Intrinsic surface reaction equilibrium constants of structurally charged amphoteric hydrotalcite-like compounds

The relative equations among intrinsic surface reaction equilibrium constants (K in 1-pK model, K(a1)(int) and K(a2)(int) in 2-pK model, and *K(Na)(int) and *K(Cl)(int) in inert electrolyte chemical binding model), points of zero charge (PZC), and structural charge density (sigma(st)) for amphoteric solids with structural charge were established to investigate the effects of sigma(st) on intrinsic equilibrium constants and PZC. The intrinsic equilibrium constants of HTlc with general formulas [(Zn,Mg)(1-x)Al(x)(OH)(2)](Cl,OH)(x) and [Mg(1-x)(Fe,Al)(x)(OH)(2)](Cl,OH)(x) were evaluated. The following main conclusions were obtained. For amphoteric solids with structural charge, a point of zero net charge (PZNC) independent of electrolyte concentration (c) exists. A common intersection point (CIP) should appear among the acid-base titration curves at different c, and the pH at the CIP is pH(PZNC). The pK, pK(a1)(int), and pK(a2)(int) may be expressed as a function of pH(PZNC) and sigma(st), and these intrinsic equilibrium constants can be directly calculated from pH(PZNC) and sigma(st). The inert electrolyte chemical binding does not exist for amphoteric surfaces with structural charge. PZNC is not equal to the point of zero net proton charge (PZNPC) when sigma(st) not equal 0. pH(PZNC) > pH(PZNPC) when sigma(st)>0; pH(PZNC) < pH(PZNPC) when sigma(st)<0; and pH(PZNC) = pH(PZNPC) only when sigma(st)=0. With increasing c, the difference between pH(PZNC) and pH(PZNPC) decreases; i.e., pH(PZNPC) moves forward to pH(PZNC) with increasing c. For the HTlc samples studied, with increasing x, the pH(PZNC) and the pK(a1)(int) and pK(a2)(int) decrease, and the pK increases. These results can be explained on the basis of the affinity of metal cations for H(+) or OH(-) and the electrostatic interaction between the charging surface and H(+) or OH(-).     

Monte Carlo modeling of ion adsorption at the energetically heterogeneous metal oxide/electrolyte interface: Micro- and macroscopic correlations between adsorption energies

Grand canonical Monte Carlo simulations are carried out for the basic Stern model of the electrical double layer formed at the energetically heterogeneous metal oxide/electrolyte interface. The effect of the global (macroscopic) and local (microscopic) adsorption energies correlations as well as the influence of the model parameter on the surface charge density curves were investigated. The linear dependence of point of zero charge (PZC) as a function of H+ ion adsorption energy proves that the acidic/basic properties of the system are mainly governed by proton uptake/release. Two kinds of systems were taken into account: one neglecting lateral interactions and the other one including them. The effect of electrolyte concentrations as well as the surface heterogeneity on the surface charge density curves were shown too. The presented simulation algorithm allows to model two experimentally observed instances of the metal oxide/electrolyte interface: one possessing a common intersection point (CIP) at pH = PZC and the other one with CIP not equal PZC.     

On derivatives of surface charge curves of minerals

Surface titrations of minerals in aqueous electrolyte solutions are used as building blocks for surface complexation modelling. However, these potentiometric data may contain less model relevant information than previously and presently assumed. In the literature, derivative analyses have been applied to experimental surface charge versus pH curves and four or more pK values were extracted for goethite or aluminium oxide. Derivative analysis of specific surface charge versus pH curves calculated for various published model variants for goethite shows that not more than the net-zero proton surface charge condition can be extracted from computer generated data. Generated data can be produced in density and precision superior to experimental data, but yield only relatively little output from such derivative analysis compared to what has previously been extracted from derivatives of experimental data. For the generated goethite data and for all model variants only the point of zero could be extracted. For the various goethite model variants tested a nearly symmetrical peak appeared at the point of zero charge in the derivative curve. A different pattern could be obtained for generic models, for which two sites with unequal sites densities and different pK values were assumed. Variation of these parameters could result in derivatives of the charging curves with two maxima or one maximum and a broad tailing. In the literature, curves with features nearly identical to these generated curves have been interpreted by up to four pK values (i.e., four different sites within a 1-pK model). It is concluded that the interpretation of the generated data is in all cases hampered by the overwhelming electrostatic contributions to the free energy of proton ad/desorption. In no case except for the one-site 1-pK model was it possible to extract the input pK value(s) from the derivatives. Plausible explanations for the discrepancy between generated data and published experimental data are discussed.     

Calculating adsorption parameters of organic substances from the capacitance curves in the absence of information on the electrode zero-charge potential in a supporting electrolyte solution

The possibility of estimating the potential of zero charge (PZC) of an electrode in a supporting electrolyte solution (SES) from the differential capacitance vs. potential dependences measured in SES and in the presence of different amounts of organic substances is analyzed by using model calculations. It is shown that the electrode PZC in SES cannot be uniquely determined in the model of two parallel capacitors and Frumkin isotherm which are commonly used for describing the organic substance adsorption. It is shown that all adsorption parameters, with the exception of the adsorption potential drop of the organic substance, can be calculated from capacitance curves in the absence of information on the electrode PZC. From the analysis carried out it follows that, under given conditions, the potential of the organic-substance maximum adsorption is an adsorption parameter on the same scale as is used for differential-capacitance measurements.     

Formal Mathematical Analysis of the Existence of the Common Intersection Point in Relation to Determining the Parameters Describing Ion Adsorption at the Oxide/Electrolyte Interface: Comparison of the Triple and Four-Layer Models

For most oxide/electrolyte systems potentiometric titration curves measured for different ionic strengths have a Common Intersection Point (CIP) which corresponds to the Point of Zero Charge (PZC). However, there are systems where a CIP exists but the surface charge at this point does not equal zero (PZC CIP). In this paper theoretical analysis of the systems in which the PZC and CIP do not coincide is presented. It is based on the well-known 2-pK surface charging approach and Triple Layer Model (TLM) as well as the Four Layer Model (FLM) of the electric double layer. The appropriate mathematical criterion for CIP existence was applied with detailed derivations, both for TLM and FLM. Having determined in this manner the parameter values, one can draw proper conclusions about the features of oxide/electrolyte adsorption systems, in which PZC and CIP do not coincide. The values of adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both of the experimental titration isotherms, and to the individual isotherms of electrolyte cation adsorption measured using radiometric methods.     

Measurements of differential capacitance in room temperature ionic liquid at mercury,glassy carbon and gold electrode interfaces

Differential capacitances were measured in 1-propyl-3-methylimidazolium tetrafluoroborate (PMIBF₄) ionic liquid at three different electrode substrates (Hg, GC (glassy carbon) and Au) as a function of potential. Essentially different capacitance–potential curves were obtained at different electrodes. From the parabolic electrocapillary curve measured at dropping Hg electrode in PMIBF_4, the potential of zero charge (PZC) was found to be −0.31 V vs. Ag/AgCl (wire). However, the capacitance–potential curve at Hg electrode was found not to show any valley related to the PZC, whereas at GC and Au electrodes a minimum was observed at 0.29 and −0.51 V, respectively. The results are in disagreement with the recent theoretical study which implies that the capacitance–potential curve should be of bell shape with the maximum value of capacitance at PZC. The parabolic capacitance–potential curve similar to those obtained in inorganic molten salts was also observed for the first time at GC electrode. Probable causes of the difference in their capacitance–potential curves were also discussed.     

Studies on intrinsic ionization constants of Fe-Al-Mg hydrotalcite-like compounds

The theoretical analysis of the intrinsic ionization constant (K(a2)(int)) of Fe-Al-Mg hydrotalcite-like compounds (HTlc) possessing permanent charges was first performed using the double extrapolation method proposed by James et al. The theoretical permanent charge density (sigma(p,T)) of the HTlc sample was calculated from the crystal structure of HTlc, and the influence of sigma(p,T) on the K(a2)(int) was also examined. From the experimental results, these conclusions can be obtained: the zero point of the charge (pH(ZPC)) of Fe-Al-Mg HTlc increases with decreased Fe3+ content and increases with increased Mg2+ in the HTlc. The pK(a2)(int) of Fe-Al-Mg HTlc also increases with decreased Fe3+ and increased Mg2+ content in the sample; furthermore, the pK(a2)(int) of Fe-Al-Mg HTlc increases with decreased sigma(p,T).     

电解液离子与炭电极双电层电容的关系

以酚醛树脂基纳米孔玻态炭(NPGC)为电极, 通过微分电容伏安曲线的测试, 研究了水相体系电解液离子与多孔炭电极双电层电容的关系. 结果表明, 稀溶液中, 多孔炭电极的微分电容曲线在零电荷点(PZC)处呈现凹点, 电容降低, 双电层电容受扩散层的影响显著；若孔径小, 离子内扩散阻力大, 电容下降更为迅速, 扩散层对双电层电容的影响增大. 而增大炭材料的孔径或电解液浓度, 可明显减弱甚至消除扩散层对电容的影响. 炭电极的单位面积微分电容高, 仅表明孔表面利用率高, 如欲获得高的电容量, 还要有大的比表面积. 离子水化对炭电极的电容产生不利影响, 选用大离子和增大炭材料的孔径, 可有效降低离子水化对炭电极电容性能的影响.     

Surface characterization of plasma sprayed oxide materials: estimation of surface acidity using mass titration

The aim of this study was to characterize the influence of plasma spraying on the point of zero charge (PZC) of Al2O3-, Cr2O3- and TiO2-based materials. PZC is one of the most important parameter, which describes the acidity of oxide material in aqueous environments. PZC values of several plasma sprayed oxides were determined using mass titration method. Studies were performed for initial spray powders and plasma sprayed coating materials. In addition, mass titration experiments were performed for water-washed and nonwashed samples. It was found that mass titration is a suitable method to estimate the surface acidity of relatively coarse sample powders. It was found for most of the studied materials that the limiting pH values (assumed to be close PZC) were close to those reported in literature for the PZC values of traditionally manufactured oxide materials. On the other hand, mass titration curves of some oxide samples showed unexpected deviation in curve shapes and limiting pH. These deviations were probably due to selective dissolution of sample contaminations or sample material.     

Enhanced anion electroadsorption into carbon molecular sieve electrodes in acidic media

We previously showed that, for neutral electrolytes of small cations and relatively larger anions, it is possible to design certain pore sizes in active carbons that are large enough to electroadsorb cations but too small to allow anion electroadsorption. This situation leads to an electrical double-layer (EDL) capacitance that is significant only at potentials that are negative to the potential of zero charge (PZC); hence, much smaller capacitance is measured at potentials positive to the PZC. It was found that when the electrolyte is a strong acid (e.g., H(2)SO(4), HCl), a considerable capacitance is observed at positive potentials, even when the average pore size is too small to allow the insertion of large anions in neutral electrolyte solutions. This effect disappears when the pore size becomes considerably larger than the size of the ions. In this case, the EDL capacitance at positive potentials for both neutral and acidic solutions is comparable. The following four-step mechanism was found to comply best with the experimental data: (1) By acid catalysis, the protons form carbonium species within the conjugated carbon network. (2) The anions react with the carbonium ions, providing uncharged species in an activated state, which are chemibound as surface groups to the walls of the pores. (3) Because these surface groups are effectively much smaller in size than are the charged ions, they can migrate by chemical bond exchange within the carbon skeleton via constrictions (known to exist in microporous and molecular sieving carbons), which are too narrow to accommodate hydrated charged species. (4) Upon reaching wider spaces, the uncharged species are reionized and solvated by water molecules, which can fill small pores. The justification for the above mechanism is thoroughly discussed and demonstrated by the experimental results.     

缓蚀剂吸附行为的电化学及AFM力曲线研究

结合极化曲线,微分电容曲线测试和AFM力曲线技术研究了直链十二胺对氯化钠溶液中铜镍合金的缓蚀行为以及吸附机理。结果表明:十二胺在合金表面形成单分子层吸附膜而起到缓蚀作用。十二胺浓度越大,吸附膜越致密,缓蚀率越高,力曲线上测得的粘附力值也越大。质子化的十二胺在荷负电的合金表面的吸附使电极零电荷电位正移,电荷屏蔽作用使得AFM力曲线上探针与试样之间的长程静电斥力减小。     

Surface charge properties of Fe2O3 in aqueous and alcoholic mixed solvents

Iron oxide (Fe2O3) was identified and characterized by surface area, X-ray diffractometry, and FTIR analyses. Surface charge densities, point of zero charge (PZC), and surface ionization constants were determined from the potentiometric titration data in various aqueous and aqueous organic mixed solvents in the temperature range 293-313 K. The surface charge densities were observed to decrease with the increase in temperature and concentration of metal ions in both the aqueous and aqueous organic mixed solvents. The absolute values of the surface charge density were found to change in the order aqueous > aqueous/methanol > aqueous/ethanol. Further, the PZC of the iron oxide was observed to shift to the higher pH values in the order ethanol > methanol > aqueous solution, which indicated a decrease in the acidity of the surface -OH groups. The pKa1 and pKa2 values of iron oxide were also determined and then used for determination of the surface potential (psi0) of the solid in aqueous and aqueous organic mixed solvents. The surface potential-surface charge curves generally supplemented the results derived from psi0-pH curves.     

Amplified potentiometric determination of pK(00), pK(0), pK(l), and pK(2) of hydrogen sulfides with Ag(2)S ISE

The chemical capacitor theory has been applied to accurately determine dissociation constants of H(2)S with the Ag(2)S ion-selective electrode (ISE). The theory's principle is based on the measurement of the change in electrode charge density as a result of protonated or unprotonated sulfide adsorbed on the electrode surface. This charge density is related to the potential. Connection of each individual capacitor in series amplifies the potential according to the equation, E(total)=E(1)+E(2)+E(3)+cdots, three dots, centeredE(n). As the charges of individual capacitors are concentrated to one capacitor area, the charge density rises, and the potential increases. The pK(00), pK(0), pK(1), and pK(2) are reported as 1.8, 2.12, 7.05, and 12.0, respectively. The pK(00) and pK(0) are reported here for the first time. The pK(1) agrees well with the literature values; however, the pK(2) differs from those reported recently under extreme conditions. Reasons for disproving the unreasonably high pK(2)>17-19 values are given based on calculations. Mainly, when pK(2)>17-19, the experimental results do not fit the equilibrium equations, pH=(pK(1)+pK(2))/2, pK(1)=(pK(0)+pK(2))/2, and pH=pK(2)+log(HS(-))/(S(2-)).     

Determination of intrinsic bacterial surface acidity constants using a donnan shell model and a continuous pK(a) distribution method

Intrinsic acidity constants (pK(a)(int)) for Bacillus subtilis (Gram+) and Escherichia coli (Gram-) cells were calculated from potentiometric titration data at different salt concentrations. Master curves were generated by replotting charge excess data as a function of pH(S) (pH at the location of surface reactive sites) where pH(S) was determined as a function of Donnan potential, Psi(DON). This potential decreased in magnitude with increasing ionic strength, from -48.5+/-0.2 to -3.5+/-0.0 mV for B. subtilis and -47.9+/-0.3 to -3.5+/-0.0 mV for E. coli at 0.01 and 0.5 M K(+), respectively, indicating an efficient surface charge neutralization by counterions. A fully optimized continuous (FOCUS) pK(a) distribution method revealed four binding sites on B. subtilis and E. coli surfaces from the master curves with pK(a)(int) values of 3.59+/-0.38, 4.33+/-0.57, 5.94+/-0.66, and 8.64+/-0.57 for B. subtilis and 3.73+/-0.44, 4.85+/-0.71, 6.56+/-0.64, and 8.79+/-0.62 for E. coli. These were assigned to functional groups according to reported pK(a) ranges of 2.0-6.0 (carboxylic acid), 3.2-3.5 (phosphodiesters), 5.6-7.2 (phosphoric acid), and 9.0-11.0 (amine groups). Average points of zero salt effect (pH(pzse)) for B. subtilis experiments were 6.63+/-0.21 and 6.42+/-0.08 as a function of pH(bulk) and pH(S), respectively. Under the same criteria, E. coli calculations yielded 5.73+/-0.23 and 5.45+/-0.05. An understanding of metal and proton reactivity on bacterial cell surfaces can be addressed quantitatively through the use of electrostatic and chemical equilibrium modeling techniques proposed in this study. The results are consistent with those of electrical force microscopy studies used to document the intrinsic electrochemical heterogeneity of bacterial cell surfaces.     

Synthesis and characterization of goethite and goethite-hematite composite: experimental study and literature survey

Aging of synthetic goethite at 140 degrees C overnight leads to a composite material in which hematite is detectable by M?ssbauer spectroscopy, but X-ray diffraction does not reveal any hematite peaks. The pristine point of zero charge (PZC) of synthetic goethite was found at pH 9.4 as the common intersection point of potentiometric titration curves at different ionic strengths and the isoelectric point (IEP). For the goethite-hematite composite, the common intersection point (pH 9.4), and the IEP (pH 8.8) do not match. The electrokinetic potential of goethite at ionic strengths up to 1 mol dm(-3) was determined. Unlike metal oxides, for which the electrokinetic potential is reversed to positive over the entire pH range at sufficiently high ionic strength, the IEP of goethite is rather insensitive to the ionic strength. A literature survey of published PZC/IEP values of iron oxides and hydroxides indicated that the average PZC/IEP does not depend on the degree of hydration (oxide or hydroxide). Our material showed a higher PZC and IEP than most published results. The present results confirm the allegation that electroacoustic measurements produce a higher IEP than the average IEP obtained by means of classical electrokinetic methods.     

Potential of zero charge as a sensitive probe for the titration of ionizable self-assembled monolayers

Potential of zero charge and interfacial capacitance values of ω-mercaptoalkanoic acid monolayers deposited on Au(1 1 1) are derived from immersion current transients, and are applied to the analysis of their acid properties. Potential of zero charge and capacitance titration curves extend over six pH units, and exhibit apparent pK_1/2 values that vary with electrolyte concentration. Electrostatic adsorption of protons at the monolayer-solution boundary provides a quantitative explanation of these observations, and leads to a diffuse layer corrected pK_a value of 4.3, which is independent of the thiol chain-length.     

Effect of background electrolyte on the estimation of protein hydrodynamic radius and net charge through capillary zone electrophoresis

Two physicochemical models are proposed for the estimation of both hydrodynamic radius and net charge of a protein when the capillary zone electrophoretic mobility at a given protocol, the set of pK of charged amino acids, and basic data from Protein Data Bank are available. These models also provide a rationale to interpret appropriately the effects of solvent properties on protein hydrodynamic radius and net charge. To illustrate the numerical predictions of these models, experimental data of electrophoretic mobility available in the literature for well-defined protocols are used. Five proteins are considered: lysozyme, staphylococcal nuclease, human carbonic anhydrase, bovine carbonic anhydrase, and human serum albumin. Numerical predictions of protein net charges through these models compare well with the results reported in the literature, including those found asymptotically through protein charge ladder techniques. Model calculations indicate that the hydrodynamic radius is sensitive to changes of the protein net charge and hence it cannot be assumed constant in general. Also, several limitations associated with models for estimating protein net charge and hydrodynamic radius from protein structure, amino acid sequence, and experimental electrophoretic mobility are provided and discussed. These conclusions also show clear requirements for further research.     

pH-dependent surface charging and points of zero charge. III. Update

The recently published points of zero charge (PZC) of various materials are compiled to update previous compilations [M. Kosmulski, Chemical Properties of Material Surfaces, Dekker, New York, 2001; M. Kosmulski, J. Colloid Interface Sci. 253 (2002) 77; M. Kosmulski, J. Colloid Interface Sci. 275 (2004) 214]. The recent results corroborate the previously found PZC with a few exceptions. The PZC of alumina obtained from the second-harmonic generation response is substantially lower than the PZC obtained by means of standard methods, while for titania the difference is less significant. PZC of Tl2O3 at pH 7.9 was reported for the first time. A surprisingly insignificant temperature effect on the IEP of rutile was found. Recent model studies aimed at explanation of the effect of the nature of 1-1 electrolytes on the course of charging curves and of discrepancies in the PZC of different materials having the same chemical formula are summarized.     

Assessment of supporting electrolyte contributions in electrochemically modulated liquid chromatography

This paper reports the results of an investigation on the role of the supporting electrolyte in separations using electrochemically modulated liquid chromatography (EMLC) with a porous graphitic carbon stationary phase. With respect to the identity of the supporting electrolyte, the elution strength of the electrolyte anion increased as F- < OH- < BF4- < ClO4- < PF6- for injections of negatively charged aromatic molecules, whereas a 10-fold increase in electrolyte concentration induced a 60% change in retention for the same solutes. Furthermore, both the concentration and composition of the supporting electrolyte affected retention in a manner that varied with the charge of the analyte and applied potential. This behavior is explained using Gouy-Chapman diffuse double layer theory, coupled with comparisons of this theory with closely related models for ion-pair chromatography. Insights into the retention mechanism reveal that an ion-exchange mechanism controls the retention of negatively charged solutes at applied potentials removed from the potential of zero charge (PZC). At potentials close to the PZC, the electrostatic model is less effective with the predominant retention mechanism likely involving hydrophobic interactions with the carbonaceous stationary phase. The combined effects of these findings are demonstrated by using a temporal gradient in supporting electrolyte concentration to optimize an EMLC separation.     

Temperature effects on the point of zero charge and isoelectric point of a red soil rich in kaolinite and iron minerals

The effects of temperature (373–1373 K) on the point of zero charge (PZC) and isoelectric point (IEP) of a red soil rich in kaolinite and iron minerals were studied. PZC values of the soil treated at 373 and 573 K indicated the presence of iron oxide. The soil calcined between 773 and 1173 K shows a PZC almost coincident with the respective values of kaolinite. At 1373 K, the PZC of the soil is nearer to the value of iron oxide. In the entire temperature range studied the PZC values were lower than the IEP values. An approach of PZC and IEP values was observed after a partial removal of iron oxide by the dithionite-citrate-bicarbonate (DCB) method. The analyses of the PZC and IEP values, of electron probe micro analysis (EPMA) data and of specific surface areas evidence a specific adsorption of iron oxide on kaolinite. Finally, the dissolution sequence of iron and aluminium contained in soil was determined using hydrochloric acid.