curve was obtained by coulometric back titration (using the Gran plot) or titrations with tris (hydroxymethyl)-aminomethane of the supernatant of acidified goethite suspension. These experimental data are to our knowledge the first high quality data at such low pHs. However, small errors in the determination of proton concentrations (1%) are shown to strongly affect the shape of the charging curve for ph < 2. Furthermore, goethite dissolution (proton consumption and iron reduction in coulometric titrations) and liquid junction effects interfere at low ph, hampering the straightforward application of coulometric Gran titrations over the whole pH range. From these experiments, it can nonetheless be ascertained that a minimum of 2.5 protons/nm2 can be adsorbed at the goethite surface from the point of zero charge (ph 9.4) to pH 0.9. Although these studies are restricted to goethite, those studies in which titrations with excess acid and base have been used for the determination of proton active site concentrations of sorbents should be reconsidered.
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| 5. |
- Lützenkirchen, Johannes, et al.
(författare)
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Net surface proton excess of smectites obtained from a combination of potentiometric acid–base, mass and electrolyte titrations
- 2012
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Ingår i: Colloids and Surfaces A. - : Elsevier. - 0927-7757 .- 1873-4359. ; 412, s. 11-19
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Tidskriftsartikel (refereegranskat)abstract
- Difficulties in the determination of absolute net proton and hydroxide surface consumption of clays from potentiometric acid–base titration are due to the fact that unlike for oxide minerals curves of relative proton related surface charge densities for several electrolyte concentrations are shifted and do not exhibit a common intersection point. Moreover, the isoelectric point (pHiep) of clays is usually in the acidic region and differs from the point of zero net proton and hydroxide consumption (pHncph). As a simple experimental procedure for clays we propose to perform potentiometric mass titrations in combination with potentiometric electrolyte titrations for the determination of the pHncph at different electrolyte concentration.Potentiometric acid–base titrations (pH of the suspension monitored as a function of the volume of added acid or base) and mass titrations (pH of the suspension monitored as a function of the mass concentration of added solid) of two pretreated clay samples (SAz-1 and SWy-2) were carried out in sodium chloride media. Due to the pre-treatment samples SAz-1 and SWy-2 contain ≥92 wt.% montmorillonitic smectite. The mass titrations were supplemented by electrolyte titrations (pH of the suspension of high mass concentration measured as a function of electrolyte concentration). The observed decrease in the mass-titration endpoint pH due to additional salt clearly corroborates previous reports that the point of zero net proton and hydroxide consumption of smectites depends on the electrolyte concentration.
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| 6. |
- Lützenkirchen, Johannes, et al.
(författare)
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Potentiometric titrations as a tool for surface charge determination
- 2012
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Ingår i: Croatica Chemica Acta. - : Croatian Chemical Society. - 0011-1643 .- 1334-417X. ; 85:4, s. 391-417
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Tidskriftsartikel (refereegranskat)abstract
- This article summarizes methods for determining proton surface charge at mineral/water interfaces. It covers conventional experimental procedures and discusses problems with the techniques. Also it involves recommendations for obtaining reasonable and comparable results. The term "comparable results" refers to comparison between results for the same solid as obtained in different laboratories. The most important parameters for the surface titrations are discussed. We also propose a reference titration procedure that would allow direct, unbiased comparisons of experimental data. The article finally includes a check-list for researchers and reviewers which should allow limiting the amount of titration data that are not useful for future uses.
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| 7. |
- Lutzenkirchen, Johannes, et al.
(författare)
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Speciation and equilibria in the H+-Al3+ - polyacrylic acid system
- 2018
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Ingår i: Monatshefte fuer Chemie. - : Springer Science and Business Media LLC. - 0026-9247 .- 1434-4475. ; 149:2, s. 423-429
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Tidskriftsartikel (refereegranskat)abstract
- Equilibria between Al3+ and the polyelectrolyte polyacrylic acid (PAA) were studied in 0.1 M NaCl medium at 25 A degrees C (M = mol dm(-3)). The measurements were performed as precise potentiometric titrations in which OH- was in most cases generated coulometrically. The total concentration of Al(III) and PAA varied within the limits 6.23 x 10(-5)-2.01 x 10(-4) M and 7.20 x 10(-4)-1.12 x 10(-3) M, resp., with a 4-18 fold excess of ligand. The value of - log{[H+]/mol dm(-3)} varied between 3 and 5-7, the upper limit set by drifting EMF potentials caused by the onset of a precipitation reaction. The constant capacitance model was utilized to model the experimental data. With the objective of finding a model as simple as possible that gives an acceptable fit to data, the two species AlPAA(2+) and Al(OH)(2)PAA are postulated. The polyelectrolyte effect resulting in pH-dependent formation constants is demonstrated. Furthermore a comparison between the stability constant of the 1:1 complexes of Al3+ with acrylate(-) and PAA(-), respectively, shows the logK value of the latter to be five units higher and is ascribed to particular properties of the polyelectrolyte.
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| 8. |
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