Aqueous Kaolinite Suspensions Research Articles

Low-frequency electric conductivity of aqueous kaolinite suspensions III: Temperature effects

The electrical conductivity of aqueous kaolinite suspensions was studied as a function of both temperature and KCl concentration and interpreted in terms of surface conductance and electrokinetic potentials. The surface conductivity in the inner part of the double layer shows a temperature behaviour, which differs significantly from that of the diffuse part. While the latter is characterized by a temperature-independent activation energy, the ionic transport in the inner double layer appears to be influenced by additional, temperature-dependent processes at temperatures below 15 °C. Possible mechanisms for this observation are discussed and it is shown that an increase of temperature leads to more pronounced ionic transport in the double layer, which is then dominated by the diffuse portion. Finally, we estimate the counterion mobility in the inner part of the double layer and find these quantities to display a different temperature dependence than the corresponding bulk mobilities.

Low-Frequency Electrical Conductivity of Aqueous Kaolinite Suspensions II: Counterion Effects and Estimating Stern Layer Mobilities of Counterions

AbstractThe electrical state of the interface between a kaolinite-dominated clay sample and aqueous electrolyte solutions was characterized using low-frequency conductance measurements. From these measurements, the ζ-potential and surface conductivity contributions from the diffuse and non-diffuse parts of the electrical double layer were obtained. The suspensions were studied as a function of volume fraction, electrolyte concentration, and electrolyte type (LiCl, NaCl, KCl, CsCl, CaCl2, SrCl2, and BaCl2). Interpretation in terms of the surface conductance revealed that a substantial part of the surface conductivity originates in the inner part of the double layer. Electrokinetic potentials and related diffuse double layer properties are highly dependent on the nature of monovalent counterions, whereas divalent counterions do not show such clear dependencies. Further presented was a simple way to estimate the order of magnitude of counterion mobilities in the inner part of the electrical double layer. All counterions were shown to have a substantial mobility in the inner part of the double layer. Finally, we suggest that the apparent ion-specific effects observed in the diffuse part of the double layer are at least in part related to the finite size of the counterions. Our findings are relevant to scenarios where fluid flow in porous media is accompanied by charged species transport, e.g., in electro-osmotic remediation, spectral-induced polarization, or permeability measurements.

Low-frequency electrical conductivity of aqueous kaolinite suspensions: surface conductance, electrokinetic potentials and counterion mobility

AbstractThe low-frequency conductivity of aqueous kaolinite suspensions has been measured as a function of volume fraction and concentration of KCl, K2SO4and BaCl2, respectively. These measurements were interpreted with a theoretical model accounting for surface conductivity and particle shape. For the first time, an internally consistent data set was established by measuring all parameters necessary to solve the relevant equations. The simultaneous availability of surface conductivity, surface charge density and diffuse layer charge density permitted the estimation of counterion mobilities in the stagnant layer and a consistency check for the evaluation procedure of the conductivity experiments. In agreement with current literature results, monovalent counterions were found to have a Stern layer mobility similar to their bulk mobility, whereas the mobility of divalent counterions in this layer is reduced by a factor of ∼2.

Effects of Polymer Molecular Weight on Adsorption and Flocculation in Aqueous Kaolinite Suspensions Dosed with Nonionic Polyacrylamides

The effects of polyacrylamide (PAM) molecular weights (MWs) on the PAM adsorption capacities and PAM-mediated flocculation of kaolinite suspensions were investigated using a series of nonionic PAMs with different MWs. Adsorption tests using aqueous kaolinite suspensions dosed with a series of PAMs with MWs of 1.5 kg/mol, 10 kg/mol, 0.6–1 Mg/mol, 5–6 Mg/mol, and 18 Mg/mol (referred to as 1.5 K, 10 K, 0.6–1 M, 5–6 M, and 18 M PAMs) indicated that the adsorption capacity of the kaolinite for PAM increased with increasing MW. However, the capacity for 18 M PAM was 20 times smaller than those for the 0.6–1 M and 5–6 M PAMs, although it has the highest MW. In steady-shear viscosity tests, a 1 g/L stock solution of 18 PAM was found to cause polymeric chain entanglements, which reduced the adsorption capacity. The 0.6–1 M and 5–6 M PAMs were further used in flocculation tests, in order to investigate the effect of PAM MW on the flocculation capability. The 5–6 M PAM was found to have higher flocculation capabilities than 0.6–1 M PAM; 5–6 M PAM was more subject to nonequilibrium flocculation, resulting in the development of unstable, stretched polymeric structures on solid surfaces and increasing particle-particle bridging and flocculation. Higher-MW PAMs are more effective flocculation agents, because of their higher adsorption capacities and flocculation capabilities. However, an extremely high-MW PAM, such as 18 M PAM, decreases adsorption/flocculation, and the preparation and handling of working solutions become difficult, because of polymeric chain entanglements.

Water-soluble acrylamide copolymers: V. Dispersion properties of poly(acrylamide-co-p-maleimidobenzoic acid) and poly(acrylamide-co-sodium N,N-diallylsulfanilate)

Eight new poly(acrylamide-co-p-maleimidobenzoic acid) (PAMMBA) water-soluble copolymers, 3, have been prepared in a synthetic study with the objective to determine the influence of ring incorporation on the surface activity of these materials.1 Sufficient sample was obtained from three of these experiments to enable measurement of the dispersion properties of these polymers using aqueous kaolinite suspensions. PAMMBA-1, -2, and -3 copolymers containing 7, 13, and 23 mole percent p-maleimidobenzoic acid (MBA) were tested. Their effectiveness as dispersants increased with increasing MBA content. But the copolymer containing 23% MBA required a dose rate four to eight times that of DISPEX N-40, a commercial dispersant, to obtain equivalent dispersion properties from a standard kaolinite suspension. In similar copolymer synthesis experiments, ten new poly(acrylamide-co-sodium N,N-diallylsulfanilate) (PAMDAS) copolymers were also prepared and fully characterized.2 Sufficient material was isolated from three of these preparations to enable testing of the dispersant activity. PAMDAS-1, -2, -3 copolymers contained 3·0, 8·4, and 19·2 mole percent sodium N,N-diallysulfanilate (DAS) and were tested as dispersants in experiments conducted so as to produce results comparable to those of both the PAMMBA copolymers and a commercial dispersant. Dispersant effectiveness increased with increasing DAS content. PAMDAS-1 (3·0% DAS) was about the same effectiveness as PAMMBA-1 (7% MBA), PAMDAS-2 (8·4% DAS) was more effective than PAMMBA-2 (13% MBA), and PAMDAS-3 (19·2% DAS) was about equivalent in effectiveness to PAMBA-3 (23% MBA). Relative to DISPEX N-40, however, PAMDAS-2, at a dose rate of 2000 ppm by weight was about half as effective as DISPEX N-40 at a 250 ppm dose rate.

The effects of added nanoparticles on aqueous kaolinite suspensions: II. Rheological effects

This series examines the effects of added silica nanoparticles on the properties and behavior of an aqueous suspension of kaolinite particles. Part I focused on the structural changes induced by the nanoparticles, primarily through scanning electron microscopy images. In this manuscript, we describe the changes in the rheological behavior of the kaolinite suspensions upon addition of the nanoparticles. In the absence of any additives, kaolinite platelets quickly aggregate and settle. When nanoparticles and salt (NaCl in these experiments) are added together, however, the suspensions begin to stabilize. When the salt and nanoparticle concentrations each exceed specific lower limits, the suspensions undergo a transition to a gel and develop a finite yield stress. Increasing the nanoparticle concentration or added salt concentration substantially increases the measured yield values, such that for the strongest samples, the yield stress exceeds the maximum for the rheometer to shear (3500 Pa). Plots of the complex viscosity, | η ∗ | , versus time suggest two different time scales for the gelation process—a short, initial time (e.g., less than 2 h) in which | η ∗ | increases rapidly, followed by a gradual rise over a much longer period. Measurement of the phase lag, δ, between the applied stress and response strain indicates that the long-term state of the suspension is either completely viscous ( δ = π / 2 ) or completely elastic ( δ = 0 ). Values of δ between 0 and π / 2 were only seen with suspensions that were transitioning from a liquid to a gel state.

The effects of added nanoparticles on aqueous kaolinite suspensions: I. Structural effects

The results of an experimental study focused on the effect of added silica nanospheres on the structure of an aqueous suspension of disc-shaped kaolinite particles are presented. In the absence of any additives, kaolinite particles rapidly aggregate and settle. When only nanoparticles were added to a 14% vol. kaolinite suspension, some stabilization was observed, although a thick, fluid-like sediment still formed. Adding both nanoparticles and salt (NaCl or KCl), however, caused the entire suspension to transition into a solid material that was strong enough to actually be sliced. A phase diagram was constructed showing the concentration of salt and nanoparticles needed to produce this transition. With smaller nanoparticles, the transition occurred at much lower nanoparticle volume fractions. Scanning electron micrographs of both the sediment and solid-like material, obtained by cryogenic drying, showed that the latter consisted of a porous, ‘sponge-like’ structure. The characteristic size of the pores decreased as the number density of the added nanoparticles increased. Although the nanoparticles were not visible in the SEM images, it is believed that they had separated into the pores of the solid-like material. While a similar type of transition could be produced in suspensions containing only the silica nanospheres, the structure and flow behavior of this material were markedly different from that obtained with the added clay.

Stabilizing of aqueous kaolinite suspensions by thermal vapour-pressure shock explosion treatment

A new device for thermal vapor-pressure shock explosion (TSE) was constructed and the dispersion of three kaolinites (KGa-1, KGa-2 and S-5, well and poorly crystallized kaolinites from Georgia, USA and from Makhtesh Ramon, Israel, respectively) in aqueous suspensions investigated. In the new device, the clay suspension is heated in a sealed glass ampoule at 300°C for a few minutes and the water vapor pressure increases in the cell to ≈100 atm. At this stage the ampoule is shattered in ice-cold water. This treatment stabilized suspensions of KGa-1 and S-5 kaolinites. Particle-size distribution analysis showed that, by this treatment, smaller particles were obtained. A suspension of KGa-2 was not stabilized by this treatment although particles became smaller. Suspensions of the three kaolinites peptized by sodium pyrophosphate, were compared with those obtained by TSE treatment. SEM images and XRD patterns of sedimented films showed that tactoids became thinner as a result of the TSE treatment. EDS analysis of Ti suggested that the small particles, after the TSE treatment were formed from the disaggregation of the large aggregates.

Particle interactions in aqueous kaolinite suspensions: II. Comparison of some laboratory and commercial kaolinite samples

Flow curves of dilute suspensions of a natural kaolinite sample and two commercial kaolinite samples have been measured as a function of pH and NaCl concentration. The results have been compared with five specially prepared batches of sodium kaolinite. Two of the untreated and two treated samples were found to be contaminated with a specifically adsorbed aluminum species, and they also had edge isoelectric points lower than the specially prepared Na kaolinites which were “aluminum free.” Possible reasons for this shift are discussed and comments are made about the preparation process. The effect of NaCl and pH upon the Bingham yield stress of the kaolinite samples can be explained in terms of the modes of particle interaction discussed previously in a detailed study of the “Al-free” Na kaolinite, but the pH values and added NaCl concentrations at which edge-edge interactions are fully developed vary according to the edge isoelectric point and degree of contamination of the clay with soluble salts. Finally, the results of this study are compared with those of others.

Particle interactions in aqueous kaolinite suspensions: I. Effect of pH and electrolyte upon the mode of particle interaction in homoionic sodium kaolinite suspensions

Flow curves of dilute homoionic sodium kaolinite suspensions have been measured as a function of pH and electrolyte concentration. The changes in Bingham yield stress have been shown to be entirely in accord with the Schofield-van Olphen model of particle interactions, and it has been possible to elucidate the conditions under which edge-face, edge-edge, and face-face coagulated structures exist. The results have also enabled the isoelectric point of the edge surface of this kaolinite sample to be located at pH 7.3 ± 0.2. The electrolyte concentration required to develop the edge-edge structure fully increases as the edge surface potential increases on either side of the edge isoelectric point; the results are in accord with the DLVO theory of colloidal stability. Comparison of the effects of 1:1, 1:2, 2:2, and 2:1 electrolytes on the Bingham yield stress suggests that the anionic species may be the more important in converting edge-face structures into the edge-edge form at low pH values. The effect of pH and electrolyte concentration on the plastic viscosity is also discussed in terms of the electrical double layers at the kaolinite/water interfaces.

The application of the verwey and overbeek theory to the stability of kaolinite-water systems

A general method for measuring the adsorption of various anions on kaolinite has been presented. Using this method, the adsorption of citrate ions and hydroxyl ions on kaolinite has been described over the alkaline pH range. It has also been shown that the adsorption of these ions does not occur primarily on either the edges or faces of the kaolinite crystal, but probably occurs uniformly over the surface of the particle. The adsorption measurements permitted the calculation of all quantities pertinent to the Verwey and Overbeek theory. This, in turn, permitted a quantitative test of the applicability of this theory to the stability of kaolinite-water systems. It was found, with no overt contradictions, that a great deal of the viscous behavior of aqueous kaolinite suspensions could be explained on the basis of this theory. The results also permitted the formulation of a mechanism for the flocculation and deflocculation of aqueous kaolinite suspensions. It has been shown that three circumstances control the repulsion between particles. These are the extent to which the diffuse parts of the double layers of the particles interact, the total charge of the particles and the potential existing between the surface of the particles and the intermicellar solution, and the distribution of charges in the diffuse part of each double layer. It has also been shown that, in most cases, the effects of charge and potential are of minor importance and that the dispersion and flocculation of aqueous kaolinite suspensions depend only on the effects of diffuse layer interaction and charge distribution in the diffuse layer.