C. Chassagne

Shear-induced flocculation of a suspension of kaolinite as function of pH and salt concentration.

The relation between the electrokinetic charge of kaolinite particles and their flocculation behavior has been investigated over a wide range of pH and added salt (for MgCl(2) and NaCl salts). All flocculation experiments have been done with a mixing jar (sediment volume concentration phi=3.84x10(-5)). The electrokinetic charge of particles in different suspensions has been assessed by electrophoresis while laser diffraction has been used to measure the floc size distribution. Mixing jar experiments can be successfully used to investigate the flocculation behavior of kaolinite at shear rates higher than or equal to G=35 s(-1), which is the shear rate used in the experiments. At lower shear rates, the floc size distribution is affected by particle settling. The electrophoretic mobility of kaolinite decreases in absolute value when the pH of the suspension decreases. This is reflected in an increase of both floc size and flocculation rate: the floc size at pH 4 is three times larger than at pH 7 and the flocculation time is one order of magnitude smaller (from 1000 to 100 min). When the ionic strength of the suspension is increased, the electrophoretic mobility and the mean floc size display the same variations. On addition of NaCl (pH 9) both the electrophoretic mobility and the floc size display an optimum around 1 mM of added salt, a feature that has been observed by other authors as well. The equilibrium floc size for a suspension (A) at 1 M of added NaCl and pH 9 is the same as for a suspension (B) at pH 2 with no added salt. However, the time needed to reach the equilibrium for suspension (A) is one order of magnitude larger than for suspension (B). This is due to edge-face Coulombic attraction in suspension (B). The equilibrium floc size obtained by addition of MgCl(2) or sea salt at pH 9 is similar to the size obtained by addition of NaCl. The flocculation rate for a suspension with added MgCl(2) is higher than for suspensions with other added salts.

Electrokinetic study of kaolinite suspensions

In this article a new formula for the electrophoretic mobility of a spheroidal colloid is given. This formula and the formula presented in Chassagne and Bedeaux [C. Chassagne, D. Bedeaux, J. Colloid Interface Sci. 326 (2008) 240-253] for the dipolar coefficient of a spheroidal colloid are intertwined. The combination of electrophoresis and complex conductivity measurements (from which the dipolar coefficient can be derived) allows to assess both the zeta potential and the Stern layer conductance. We will in particular show that the values found for the zeta potential from both techniques are similar in the case of the kaolinite suspension studied. Electrophoretic mobility data are also presented and discussed for a wide range of ionic strengths, different types of salt and various pH. This data has been used in flocculation studies on the same kaolinite samples [F. Mietta, C. Chassagne, J.C. Winterwerp, J. Colloid Interface Sci., accepted for publication, doi:10.1016/j.jcis.2009.03.044].

Electrophoretic mobility of latex nanospheres in electrolytes: Experimental challenges

The electrophoretic mobility of sulfate latex nanospheres (radius 300 ± 10 nm) was measured as a function of ionic strength for different salts. The results were obtained from two similar instruments (Malvern ZetaSizer 3000 HSa and Malvern ZetaSizer Nano) using the same dispersions, in the same conditions. The values predicted from the standard electrokinetic model for constant surface charge were in good agreement with the data over a large range of ionic strength. The influence of the protocol used to fill the cells appears to be of importance between 1–10 mM of added monovalent salt. There, the capillary wall properties seem to influence the electrophoretic measurements, even at fast field reversal (FFR), where electroosmosis should be absent. We found that during a series of measurements with monovalent salts, it was best to fill the cell starting from high ionic strength and decreasing the ionic strength during the series. The measurements with divalent salts were not sensitive to the filling procedure.

Theory of electrode polarization: application to parallel plate cell dielectric spectroscopy experiments

An extension of the model for electrode polarization of Cirkel et al. [Physica A 235 (1997) 269] is given. The problem is solved using both classical boundary conditions and the new boundary conditions using excess densities presented in a previous paper [J. Phys. Chem. B 105 (2001) 11743]. In the present paper, the electrodes are supposed to be ideal, meaning that charge transfer or adsorption are not considered. The advantage of the new boundary conditions lies in the possibility to extend to more complicated situations including for instance specific ion adsorption. We prove that the new boundary conditions and classical ones give the same results. A comparison of the model predictions, involving no adjustable parameters, experimental dielectric spectroscopy data is performed and fairly good agreement is found.

Dielectric and electrophoretic response of montmorillonite particles as function of ionic strength.

Montmorillonite is a sheet-like clay mineral. The surface charge of the faces is always negative, whereas the surface charges of the edges depend on pH. In this study, pH is around 6.5 implying that the edges are slightly positive; however, the overall charge of the particle appears to be negative as the surface of the faces is 50 times larger than the edges. In the presence of an applied electric field, montmorillonite particles and their double layer will polarize. This polarization affects the electrokinetic response of the particles. In this article, we investigated the effect of ionic strength on the electrokinetic response of montmorillonite particles using the dielectric spectroscopy and electrophoretic mobility. The experimental dipole coefficient found by dielectric spectroscopy was compared to the semi-analytical formula presented by Chassagne [C. Chassagne, J. Colloid Interface Sci. 326 (2008)]. The amplitude of the dipole coefficient of montmorillonite particles increased and the relaxation frequency shifted to lower frequencies with decreasing ionic strength. This tendency is in qualitative agreement with the theoretical prediction. A better agreement between the experimental and theoretical amplitudes of the dipole coefficient and between the high-frequency experimental and theoretical mobilities was obtained when a Stern layer conductivity is introduced. The same values for the zeta potential and Stern layer conductivities were used in both measurement sets. The relaxation frequencies were not changed by addition of a Stern layer. This discrepancy between experimental and theoretical relaxation frequencies are due to the limitation of the theory that is not valid at low κa, as discussed in the conclusion.

Comparison between the electrokinetic properties of kaolinite and montmorillonite suspensions at different volume fractions

We have investigated the electrokinetic responses of two different kinds of clay particles, kaolinite and montmorillonite. The dielectric permittivity of kaolinite suspensions is linearly proportional to volume fraction up to volume fractions of 20%, whereas that of montmorillonite is deviating from a linear relationship, for volume fractions below 0.5%. This indicates that the montmorillonite particles experience a particle-particle interaction at these low volume fractions. The complex dipole coefficients of both clays estimated by experimental data are, however, within experimental error in good approximation independent on volume fraction and agree with the theoretical predictions. The relaxation frequency in clay-water system at low ionic strength is almost determined by the relaxation of the double layer for both kaolinite and montmorillonite. For volume fractions larger than 0.5% for montmorillonite, we find that the zeta potential measured by electroacoustic methods starts to depend strongly on volume fraction. It is expected that for these high volume fractions, the dipole coefficients will also become volume-fraction dependent.

Compensating for electrode polarization in dielectric spectroscopy studies of colloidal suspensions : Theoretical assessment of existing methods

Dielectric spectroscopy can be used to determine the dipole moment of colloidal particles from which important interfacial electrokinetic properties, for instance their zeta potential, can be deduced. Unfortunately, dielectric spectroscopy measurements are hampered by electrode polarization (EP). In this article, we review several procedures to compensate for this effect. First EP in electrolyte solutions is described: the complex conductivity is derived as function of frequency, for two cell geometries (planar and cylindrical) with blocking electrodes. The corresponding equivalent circuit for the electrolyte solution is given for each geometry. This equivalent circuit model is extended to suspensions. The complex conductivity of a suspension, in the presence of EP, is then calculated from the impedance. Different methods for compensating for EP are critically assessed, with the help of the theoretical findings. Their limit of validity is given in terms of characteristic frequencies. We can identify with one of these frequencies the frequency range within which data uncorrected for EP may be used to assess the dipole moment of colloidal particles. In order to extract this dipole moment from the measured data, two methods are reviewed: one is based on the use of existing models for the complex conductivity of suspensions, the other is the logarithmic derivative method. An extension to multiple relaxations of the logarithmic derivative method is proposed.

Polarization between concentric cylindrical electrodes

We consider an asymmetric electrolyte between two cylindrical concentric electrodes that are uncharged in the absence of an applied voltage difference. We calculate the dielectric response of this capacitor to an alternating voltage difference. The problem is solved using both classical boundary conditions and the new boundary conditions using excess densities to describe the charge build-up near the condensator plates as given in a previous article (C. Chassagne, D. Bedeaux and G.J.M. Koper, Colloid Surf. A 210 (2002) 137.). We verify that both boundary conditions give the same results. The advantage of the new boundary conditions lies in the possibility to extend in the future the analysis to real electrodes including reactions and specific ion adsorption. A comparison of the model predictions, involving no adjustable parameters with experimental dielectric spectroscopy data, is performed and excellent agreement is found.

Electrically induced anisotropy in nanospheres dispersions

Significant anisotropy can be reversibly induced in aqueous suspensions of highly charged latex spheres of about 10 and 20nm radius by the application of an electric field. For the first time, information is extracted from the rise time of the induced birefringence. The stationary value of the induced birefringence is found to increase with ionic strength, contrasting with what has been observed for non-spherical particles. These results, unexpected on the basis of commonly used particle–field interaction theories, are interpretated as the consequence of a collective reorganization of the system resulting from the electrically induced anisotropy in the particle–particle interactions.

THE ROLE OF MONO- AND DIVALENT IONS IN THE STABILITY OF KAOLINITE SUSPENSIONS AND FINE TAILINGS

A major issue for the oil sand industry is the settling of thin fine tailings (TFT) which are a byproduct of the oil sand extraction process. These tailings are deposited in large ponds and settling takes decades. The aim of the present study was to increase understanding of the role of specific ion types (monovalent/divalent) present in the water in flocculation behavior, and hence the settling of flotation fine tailings of the Athabasca oil sands (which consist predominantly of kaolinite). In this study, two series of measurements were conducted and compared: one with TFT and with varying pH and salinity, and another with kaolinite suspensions with varying pH, salinity, and volume fraction. The volume fraction of kaolinite and TFT used was in the range 0.01–1% volume fraction for any ionic strength or ion. In this range the electrophoretic mobility was constant indicating that there were no particle-particle interactions, a required condition for electrophoretic mobility measurements. Electrokinetic measurements were made as a function of concentration of salt added and pH. The flocculation behavior of both TFT and kaolinite can be linked to the electrokinetic mobility at high ionic strength. The electrophoretic mobility values and therefore the electrokinetic charge of the particles were smaller for divalent salt than for monovalent salt. As a consequence, both kaolinite and fine tailings should and do flocculate more quickly in the presence of a divalent electrolyte during settling-column experiments. The electrophoretic mobility of kaolinite and tailings in electrolytes containing a majority of monovalent ions (NaCl) decreased in absolute values with decreasing pH while their electrophoretic mobility in electrolytes containing a majority of divalent ions (MgCl2) did not depend on pH. The flocculation of the fine tailings in an electrolyte where divalent ions are predominant is therefore not expected to be influenced by pH.