Jan 2, 2007 - Electroacoustics was employed for the measurement of zeta potentials for the highest possible kaolin content in suspension and the effect.
c 2007 Institute of Chemistry, Slovak Academy of Sciences DOI: 10.2478/s11696-007-0003-x
The Zeta Potential of Kaolin Suspensions Measured by Electrophoresis and Electroacoustics a
R. GREENWOOD, b B. LAPČÍKOVÁ, b M. SURÝNEK, a K. WATERS, and b L. LAPČÍK, Jr.* a Chemical
Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
b Department
of Physics and Materials Engineering, Tomas Bata University in Zlín, Nad Stráněmi 4511, CZ-760 05 Zlín, Czech Republic
Received 29 March 2006; Revised 26 September 2006; Accepted 8 October 2006
The zeta potentials of kaolin dilute and concentrated suspensions were monitored using the techniques of electrophoresis and electroacoustics, respectively. The effect of addition of salt (KCl), a polymer material (Triton X-100), and an anionic surfactant (sodium dodecyl sulphate, SDS) on the suspension properties was investigated by electrophoresis. Electroacoustics was employed for the measurement of zeta potentials for the highest possible kaolin content in suspension and the effect of dilution. The effect of aging of a freshly prepared sample and kaolin isoelectric point was also studied. Using both techniques it was noted that there was no isoelectric point, just a maximum value in the magnitude of the kaolin suspension zeta potential. These maxima were observed also in the presence of Triton X-100 and SDS. An increase of the concentration of KCl and SDS in suspension shifted the maxima towards more acidic values, while in the presence of Triton X-100 the position of the zeta potential maxima remained constant. Electroacoustic techniques revealed that a freshly prepared concentrated suspension requires about six hours to equilibrate to achieve a steady zeta potential. Diluting the concentrated suspensions led to decrease of the zeta potential as ions bound to the surface desorbed and screened the surface charge. The zeta potential maxima remained unchanged even after heating the powder in an oven at 200 ◦C (to remove any organic material) thereby suggesting that the most likely explanation for the maxima is isomorphic substitution. Keywords: kaolinite, kaolin, concentrated aqueous suspensions, electrophoresis, electroacoustics, zeta potential, isomorphic substitution
INTRODUCTION Kaolin is a clay mineral also known as China Clay or Paper Clay. The chief constituent of kaolin (Al2 O3 · 2SiO2 · H2 O), is kaolinite, which consists of successive layers of octahedral alumina and tetrahedral silica, which alternate to form plate like hexagonal particles. The particles are flat disks or plate-like in shape, with the disk radius of the order 5—10 times larger than the thickness. Thus a typical crystal is a disk of 0.5—1 µm in diameter and 0.1 µm thick. The flat surface is negatively charged over the entire pH range, whilst the edges are positive at low pH but negative at high pH, with an isoelectric point at about pH 7. These edge effects arise due to the fracture of the lattice network and contain silica- and alumina-like sites [1]. The electrokinetic behaviour of kaolinite should
represent an average of the surface and edge properties. However, the initial experiments revealed that the zeta potentials of kaolin suspensions were negative over the entire pH range, analogously to the silica zeta potentials. This phenomenon was explained by the large face to edge surface area ratio [1]. The mining of kaolin begins with blasting highpressure water through jets against the wall of clay pits. The resulting clay suspension then passes through a series of mechanical systems to remove any unwanted sand and mica before being pumped into large tanks where it is allowed to settle and thicken. Then excess water is drained off, leaving high-density slurry which is pumped on for blending. The resulting product is a soft white mineral used primarily in the ceramics industry. Its whiteness, opaqueness, large internal surface area, and non-abrasiveness make it an
*The author to whom the correspondence should be addressed.
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R. GREENWOOD, B. LAPČÍKOVÁ, M. SURÝNEK, K. WATERS, L. LAPČÍK, Jr.
ideal filler material for paper production [2], where it provides a smooth, opaque surface with good printability and ink retention. Kaolin also delivers gloss smoothness and excellent print quality. Other uses include fillers for rubber, plastic, paint, and adhesives. Currently novel types of resin matrix composites based on organoclay-epoxy nanocomposites with improved mechanical storage modulus, solvent resistance, fire retardancy, and limited gas permeability are being developed for advanced aerospace applications [3—7]. The aim of this paper is to investigate the zeta potential of two different kaolin suspensions using electrophoresis and electroacoustics techniques. THEORETICAL The technique of electrophoresis is well known in colloid chemistry and will not be discussed here. Electroacoustics is a newer technique based on the application of high-frequency alternating electric fields to a suspension of particles. As a result, a sound wave is formed, the magnitude and phase of which is used to deduce the sample zeta potential and a particle size distribution. For more details on the actual technique see the papers by O’Brien [8—10] and for a review of the applications of the technique see Hunter [11] or Greenwood [12]. The electroacoustics method [13, 14] can be applied to disk-shaped particles as the particle shape does not have a great effect on the sound wave. Disks behave like spheres that are smaller than the disk radius but larger than its thickness. For kaolinite particles (aspect ratio 10 : 1) the apparent size is reduced by 1 : 101/2 , i.e. a 1 µm disk would yield an apparent size of 0.3 µm. Measurements of the zeta potential are not difficult if the particles are above a micron in size and the electrolyte concentration is reasonably large (> 20 mM), i.e. the double layer thickness is comparable with the particle size. If the solid particles are smaller and the double layer is not thin enough, then the situation becomes complicated as the clay mineral has a large number of bound ions. These bound ions contribute to the conduction of the electric current by the ions in the region around the particle. This extra conduction cannot be ignored and is termed anomalous surface conduction [14—16]. However, the problem can be circumvented by working at increased electrolyte concentrations so the contribution of surface conduction becomes minute compared to the conduction through the bulk solution. Much of the work on measuring the zeta potential of kaolinite suspensions has been carried out at the Ian Wark Institute in Australia. In a series of three papers Mpofu et al. [17—19] studied the flocculation of kaolinite with polyethylene oxide, polyacrylamide, and a polyacrylamide—acrylate copolymer using the techniques of electroacoustics in conjunction with rheological measurements (especially yield values). The 84
authors also noted that the adsorption of flocculants could be enhanced by the addition of metal ions. Specifically improved clarification and settling rates could be obtained with Mn2+ at pH 7.5 and Ca2+ at pH 10. For each metal ion there was a critical pH range, about one unit wide over which hydrolysis and adsorption of metal ion species was increased from almost 0 % to 100 %. Similar work with Al3+ and kaolinite has been published by Johnson and co-workers [1]. Another Australian paper investigated the effects of cadmium and cobalt on the properties of kaolinite suspensions [20], whilst Taylor et al. [21] studied the kinetics of adsorption of high-molecular-mass anionic polyelectrolyte onto kaolinite. Cationic surfactant (cetyltrimethylammonium chloride, CTMAC) inducing coagulation has been studied by Janek and Lagaly [22]. There are numerous papers reporting on the use of electrophoresis to measure the zeta potential of kaolinite suspensions [23—29]. Kretzschmar et al. [25] studied the effect of pH and humic acid on the coagulation kinetics of kaolinite in conjunction with a dynamic light technique. They noted a point of zero charges for the edges of pH 5.8, which is slightly lower than the value reported in [1]. In a series of three papers Kaya and coworker [26—28] studied the zeta potential of kaolinite, montmorillonite, and quartz in the presence of various metal ions [27, 28] and ionic and non-ionic surfactants [26] with the aim to investigate the electrokinetic decontamination/remediation of contaminated soils. In [27] the authors list the wide range of the points of zero charge reported over the years and pointed out that Hotta and Stephan as well as Chase were not able to detect pH value at which the zeta potential was zero. Recently, also the influence of pH, temperature, and ionic strength on the kaolinite surface properties in suspensions was investigated based on the adsorption of polyacrylamide onto kaolinite [29]. EXPERIMENTAL For the electrophoresis experiments two powders termed Kolloid and OT80 were used, whilst for the electroacoustics work a series of concentrated kaolinite suspensions stabilised with a polyacrylate dispersant (Supragloss 95) were used as supplied by Imerys. Fresh samples for the aging experiments were prepared from the Imerys SPS China clay powder. The Imerys powder was characterised by XRF and XRD for composition and by BET for surface area analysis. Electrophoresis Experiments Kolloid and OT80 kaolin powders were supplied by Sedlecký kaolín (Czech Republic). The powder denoted as Kolloid was dispersed in the original form as obtained from the supplier. Powder denoted as
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ZETA POTENTIAL OF KAOLIN SUSPENSIONS
OT80 was milled in a bead mill (Fritsch Pulner Sette 6) at 450 min−1 for 1 min prior to dispersing. Dispersion was performed in the ultrasonic bath for 15 min (Bandelin SONOREX TK 52). For all electrophoretic measurements the content of kaolin in suspension was 0.1 mass %. The measurements were performed in distilled water, and in aqueous solutions of KCl (Aldrich), sodium dodecyl sulphate (CH3 (CH2 )11 OSO3 Na, Aldrich), and Triton X-100 (4(C8 H17 )C6 H4 (OCH2 CH2 )n OH, n ∼ 10, Sigma) with concentrations of 1 × 10−6 M, 1 × 10−5 M, 1 × 10−4 M, and 1 × 10−3 M. Samples were then placed in quartz cells (optical width of 10 mm) and measured using a laser-Doppler electrophoretic instrument ZetaPlus (Brookhaven Instruments, Hotsville, NY) equipped with a Kevlar electrode holder. Firstly, the instrument was set to work in the particle size measuring mode, where the mean particle size and particle distribution function were determined by means of dynamic light scattering technique using the builtin correlator BI-9000 AT (Brookhaven Instruments, Hotsville, NY) working in homodyne detection mode. Then, the instrument was switched into the zeta potential measurement mode. The first set of experiments was performed to study the effect of pH and KCl concentration on the particle size and zeta potential of both powders. The second set of experiments investigated the pH and sodium dodecyl sulphate concentration effects. The third set of experiments was aimed to study the effect of pH and polymer-based surfactant Triton X-100 on the physical properties of kaolin samples (mean particle size and zeta potential) expecting that the shift of the shear plane was the principal mechanism of the zeta potential changes. Each measurement (both zeta potential and particle size experiments) was repeated 10 times, allowing to calculate the respective statistical parameters. The results were fitted using the gradient method to the peak category models by means of Sigma Plot 2002, Ver. 8.0 (SPSS Science) software. Electroacoustics Experiments Four samples of 2 L of kaolin suspension each were prepared by Imerys (Par, St Austell, Cornwall) with the nominal kaolin content of 68 mass %. The suspensions were stabilised with a sodium polyacrylatebased dispersant at four different levels, namely 0.025 mass %, 0.15 mass %, 0.30 mass %, and 0.45 mass %. 180 mL of the suspensions was poured in the measuring reservoir of the Acoustosizer II (Colloidal Dynamics, USA). The software requires only three inputs to calculate the zeta potential, namely the mass fraction, particle density, and the dielectric constant of the particles. The mass fraction of each suspension was measured by drying a small sample in a Petri dish placed in an oven for several hours at 80 ◦C. The den-
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sity of the particles was taken to be 2.65 g cm−3 and the dielectric constant 11.8 [30]. Prior to all measurements, the machine was calibrated using a solution of K4 SiW12 O40 · nH2 O. Stability of the supplied suspensions was assessed on the basis of the measurement of the sample zeta potential every 150 s for approximately 20—25 min. Then, the effect of diluting of concentrated suspensions with small amounts of distilled water was investigated using the same experimental procedure. Consecutively, the zeta potential variation with time was studied using kaolinite sample freshly suspended in a background solution of NaCl with the electrolyte concentration of 10−2 M. Finally, an experiment was performed to estimate the isoelectric point of the kaolinite sample. It was decided not to use an extremely concentrated suspension of kaolin as the peristaltic pump could not cope with the suspension high viscosity. Thus, a suspension containing 10 mass % of the solid matter was studied instead. The pH of the suspension was adjusted to 10 with 1 M-NaOH (Fisher). The automatic titration software was programmed to perform titration of the suspension back to pH 2.75 in steps of 0.25 pH units using 1 M-HCl solution diluted from a stock solution supplied by Fisher. The experiment was carried out in background electrolytes containing 10−3 M and 10−2 M NaCl and KCl and distilled water. The pH values recorded by the Acoustosizer represent an average value recorded over the 150 s required to carry out the experiment. Each experimental point was taken after 120 s, which was the preset equilibrium time. RESULTS AND DISCUSSION Kaolin Samples Characterisation The XRF characterisation of the Imerys kaolinite allowed to assess the sample composition. As expected, the major components of the kaolinite sample were silica (49.0 mass %) and alumina (35.9 mass %). Further constituents were K2 O (1.80 mass %) and Fe2 O3 (0.75 mass %) as well as MgO, Na2 O, CaO, and TiO2 . On the other hand, one should take into account also a large mass loss on ignition possibly due to the removal of water or organic material present in the clay. The XRD data revealed that Imerys powder contained 88 % of kaolinite, 9 % of mica, 1 % of quartz, 2 % of feldspar, and 0 % of both tourmaline and albite. The surface area of the Imerys powder was 11.2 m2 g−1 , whilst that of the Kolloid and OT80 kaolin powders was 10.5 m2 g−1 and 10.8 m2 g−1 , respectively, as determined by the BET measurements. Electrophoresis Experiments In this paper, the convention is adopted that the isoelectric point represents the pH value, at which the 85
R. GREENWOOD, B. LAPČÍKOVÁ, M. SURÝNEK, K. WATERS, L. LAPČÍK, Jr.
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Fig. 1. Variation of the zeta potential (a) and mean particle diameter (b) of suspension of Kolloid powder in distilled water (opened triangles), and in solutions containing 10−6 M (circles), 10−5 M (squares), 10−4 M (triangles), and 10−3 M (diamonds) KCl with pH.
zeta potential of dispersed material is zero, whereas, the point of zero charge is the pH value, at which the particles possess no charge. Normally, these two values are exactly the same, but in the presence of adsorbable ions they can differ widely. Moreover, under certain circumstances two apparent points of zero charge can be detected (strictly speaking isoelectric points) due to the precipitation of ions in highly basic solutions. Fig. 1a shows the variation of zeta potential of the 0.1 mass % Kolloid suspension with pH in distilled 86
water and in solutions with increasing concentration of KCl. Firstly, it is important to note that no isoelectric point (IEP) could be deduced for all suspending media, similarly to the results presented in [1, 21, 27, 31]. Small negative values of the zeta potential were recorded but no positive ones. In fact, a maximum of the zeta potential can be seen at pH 5 for the kaolin sample suspended in distilled water. As the KCl concentration was increased the position of the maxima moved to lower pH values down to pH 4 for the
Chem. Pap. 61 (2) 83—92 (2007)
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Fig. 2. Variation of the zeta potential of suspensions of OT80 powder in distilled water (opened triangles), and in solutions containing 10−6 M (circles), 10−5 M (squares), 10−4 M (triangles), and 10−3 M (diamonds) KCl with pH.
Kolloid sample suspended in the KCl solution with the highest studied concentration. The maxima corresponded to the zeta potential increase between −5 mV and −25 mV, not expected for the KCl addition to kaolin—water suspension. Fig. 1b shows the corresponding mean diameter of the Kolloid powder particles under the same conditions. The particles were best dispersed, i.e. with no aggregates present, at higher
Chem. Pap. 61 (2) 83—92 (2007)
pH values when the repulsive forces are greater. Fig. 2 shows the zeta potential pH curves for the suspension containing 0.1 mass % of the OT80 powder measured at different KCl concentrations. Similarly to the Kolloid suspensions, there could not be established the sample IEP, however, the zeta potential maxima were observed, shifting towards the lower pH value with the increasing KCl concentration. Unexpectedly, the magnitudes of the zeta potential maxima appeared to change with the increasing salt concentration from about −10 mV to −30 mV. Moreover, it can be concluded that the particles of OT80 kaolin were better dispersed at higher pH values. Fig. 3 presents variation of the zeta potential with pH for the Kolloid powder suspended in the solution of sodium dodecyl sulphate. The effect of the increasing SDS concentration on the kaolin zeta potential was 87
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R. GREENWOOD, B. LAPČÍKOVÁ, M. SURÝNEK, K. WATERS, L. LAPČÍK, Jr.
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Fig. 4. Variation of the zeta potential of suspension of OT80 powder in solutions containing 10−6 M (circles), 10−5 M (squares), 10−4 M (triangles), and 10−3 M (diamonds) SDS with pH.
Fig. 5. Variation of the zeta potential of suspension of Kolloid powder in solutions containing 10−6 M (circles), 10−5 M (squares), 10−4 M (triangles), and 10−3 M (diamonds) Triton X-100 with pH.
similar to that of the addition of KCl with the maxima shifting from pH 5.6 to 4.4. The zeta potential curves become shallower with the increasing SDS concentration, which can be explained by the adsorption of the dodecyl sulphate anion onto the kaolin particles. Taking into account that the kaolin aggregates were present in suspensions at low pH values only, whilst the zeta potential was not changed significantly over the whole range of pH, one can assume that SDS imparts stability to the suspensions not only via electrostatic mechanism. By increasing the suspension pH the SDS solubility is changed so that the distance between the stabiliser molecule and particle surface becomes longer and SDS can start to act as a steric stabiliser, too. Thus, the mechanism of kaolin suspension stabilisation by SDS could be denoted as electrosteric one.
Fig. 4 summarizes the zeta potential curves found for the suspensions of OT80 powder containing different amounts of SDS. The usual shape of curves can be seen with maxima shifted towards lower pH values with increasing the SDS content in suspension. Also in this case the electrosteric effect of SDS on the kaolin suspension stability is suspected. Figs. 5 and 6 show the zeta potential curves of suspensions of the two kaolin powders containing various concentrations of Triton X-100. In both cases the position of the zeta potential maximum remained approximately the same regardless of the amount of Triton X-100 added. This observation correlates with the fact that Triton X-100 is a polymer substance, not a polyelectrolyte one and, therefore, it does not contain any dissociable groups, which would be affected by the pH value. Furthermore, the magnitude of the
88
Chem. Pap. 61 (2) 83—92 (2007)
ZETA POTENTIAL OF KAOLIN SUSPENSIONS
zeta potential at the maxima did not change significantly with the addition of the steric stabiliser. Interestingly, at pH 4.4 and Triton X-100 concentration of 1 × 10−5 M a vigorous change of both the zeta potential as well as mean diameter curves was observed in the case of OT80 dispersions. It was assumed that at given pH value the macromolecular chains of Triton X were present in their most extended conformation, which is favourable for inducing the bridging flocculation of kaolin particles. This assumption was supported by the observed sharp increase of the mean particle size and the maxima in both the zeta potential and mean diameter curves. The maximum of the zeta potential at pH 5.6 reflects the strong shift of the shear plane out from the particle surface due to the extended Triton X macromolecular chain conformation.
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Fig. 6. Variation of the zeta potential of suspension of OT80 powder in solutions containing 10−6 M (circles), 10−5 M (squares), 10−4 M (triangles), and 10−3 M (diamonds) Triton X-100 with pH.
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Fig. 7. The electroacoustic measurement of the zeta potential of a concentrated Imerys kaolin suspension containing 0.025 % (diamonds), 0.15 % (squares), 0.30 % (triangles), and 0.45 % (circles) of sodium polyacrylate dispersant.
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Fig. 7 reports the stability of Imerys kaolin suspensions containing different concentrations of dispersant. Relatively constant value of the zeta potential over the whole monitored time interval indicates that no material was dissolving out of the powder, thus altering the particle surface properties. Slight variation of the zeta potential can be explained by the effect of the peristaltic pump breaking down the aggregates present in the suspension. The mean zeta potential, pH, and conductivity of these samples are given in Table 1. The particle size distribution for the suspension was approximately as follows: d15 = 0.3 µm, d50 = 0.4 µm, and d85 = 0.7 µm. Thus, according to [13] and [14], the real d50 ≈ 1.3 µm. By increasing the dispersant dosage the zeta potential of dispersed solid became more negative due to the adsorption of polyacrylic acid onto the kaolin particles. Once the particles were completely covered by polyelectrolyte the zeta potential did not increase further [12, 32]. Hence it is possible to identify an optimum amount of dispersant required to stabilise the suspension against aggregation, which appears to be about 0.15 % (Table 1). Viscosity of concentrated kaolin suspensions containing no polyelectrolyte restricted the use of experimental equipment; hence, the lowest amount of dispersant, which enabled the measurement, was 0.025 %. The pH value of the suspensions remained practically constant once the particles were fully coated by dispersant. On the other hand, the suspension conductivity was slightly increased on addition of 0.45 % of dispersant due to the presence of free polyelectrolyte in the suspension. The values of pH and conductivity measured for the suspension containing 0.025 % of dispersant were much higher than those of the remaining three samples as a solution of NaOH was added to them in order to improve dispersion of the kaolin particles. Table 2 summarizes the effect of diluting the kaolin suspensions containing different concentration of dis89
R. GREENWOOD, B. LAPČÍKOVÁ, M. SURÝNEK, K. WATERS, L. LAPČÍK, Jr.
Table 1. Effect of Dispersant Content on the Properties of Kaolin Suspension Dispersant content
Kaolin content
Zeta potential
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mass %
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64.9 68.8 68.5 63.9
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Table 2. Effect of Dilution on the Properties of Kaolin Suspension Dispersant content
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Fig. 8. Aging of a freshly prepared suspension of Imerys kaolin: a) variation of the suspension electroacoustic zeta potential (solid line) and pH (dotted line) and b) conductivity (dashed line) and temperature (dotted-dashed line) with time.
persant on the sample zeta potential, pH, and conductivity. In all three cases the zeta potential of kaolin suspensions was increased by 5—7 mV. Similarly, an increase of the sample conductivity and pH was observed. The only exception was the pH value of the sample containing 0.15 % of dispersant. Above the IEP an increasing negative zeta potential would be expected due to the increasing pH value. By diluting the concentrated suspensions the ions bound to the particle surface are released into the solution thereby increasing its conductivity and decreasing the particle zeta potential due to the shielding of the particle surface. Figs. 8a and 8b present the kaolin suspension aging over a period of about 15—16 h. It was found that the 90
zeta potential of suspended kaolin reached a steady value of approximately −42 mV first after about 6 h. The suspension pH value also changed rapidly from the initial value of 6.05 and reached the value around 6.3 after the same period of time. The temperature of the measured suspension was kept at about 30 ◦C over the chosen time period with a small warm-up period, long plateau period and then cooling off slightly towards the end of experiment. The same trend is also reflected in the conductivity results, when the sample conductivity increased from 0.128 S m−1 to 0.131 S m−1 during the first hour and then it remained fairly constant until the last four hours of the experimental run. Hence, when studying concentrated suspensions of kaolin, it is important to allow the suspensions to
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Fig. 9. Variation of the electroacoustic zeta potential of a concentrated Imerys kaolin suspension in KCl (a) or NaCl (b) solution with pH. Background electrolyte concentration of 10−3 M (full squares) and 10−2 M (triangles). Results were compared with those obtained for the Imerys kaolin suspension in pure water (empty squares).
Chem. Pap. 61 (2) 83—92 (2007)
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age for several hours prior to any measurements. This observation is in good agreement with the results of Alkan et al. [33], who noted that the pH of kaolinite suspension reached equilibrium first after about six hours. Fig. 9 shows typical zeta potential curves for kaolin suspension aged for 6 h and doped with KCl and NaCl, respectively. For comparison, the curve is added representing the zeta potential variation with pH for kaolin particles dispersed in pure water. Even using different technique, IEP of the kaolin suspension could not be measured, while two extremes of the zeta potential were found at slightly lower pH values (pH 3.5) than those observed during the electrophoresis experiments (pH 4). Measurement of the suspension zeta potentials at the pH values lower than 3.25 could not be carried out as increasing volumes of acid had to be added to alter the suspension pH. The zeta potential maxima observed at low pH could be attributed to the presence of the polyacrylic acid dispersant or humic acids present in kaolinite [21]. It was found that already extremely small amounts of humic acid adsorbed on the surface of kaolin particle at pH 4 can cause charge reversal of the kaolinite edges from positive to negative. In order to exclude the humic acid interference, the experiments were repeated under the same conditions using calcined kaolin. Fig. 10 shows the comparison of zeta potentials of original and calcined kaolin powders dispersed in both 10−2 M-NaCl and KCl solutions. The magnitude of the zeta potential of untreated and calcined powders was very similar for the powders dispersed in the NaCl solutions. In the case of suspensions prepared in the KCl solution, these values varied slightly, especially at higher pH values. These differences were attributed to the batch variability. Calcination of the original kaolin sample did not remove the maximum of the suspension zeta potentials. Therefore, another explanation for this phenomenon should be found. One of the possible explanations is that the background ESA signal is growing over the course of the suspension titration and especially at the lower pH values where relatively large volumes of acid
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pH
Fig. 10. Variation of the electroacoustic zeta potential of a concentrated calcined (opened symbols) and untreated (full symbols) Imerys kaolin suspended in 10−2 MKCl (triangles) or 10−2 M-NaCl (circles) with pH.
are being added. It is then likely that the background signal is swamping the one corresponding to the particles itself. However, as the electrophoresis data show the same trends, another explanation is more probable, namely that of isomorphic substitution previously suggested by Hussain et al. [31] and Ferris and Jepson [34]. Recently, the zeta potentials of suspensions of concentrated kaolin have also been determined as a function of calcination temperatures by Waters et al. [35], showing maxima at higher values of pH. Torres-Sanchez and coworkers [36] attributed the shift of kaolin suspension IEP to higher values with respect to temperature to the formation of mullite and metakaolinite. However, it is improbable that such phase changes could occur while heating kaolin at 200 ◦C for two hours. Acknowledgements. The authors would like to express their gratitude for partial financing of this research by the Socrates/Erasmus EU exchange program and to the Ministry of Education, Youth, and Physical Training of the Czech Republic (Grants No. VZ MSM7088352101 and TP150201056/1120).
REFERENCES 1. Johnson, S. B., Dixon, D. R., and Scales, P. J., Colloids Surf., A 146, 281 (1999).
91
R. GREENWOOD, B. LAPČÍKOVÁ, M. SURÝNEK, K. WATERS, L. LAPČÍK, Jr.
2. Lapčík, L., Alince, B., and van de Ven, T. G. M., J. Pulp Pap. Sci. 21, J19 (1995). 3. Rice, B. P., Chen, C. G., Cloos, L., and Curliss, D., SAMPE J. 37, 7 (2001). 4. Chen, C. G. and Curliss, D., SAMPE J. 37, 11 (2001). 5. Jama, C. and Delobel, R., in Proceedings of ICCE-12, Tenerife, Spain, 2005. 6. Martínez-Vilari˜ no, S., Hui, D., Miller, S. G., and Daniel, L., in Proceedings of ICCE-12, Tenerife, Spain, 2005. 7. Ophir, A., Dotan, A., Dodiuk, H., Belinsly, I., and Kenig, S., in Proceedings of ICCE-12, Tenerife, Spain, 2005. 8. O’Brien, R. W., J. Fluid Mech. 212, 81 (1990). 9. O’Brien, R. W., Midmore, B. R., Lamb, A., and Hunter, R. J., Faraday Discuss. Chem. Soc. 90, 301 (1990). 10. O’Brien, R. W., J. Colloid Interface Sci. 171, 495 (1995). 11. Hunter, R. J., Colloids Surf., A 141, 37 (1998). 12. Greenwood, R., Adv. Colloid Interface Sci. 106, 55 (2003). 13. www.colloidal–dynamics.com. Application note. 14. O’Brien, R. W. and Rowlands, W. N., J. Colloid Interface Sci. 159, 471 (1993). 15. Rowlands, W. N. and O’Brien, R. W., J. Colloid Interface Sci. 175, 190 (1995). 16. Hunter, R. J. and James, M., Clays Clay Miner. 40, 644 (1992). 17. Mpofu, P., Addai-Mensah, J., and Ralston, J., J. Colloid Interface Sci. 271, 145 (2004). 18. Mpofu, P., Addai-Mensah, J., and Ralston, J., Int. J. Miner. Process. 71, 247 (2003). 19. Mpofu, P., Addai-Mensah, J., and Ralston, J., J. Colloid Interface Sci. 261, 349 (2003). 20. Angove, M. J., Wells, J. D., and Johnson, B. B., Colloids Surf., A 146, 243 (1999).
92
21. Taylor, M. L., Morris, G. E., Self, P. G., and Smart, R. S., J. Colloid Interface Sci. 250, 28 (2002). 22. Janek, M. and Lagaly, G., Colloid Polym. Sci. 281, 293 (2003). 23. Hunter, R. J. and Alexander, A. E., J. Colloid Sci. 18, 820 (1963). 24. Williams, D. J. A. and Williams, K. P., J. Colloid Interface Sci. 65, 79 (1978). 25. Kretzschmar, R., Holthoff, H., and Sandticher, H., J. Colloid Interface Sci. 202, 95 (1998). 26. Kaya, A. and Yukselen, Y., J. Hazard. Mater. 120, 119 (2005). 27. Yukselen, Y. and Kaya, A., Water, Air, Soil Pollut. 145, 155 (2003). 28. Kaya, A. and Yukselen, Y., Can. Geotech. J. 42, 1280 (2005). 29. Tekin, N., Demirbas, O., and Alkan, M., Microporous Mesoporous Mater. 85, 340 (2005). 30. Olhoeft, G. R., Tables of Room Temperature Electrical Properties for Selected Rocks and Minerals with Dielectric Permittivity Statistics, p. 24. US Geological Survey Open File Report 77-993, 1979. 31. Hussain, S. A., Demirci, S., and Ozbayoglu, G., J. Colloid Interface Sci. 184, 535 (1996). 32. Greenwood, R. and Bergstr¨ om, L., J. Eur. Ceram. Soc. 17, 537 (1997). 33. Alkan, M., Demirbas, O., and Dogan, M., Microporous Mesoporous Mater. 83, 51 (2005). 34. Ferris, A. P. and Jepson, W. B., J. Colloid Interface Sci. 51, 245 (1975). 35. Waters, K. E., Greenwood, R. W., Rowson, N. A., Lapčík, L., Jr., and Lapčíková, B., Paper No. 159E, World Congress on Particle Technology 5. Orlando, Florida, 2006. 36. Torres-Sanchez, R. M., Basaldella, E. I., and Marco, J. F., J. Colloid Interface Sci. 215, 339 (1999).
Chem. Pap. 61 (2) 83—92 (2007)
