Marcelo J. Avena

Methylene blue dimerization does not interfere in surface-area measurements of kaolinite and soils

Methylene blue (MB) was adsorbed from aqueous solutions onto a kaolinite and four soil samples to determine the effects of MB dimerization on the measured surface area. Adsorption isotherms were prepared using four adsorbing solutions containing, respectively, 9, 46, 71, and 83% of MB molecules in the dimeric state. Langmuir-type isotherms were obtained in each case. The results indicate that equilibration occurs quickly. The aggregation state of MB molecules at the surface does not depend on the aggregation state in the initial adsorbing solutions, but on the final equilibrium concentration of MB. A comparison with the specific surface area measured by adsorption of ethylene glycol monoethyl ether indicates that MB adsorbs as a monomer, regardless of the aggregation number in solution. This result occurs owing to the strength of monomer-surface and monomer-monomer interactions. If monomer-surface interactions are favored, the MB dimer adsorbs in the monomeric form. If monomer-monomer interactions are favored, dimer adsorption may occur. The visible spectra of adsorbed molecules indicated that MB was present at the surface as a mixture of monomeric and dimeric species. These results suggest that dimers are formed in the contact region between two aggregating particles.

Modeling the interfacial properties of an amorphous aluminosilicate dispersed in aqueous NaCl solutions

Abstract The structural and interfacial properties of an amorphous aluminosilicate prepared from aluminum chloride and sodium silicate solutions are reported. IR and 27 Al NMR spectroscopy revealed that the structure of the studied sample was similar to that of (hydrous) feldspathoids and amorphous aluminosilicate catalysts, which are characterized by the presence of negatively charged SiOAl groups (SiAlO − ) and hydroxylated surface groups. The particles were non-porous and dissolved appreciably at pH + , Cl − and Na + adsorption data ( Γ H , Γ Cl and Γ Na ), together with electrophoretic measurements, revealed, however, that the point of zero saline effect (PZSE) was different from the isoelectric point (IEP) and from the pH where Γ Cl − Γ Na = 0. This behavior differs from that of (hydr)oxides that contain only variable charge (amphoteric) surface sites. A simple model of the solidNaCl solution interface is proposed to account for the cation exchange properties of SiAlO − sites and for the acid-base character of the hydroxylated ones. The lack of coincidence between the PZSE and the IEP and the invariability of the IEP with the electrolyte concentration is attributed to the presence, and cation exchange properties, of SiAlO − sites. However, the model suggests that a high proportion of structural charge sites are neutralized by sodium ions in the particle bulk and cannot be assessed for cation exchange. The ability of the model to fit experimental adsorption data suggests that it can be used to improve the understanding of the amorphous aluminosilicate-electrolyte solution interface.

Open circuit potential measurements with Ti/TiO2 electrodes

The open circuit potential (OCP) vs pH response of Ti/TiO2 electrodes prepared by thermal and electrochemical oxidation of metallic titanium was measured in KNO3 aqueous solutions in order to establish whether the slopes of the OCP-pH curves can be used as a measure of the variation of surface potential (ψo) of TiO2 with the pH of the aqueous solution. For comparison purposes, ψo—pH slopes were also evaluated from surface charge-pH data obtained by acid-base potentiometric titrations of TiO2 dispersions. Ti/TiO2 electrodes showed a linear OCP-pH response in the range of ph 5–10 with slopes of −0.039 ± 0.005 V ph−1. The OCP-pH dependence of Ti/TiO2 electrodes was lower than that predicted thermodynamically. According to the triple-layer model, used to describe the oxide/aqueous solution interface, the OCP-pH slopes obtained for Ti/TiO2 electrodes cannot be identified with the variation of ψo with the pH. Calculations with this model indicated that neither the single nor the double extrapolation methods used to evaluate the intrinsic ionization constant of oxide surface sites render realistic values of these parameters for the TiO2 surface. The analysis of surface charge—pH data and the use of the triple-layer model to predict experimental values indicated that the ψo—pH slopes of TiO2 surfaces must be greater (in absolute magnitude) than 0.041 V pH−1.

A simple and novel method for preparing Ni(OH)2 Part I: Structural studies and voltammetric response

Ni/Ni(OH)2 electrodes were prepared by in situ precipitation of nickel hydroxide into porous nickel substrates. The thermal decomposition of urea in solutions containing Ni2+ was used for the synthesis of the active material. The compound prepared by this method was a poorly crystallized β(II)-type phase. The electrochemical behaviour of Ni/Ni(OH)2 electrodes thus prepared was studied by cyclic voltamperommetry and the efficiency of the in situ precipitation was followed by the measurement of the cathodic charge obtained from the j-E response. The amount of incorporated material depends on the urea concentration and the electrochemical response of the electrodes was improved by increasing the number of impregnation cycles.

Dehydration process on nickel hydroxide: Its influence on the electrochemical behaviour of Pt/Ni(OH)2 electrodes

Abstract The effects of dehydration on the structural and electrochemical properties of Ni(OH) 2 , prepared by massive precipitation from NiSO 4 and KOH solutions or by precipitation on Pt substrates were studied. The so prepared materials showed gel characteristics. They were composed by an approximately 5% of Ni(OH) 2 and a 95% of water. Dehydration of these materials resulted in a gel shrinkage until the final volume was a 4–10% of the initial one. Dehydration treatments produced a significant increase in the voltammetric discharge efficiency of Ni(OH) 2 coated Pt electrodes. Such improvement can be qualitatively explained by an increase in the concentration of Ni(OH) 2 particles near the substrate surface due to the gel shrinkage.

H+-promoted dissolution of Ni(OH)2: a turbidimetric study

Abstract Results related to the dissolution mechanism of β-Ni(OH)2 particles in acidic (HClO4) media are reported. The influence of the H+ concentration and temperature on the dissolution kinetics of these particles was followed using a turbidimetric method, which appeared to be a useful tool for this kind of study. Surface charge (H+ adsorption) and electrokinetic measurements were also used to characterize the adsorption and charging behavior of the Ni(OH)2-aqueous solution interface. The point of zero charge was in the pH 10.4±0.2 range. Protons as well as temperature promoted the dissolution of Ni(OH)2. All the studied samples exhibited a fractional reaction order of 0.35±0.03 when the dissolution rate law was expressed in terms of the H+ concentration in the aqueous solution; conversely, an integer reaction order of 2 was found when the rate law was expressed in terms of protonated surface site density. The results suggest that Ni(OH)2 dissolves through a surface-controlled reaction in which two proton adsorption steps precede the detachment of Ni(II) species. The apparent activation energy of the process was 9–10 kcal mol−1.

Correlation between the electrochemical properties of colloidal oxides and supported oxides on metallic substrates

Mixed oxide electrodes of Ti(IV) and Cr(III) were prepared by calcining Cr(OH)3 layers deposited on metallic titanium supports. This treatment produced a mixed oxide film of TiO2−Cr2O3 covered with a layer of pure Cr2O3. The electrochemical response (cyclic voltammetry) shows the presence of two or three oxidation peaks depending on the electrode preparation conditions. One peak may be interpreted as the oxidation of Cr(III) to Cr(VI) species and the appearance of other peaks is due to the presence of chromium atoms in oxidation states higher than (III). The results of chemical analyses, electrophoretic mobilities and acid-base potentiometric titrations on calcined Cr(OH)3 powders shows that the calcination step in air produced the decomposition of the Cr(III) hydroxide to the Cr(VI) oxide. Soluble Cr(VI) compounds were found in equilibrium with the suspended powder oxide which markedly affected the shape of the titrations curves. From the amount of Cr(VI) present in solution it was possible to correct the experimental σ0-pH curves. These corrected data indicated that Cr(VI) soluble species adsorbed at the Cr2O3/electrolyte interface.