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Correlation of electrophoretic mobility with exfoliation of montmorillonite platelets in aqueous solutions
Colloids and Surfaces A 525 (2017) 1–6
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Correlation of electrophoretic mobility with exfoliation of montmorillonite platelets in aqueous solutions Tianxing Chena,b,c, Yunliang Zhaoa,b,c, Shaoxian Songa,b,c,
MARK
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a
School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China c Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Montmorillonite Electrokinetic property Exfoliation Electrophoretic mobility distribution
In this work, the effect of the exfoliation degree of montmorillonite (MMT) platelets on their electrophoretic mobility distributions in aqueous suspensions was investigated in order to approach into the correlation of MMT exfoliation with electrokinetic property. The experimental results have shown that the negative mobility increased with the increase of exfoliation degree or with the decrease of MMT thickness. This observation might be attributed to the “exposing” of permanent layer negative charge during the exfoliation processing and the “spillover” of electrostatic potential from basal surfaces onto edges. The former would increase the permanent negative charge of the MMT platelets because of the large amounts of layer charge; the latter would decrease the positive edge charge. It is demonstrated that MMT exfoliation would change the electrokinetic property, which might be helpful of the applications of MMT in the related industries.
1. Introduction Montmorillonite (MMT), a smectite type clay mineral, has been used in many industries and has received many interests in terms of practical application due to its swelling, colloidal, rheological, and electrical properties [1–3]. It is known that a unit of this clay (a primary particle) consists of a quite thin platelet of thickness of 10 Å. The platelet consists of two kinds of sheets: two tetrahedral layers of silicon oxide between which one octahedral layer formed by aluminum, magnesium, or iron oxide is sandwiched. (Fig. 1). Its general formula is (Na)0.7(Al3.3Mg0.7) Si8O20(OH)4.nH2O. MMT has very small particle size, a high specific surface area and a cation exchange capacity values. The particles have permanent negative charges on their faces due to isomorphic substitutions which are Al3+ for Si4+ substitution in tetrahedral sites and Mg2+ for Al3+ substitution in octahedral sites. The broken bonds located at the edges of the platelet (alumina sheet) have a capacity to adsorb H+ or OH−, depending on pH value [4]. Due to these characteristics, MMT can show complex electrokinetic properties when they are dispersed in aqueous media, especially with electrolyte species. Electrokinetic properties govern the flotation, coagulation and dispersion properties in suspension systems and also identify the optimal conditions of a well dispersed system [5,6]. The first step in
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obtaining clay-based products with homogeneous structure is the preparation of stable suspensions [7]. Clay suspensions are the first step to obtain commercial products which are diverse size, shape, material composition and cost. The stability properties of clay suspensions are very important in the manufacture of various products, since the final property and formulation of product, economic aspects of the process and storage stability of product depend on these properties [8,9]. In recent years, great attention has been paid to exfoliation of MMT minerals. The delamination nature of MMT induces the production of a large amount of slimes. The presence of slimes has a negative impact on slurry rheology, with detrimental effects on both flotation and comminution [10–12]. Moreover, after exfoliation of lamellar MMT, the specific surface area and cation exchange capacity (CEC) values will increase by a substantial margin, providing the fundament of for syntheses and applications of layered silicate nanocomposites [13,14]. Despite the importance of exfoliated MMT for the mineral processing and the synthesis of successful polymer nanocomposites, very little work has been done to directly quantify of their physical characteristics. Therefore, it is necessary to investigate the property of exfoliated MMT, especially the electrokinetic property. There are some studies published in the literature related with the electrokinetic properties of MMT suspensions. Duman et al. [15]
Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected], [email protected] (S. Song).
http://dx.doi.org/10.1016/j.colsurfa.2017.04.057 Received 14 March 2017; Received in revised form 19 April 2017; Accepted 25 April 2017 Available online 02 May 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
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diffraction (XRD) image of the sample, showing that the colloidal MMT particles were very high grade and contained negligible impurities. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) for adjusting pH were from the Sinopharm Chemical Reagent Co., Ltd (China). All of them were of analytical purity. The water used in this work was produced using a Millipore Milli-Q Direct 8/16 water purification system with 18.2 MΩ. 2.2. Preparation of exfoliated MMT platelets
Fig. 1. Schematic MMT platelet structure.
The exfoliated MMT platelets were got according to a ultrasonic process rather similar to that indicated by Song et al. [19] 0.7 g NaMMT was added into 70 mL liquid medium (water) in a beaker and then mixed for 30 min with a magnetic stirrer. The supernatant was treated by a Vernon Hills Illinois CP505 ultrasonic dispersion instrument (40 kHz) at different ultrasonic intensity for 4 min separately. Then a Fluko FA25 high speed mixer was used to disperse the supernatant with 10000 r/min for 3 min. The ultrasonic and shearing treatment was done to exfoliate the MMT.
performed a series of systematic zeta potential measurements to determine the effect of various electrolyte solutions on the zeta potential of Na-bentonite. They reported that divalent cations (Cu2+, Mn2+, Ca2+, Ba2+ and Ni2+) and trivalent cation (Al3+) were potential determining cations for the Na-bentonite suspensions. Monovalent counter-cations and mono-, di- and tri-valent anions were indifferent ions for the Na-bentonite suspensions. Durán et al. [16] studied that the zeta potential of sodium MMT is based on the assumption of constant surface charge of faces and pH-dependent charge of edges. They assumed that surface properties of edges can be considered as a weighted average of that of silica and alumina, this yielding an isoelectric point of edges at pH∼7. Thomas et al. [17] determined the effect of the layer charge on the electrophoretic mobility of smectites, using electrophoresis measurements. Tsujimoto et al. [18] investigated the electrokinetic properties of MMT suspensions at different volume fractions. They noted that the acoustic zeta potential is extremely dependent on volume fraction around the volume fraction (2% and 4% volume fractions). The question remains, however, how the electrokinetic property of particles MMT behaves after exfoliation. In this study, an attempt has been made to investigate the effect of exfoliation on the electrokinetic property of MMT platelets. The objective is to establish the relationship between electrokinetic characteristics and the thickness of lamellar MMT in aqueous solutions, in order to obtain more understandings of the electrokinetic property of the exfoliated MMT platelets.
2.3. Measurements The Stokes size distribution of the MMT platelets was estimated by using centrifugal sedimentation with a Thermo Fisher Sorvall ST16 centrifuge. Sample aliquots were placed in an ultrasonic bath for 1 min before measurement. After the centrifugation, the supernatants and sediments were dried at 60 °C and then weighed. The weight of supernatant divided by the total weight was the percentage of correspondent particles. The atomic force microscope (AFM) images of MMT platelets were obtained by using a Bruker MultiMode 8 AFM with peak force tappingmode. The sample for AFM measurement was prepared by dropping MMT dispersion on a freshly cleaved mica surface. The mica substrate with MMT sample was dried at 60 °C for 2 h in an automatic thermostatic blast air oven. In order to obtain the distribution of sheet thickness, 50 sheets of each MMT sample in the AFM images were determined for the topographic height. A Malvern Zetasizer Zeta-Nano was used to determine the electrophoretic mobility of the MMT platelets in aqueous solutions. This instrument works with the technique of laser doppler electrophoresis. Then, the suspension was poured into the measuring cell of zeta meter. The temperature was kept at 25 ± 1 °C throughout the measurement. Every individual measurements can get the distribution of electrophoretic mobility. All the measurements were performed with 1 mM KCl background electrolyte concentration. The determination of the cation exchange capacity (CEC) of clays were performed by exchange with the cationic copper complexes [Cu (trien)]2+ according to the procedure described previously [20]. The 0.02 M solution of [Cu(trien)]2+ was prepared by dissolving 0.02 mol of triethylenetetramine (2.926 g) and 0.02 mol of CuSO4 (3.192 g) in water and filling up to 1 L (pH = 8.4). For the CEC determination, 4 mL MMT suspension (25 g/L) were added in 25 mL centrifugal tubes and 4 mL of the 0.02 M complex solution [Cu(trien)]2+ were added. The samples were shaken for at least 30 min and then centrifuged at a relative centrifugal field of 3000g for 10 min 3 mL of the supernatant were transferred into cuvettes, and the absorption was measured at 577 nm for [Cu(trien)]2+ using calibration curves. The amount of copper complex adsorbed was calculated from the concentration differences. Every determination was carried out with at least three parallel samples. The titration curves were recorded by using a computer-controlled titrator (907 Titrino, Metrohm) and a low-temperature thermostat bath to maintain the constant temperature. All titration were conducted under nitrogen atmosphere on 50 mL clay suspensions and at a constant temperature of 25 °C. The MMT suspension (50 mL, 4 g/L) diluted in
2. Experimental 2.1. Materials The original MMT used in the present study was obtained from Sanding Technology Co., Ltd, Zhejiang province, China. A common method for obtaining purified colloidal MMT is fractionation by sedimentation after removal of carbonates, oxides, and organic materials and smashed by the ultrasonic grinder. Fig. 2 gave the X-ray
Fig. 2. XRD trace of the MMT.
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the corresponding KCl electrolyte concentration (1 mM) was placed in a closed Teflon vessel. Hence, the titration curves on the whole pH domain were carried out from two sets of experiment starting both from the initial pH of the suspension up to acid or basic domain. The time interval between successive 10-μL increments of acid (0.1 mol/L HCl) or base (0.1 mol/L NaOH) was fixed at 10 min to avoid significant contribution of dissolution processes. The raw titration curves are represented as pH vs the concentration of added titrant(in mol L−1), the acid being set positive and the base negative (Ca–Cb). 3. Results and discussions 3.1. Degree of exfoliation of MMT Exfoliation was defined as the decomposition of large aggregates (booklets) into smaller particles. Exfoliated MMT platelets are formed by the complete separation of the individual layers of the MMT mineral, resulting in a disordered dispersion of the individual MMT platelet. The platelet size of exfoliated MMT can often be measured by dynamic light scattering (DLS) or transmission electron microscopy (TEM) in a suitable solvent. But DLS provides a fast, convenient means of measuring an “effective spherical” particle diameter, its relationship to platelet aspect ratio is not straightforward. TEM permits direct visualization of the lateral dimensions of platelets deposited on a surface; however, we still cannot discern whether platelets are fully exfoliated from TEM images. In this study, we provide a rigorous analysis on the degree of exfoliation of MMT through the Stokes size analysis and atomic force microscopy (AFM) technique. Optical size is derived from light scattering of particles and depended on the scattered area of the particles, while Stokes size is derived from sedimentation and depended on the mass of the particle. In the case of plate-like particles, the Stokes size of exfoliated MMT particles might be more appropriate. Fig. 3 illustrated the Stokes size distribution of exfoliated MMT in presence of different ultrasonic intensity. It shows that the particles size of exfoliated MMT decreased monotonously as the increase of the ultrasonic intensity. At low ultrasonic intensity a large particle size system is observed, which would indicate the degree of exfoliation of MMT is low. Fig. 4a–d shows the representative AFM images with deposition of the as-prepared exfoliated MMT. For every particle, we obtain the average vertical dimension, or mean height, of that individual particle, relative to the mean height of the mica background. It was noteworthy that most sheets in each image had different color contrast, indicating that they had different thickness. The typical thickness was about 1.9 nm for exfoliated MMT at 60% ultrasonic intensity, 7.3 nm for exfoliated MMT at 40% ultrasonic intensity, 14 nm for exfoliated MMT
Fig. 4. Typical tapping mode AFM images (a: No ultrasonic intensity, b:20% ultrasonic intensity, c:40% ultrasonic intensity, d: 60% ultrasonic intensity) of MMT suspensions and thickness cumulative distribution (e) of MMT platelets as a function of ultrasonic intensity.
at 20% ultrasonic intensity and 19 nm for unexfoliated MMT, which represent the different degree of exfoliation. The platelets are wellseparated on the mica surface, so we must use a large scan area (7.5 μm × 7.5 μm) and gather many images to have enough platelets for an accurate measure of the distribution. As shown in Fig. 4e, the thickness of exfoliated MMT less than 2 nm (approximately 1–2layers) was 43% for 60% ultrasonic intensity but only 1% for unexfoliated MMT, suggesting that the degree of exfoliation of MMT increase with increasing ultrasonic intensity. This phenomenon also corresponded well with the results for the Stokes size distribution. So we use the Stokes mean particle size (D50) to express the exfoliation degree in the following article. 3.2. Electrokinetic property of exfoliated MMT Electrokinetic measurements, especially the electrophoresis, often give some information about the surface charge of clays. Electrokinetic methods yield the potential of the diffuse layer, the ζ potential. According to the Stern model of the double layer, the ζ potential corresponds to the part of the surface potential which is not neutralized by the counterions adsorbed in the Stern plane. The electrophoretic mobility distributions of the exfoliated MMT as a function of mean particle size were measured below in a 1 mM KCl solution at pH 3. The typical observations were photographed as shown in Fig. 6. It shows that the value of electrophoretic mobility of exfoliated MMT platelets decreased monotonously as the decrease of mean particle size. The electrophoretic mobility peak of the unexfoliated MMT was located at
Fig. 3. Stokes size distribution of MMT particles as a function of ultrasonic intensity.
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platelets, the potential of the diffuse layer values (ζ potential) will decrease by a substantial margin. Therefore, a significant relationship between electrokinetic property and exfoliation for MMT platelets is likely to confirm by the electrophoretic mobility distributions measurements. According to the Stern model of the double layer, the ζ potential corresponds to the part of the surface potential which is not neutralized by the counterions adsorbed in the Stern plane. And the origin of the electrical charge of montmorillonite is of two kinds: on one hand substitutions in the crystalline network, and on the other hand broken-bond atoms at the edge surfaces. So the relationship between electrokinetic property and exfoliation for MMT platelets is attributed to two aspects: (1) the “exposing” of permanent layer negative charge during the exfoliation processing, and (2) the “spillover” of electrostatic potential from basal onto edge surfaces. 3.2.1. “exposing” of permanent layer negative charge Charge-imbalanced substitutions in the crystalline network, e.g., Al3+ for Si4+ in tetrahedral coordination, or Mg2+ for Al3+ in octahedral coordination, result in positive charge defects, i.e., net negative charges, the amount of which is directly related to the amount of substitutions. The location and amount of substitutions are the key to the mineralogical classification of the 2:1 phyllosilicates. The structural charges of clays are called permanent layer charges because they are independent of the physicochemical conditions in the surrounding medium. Although the structural charges are not in direct contact with the solution, they give rise to a negative electric potential, which is compensated for by an electrostatically attracted interlayer of exchangeable cations. Their valency and hydration properties control the swelling (or lattice expansion) and colloidal behaviour of these clays. Among the numerous electrokinetic measurements on MMT platelets, the electrokinetic potential is dominated by the permanent layer charge, which is by nature independent from the external conditions [22–24]. Fig. 7 shows the sketch map of exfoliation of MMT platelets in aqueous dispersions. It suggested that more permanent layer negative charge will expose after the exfoliation, making the value of electrophoretic mobility of exfoliated MMT platelets decreased. To confirm the “exposing” of permanent layer negative charge, the cation exchange capacity (CEC) measurements were performed at pH 4.5 for illustrative purpose. Christidis et al. [25] suggests the measurement of the CEC at the isoelectric point of the edges in order to minimize the pH-dependent charge and to measure only the permanent charge. The CEC values of the exfoliated MMT platelets as a function of mean particle size are showed in Table 1. It can be seen from these results that after exfoliation of lamellar MMT platelets, the CEC value of MMT will increase by a substantial margin. This phenomenon corresponded well with the interpretation that more permanent layer negative charge will expose after the exfoliation. This phenomenon will change the effects on slurry rheology and the energy and entropy of polymer molecules located in the “interphase” near the platelet surface in the polymer nanocomposites.
Fig. 5. Electrophoretic mobility distribution of MMT platelets as a function of mean particle size in 1 mM KCl solution at pH 3.
Fig. 6. Electrophoretic mobility distribution of MMT particles as a function of mean particle size in 1 mM KCl solution at pH 10.
−1.3 μm cm/V s, but the exfoliated MMT (D50 = 0.06 μm) was located at −1.75 μm cm/V s. The electrophoretic mobility result was similar with other reports that the electrophoretic mobility of smectites ranges from −3 to −0.4 μm cm/V s at acidic pH [17,21]. In other words, the value of electrophoretic mobility of exfoliated MMT decreased as the increase of the degree of exfoliation (Fig. 5). At pH 9 in Fig. 7, the electrophoretic mobility distributions of the exfoliated MMT platelets as a function of mean particle size show the same result. The electrophoretic mobility peak of the unexfoliated MMT platelets was located at −2.6 μm cm/V s, but the exfoliated MMT platelets (D50 = 0.06 μm) was located at −3.6 μm cm/V s. It can be seen from these experiments that after exfoliation of lamellar MMT
3.2.2. “spillover” of electrostatic potential Experimental determination of the electric potential of the diffuse layer of counterions of clays is usually based on electrophoretic mobility, and potentiometric titration which give access to the dissociable charge. For MMT, the contribution of the edge surface charge to the overall surface charge is a priori negligible, since the edges represent 5–10% of the total surface area because of the strong anisotropy of the particles. The edge surface charge of clays is experimentally determined by acid–base potentiometric titration, after the methodology developed in early works [26,27]. Proton adsorption on MMT occurs mainly at oxide-type reactive sites on the edge surfaces through equilibrium reactions of the form [28] (if^S represents the structural cation(s) to which a surface O atom is attached and edge site
Fig. 7. Exfoliation of MMT platelets in aqueous dispersions.
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Table 1 CEC values of MMT particles as a function of mean particle at pH 4.5.
CEC(cmol/kg)
D50 = 0.25 μm
D50 = 0.20 μm
D50 = 0.14 μm
D50 = 0.06 μm
92.8
97.2
101.3
105.6
On one hand, the permanent layer negative charge will exposing during the exfoliation processing and increase the permanent negative charge of the MMT platelets. On the other hand, the “spillover” of electrostatic potential from basal onto edge surfaces will decrease the positive edge charge. It is demonstrated that the electrokinetic property of exfoliated MMT platelets is different with the unexfoliated MMT, which provides a fundamental theoretical guidance in industry. Acknowledgements The financial support for this work from the National Natural Science Foundation of China under the project Nos. 51474167 and 51674183 is gratefully acknowledged. References [1] C.C. Wang, L.C. Juang, C.K. Lee, T.C. Hsu, J.F. Lee, H.P. Chao, Effects of cation exchange on the pore and surface structure and adsorption characteristics of montmorillonite, J. Colloid Interface Sci. 280 (2004) 27–35. [2] Q. Chen, R. Zhu, W. Deng, Y. Xu, J. Zhu, Q. Tao, et al., From used montmorillonite to carbon monolayer?montmorillonite nanocomposites, Appl. Clay Sci. 100 (2014) 112–117. [3] Y. Zhao, J. Wang, Y. Du, C. Pan, W. Zhao, Enhanced application properties of EPDM/Montmorillonite composites, Polym. Polym. Compos. 22 (2014) 799–808. [4] E.J. Teh, Y.K. Leong, Y. Liu, A.B. Fourie, M. Fahey, Differences in the rheology and surface chemistry of kaolin clay slurries: the source of the variations, Chem. Eng. Sci. 64 (2009) 3817–3825. [5] S. Tunç, O. Duman, Effects of electrolytes on the electrokinetic properties of pumice suspensions, J. Dispers. Sci. Technol. 30 (2009) 548–555. [6] O. Duman, S. Tunç, A. Çetinkaya, Electrokinetic and rheological properties of kaolinite in poly(diallyldimethylammonium chloride), poly(sodium 4-styrene sulfonate) and poly(vinyl alcohol) solutions, Colloids Surf. A Physicochem. Eng. Asp. 394 (2012) 23–32. [7] S. Tunç, O. Duman, B. Kancı, Rheological measurements of Na-bentonite and sepiolite particles in the presence of tetradecyltrimethylammonium bromide, sodium tetradecyl sulfonate and Brij 30 surfactants, Colloids Surf. A Physicochem. Eng. Asp. 398 (2012) 37–47. [8] S. Tunç, O. Duman, R. Uysal, Electrokinetic and rheological behaviors of sepiolite suspensions in the presence of poly(acrylic acid sodium salt)s polyacrylamides, and poly(ethylene glycol)s of different molecular weights, J. Appl. Polym. Sci. 109 (2008) 1850–1860. [9] S. Tunç, O. Duman, A. Çetinkaya, Electrokinetic and rheological properties of sepiolite suspensions in the presence of hexadecyltrimethylammonium bromide, Colloids Surf. A Physicochem. Eng. Asp. 377 (2011) 123–129. [10] E. Forbes, K.J. Davey, L. Smith, Decoupling rehology and slime coatings effect on the natural flotability of chalcopyrite in a clay-rich flotation pulp, Miner. Eng. 56 (2014) 136–144. [11] Y. Wang, Y. Peng, T. Nicholson, R.A. Lauten, The different effects of bentonite and kaolin on copper flotation, Appl. Clay Sci. 114 (2015) 48–52. [12] L. Huynh, A. Feiler, A. Michelmore, J. Ralston, P. Jenkins, Control of slime coatings by the use of anionic phosphates: a fundamental study, Miner. Eng. 13 (2000) 1059–1069. [13] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Prog. Polym. Sci. 28 (2003) 1539–1641. [14] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng. R Rep. 28 (2000) 1–63. [15] O. Duman, S. Tunç, Electrokinetic and rheological properties of Na-bentonite in some electrolyte solutions, Microporous Mesoporous Mat. 117 (2009) 331–338. [16] J. Durán, M. Ramos-Tejada, F. Arroyo, F. González-Caballero, Rheological and electrokinetic properties of sodium montmorillonite suspensions, J. Colloid Interface Sci. 229 (2000) 107–117. [17] F. Thomas, L.J. Michot, D. Vantelon, E. Montargès, B. Prélot, M. Cruchaudet, et al., Layer charge and electrophoretic mobility of smectites, Colloids Surf. A Physicochem. Eng. Asp. 159 (1999) 351–358. [18] Y. Tsujimoto, C. Chassagne, Y. Adachi, Comparison between the electrokinetic properties of kaolinite and montmorillonite suspensions at different volume fractions, J. Colloid Interface Sci. 407 (2013) 109–115. [19] Y. Zhao, H. Yi, F. Jia, H. Li, C. Peng, S. Song, A novel method for determining the thickness of hydration shells on nanosheets: a case of montmorillonite in water, Powder Technol. 306 (2016) 74–79. [20] L. Ammann, F. Bergaya, G. Lagaly, Determination of the cation exchange capacity
Fig. 8. Effect of the mean particle size on the continuous titration curves of MMT suspension (4 g/L) in 1 mmol/L KCl.
valence and additional surface protons are omitted for convenience): ^SeOH ↔ ^SeO + H+ At low pH and ionic strength, proton uptake by MMT also occurs by cation exchange on structural charge sites (^X−) [29]: ^XNa + H+ ↔ ^XH + Na+ The titration curves recorded in continuous mode at different ultrasonic intensity (Fig. 8) show that the increase of mean particle size increases the net proton/hydroxide consumptions (Ca-Cb) especially in basic domain. Indeed, above pH 8 the titration curves reach steep slope and the consumption of unexfoliated MMT at pH 10 is 1.5 times higher than the exfoliated MMT platelets (D50 = 0.06 μm). It may be due to that the “spillover” of electrostatic potential from basal onto edge surfaces will strengthen with the increasing of the degree of exfoliation. For disc-shaped clay particles, charge densities on their basal and edge surfaces are unequal and edge thickness is small relative to the Debye length, the negative double layer extending from the particle faces may spillover into the edge region, and possibly overpower the positive edge charge that contribute to the consumption of H+ and OH− ions. The spillover effect was determined by the experimental data on anion exclusion [30] and edge-to-face particle aggregation [31] in MMT suspensions. In the opposite, this trend is significantly attenuated in the acid domain below pH 5. It has been well accepted that proton adsorption on MMT occurs mainly at oxide-type reactive sites on the edge surfaces at alkaline pH. After exfoliation of lamellar MMT, the edge thickness will decrease and more negative charge possibly will overpower the positive edge charge. Based on the above-noted concept, it is clear that the electrokinetic property of layered silicate clays such as MMT will change after exfoliation.
4. Conclusions The experimental results from this study have shown that there is a close relationship between the exfoliation and the electrokinetic property of MMT platelets in aqueous suspensions. The overall charge decreased significantly with the increasing of the degree of exfoliation. 5
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[26] G.H. Bolt, Determination of the charge density of silica sols, J. Phys. Chem. 61 (1957) 1166–1169. [27] G.A. Parks, P.L. De Bruyn, The zero point of charge of oxides, J. Phys. Chem. 66 (1962) 967–973. [28] B.H. Wanner, Y. Albinsson, O. Karnland, E. Wieland, P. Wersin, L. Charlet, The acid/base chemistry of montmorillonite, Radiochim. Acta 66/67 (1994) 157–162. [29] M.J. Avena, C.P. De Pauli, Proton adsorption and electrokinetics of an argentinean montmorillonite, J. Colloid Interface Sci. 202 (1998) 195–204. [30] D.G. Edwards, A.M. Posner, J.P. Quirk, Repulsion of chloride ions by negatively charged clay surfaces. Part 2.—monovalent cation montmorillonites, Trans. Faraday Soc. 61 (1965) 2816–2819. [31] E. Tombácz, M. Szekeres, Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes, Appl. Clay Sci. 27 (2004) 75–94.
of clays with copper complexes revisited, Clay Miner. 40 (2005) 441–453. [21] E.E. Saka, C. Güler, The effects of electrolyte concentration, ion species and pH on the zeta potential and electrokinetic charge density of montmorillonite, Clay Miner. 41 (2006) 853–861. [22] A. Delgado, F. González-Caballero, J.M. Bruque, On the zeta potential and surface charge density of montmorillonite in aqueous electrolyte solutions, J. Colloid Interface Sci. 113 (1986) 203–211. [23] I. Sondi, J. Biscan, V. Pravdic, Electrokinetics of pure clay minerals revisited, J. Colloid Interface Sci. 178 (1996) 514–522. [24] S. Miller, P. Low, Characterization of the electrical double layer of montmorillonite, Langmuir (1990) 572–578. [25] G.E. Christidis, Validity of the structural formula method for layer charge determination of smectites: a re-evaluation of published data, Appl. Clay Sci. 42 (2008) 1–7.
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Correlation of electrophoretic mobility with exfoliation of montmorillonite platelets in aqueous solutions Tianxing Chena,b,c, Yunliang Zhaoa,b,c, Shaoxian Songa,b,c,
MARK
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a
School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China c Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Montmorillonite Electrokinetic property Exfoliation Electrophoretic mobility distribution
In this work, the effect of the exfoliation degree of montmorillonite (MMT) platelets on their electrophoretic mobility distributions in aqueous suspensions was investigated in order to approach into the correlation of MMT exfoliation with electrokinetic property. The experimental results have shown that the negative mobility increased with the increase of exfoliation degree or with the decrease of MMT thickness. This observation might be attributed to the “exposing” of permanent layer negative charge during the exfoliation processing and the “spillover” of electrostatic potential from basal surfaces onto edges. The former would increase the permanent negative charge of the MMT platelets because of the large amounts of layer charge; the latter would decrease the positive edge charge. It is demonstrated that MMT exfoliation would change the electrokinetic property, which might be helpful of the applications of MMT in the related industries.
1. Introduction Montmorillonite (MMT), a smectite type clay mineral, has been used in many industries and has received many interests in terms of practical application due to its swelling, colloidal, rheological, and electrical properties [1–3]. It is known that a unit of this clay (a primary particle) consists of a quite thin platelet of thickness of 10 Å. The platelet consists of two kinds of sheets: two tetrahedral layers of silicon oxide between which one octahedral layer formed by aluminum, magnesium, or iron oxide is sandwiched. (Fig. 1). Its general formula is (Na)0.7(Al3.3Mg0.7) Si8O20(OH)4.nH2O. MMT has very small particle size, a high specific surface area and a cation exchange capacity values. The particles have permanent negative charges on their faces due to isomorphic substitutions which are Al3+ for Si4+ substitution in tetrahedral sites and Mg2+ for Al3+ substitution in octahedral sites. The broken bonds located at the edges of the platelet (alumina sheet) have a capacity to adsorb H+ or OH−, depending on pH value [4]. Due to these characteristics, MMT can show complex electrokinetic properties when they are dispersed in aqueous media, especially with electrolyte species. Electrokinetic properties govern the flotation, coagulation and dispersion properties in suspension systems and also identify the optimal conditions of a well dispersed system [5,6]. The first step in
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obtaining clay-based products with homogeneous structure is the preparation of stable suspensions [7]. Clay suspensions are the first step to obtain commercial products which are diverse size, shape, material composition and cost. The stability properties of clay suspensions are very important in the manufacture of various products, since the final property and formulation of product, economic aspects of the process and storage stability of product depend on these properties [8,9]. In recent years, great attention has been paid to exfoliation of MMT minerals. The delamination nature of MMT induces the production of a large amount of slimes. The presence of slimes has a negative impact on slurry rheology, with detrimental effects on both flotation and comminution [10–12]. Moreover, after exfoliation of lamellar MMT, the specific surface area and cation exchange capacity (CEC) values will increase by a substantial margin, providing the fundament of for syntheses and applications of layered silicate nanocomposites [13,14]. Despite the importance of exfoliated MMT for the mineral processing and the synthesis of successful polymer nanocomposites, very little work has been done to directly quantify of their physical characteristics. Therefore, it is necessary to investigate the property of exfoliated MMT, especially the electrokinetic property. There are some studies published in the literature related with the electrokinetic properties of MMT suspensions. Duman et al. [15]
Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected], [email protected] (S. Song).
http://dx.doi.org/10.1016/j.colsurfa.2017.04.057 Received 14 March 2017; Received in revised form 19 April 2017; Accepted 25 April 2017 Available online 02 May 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
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diffraction (XRD) image of the sample, showing that the colloidal MMT particles were very high grade and contained negligible impurities. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) for adjusting pH were from the Sinopharm Chemical Reagent Co., Ltd (China). All of them were of analytical purity. The water used in this work was produced using a Millipore Milli-Q Direct 8/16 water purification system with 18.2 MΩ. 2.2. Preparation of exfoliated MMT platelets
Fig. 1. Schematic MMT platelet structure.
The exfoliated MMT platelets were got according to a ultrasonic process rather similar to that indicated by Song et al. [19] 0.7 g NaMMT was added into 70 mL liquid medium (water) in a beaker and then mixed for 30 min with a magnetic stirrer. The supernatant was treated by a Vernon Hills Illinois CP505 ultrasonic dispersion instrument (40 kHz) at different ultrasonic intensity for 4 min separately. Then a Fluko FA25 high speed mixer was used to disperse the supernatant with 10000 r/min for 3 min. The ultrasonic and shearing treatment was done to exfoliate the MMT.
performed a series of systematic zeta potential measurements to determine the effect of various electrolyte solutions on the zeta potential of Na-bentonite. They reported that divalent cations (Cu2+, Mn2+, Ca2+, Ba2+ and Ni2+) and trivalent cation (Al3+) were potential determining cations for the Na-bentonite suspensions. Monovalent counter-cations and mono-, di- and tri-valent anions were indifferent ions for the Na-bentonite suspensions. Durán et al. [16] studied that the zeta potential of sodium MMT is based on the assumption of constant surface charge of faces and pH-dependent charge of edges. They assumed that surface properties of edges can be considered as a weighted average of that of silica and alumina, this yielding an isoelectric point of edges at pH∼7. Thomas et al. [17] determined the effect of the layer charge on the electrophoretic mobility of smectites, using electrophoresis measurements. Tsujimoto et al. [18] investigated the electrokinetic properties of MMT suspensions at different volume fractions. They noted that the acoustic zeta potential is extremely dependent on volume fraction around the volume fraction (2% and 4% volume fractions). The question remains, however, how the electrokinetic property of particles MMT behaves after exfoliation. In this study, an attempt has been made to investigate the effect of exfoliation on the electrokinetic property of MMT platelets. The objective is to establish the relationship between electrokinetic characteristics and the thickness of lamellar MMT in aqueous solutions, in order to obtain more understandings of the electrokinetic property of the exfoliated MMT platelets.
2.3. Measurements The Stokes size distribution of the MMT platelets was estimated by using centrifugal sedimentation with a Thermo Fisher Sorvall ST16 centrifuge. Sample aliquots were placed in an ultrasonic bath for 1 min before measurement. After the centrifugation, the supernatants and sediments were dried at 60 °C and then weighed. The weight of supernatant divided by the total weight was the percentage of correspondent particles. The atomic force microscope (AFM) images of MMT platelets were obtained by using a Bruker MultiMode 8 AFM with peak force tappingmode. The sample for AFM measurement was prepared by dropping MMT dispersion on a freshly cleaved mica surface. The mica substrate with MMT sample was dried at 60 °C for 2 h in an automatic thermostatic blast air oven. In order to obtain the distribution of sheet thickness, 50 sheets of each MMT sample in the AFM images were determined for the topographic height. A Malvern Zetasizer Zeta-Nano was used to determine the electrophoretic mobility of the MMT platelets in aqueous solutions. This instrument works with the technique of laser doppler electrophoresis. Then, the suspension was poured into the measuring cell of zeta meter. The temperature was kept at 25 ± 1 °C throughout the measurement. Every individual measurements can get the distribution of electrophoretic mobility. All the measurements were performed with 1 mM KCl background electrolyte concentration. The determination of the cation exchange capacity (CEC) of clays were performed by exchange with the cationic copper complexes [Cu (trien)]2+ according to the procedure described previously [20]. The 0.02 M solution of [Cu(trien)]2+ was prepared by dissolving 0.02 mol of triethylenetetramine (2.926 g) and 0.02 mol of CuSO4 (3.192 g) in water and filling up to 1 L (pH = 8.4). For the CEC determination, 4 mL MMT suspension (25 g/L) were added in 25 mL centrifugal tubes and 4 mL of the 0.02 M complex solution [Cu(trien)]2+ were added. The samples were shaken for at least 30 min and then centrifuged at a relative centrifugal field of 3000g for 10 min 3 mL of the supernatant were transferred into cuvettes, and the absorption was measured at 577 nm for [Cu(trien)]2+ using calibration curves. The amount of copper complex adsorbed was calculated from the concentration differences. Every determination was carried out with at least three parallel samples. The titration curves were recorded by using a computer-controlled titrator (907 Titrino, Metrohm) and a low-temperature thermostat bath to maintain the constant temperature. All titration were conducted under nitrogen atmosphere on 50 mL clay suspensions and at a constant temperature of 25 °C. The MMT suspension (50 mL, 4 g/L) diluted in
2. Experimental 2.1. Materials The original MMT used in the present study was obtained from Sanding Technology Co., Ltd, Zhejiang province, China. A common method for obtaining purified colloidal MMT is fractionation by sedimentation after removal of carbonates, oxides, and organic materials and smashed by the ultrasonic grinder. Fig. 2 gave the X-ray
Fig. 2. XRD trace of the MMT.
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the corresponding KCl electrolyte concentration (1 mM) was placed in a closed Teflon vessel. Hence, the titration curves on the whole pH domain were carried out from two sets of experiment starting both from the initial pH of the suspension up to acid or basic domain. The time interval between successive 10-μL increments of acid (0.1 mol/L HCl) or base (0.1 mol/L NaOH) was fixed at 10 min to avoid significant contribution of dissolution processes. The raw titration curves are represented as pH vs the concentration of added titrant(in mol L−1), the acid being set positive and the base negative (Ca–Cb). 3. Results and discussions 3.1. Degree of exfoliation of MMT Exfoliation was defined as the decomposition of large aggregates (booklets) into smaller particles. Exfoliated MMT platelets are formed by the complete separation of the individual layers of the MMT mineral, resulting in a disordered dispersion of the individual MMT platelet. The platelet size of exfoliated MMT can often be measured by dynamic light scattering (DLS) or transmission electron microscopy (TEM) in a suitable solvent. But DLS provides a fast, convenient means of measuring an “effective spherical” particle diameter, its relationship to platelet aspect ratio is not straightforward. TEM permits direct visualization of the lateral dimensions of platelets deposited on a surface; however, we still cannot discern whether platelets are fully exfoliated from TEM images. In this study, we provide a rigorous analysis on the degree of exfoliation of MMT through the Stokes size analysis and atomic force microscopy (AFM) technique. Optical size is derived from light scattering of particles and depended on the scattered area of the particles, while Stokes size is derived from sedimentation and depended on the mass of the particle. In the case of plate-like particles, the Stokes size of exfoliated MMT particles might be more appropriate. Fig. 3 illustrated the Stokes size distribution of exfoliated MMT in presence of different ultrasonic intensity. It shows that the particles size of exfoliated MMT decreased monotonously as the increase of the ultrasonic intensity. At low ultrasonic intensity a large particle size system is observed, which would indicate the degree of exfoliation of MMT is low. Fig. 4a–d shows the representative AFM images with deposition of the as-prepared exfoliated MMT. For every particle, we obtain the average vertical dimension, or mean height, of that individual particle, relative to the mean height of the mica background. It was noteworthy that most sheets in each image had different color contrast, indicating that they had different thickness. The typical thickness was about 1.9 nm for exfoliated MMT at 60% ultrasonic intensity, 7.3 nm for exfoliated MMT at 40% ultrasonic intensity, 14 nm for exfoliated MMT
Fig. 4. Typical tapping mode AFM images (a: No ultrasonic intensity, b:20% ultrasonic intensity, c:40% ultrasonic intensity, d: 60% ultrasonic intensity) of MMT suspensions and thickness cumulative distribution (e) of MMT platelets as a function of ultrasonic intensity.
at 20% ultrasonic intensity and 19 nm for unexfoliated MMT, which represent the different degree of exfoliation. The platelets are wellseparated on the mica surface, so we must use a large scan area (7.5 μm × 7.5 μm) and gather many images to have enough platelets for an accurate measure of the distribution. As shown in Fig. 4e, the thickness of exfoliated MMT less than 2 nm (approximately 1–2layers) was 43% for 60% ultrasonic intensity but only 1% for unexfoliated MMT, suggesting that the degree of exfoliation of MMT increase with increasing ultrasonic intensity. This phenomenon also corresponded well with the results for the Stokes size distribution. So we use the Stokes mean particle size (D50) to express the exfoliation degree in the following article. 3.2. Electrokinetic property of exfoliated MMT Electrokinetic measurements, especially the electrophoresis, often give some information about the surface charge of clays. Electrokinetic methods yield the potential of the diffuse layer, the ζ potential. According to the Stern model of the double layer, the ζ potential corresponds to the part of the surface potential which is not neutralized by the counterions adsorbed in the Stern plane. The electrophoretic mobility distributions of the exfoliated MMT as a function of mean particle size were measured below in a 1 mM KCl solution at pH 3. The typical observations were photographed as shown in Fig. 6. It shows that the value of electrophoretic mobility of exfoliated MMT platelets decreased monotonously as the decrease of mean particle size. The electrophoretic mobility peak of the unexfoliated MMT was located at
Fig. 3. Stokes size distribution of MMT particles as a function of ultrasonic intensity.
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platelets, the potential of the diffuse layer values (ζ potential) will decrease by a substantial margin. Therefore, a significant relationship between electrokinetic property and exfoliation for MMT platelets is likely to confirm by the electrophoretic mobility distributions measurements. According to the Stern model of the double layer, the ζ potential corresponds to the part of the surface potential which is not neutralized by the counterions adsorbed in the Stern plane. And the origin of the electrical charge of montmorillonite is of two kinds: on one hand substitutions in the crystalline network, and on the other hand broken-bond atoms at the edge surfaces. So the relationship between electrokinetic property and exfoliation for MMT platelets is attributed to two aspects: (1) the “exposing” of permanent layer negative charge during the exfoliation processing, and (2) the “spillover” of electrostatic potential from basal onto edge surfaces. 3.2.1. “exposing” of permanent layer negative charge Charge-imbalanced substitutions in the crystalline network, e.g., Al3+ for Si4+ in tetrahedral coordination, or Mg2+ for Al3+ in octahedral coordination, result in positive charge defects, i.e., net negative charges, the amount of which is directly related to the amount of substitutions. The location and amount of substitutions are the key to the mineralogical classification of the 2:1 phyllosilicates. The structural charges of clays are called permanent layer charges because they are independent of the physicochemical conditions in the surrounding medium. Although the structural charges are not in direct contact with the solution, they give rise to a negative electric potential, which is compensated for by an electrostatically attracted interlayer of exchangeable cations. Their valency and hydration properties control the swelling (or lattice expansion) and colloidal behaviour of these clays. Among the numerous electrokinetic measurements on MMT platelets, the electrokinetic potential is dominated by the permanent layer charge, which is by nature independent from the external conditions [22–24]. Fig. 7 shows the sketch map of exfoliation of MMT platelets in aqueous dispersions. It suggested that more permanent layer negative charge will expose after the exfoliation, making the value of electrophoretic mobility of exfoliated MMT platelets decreased. To confirm the “exposing” of permanent layer negative charge, the cation exchange capacity (CEC) measurements were performed at pH 4.5 for illustrative purpose. Christidis et al. [25] suggests the measurement of the CEC at the isoelectric point of the edges in order to minimize the pH-dependent charge and to measure only the permanent charge. The CEC values of the exfoliated MMT platelets as a function of mean particle size are showed in Table 1. It can be seen from these results that after exfoliation of lamellar MMT platelets, the CEC value of MMT will increase by a substantial margin. This phenomenon corresponded well with the interpretation that more permanent layer negative charge will expose after the exfoliation. This phenomenon will change the effects on slurry rheology and the energy and entropy of polymer molecules located in the “interphase” near the platelet surface in the polymer nanocomposites.
Fig. 5. Electrophoretic mobility distribution of MMT platelets as a function of mean particle size in 1 mM KCl solution at pH 3.
Fig. 6. Electrophoretic mobility distribution of MMT particles as a function of mean particle size in 1 mM KCl solution at pH 10.
−1.3 μm cm/V s, but the exfoliated MMT (D50 = 0.06 μm) was located at −1.75 μm cm/V s. The electrophoretic mobility result was similar with other reports that the electrophoretic mobility of smectites ranges from −3 to −0.4 μm cm/V s at acidic pH [17,21]. In other words, the value of electrophoretic mobility of exfoliated MMT decreased as the increase of the degree of exfoliation (Fig. 5). At pH 9 in Fig. 7, the electrophoretic mobility distributions of the exfoliated MMT platelets as a function of mean particle size show the same result. The electrophoretic mobility peak of the unexfoliated MMT platelets was located at −2.6 μm cm/V s, but the exfoliated MMT platelets (D50 = 0.06 μm) was located at −3.6 μm cm/V s. It can be seen from these experiments that after exfoliation of lamellar MMT
3.2.2. “spillover” of electrostatic potential Experimental determination of the electric potential of the diffuse layer of counterions of clays is usually based on electrophoretic mobility, and potentiometric titration which give access to the dissociable charge. For MMT, the contribution of the edge surface charge to the overall surface charge is a priori negligible, since the edges represent 5–10% of the total surface area because of the strong anisotropy of the particles. The edge surface charge of clays is experimentally determined by acid–base potentiometric titration, after the methodology developed in early works [26,27]. Proton adsorption on MMT occurs mainly at oxide-type reactive sites on the edge surfaces through equilibrium reactions of the form [28] (if^S represents the structural cation(s) to which a surface O atom is attached and edge site
Fig. 7. Exfoliation of MMT platelets in aqueous dispersions.
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Table 1 CEC values of MMT particles as a function of mean particle at pH 4.5.
CEC(cmol/kg)
D50 = 0.25 μm
D50 = 0.20 μm
D50 = 0.14 μm
D50 = 0.06 μm
92.8
97.2
101.3
105.6
On one hand, the permanent layer negative charge will exposing during the exfoliation processing and increase the permanent negative charge of the MMT platelets. On the other hand, the “spillover” of electrostatic potential from basal onto edge surfaces will decrease the positive edge charge. It is demonstrated that the electrokinetic property of exfoliated MMT platelets is different with the unexfoliated MMT, which provides a fundamental theoretical guidance in industry. Acknowledgements The financial support for this work from the National Natural Science Foundation of China under the project Nos. 51474167 and 51674183 is gratefully acknowledged. References [1] C.C. Wang, L.C. Juang, C.K. Lee, T.C. Hsu, J.F. Lee, H.P. Chao, Effects of cation exchange on the pore and surface structure and adsorption characteristics of montmorillonite, J. Colloid Interface Sci. 280 (2004) 27–35. [2] Q. Chen, R. Zhu, W. Deng, Y. Xu, J. Zhu, Q. Tao, et al., From used montmorillonite to carbon monolayer?montmorillonite nanocomposites, Appl. Clay Sci. 100 (2014) 112–117. [3] Y. Zhao, J. Wang, Y. Du, C. Pan, W. Zhao, Enhanced application properties of EPDM/Montmorillonite composites, Polym. Polym. Compos. 22 (2014) 799–808. [4] E.J. Teh, Y.K. Leong, Y. Liu, A.B. Fourie, M. Fahey, Differences in the rheology and surface chemistry of kaolin clay slurries: the source of the variations, Chem. Eng. Sci. 64 (2009) 3817–3825. [5] S. Tunç, O. Duman, Effects of electrolytes on the electrokinetic properties of pumice suspensions, J. Dispers. Sci. Technol. 30 (2009) 548–555. [6] O. Duman, S. Tunç, A. Çetinkaya, Electrokinetic and rheological properties of kaolinite in poly(diallyldimethylammonium chloride), poly(sodium 4-styrene sulfonate) and poly(vinyl alcohol) solutions, Colloids Surf. A Physicochem. Eng. Asp. 394 (2012) 23–32. [7] S. Tunç, O. Duman, B. Kancı, Rheological measurements of Na-bentonite and sepiolite particles in the presence of tetradecyltrimethylammonium bromide, sodium tetradecyl sulfonate and Brij 30 surfactants, Colloids Surf. A Physicochem. Eng. Asp. 398 (2012) 37–47. [8] S. Tunç, O. Duman, R. Uysal, Electrokinetic and rheological behaviors of sepiolite suspensions in the presence of poly(acrylic acid sodium salt)s polyacrylamides, and poly(ethylene glycol)s of different molecular weights, J. Appl. Polym. Sci. 109 (2008) 1850–1860. [9] S. Tunç, O. Duman, A. Çetinkaya, Electrokinetic and rheological properties of sepiolite suspensions in the presence of hexadecyltrimethylammonium bromide, Colloids Surf. A Physicochem. Eng. Asp. 377 (2011) 123–129. [10] E. Forbes, K.J. Davey, L. Smith, Decoupling rehology and slime coatings effect on the natural flotability of chalcopyrite in a clay-rich flotation pulp, Miner. Eng. 56 (2014) 136–144. [11] Y. Wang, Y. Peng, T. Nicholson, R.A. Lauten, The different effects of bentonite and kaolin on copper flotation, Appl. Clay Sci. 114 (2015) 48–52. [12] L. Huynh, A. Feiler, A. Michelmore, J. Ralston, P. Jenkins, Control of slime coatings by the use of anionic phosphates: a fundamental study, Miner. Eng. 13 (2000) 1059–1069. [13] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Prog. Polym. Sci. 28 (2003) 1539–1641. [14] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng. R Rep. 28 (2000) 1–63. [15] O. Duman, S. Tunç, Electrokinetic and rheological properties of Na-bentonite in some electrolyte solutions, Microporous Mesoporous Mat. 117 (2009) 331–338. [16] J. Durán, M. Ramos-Tejada, F. Arroyo, F. González-Caballero, Rheological and electrokinetic properties of sodium montmorillonite suspensions, J. Colloid Interface Sci. 229 (2000) 107–117. [17] F. Thomas, L.J. Michot, D. Vantelon, E. Montargès, B. Prélot, M. Cruchaudet, et al., Layer charge and electrophoretic mobility of smectites, Colloids Surf. A Physicochem. Eng. Asp. 159 (1999) 351–358. [18] Y. Tsujimoto, C. Chassagne, Y. Adachi, Comparison between the electrokinetic properties of kaolinite and montmorillonite suspensions at different volume fractions, J. Colloid Interface Sci. 407 (2013) 109–115. [19] Y. Zhao, H. Yi, F. Jia, H. Li, C. Peng, S. Song, A novel method for determining the thickness of hydration shells on nanosheets: a case of montmorillonite in water, Powder Technol. 306 (2016) 74–79. [20] L. Ammann, F. Bergaya, G. Lagaly, Determination of the cation exchange capacity
Fig. 8. Effect of the mean particle size on the continuous titration curves of MMT suspension (4 g/L) in 1 mmol/L KCl.
valence and additional surface protons are omitted for convenience): ^SeOH ↔ ^SeO + H+ At low pH and ionic strength, proton uptake by MMT also occurs by cation exchange on structural charge sites (^X−) [29]: ^XNa + H+ ↔ ^XH + Na+ The titration curves recorded in continuous mode at different ultrasonic intensity (Fig. 8) show that the increase of mean particle size increases the net proton/hydroxide consumptions (Ca-Cb) especially in basic domain. Indeed, above pH 8 the titration curves reach steep slope and the consumption of unexfoliated MMT at pH 10 is 1.5 times higher than the exfoliated MMT platelets (D50 = 0.06 μm). It may be due to that the “spillover” of electrostatic potential from basal onto edge surfaces will strengthen with the increasing of the degree of exfoliation. For disc-shaped clay particles, charge densities on their basal and edge surfaces are unequal and edge thickness is small relative to the Debye length, the negative double layer extending from the particle faces may spillover into the edge region, and possibly overpower the positive edge charge that contribute to the consumption of H+ and OH− ions. The spillover effect was determined by the experimental data on anion exclusion [30] and edge-to-face particle aggregation [31] in MMT suspensions. In the opposite, this trend is significantly attenuated in the acid domain below pH 5. It has been well accepted that proton adsorption on MMT occurs mainly at oxide-type reactive sites on the edge surfaces at alkaline pH. After exfoliation of lamellar MMT, the edge thickness will decrease and more negative charge possibly will overpower the positive edge charge. Based on the above-noted concept, it is clear that the electrokinetic property of layered silicate clays such as MMT will change after exfoliation.
4. Conclusions The experimental results from this study have shown that there is a close relationship between the exfoliation and the electrokinetic property of MMT platelets in aqueous suspensions. The overall charge decreased significantly with the increasing of the degree of exfoliation. 5
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of clays with copper complexes revisited, Clay Miner. 40 (2005) 441–453. [21] E.E. Saka, C. Güler, The effects of electrolyte concentration, ion species and pH on the zeta potential and electrokinetic charge density of montmorillonite, Clay Miner. 41 (2006) 853–861. [22] A. Delgado, F. González-Caballero, J.M. Bruque, On the zeta potential and surface charge density of montmorillonite in aqueous electrolyte solutions, J. Colloid Interface Sci. 113 (1986) 203–211. [23] I. Sondi, J. Biscan, V. Pravdic, Electrokinetics of pure clay minerals revisited, J. Colloid Interface Sci. 178 (1996) 514–522. [24] S. Miller, P. Low, Characterization of the electrical double layer of montmorillonite, Langmuir (1990) 572–578. [25] G.E. Christidis, Validity of the structural formula method for layer charge determination of smectites: a re-evaluation of published data, Appl. Clay Sci. 42 (2008) 1–7.
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