Robert O. James

Adsorption of hydrolyzable metal ions at the oxide—water interface. III. A thermodynamic model of adsorption

Abstract A detailed quantitative model for the adsorption of hydrolyzable metal ions at the oxide—water interface is presented in terms of the competition between the free energy changes faavorble to adsorption, i.e., coulombic and chemical energy changes, and the unfavorable change in solvation energy. Because of the quadratic dependence of solvation energy changes on the charge of the ion, this term decreases the adsorption of highly charged species. As hydrolysis occurs and the ionic charge is lowered, the coulombic and chemical energy contributions dominate the adsorption energy change and adsorption is abruptly enhanced. If the oxide has a dielectric constant similar to that of the solvent then solvation energy changes are minimal and do not prevent adsorption of unhydrolyzed species.

Adsorption of hydrolyzable metal ions at the oxide—water interface. I. Co(II) adsorption on SiO2 and TiO2 as model systems

Complete isotherms of adsorption density vs. pH and concentration have been determined for Co(II) adsorption on SiO2 (point-of-zero charge pHPZC = 2.0) and TiO2 (pHPZC = 5.6). Isotherms for Fe(III), Cr(III), and Ca(II) on SiO2 have also been determined as a function of pH at fixed added concentration. The adsorption data indicate that the primary hydration sphere of free or hydrolyzed metal ions is not altered in the adsorption process. While a specific adsorption potential of ca. −5 kcal mole−1 exists for Co2+ adsorption on TiO2, for SiO2, it is −2 kcal mole−1, or less. Adsorption does not show a simple plateau monolayer behavior, but rather a steady trend with pH and concentration to saturation at the precipitation condition. The qualitative correlation between adsorption and hydrolysis is shown to break down when examined in detail. A complete description is shown to require many interactions for all hydrolysis products and the free ion.

Adsorption of hydrolyzable metal ions at the oxide—water interface. II. Charge reversal of SiO2 and TiO2 colloids by adsorbed Co(II), La(III), and Th(IV) as model systems

Abstract The detailed electrophoretic mobility behavior, supplemented by streaming potential data, for Si02 in aqueous solutions of Co(II), La(III), and Th(IV) has been determined as a function of metal ion concentration, pH, ionic strength, and percentage solids. The several charge reversals (CR) observed are, in order of increasing pH, shown to represent the point-of-zero charge (PZC) on the Si02 substrate (CR 1), the pH of surface nucleation of metal hydroxide (CR 2), and at high pH, the PZC of the metal hydroxide coating (CR 3).

Preparation and characterization of amphoteric polystyrene latices

The experimental investigation of the interaction of dissimilar colloids has often been complicated by the solubility of one colloid and subsequent resorption on the other. When this occurs, the observed interaction may in fact be homocoagulation rather than heterocoagulation. This problem is common in aqueous mixed oxide dispersions. To overcome this problem, we have prepared polystyrene latices, which are essentially insoluble, with a variety of surface functional groups, for example, −COOH and −NH2, which ionize to form charged sites. For the case of mixed ionized functional sites, e.g., −COO− and −NH3+, the surface is amphoteric and has surface charge-pH and zeta potential-pH dependence similar to oxides. By variation of the mole fraction of surface sites, e.g., m−COOHm−COOH + m−NH2, it has been possible to vary the pzc and icp of the colloid. Thus these latices provide a method of preparing identical particles with dissimilar double layers, low solubility, and pH-dependent double-layer structure for experimental study of heterocoagulation.

Analysis of models of adsorption of metal ions at oxide/water interfaces

The uptake of hydrolysable metal ions at solid/aqueous solution interfaces may be represented by a variety of models. These models define uptake as due to: Studies of adsorption of the relatively simple metal aquohydroxo complexes do not distinguish unambiguously between the importance of each of the mechanisms. However, in the future it should be possible to exploit some of the differences in algebraic expression of the models to further improve our understanding of the uptake of hydrolysable metal ions.

Coagulation of Amphoteric Latex Colloids Reversibility and Specific Ion Effects

Coagulation studies of amphoteric latex sols of various i.e.p. values in LiNO3, KNO3 and CsNO3 solutions are reported for a wide range of salt concentrations and pH. Three different techniques for the study of coagulation phenomena all indicate reversibility, in that sols coagulated by pH or salt are able to be redispersed. K+ and Li+ counter ions are able to stabilize these sols in the high salt concentration region. With NO3– and Cs+, the expected narrow zone of coagulation at low salt (i.e.p. coagulation) expands into the usual broad coagulation zone at high salt, where coagulation is observed at all pH values. The stabilizing effect of Li+ and K+ counter ions is attributed to a hydration barrier at the interface.

Adsorption, precipitation, and electrokinetic processes in the iron oxide (Goethite)—oleic acid—oleate system

Detailed adsorption isotherms based on radiochemical analyses for adsorption of oleate aqueous species on α-FeOOH (goethite) are reported. The range of total oleate and pH conditions are shown, from detailed analysis of oleate solution chemistry, to be in a domain free of liquid oleic acid or metal oleate precipitation. The form of the isotherms indicates that down to 10 −7 mole·dm 3 the adsorption is controlled by coulombic plus hydrophobic effects together with a specific adsorption component that is considered to be neutral oleic acid coadsorption with oleate anions, or acid soap dimer adsorption. The electrokinetic data also indicate a specific contribution beyond coulombic plus hydrophobic effects. For regions where precipitation of liquid oleic acid occurs a model of the surface film akin to a two-dimensional microemulsion is proposed.

Heterocoagulation of amphoteric latex colloids

Amphoteric latex colloids with permanently bound carboxyl and amine groups have been used to test the general theory of heterocoagulation. By varying the carboxyl-to-amine ratio, particles with isoelectric points between pH 5 and 8 were prepared and turbidimetric methods used to monitor both homo- and heterocoagulation behaviour. Close agreement between theory and experiment was observed, particularly in the case where the ratio of particle numbers was varied. Heterocoagulation between an amphoteric latex and a conventional latex was examined and evidence of regulation-type behaviour was obtained.

Titanium dioxide–electrolyte interface. Part 1.—Gas adsorption and tritium exchange studies

In order to facilitate analysis of potentiometric titration data on colloidal TiO2 dispersions, a detailed gas adsorption and tritium exchange study of the TiO2 colloidal material has been undertaken. Gas adsorption data revealed no significant porosity. The weight loss and tritium exchange yield the number of surface protons as 12.5 nm–2, which is consistent with the crystal being made up of exposed crystal planes. This concordance substantiates the essential non-porosity and absence of a gel layer at the TiO2–water interface.

A multiple equilibria model of the adsorption of oleate aqueous species at the goethite-water interface

Abstract The interaction between monomeric and dimeric aqueous oleate species and ionizing groups at the goethite—water interface has been constructed to analyze experimental surface change titration and oleate adsorption isotherms. The binding constants that emerge and the pH—concentration distribution of surface oleate complexes support the concept of anion-neutral molecule coadsorption or acid soap dimer adsorption in the region where precipitation of oleic acid and/or ferric oleate is precluded.