Dudley W. Thompson

The nature of laponite and its aqueous dispersions

A study has been made of the physical properties and aqueous dispersion characteristics of the synthetic magnesium silicate Laponite XLG. Results obtained from electron microscopy, electron diffraction, ultracentrifuge analysis, and small angle X-ray scattering studies indicate that the primary particles of this material are thin plate-like crystals of reasonably uniform size, having (plate) diameters of ca. 20 nm. In dilute, colloidally stable aqueous dispersions the average thickness of discrete crystals is within the range 2–4 nm. Significant dissolution of this Laponite material occurs in aqueous solutions of pH < 9 with solution concentrations of magnesium exceeding 10−3 mol dm−3 at pH ⩽ 7. In the region of pH 7–8 precipitation of colloidal silica results in a marked increase in the cation-exchange capacity (CEC) of Laponite dispersions. CEC values obtained for dispersions having pH values within the range 8–10 indicate that the Laponite particles develop no significant pH-dependent surface charge in aqueous electrolyte (NaCl) solutions. The results of this investigation indicate that with regard to particle morphology and high pH surface charge characteristics Laponite XLG constitutes a reasonable model material for studies of the behavior of natural smectitie clay minerals. It is clear however that the use of Laponite for such studies requires careful attention to dispersion preparation and “purification” methods and to the choice and control of experimental pH ranges.

Surface electrical properties of calcite

The electrical state at the calcite aqueous solution interface has been investigated using a streaming potential method applied to systems containing no gas phase. Experimental results have allowed a clear and unambiguous identification of the major surface ions as Ca2+ and CO32− species and have also shown that other solution species, including H+ and OH−, have no significant direct influence on the surface charge of calcite over the range pH 7–12. Induced changes in the pH of aqueous calcite dispersions in the directions of increasing and of decreasing calcite solubility have been shown to result in the formation of a new solid phase at the calcite crystal surface. Formation of this surface material leads to more positive values of the calcite ζ-potential and it is suggested that this surface precipitation phenomenon could be at least partly responsible for the diversity of experimental results which have been obtained from electrokinetic studies of aqueous calcite dispersions.

The adsorption of small particles onto larger particles of opposite charge direct electron microscope studies

Abstract A new thin-film, freeze-drying/scanning electron microscopy technique for directly observing the adsorption of small colloidal particles onto larger colloidal particles of opposite charge, is described. Independent establishment of adsorbed amounts using a centrifugal/analytical technique gives confidence that no artefacts exist in the freeze-drying method. Adsorption isotherms are presented for small, negative latex particles adsorbed onto larger, positive latex particles, at varying background electrolyte concentrations, and in the presence and absence of preadsorbed layers of poly(vinyl alcohol—co-vinyl acetate) on both sets of particles. It is seen that the presence of this preadsorbed polymer layer only affects the adsorbed amount at electrolyte concentrations greater than some critical value. Kinetic data for the small particle adsorption are also presented. These give further support for the implication that long range interactions between the particles play a significant role in determining the adsorption rate coefficient, as well as the final amount adsorbed.

Electrical properties of the gold—aqueous solution interface

Abstract The electrokinetic properties of gold particles dispersed in aqueous inorganic electrolyte solutions have been studied using the technique of microelectrophoresis. Results obtained for dispersions formed from vacuum-evaporated metal indicate that electrical charge development at the “pristine” surface of gold in contact with aqueous solutions arises from the presence of a surface oxide layer. The influence of halide ions on the electrokinetic properties of these particles is consistent with metal—halide interactions involving both ligand exchange and specific anion adsorption. Evidence has been obtained which indicates that in the case of dispersions formed by citrate reduction of chloroauric acid, strongly adsorbed citrate ions modify the metal surface and inhibit interactions with halide species.

The aggregation of silica and haematite particles dispersed in natural water samples

Abstract Aggregation rates for quartz and haematite particles dispersed in two different river waters have been determined using a direct particle counting technique. The river water samples were obtained from the Sherston Avon (a basic, hard water) and Whitray Beck (an acidic, soft water), and the influence of ionic strength on the aggregation processes in these waters was investigated using samples to which indifferent electrolyte had been added. The homoaggregation characteristics of quartz and haematite differ substantially between the different river waters. For both oxides, electrostatic interactions play a role in determining the particle aggregation behaviour in both river waters, although in the case of quartz particles dispersed in Sherston Avon water, non-electrostatic interactions assume more significance. Experimental evidence indicates that for a particular oxide in a particular river water, the magnitude of the electrophoretic mobility of the particles provides a reasonable qualitative assessment of aggregation rate. When comparing the aggregation behaviour of a particular oxide in the different river waters, or the different oxides in the same river water, however, results obtained in this work indicate that electrophoretic mobility does not necessarily provide a good indication of colloidal stability. Results obtained for mixed dispersions of quartz and haematite provide a clear indication that heteroaggregation of the two oxides occurs in both river water systems. Particle aggregation in these systems can be described in terms of the appropriate homoaggregation rate constants together with a single heteroaggregation rate constant. Heteroaggregation occurs at a significant rate even under conditions where the quartz particles are essentially colloidally stable.

The influence of a smectite clay on the hydrolysis of iron(III)

Abstract The hydrolysis and precipitation of Fe(III) in simple inorganic electrolyte solutions and in aqueous dispersions of the clay mineral montmorillonite have been investigated by UV-absorption spectrophotometry, pH-titration, chemical analysis, electron microscopy, electron-probe microanalysis and microelectrophoresis. Experimental evidence indicates that during the neutralization of acid-Fe(III) solutions in perchlorate, nitrate, chloride and sulphate media, Fe(III) speciation is essentially confined to the simple hexa-aquo ion Fe 3+ , the first hydrolysis product, FeOH 2+ , and the 1:1 complex species FeCl 2+ and FeSO + 4 . Thermodynamically irreversible precipitation appears to occur in these systems under well-defined solution conditions, yielding compounds which conform to the stoichiometry Fe(OH) x A (3- x )/ n where the value of x depends on the identity of the anion species A n− . The presence of sulphate at SO 2− 4 /Fe(III) mole ratios > 1 increases the pH value required for the onset of precipitation owing to formation of the relatively stable complex FeSO + 4 . Formation function data have been compared with previously proposed hydrolysis/precipitation mechanisms. In the presence of montmorillonite, hydrolysis of Fe(III) is promoted, prior to precipitation, owing to preferential adsorption of FeOH 2+ at the clay surface. Whilst the presence of the clay appears to have no significant influence on the conditions required for hydrous oxide precipitation, experimental evidence shows a marked effect of the clay are the stoichiometry and growth of the initially precipitated Fe(III) phase.

Electron microscope studies of the surface microstructures of layer-lattice silicates

Microstructural features of crystal surfaces of the silicate minerals muscovite, margarite, montmorillonite, beidellite, and kaolinite have been investigated by studying the way in which gold atoms, deposited from the vapor phase, coalesce on these surfaces. Although detailed interpretation of the experimental data is in some cases open to speculation, the results obtained from these investigations show marked differences between the surface structures of the different silicate minerals and reveal microstructural features which would be difficult to detect by any other means. It would seem that this experimental technique can be used to obtain quantitative information concerning the composition of silicate surfaces provided the precautions are taken to preclude postdepositional migration and coalescence of the metal particles. The coalescence of gold particles partially embedded in a carbon film is promoted by aqueous salt solutions and acids. This phenomenon imposes serious practical restrictions on the way in which this type of study can be carried out in order to obtain quantitative surface structural information and does not seem to have been taken into account in many previous experimental investigations.

Electrokinetic properties of oxide particles in natural waters

Abstract Microelectrophoresis measurements have been made on haematite and on quartz particles equilibrated with and dispersed in two different river waters; a basic hard water obtained from the Sherston Avon (SA) and an acidic soft water obtained from Whitray Beck (WB). Information concerning the reversibility of surface-modifying adsorption processes has been obtained from studies of the effects of extensive washing of river-water-equilibrated particles with distilled water and with indifferent electrolyte solutions. Additional information concerning the influences of specific river water constituents on the oxide surface properties has been obtained from electrokinetic studies of particles dispersed in synthetic electrolyte solutions. The results obtained indicate that in the case of haematite, the surface electrical properties are controlled by a combination of inorganic cation and organic anion interactions with the particle surfaces. Whilst adsorbed cation species were found to be readily and completely removed from the particle surfaces by washing the particles with simple electrolyte solutions, adsorbed organic anion materials were found to be held far more tenaciously. This evidence suggests that the combined effects of cation and anion species on the surface electrical properties of haematite dispersed in these river waters arises either by the adsorption of these species at different surface sites or by adsorbed anions providing surface sites for cation species. Organic anion interactions with haematite particles appear to occur to essentially the same extent in the different river waters in spite of the significantly different pH values (pH ≈ 8.4 for SA pH ≈ 5.5 for WB) and humic concentrations (3 mg dm −3 for SA, 7 mg dm −3 for WB) of the two waters. In the case of quartz particles, surface interactions with organic anion species present in the river waters are of negligible importance with regard to modification of the surface electrical properties. The quartz surface properties appear to be determined essentially by interactions with inorganic cations, principally Ca 2+ and Mg 2+ in the basic SA water, and additionally, Al species in the acidic WB water.

An electron microscope study of the surface electrical properties of silicate clay crystals

Abstract A particle “decoration” technique has been used to investigate the distribution of electrical charge on the basal surfaces of kaolinite and montmorillonite crystals. The mode of interaction of kaolinite with dispersions of positively charged colloidal particles was found to be consistent with the presence of a pH-independent negative charge uniformly distributed over both basal surfaces of the clay crystals. Ultrasonic treatment of dilute aqueous dispersions of kaolinite resulted in the generation of basal surfaces which developed a positive charge in acid solutions. In aqueous dispersions these surfaces persisted for only short periods of time ( Evidence is presented for significant variations in the magnitude of the isomorphous substitution charge of individual crystals of montmorillonite.

The hydrolytic precipitation of iron in aqueous dispersions of mineral particles

Abstract The hydrolysis of Fe(II) in deoxygenated dispersions of the smectite clay mineral montmorillonite and of TiO2, (rutile) particles has been investigated using a pH-titration method together with chemical analysis, electron probe microanalysts and microelectrophoresis. Formation function data obtained for FeOH interactions in these systems indicate a substantial influence of these minerals on the hydrolysis and precipitation processes. In the case of montmorillonite, oxidation of adsorbed Fe(II) by structural Fe3+ ions in the clay crystal lattice promotes the formation of Fe(III), and mixed oxidation state hydrous oxides at the clay surface, the oxidation process being reversible with respect to solution pH. TiO2, was found to inhibit the precipitation of Fe(II) owing to the specific adsorption of FE(II) species at the oxide surface. The pH dependence of the adsorption process suggests that these interactions involve the unidentate binding of Fe2+ and FEOH+ species. The precipitation of Fe(OH)2, in systems containing low TiO2/Fe(II) concentration ratios would seem to occur under conditions where the “normal” solubility product of this compound is just exceeded in the solution phase, indicating no significant influence of the oxide on this precipitation process.