A Thermodynamic Analysis of Ion Adsorption in the Metal Oxide/Electrolyte Systems in which PZC and CIP do not Coincide

A theoretical-numerical analysis of two adsorption systems composed of the same kind of oxide- TiO₂, and of two different electrolytes, NaCl and CsCl is presented. For one kind of the electrolyte (NaCl), PZC and CIP coincide, whereas they are different for the other (CsCl) electrolyte. The analysis is carried out by applying the popular TLM model, and by drawing formal-mathematical consequences of CIP existence in both kinds of adsorption systems. The values of the adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both experimental titration isotherms, and to the individual isotherms of cation adsorption measured using radiometric methods. That theoretical-numerical analysis suggests, that the inequality PZC (pK ^int _a1 + pK ^int _a2) may be a general feature of the oxide/electrolyte systems including the systems in which PZC and CIP coincide.     

1pK and 2pK protonation models in the theoretical description of simple ion adsorption at the oxide/electrolyte interface: the analysis of temperature dependence of potentiometric titration curves

Two models of oxide surface charging (1pK and 2pK) were used to describe the potentiometric titration curves measured by Blesa et al. (J. Colloid Interface Sci.101, 410 (1984)) at three temperatures and at three concentrations of electrolyte. Rudzinski et al. (Langmuir13, 483 (1997)) have applied the 2pK Triple Layer Model to analyze the above system earlier. Two calculation procedures based on the 1pK Basic Stern Model were developed to described Blesa's data. Since the experimental curves have the CIP (common intersection point) it was assumed that heat of electrolyte ion adsorption was equal to zero. We assumed two different values of the double layer innermost capacitance on both sides of PZC (point of zero charge), which was followed from asymmetry of surface charge curves relative to PZC. Moreover, it was necessary to take into consideration the dependence of capacitance on ionic strength and temperature. The quality of fit given by two models was comparable. Since the 1pK BSM is simpler than the 2pK TLM and includes not so many best-fit parameters it seems to be a better choice in this case. Discussion of the results obtained by other authors concerning the subject under consideration is also presented.     

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.     

Adsorption of alkali metal cations and halide anions on metal oxides: prediction of Hofmeister series using 1-pK triple layer model

Adsorption of electrolyte ions on metal oxides significantly affects the interfacial charge distribution. The general procedure for the prediction of surface charge on oxides in salt solutions was given by Sverjensky for the 2-pK Triple Layer Model (2-pK TLM) (Sverjensky, Geochim. Cosmochim. Acta 69:225–257, 2005). Based on his parameters values and by assuming parameters transferability (Piasecki, J. Colloid Interface Sci. 302:389–395, 2006) we have predicted the adsorption constants for three monovalent ions (Rb⁺, F^?, Br^?) for eight oxides within the framework of the 1-pK Triple Layer Model (1-pK TLM). The obtained parameters values along with the previously reported ones (Piasecki, J. Colloid Interface Sci. 302:389–395, 2006) allowed us to compare the adsorption affinities of alkali metal cations and halide anions, and construct the following Hofmeister series for the cations (Cs⁺≈ Rb⁺≈ K⁺< Na⁺< Li⁺) and for the anions (F^?? Cl^?≈ Br^?< I^?) for investigated oxides. The same lyotropic series was predicted by the 2-pK TLM. It indicates that Hofmeister series is invariable during parameter transfer between surface complexation models.     

金属│固体电解质│含水凝胶│石墨体系的研究

应用点缺陷模型和双层模型分析了金属│固体电解质│含水凝胶│石墨体系贮藏和放电时金属表面氧化膜的变化.在固体电解质含有微量水分的情况下,微量水分有助于金属表面形成松层,但由于水分子与紧密钝化膜的持续作用,金属表面松层厚度会不断增长,孔率降低,使得电池的电压和放电电流下降.松层的孔率强烈影响膜溶解行为,与紧密钝化膜的电学特性一样,松层也是一种半导体.     

Adsorption of human serum albumin (HSA) onto colloidal TiO2 particles, Part I

The adsorption of human serum albumin (HSA) onto colloidal TiO2 (P25 Degussa) particles was studied in NaCl electrolyte at different solution pH and ionic strength. The HSA-TiO2 interactions were studied using adsorption isotherms and the electrokinetic properties of HSA-covered TiO2 particles were monitored by electrophoretic mobility measurements. The adsorption behavior shows a remarkable dependence of the maximum coverage degree on pH and was almost independent of the ionic strength. Other characteristic features such as maximum adsorption values at the protein isoelectric point (IEP approximately 4.7) and low-affinity isotherms that showed surface saturation even under unfavorable electrostatic conditions (at pH values far away from the HSA IEP and TiO2 PZC) were observed. Structural and electrostatic effects can explain the diminution of HSA adsorption under these conditions, assuming that protein molecules behave as soft particles. Adsorption reactions are discussed, taking into account acid-base functional groups of the protein and the surface oxide in different pH ranges, considering various types of interactions.     

Effects of Energetic Surface Heterogeneity on Ion Adsorption at the Electrolyte/Oxide Interfaces: Non-symmetrical Binding-to-Surface Energy Dispersions

The equations developed by us for the surface complexation models, taking into account energetic heterogeneity of surface oxygens, are applied here to study the effects of the shape of adsorption energy distribution on the ion adsorption at the oxide/electrolyte interface. The paper presents comparison of two models: one assuming that the energetical heterogeneity of oxide surface is described by a symmetrical and next a non-symmetrical adsorption energy dispersion for the formation of various surface complexes. The comparison of these two models was based on the obvious assumption that the variances of both the symmetrical and the non-symmetrical distributions are equal. The potentiometric titration data are not sensitive enough to choose the right adsorption model. So, in addition the individual isotherms of adsorption of cation and anion, of the inert electrolyte measured radiometrically, have been taken into consideration. The symmetrical adsorption energy distribution seems to represent the features of these adsorption systems better.     

Influence of PZC (Point of Zero Charge) on the Static Adsorption of Anionic Surfactants on a Malaysian Sandstone

Static adsorption studies of two anionic surfactants produced in our lab are reported. The adsorption of surfactants on the rock samples was investigated with and without the presence of alkali. The point of zero charge (PZC) values were determined for the sandstone samples employing three titrimetric methods and it was found to be at pH 7.98. The relationship between the adsorption degree and pH value of brine below and above the PZC is discussed. It was found that at the pH of solution exceeds the PZC of the rock, the adsorption was 0.43 and 0.86mg/g of rock for the two surfactants. However, at pH values below PZC, the adsorption as high as 3.66 and 4.49mg/g for the two surfactants. The synthesized surfactants are found to be suitable for the EOR applications at pH values higher than the PZC of the rock sample.     

Effect of the pore geometry in the characterization of the pore size distribution of activated carbons

In this work, the characterization of Activated Carbons (AC) by using the independent pore models is discussed, with special emphasis on the issue of how the assumed pore geometry can affect the resulting Pore Size Distribution (rPSD) and on the problem of the unicity of the PSD when different probe molecules are used in adsorption experiments. A theoretical test was performed using virtual solids based in the so-called Mixed Geometry Model (MGM) (Azevedo et al. 2010). The MGM uses a kernel of adsorption isotherms generated by GCMC for different pore sizes and two pore geometries: slit and triangular. The adsorption isotherms of a virtual MGM solid were fitted with both the traditional Slit Geometry Model (SGM) and the Mixed Geometry Model (MGM). It is demonstrated that, by assuming a different pore geometry model from that of the real sample, different PSDs may be obtained by fitting adsorption isotherms of different probe gases. Finally, experimental results are shown which both point toward the MGM as an acceptable extension of the SGM and confirm that the MGM is a closer representation of the actual porous structure of most activated carbons.     

Adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide from simple (1-1) electrolyte solutions

The adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide (delta-MnO2) has been studied using two methods, viz., isotherms at constant pH in the presence of buffer solution and pH variation in the absence of buffer solution from a fixed metal ion concentration. While the adsorption isotherm experiments were carried out in 0.5 M NaCl only, pH variation or batch titration experiments were carried out in 0.5 M NaCl, 0.01 M NaCl, and 0.01 M KNO3 solutions. The complex nature of adsorption isotherms at constant pH values indicates that adsorption of all the cations is non-Langmuirian (Freundlich) and takes place on the highly heterogeneous oxide surface with different binding energies. The proton stoichiometry derived from isotherms at two close pH values varies between 0.3 and 0.8. The variation of fractional adsorption with pH indicates that the background electrolyte solution influences the adsorption of cations through either metal-like or ligand-like complexes with Cl-, the former showing a low adsorption tendency. The proton stoichiometry values derived from the Kurbatov-type plot varies not only with the electrolyte solution but also with the adsorbate/adsorbent ratio. The variation of fractional adsorption with pH can be modeled either with the formation of the SOM+ type or with a combination of SOM+ and SOMOH type complexes, depending upon the cation and electrolyte medium. The equilibrium constants obtained from Kurbatov-type plots are found to be most suitable in these model calculations. Adsorption calculated on the basis of ternary surface metal-chlorocomplex formation exhibits very low values.     

Adsorption kinetics,isotherms and mechanisms of three-component covalent organic polymer/sodium alginate beads for ciprofloxacin removal from aqueous solution

Ciprofloxacin (CIP), an important representative fluoroquinolone antibiotic, has been frequently detected in water sources, thus threatening aquatic organisms and human health. In this work, a porous three-component covalent organic polymer (SLEL-6) was synthesized through multi-component (MC) reaction systems for adsorptive removal of CIP from aqueous solution, followed by an encapsulation process to achieve SLEL-6/sodium alginate (SA) beads with boosted adsorption ability, reusability and recyclability. By virtue of the hierarchical porous natures, functional groups as well as π-rich skeletons, SLEL-6 and SLEL-6/SA beads could deal with CIP contamination effectively. Moreover, the adsorption isotherms of CIP by SLEL-6 and SLEL-6/SA beads follow the Langmuir model showing high theoretical maximum adsorption capacities of 57.47 and 163.93 mg g⁻¹, respectively. Furthermore, the plausible mechanisms are proposed according to experimental studies of influencing factors, coupled with characterizations before and after adsorption. This work therefore highlighting the immense potential of COP-based SA composite beads as new-type globular adsorbents for eliminating fluoroquinolones from aqueous solution.     

Oxide-dependent adsorption of a model membrane phospholipid, dipalmitoylphosphatidylcholine: bulk adsorption isotherms

The importance of substrate chemistry and structure on supported phospholipid bilayer design and functionality is only recently being recognized. Our goal is to investigate systematically the substrate-dependence of phospholipid adsorption with an emphasis on oxide surface chemistry and to determine the dominant controlling forces. We obtained bulk adsorption isotherms at 55 degrees C for dipalmitoylphosphatidylcholine (DPPC) at pH values of 5.0, 7.2, and 9.0 and at two ionic strengths with and without Ca(2+), on quartz (alpha-SiO(2)), rutile (alpha-TiO(2)), and corundum (alpha-Al(2)O(3)), which represent a wide a range of points of zero charge (PZC). Adsorption was strongly oxide- and pH-dependent. At pH 5.0, adsorption increased as quartz < rutile approximately corundum, while at pH 7.2 and 9.0, the trend was quartz approximately rutile < corundum. Adsorption decreased with increasing pH (increasing negative surface charge), although adsorption occurred even at pH > or = PZC of the oxides. These trends indicate that adsorption is controlled by attractive van der Waals forces and further modified by electrostatic interactions of oxide surface sites with the negatively charged phosphate ester (-R(PO(4)-)R'-) portion of the DPPC headgroup. Also, the maximum observed adsorption on negatively charged oxide surfaces corresponded to roughly two bilayers, whereas significantly higher adsorption of up to four bilayers occurred on positively charged surfaces. Calcium ions promote adsorption beyond a second bilayer, regardless of the sign of oxide surface charge. We develop a conceptual model for the structure of the electric double layer to explain these observations.     

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite-water interface

The surface properties of a well-crystallized synthetic goethite have been studied by acid-base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3+/-0.1 and 9.3+/-0.2, respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO(3) solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite-water interface.     

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.     

Interpretation of negative values of the interaction parameter in the adsorption equation through the effects of surface layer heterogeneity

The problem of the negative values of the interaction parameter in the equation of Frumkin has been analyzed with respect to the adsorption of nonionic molecules on energetically homogeneous surface. For this purpose, the adsorption states of a homologue series of ethoxylated nonionic surfactants on air/water interface have been determined using four different models and literature data (surface tension isotherms). The results obtained with the Frumkin adsorption isotherm imply repulsion between the adsorbed species (corresponding to negative values of the interaction parameter), while the classical lattice theory for energetically homogeneous surface (e.g., water/air) admits attraction alone. It appears that this serious contradiction can be overcome by assuming heterogeneity in the adsorption layer, that is, effects of partial condensation (formation of aggregates) on the surface. Such a phenomenon is suggested in the Fainerman-Lucassen-Reynders-Miller (FLM) "Aggregation model". Despite the limitations of the latter model (e.g., monodispersity of the aggregates), we have been able to estimate the sign and the order of magnitude of Frumkin's interaction parameter and the range of the aggregation numbers of the surface species.     

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.     

Substantiation of the method of an “ideal adsorption solution” for calculating the adsorption of binary vapor mixtures from the individual isotherms

Conclusions Substantiation was offered for the method of an ideal adsorption solution, which was proposed by Myers and Prausnitz to calculate the adsorption equilibrium of binary vapor mixtures with a solid adsorbent from the individual isotherms, for the particular case of adsorption systems for which is fulfilled the condition of an affinity of the adsorption isotherms of the vapor mixtures at a constant composition of the adsorption solution, and the condition of the additivity of the affinity coefficient of such a solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya, Khimicheskaya, No. 1, pp. 171–173, January, 1972.     

Structural-chemical disorder of manganese dioxides 1. Influence on surface properties at the solid-electrolyte interface

Relationships between lattice parameters of manganese dioxides and their surface properties at the solid-aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr=0, Tw=0, PZC=1 for ramsdellite; Pr=1, Tw=0, PZC=7.3 for pyrolusite; gamma-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn(3+)/Mn4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure.     

Polydisperse fractal aggregate formation in clay mineral and iron oxide suspensions, pH and ionic strength dependence

pH- and ionic-strength-dependent aggregation of permanently and conditionally charged clay mineral (montmorillonite) and iron oxide (magnetite) particles was investigated by means of dynamic light scattering and rheology. An indifferent electrolyte (NaCl) was used. The surface charging of solids was determined by acid–base titration. The point of zero charge (PZC) of magnetite seemed to be at pH 8.0 ± 0.1. The permanent negative charges on the basal plane of montmorillonite influence the interfacial distribution of H⁺ and Na⁺ ions. The pH dependence of the electrophoretic mobility showed directly the dominance of negative charges on montmorillonite lamellae independently of pH, while for magnetite the sign of the mobility reversed at pH ˜ 8.0. Montmorillonite particles formed stable suspensions; coagulation did not take place below 35 mM 1:1 electrolyte independently of pH. The aggregation of magnetite sol becomes significant near the pH of the PZC even at low ionic strength. Colloidal stability in composite systems was investigated at pH ˜ 4, where oxide and clay mineral particles are oppositely charged. At the lowest NaCl concentrations (1, 5 mM) the mixed systems remained stable and aggregation of oppositely charged particles could not be observed at all. Heterocoagulation of dissimilar particles needed a definite amount of dissolved electrolytes (about 8 mM). Mixed clay mineral and oxide systems are more sensitive to electrolyte under acidic conditions than those separately. Rheological investigation of the mixed clay mineral–oxide suspensions at pH ˜ 4 provided proof for the absence of attractive particle interaction at low ionic strength (2 mM). A physical network of oppositely charged particles could form only at higher salt concentration, for example, in the presence of 10 mM NaCl. The yield value of plastic systems showed a significant maximum at 1:15 magnetite/montmorillonite mass ratio. Received: 21 November 2000 Accepted: 20 December 2000     

Adsorption of nonionic mixtures at the air-water interface: effects of temperature and electrolyte

Specular neutron reflection has been used to investigate the effects of temperature and added electrolyte on the adsorption of nonionic surfactants and nonionic surfactant mixtures at the air-water interface. For the alkyl poly-oxyethylene oxide nonionic surfactants, C(n)EO(m), the adsorption at the air-water interface is independent of temperature for surfactants with shorter ethylene oxide groups, whereas there is an increasing tendency for increased adsorption with temperature for surfactants with longer ethylene oxide groups. The addition of "salting in" (sodium thiocyanate, NaSCN) and "salting out" (sodium chloride, NaCl, sodium sulphate, Na2SO4) electrolyte results in reduced and enhanced adsorption, respectively, for C12EO8, whereas both types of electrolyte result in enhanced adsorption for C12EO12. The addition of electrolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mixtures of C12EO3/C12EO8 increasing temperature results in a surface richer in the least surface-active component, C12EO8. For the same nonionic mixture, the addition of "salting in" and "salting out" electrolyte results in an reduced and increased adsorption, respectively. The addition of "salting in" electrolyte results in a surface more rich in C12EO3, whereas for the addition of both "salting in" and "salting out" electrolyte the surface composition is essentially unaltered.