Article pubs.acs.org/jced
Electrokinetic and Colloidal Properties of Homogenized and Unhomogenized Palygorskite in the Presence of Electrolytes Jixiang Xu and Aiqin Wang* Center of Xuyi Attapulgite Applied Technology, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, P. R. China, and Graduate University of the Chinese Academy of Science, Beijing 100049, P. R. China ABSTRACT: A series of palygorskite samples modified with electrolytes containing LiCl, NaCl, KCl, CsCl, MgCl2, CaCl2, AlCl3, and FeCl3 were prepared with the aid of high-pressure homogenization and mechanical stirring techniques. The changes of electrokinetic and colloidal properties of modified palygorskite were investigated in detail through the measurement of the zeta potential, yield stress, and sedimentation volume. The influences of charge, concentration, and type of ions on the surface potential of palygorskite were compared. The correlation between the zeta potential and the colloidal properties of obtained palygorskite was discussed. The homogenization process could favor the adsorption of ions onto palygorskite surface and enhance the colloidal properties of suspension. It was found that the 0.05 mol·kg−1 AlCl3 and FeCl3 reversed the surface charge of palygorskite from negative to positive. The yield stress measurements demonstrated that the association capacity of rods depended on the surface potential and interparticle interactions of palygorskite. A stable suspension was obtained when palygorskite was dispersed in MgCl2 solutions and homogenized at 30 MPa.
1. INTRODUCTION Palygorskite is widely used in many industrial applications such as paints, drilling fluids, and papers because of its special rodlike crystals which are prone to associate with each other and give unique colloidal properties in suspension.1 The formulation of palygorskite suspensions is important to meet performance demands with respect to nonleveling, slip, crack resistance, and water-retention characteristics and to retain these properties over a reasonable shelf life.2 These properties are governed by the microstructure of the suspension, which would be changed with the nature and strength of the interparticle forces, and can be evaluated by rheological properties of suspension.3,4 Therefore, a good understanding of the relationships between the interparticle forces and colloidal properties of palygorskite is required to evaluate and control the quality and storage stability of products. Generally, the interparticle forces in suspension depend on the surface charge of the mineral, and could be varied with many parameters like the solid-water ratio, pH, ion strength,5 the type of clay,6,7 additives including surfactants8 and electrolytes,9−14, and so forth. Among them, inorganic electrolytes play an important role in controlling the macroscopic behavior of colloidal suspensions.15 The added ions could attach to the particle surface by electrostatic interactions, rendering extra hydration interaction and changing the strength of the attraction and repulsion between particles. In particular, a prudent choice of electrolyte allows the manipulation of the net interparticle force and the suspension stability, the flow behavior, and the consolidation densities of suspensions. Even though electrolytes have significant effects on the properties of clay systems, the surface charge of particle would © 2012 American Chemical Society
be varied with the concentration, type, and charge of ions. To date, many researches have been made to investigate the electrokinetic and rheological properties of clay suspensions in the presence of various electrolytes. Duman and Tunç,9 Penner and Lagaly,10 and Saka and Güler11 have reported the electrokinetic and rheological properties of bentonite suspensions in the presence of different mono- and multivalent electrolytes. It was found that the divalent cations (Cu2+, Mn2+, Ca2+, Ba2+, and Ni2+) and trivalent cation (Al3+) were potential determining cations, while monovalent counter-cations and mono-, di-, and trivalent anions were indifferent ions for the suspensions. Marouf et al.12 have studied the electrokinetic properties of raw and thermally treated dolomite in the presence of NaCl, KCl, MgCl2, BaCl2, AlCl3, and Na2HPO4. The experimental results indicated that monovalent cations were indifferent electrolytes for dolomitic solid samples, whereas MgCl2 and BaCl2 reversed the sign of surface charge of dolomite. Alkan et al.13,14 have discussed the electrokinetic properties of kaolinite and sepiolite as a function of solid− liquid ratio, pH, type, and concentration of electrolytes. It was also found that monovalent cations were indifferent ions, whereas di- and trivalent cations were potential determining ions. Somewhat surprisingly, few works have been done to study systematically the influences of electrolytes on the electrokinetic and colloidal properties of palygorskite suspensions. Received: February 19, 2012 Accepted: March 27, 2012 Published: April 5, 2012 1586
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where η is the solution viscosity, ε is the solution permittivity, κ is the Debye−Huckel parameter, and a is the radius of the charged particle. Before measurements, 0.50 g of the obtained palygorskite was fully dispersed in 100 cm−3 distilled water under high-speed stirring at 11 000 r·min−1 for 20 min. Three parallel measurements were conducted, and the averages were reported. 2.4. Measurement of Colloidal Properties. Steady shear measurements were made using an Anton Paar Physica MCR301 Rheometer. A cone−plate with a water bath was used for all measurements. The shear rate range was from (0.1 to 200) s−1. Before measurements, 5.0 g of obtained palygorskite samples were dispersed in 45 cm−3 of distilled water and intensively stirred at 11 000 r·min−1 for 20 min as in the zeta potential measurement. All tests were performed at 25 °C. The colloidal stability of the palygorskite suspensions was evaluated using the conventional sedimentation technique in a graduated cylinder. A sample of 2.0 g of obtained palygorskite was dispersed in 120 cm−3 of deionized water with a high-speed mixer at 11 000 r·min−1 for 10 min and then transferred to 100 cm−3 graduated cylinder, where it was allowed to stand undisturbed for some time. The sedimentation volume was read directly from the graduated cylinder at fixed time intervals.
High-pressure homogenization is a liquid−liquid emulsification or liquid−solid dispersion technology commonly employed in the chemical, pharmaceutical, and food industries. During the homogenization process, the almost insoluble twophase mixtures were subjected to intense turbulence and shear in a high pressure homogenizer, leading to the breakup of particles into small units distributed uniformly in the medium.16 In comparison with the conventional mechanical mixing process, the homogenization treatment can provide a high temperature and high pressure environment in suspension. In previous work, we have found that homogenization palygorskite suspension at 30 MPa could effectively disaggregate the crystal bundles.17 From the literature, it was found that the rheological and electrokinetic properties of clay suspensions in the presence of inorganic ions were studied by mechanical mixing clay and electrolyte solutions,9−14 whereas, in the current work, palygorskite modified with LiCl, NaCl, KCl, CsCl, MgCl2, CaCl2, AlCl3, and FeCl3 were obtained via mechanical agitation followed by high-pressure homogenization. The influences of homogenization process, the type and concentration of electrolytes on the surface charge, yield stress, and colloidal stability of obtained palygorskite were examined and compared.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Palygorskite, with a chemical composition of mass fraction of SiO2, 64.31; Al2O3, 10.47; Fe2O3, 0.87; MgO, 20.41; Na2O, 1.52; K2O, 0.13; CaO, 1.29, was obtained from Jiuchuan Technology Co. (Jiangsu, China). LiCl, NaCl, KCl, CsCl, MgCl2, CaCl2, AlCl3, and FeCl3 were purchased from Shanghai Guoyao Chemical Reagent Co. (Shanghai, China). All of them were of analytical grade with the mass fraction purity of 0.99 and used as received without further purification. Deionized water was used in all experiments. 2.2. Preparation of Electrolyte-Modified Palygorskite. A sample of 50.0 g of palygorskite was dispersed in 500 cm−3 of LiCl, NaCl, KCl, CsCl, MgCl2, CaCl2, AlCl3, and FeCl3 solutions with given concentrations, respectively, and stirred at 800 r·min−1 for 2 h at ambient temperature. The obtained suspension was filtered through the sieve of 74 μm (200 mesh) screen to remove quartz (with mass fraction of 1.0, based on the weight of palygorskite). Subsequently, the filtered suspension was homogenized at 30 MPa using a high-pressure homogenizer (GJB 8-20, Changzhou Homogenizer Machinery Corporation, Ltd., Jiangsu, China). The homogenized suspensions were centrifuged at 5000 r·min−1 for 10 min, and the solid products were dried at 105 °C for 4 h. Finally, the dry products were grinded and passed through a 74 μm screen. The unhomogenized samples were prepared according to the similar procedure but without high-pressure homogenization. All samples with a particle size smaller than 74 μm were used for further experiments. 2.3. Measurement of Zeta Potential. Zeta potential was measured on a Malvern Zetasizer Nano system with irradiation from a 633 nm He−Ne laser (ZEN3600) at 25 °C, using a folded capillary cell. The electrophoretic mobility (UE) was measured using a combination of electrophoresis and laser Doppler velocimetry techniques. The zeta potential (ζ) was calculated from the electrophoretic mobility using the Henry equation: 3ηUE 1 ς= 2ε f (κa) (1)
3. RESULTS AND DISCUSSION 3.1. Zeta Potential. The zeta potentials of homogenized palygorskite modified with various electrolytes (0.05 mol·kg−1 in all cases) are shown in Figure 1. The pure palygorskite has a
Figure 1. Zeta potentials ζ of homogenized (■) and unhomogenized (□) palygorskite in the presence of LiCl (b), NaCl (c), KCl (d), CsCl (e), MgCl2 (f), CaCl2 (g), AlCl3 (h), and FeCl3 (l) electrolytes. a, pure palygorskite.
zeta potential of −23.6 mV. The magnitude of the zeta potential decreases in the order of Li+ > Na+ > K+ > Cs+ for palygorskite dispersed in LiCl, NaCl, KCl, and CsCl solutions. The suspension based on CsCl-modified palygorskite shows a zeta potential of −10.4 mV. The results can be explained in the following ways: first, with the addition of small amounts of alkali metal chlorides, the cations were attracted to the inner part of electrical double layer of particles, while the Cl− anions hardly migrated into the electrical double layer due to the strong repulsion interactions. So one can suppose that the sign and magnitude of the surface charge of palygorskite were governed largely by the Li+, Na+, K+, and Cs+ cations. Second, the alkali metal ions presented in aqueous solutions in the form 1587
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and homogenized at 30 MPa. No obvious variation in the negative zeta potential is observed for homogenized and unhomogenized palygorskite modified with MgCl2. The palygorskite particle can be considered as an anion with a large size and high charge density,22 which attracts the ions of opposite sign (counter-ions) and repels the ions of the same sign (co-ions). The surface charge is mainly determined by the ions of the opposite sign of the particle. The high-pressure homogenization can induce the formation and fracture of microbubbles in the liquid medium, leading to a sudden energy release and creation of local high temperatures, high pressures, and formation of strong cavitation, shear, and turbulence forces, which will then help to disperse, expand, and split the crystal bundles of palygorskite. Once spaces or gaps between the bundles and the specific surface area of palygorskite increased, the added ion species were easily migrated and attached onto the negatively charged surface through electrostatic interactions under the help of Brownian motion, ultimately neutralizing the surface negative charge and changing the surface potential. So high-pressure homogenization can facilitate the adsorption of cation species onto palygorskite (Figure 2).
of solvation ions, and the degree of solvation affected their adsorption onto palygorskite surfaces.11 For these four alkali metal ions, because all of them contain the same charge of +1, the inherent electronegativities of the corresponding alkali metal atoms are not important to solvation. Instead, the most important factor affecting the solvation strength is the ionic radius of alkali metal ions.18 A small ion with concentrated charge should be more strongly hydrated than a larger one. According to the literature, the ionic radii of Li+, Na+, K+, and Cs+ ions are (0.078, 0.102, 0.151, and 0.174) nm, respectively. Correspondingly, the solvation strength by water for the alkali metal ions follows the sequence of Li+ > Na+ > K+ > Cs+. It was reasonable to conjecture that a small quantity of more strongly hydrated Li+ and Na+ cations were bound to the surface of palygorskite and therefore gave rise to greater zeta potentials (Figure 1). In contrast, a large number of less hydrated Cs+ cations were bound to the surface and gave rise to a smaller zeta potential (Figure 1). The zeta potentials are −(7.88 and 8.77) mV for MgCl2- and CaCl2-modified palygorskite, respectively. The higher zeta potential of MgCl2-modified palygorskite compared with CaCl2 was due to the specific adsorption of Mg2+ ions to palygorskite surfaces.19 The surface charge of palygorskite changed from negative (−23.6 mV) to positive when palygorskite was dispersed in AlCl3 (+20.2 mV) and FeCl3 (+47.6 mV) solutions. The Fe3+ ions have more unoccupied orbitals and strong acidity compared with Al3+. It meant that Fe3+ ions could be specifically adsorbed onto clay surface by accepting the lone pairs of electrons. The stronger the interaction between a given cation and clay, the larger will be its effect on the particle charge.11 Therefore, the more positive zeta potential of palygorskite modified with Fe3+ might be due to its specific adsorption on the clay surface. As can be seen from Figure 1, the zeta potential values of palygorskite modified with LiCl, MgCl2, and AlCl3 electrolytes followed the order of Al3+ > Mg2+ > Li+. The structure of palygorskite is crossed by nanotunnels, which run parallel to the fiber axis and are filled by zeolitic water and exchangeable cations such as Na+, Ca2+, and Mg2+.20 When palygorskite was dispersed in electrolyte solution, the inorganic ions could incorporate into the tunnels of palygorskite and exchange with the Na+, Ca2+, and Mg2+ species. A similar result was reported by Melo et al.21 Moreover, the produced shear and cavitation during homogenization at 30 MPa could favor this process. However, the ion-exchange process is a stoichiometric reaction and is generated by the equivalent amounts of cations to maintain the total electroneutrality at the solid−solution interface. Consequently, the exchange of Al3+ with Na+, Ca2+, and Mg2+ in palygorskite required fewer Al3+ for electroneutrality than Mg2+ and Li+ did. In addition, the Al3+ ions have a small hydration radius, a small distance of closest approach, and large polarizability compared to Li+ and Mg2+ ions. These properties allowed more Al3+ ions to readily approach the tunnels and surface of the clay and became specifically adsorbed. Therefore, the zeta potentials of palygorskite shifted to more positive values as the charge of the cation increased. The zeta potentials of the samples without homogenization were also measured to study the influence of high-pressure homogenization process on the adsorption of inorganic ions onto the mineral surface (Figure 1). Unhomogenized samples dispersed by LiCl, NaCl, KCl, CsCl, and CaCl2 solutions have smaller zeta potentials than the corresponding homogenized samples, whereas the clay surfaces become more positively charged for palygorskite dispersed in AlCl3 and FeCl3 solutions
Figure 2. Schematic illustration of the adsorption of inorganic ions onto the surface of palygorskite after mechanical stirring or homogenization at 30 MPa.
The variations of the zeta potentials of homogenized and unhomogenized palygorskite as a function of LiCl, MgCl2, and AlCl3 concentrations are given in Figure 3. The surface charge of palygorskite become less negative with the increase in LiCl concentration, and the zeta potential values of homogenized samples are higher than that of unhomogenized samples. This reduction can be explained by the adsorption of more Li+ ions onto palygorskite as the LiCl concentration increased in the solution. Similar zeta potentials are obtained for the homogenized and unhomogenized samples in the presence of MgCl2 as shown in Figure 3b, and the zeta potential decreased to −0.60 mV when palygorskite was dispersed in 0.25 mol·kg−1 MgCl2 solution. The zeta potentials are −(12.4 and 14.6) mV, respectively, for homogenized and unhomogenized samples for palygorskite dispersed by a 0.01 mol·kg−1 AlCl3 solution. A charge reversal occurred at about 0.035 mol·kg−1 AlCl3, after which the palygorskite surface became positively charged with increasing AlCl3 concentration. It was clear that Al3+ ions were able to reverse the surface sign of palygorskite, which was due to the specific adsorption of Al3+ ions in the inner Helmhotz plane.13 3.2. Yield Stress. The colloidal properties of palygorskite suspension depend on the association strength of network structure, which is related to the particle interactions.4 According to the DLVO theory,23,24 the net interparticle force in clay suspension is governed by the sum of the attractive 1588
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Figure 4. Yield stresses τ0 of homogenized (■) and unhomogenized (□) palygorskite suspensions in the presence of LiCl (b), NaCl (c), KCl (d), CsCl (e), MgCl2 (f), CaCl2 (g), AlCl3 (h), and FeCl3 (l) electrolytes. a, pure palygorskite.
on the interparticle interactions and association of palygorskite rods. Based on the results of zeta potential shown in Figure 1, the addition of KCl to palygorskite made the surface less negative (−16.7 mV). The electrostatic repulsion of interparticle was less strong, but the association tendency increased due to an increase in van der Waals attraction forces.28 So the yield stress increased from 34.5 MPa of pure palygorskite to 39 MPa. The increase of yield stress of AlCl3-modified palygorskite was also related to the reduced electrostatic repulsion of the interparticle. The nuance in zeta potential was negligible when palygorskite was dispersed in LiCl and NaCl solutions, which may be the reason that no obvious variation in yield stress of corresponding suspensions was observed.29 The negative charge of palygorskite largely decreased in the presence of CsCl (−10.4 mV) and CaCl2 (−8.77 mV). It meant that the electrostatic repulsion between rods was largely reduced. Generally, particles with a zeta potential more than −15.0 mV or less than 15.0 mV are expected to be unstable from electrostatic considerations.30 So the reduction of yield stresses for CsCl (31 MPa) and CaCl2-modified (24.5 MPa) palygorskite suspensions was attributed to the decreased electrostatic repulsion between rods which demolished the continuity of network structure.18 Interestingly, the yield stress was largely enhanced for MgCl2-modified palygorskite although the zeta potential of corresponding sample decreased to a minimum (Figure 1). This was because the added Mg2+ ions built strong bridges through electrostatic forces between rods and linked them together to form a larger gel network. Similar behavior was observed by Ç ınar et al.31 and Desai et al.29 about sepiolite and pyrophyllite suspensions, respectively. The zeta potential became +47.6 mV for palygorskite dispersed by 0.05 mol·kg−1 FeCl3 solution. The strongly increased positive charge density could cause the interparticle distance to increase and the strength of association between rods to decrease, and then more entrapped water was released into suspension owing to increased electrostatic repulsion forces. The space available for the particles to move increased with a raising volume fraction of bulk water, and the yield stress of the suspension decreased. Similar findings have been reported in a number of other studies of concentrated aqueous suspensions.32,33 The yield stresses of the unhomogenized palygorskite suspensions in the presence of various electrolytes are also
Figure 3. Zeta potentials of homogenized (■, ●, ▲) and unhomogenized (□, ○, △) palygorskite in the presence of LiCl, MgCl2, and AlCl3 with various concentrations C.
van der Waals and the repulsive electrical double layer forces in the simplest case. However, in more complex systems, a number of other forces may further influence the interaction of colloidal particles, such as structural (“hydration”) and bridging forces. So the macroscopic behavior (viscosity, modulus, stability) of colloidal suspensions is not only governed by the balance of a range of different forces but also depended on the summation of the same particle−particle interactions.25 In this work, the influences of various electrolytes on yield stresses of the homogenized and unhomogenized palygorskite (about with mass fraction of 10) were studied. The flow curves of palygorskite suspensions were described by the H−B model to obtain the yield stress.26 As can be seen from Figure 4, the yield stresses of the suspensions were strongly dependent on the types of the added electrolytes. An increase in yield stresses was observed when KCl, MgCl2, and AlCl3 electrolytes were added into palygorskite as compared with pure palygorskite, while the yield stresses decreased for suspensions containing CsCl, CaCl2, and FeCl3 electrolytes. The palygorskite rods carry a negative charge on the basal surface originated from the substitution of Si4+ in the tetrahedral sheet with Al3+ with a consequent imbalance of negative charge. The edges of rods, where imperfections necessarily occur because of bond breakage, carry a little positive charge.27 So there are two kinds of interparticle interactions: electrical attraction between positively charged edges and negatively charged surfaces, and electrical repulsion between edges and/or surfaces of like charge. In suspensions, the rods associate with each other to form flocculated clusters. The surface charge of palygorskite has a remarkable influence 1589
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0.25 mol·kg−1 MgCl2 solution. The yield stresses reduce with increasing AlCl3 concentration for homogenized samples, whereas the yield stresses first increase with the increase in AlCl3 concentration to 0.025 mol·kg−1 and then decrease, reaching the minimum value of 3 MPa for unhomogenized palygorskite. The colloidal properties of the palygorskite suspensions are highly related to the morphologies such as the dispersion and aspect ratios of the rods.19 The colloidal nature of palygorskite is not apparent until the crystal bundles are separated into individual rods.34 In previous study, we have proved that the homogenization process could favor the dispersion of palygorskite aggregates and help the construction of a gel network structure.17 So the enhancement of yield stresses of homogenized samples compared with unhomogenized systems was due to good dispersion of palygorskite rods, which could form large clusters and entrap more water in them. Once palygorskite was dispersed in electrolyte solutions, the palygorskite rods, to some extent, remain dispersed upon increasing the electrolyte concentration. The network structure of suspension was not destroyed. So a relatively high yield stress was generated. Further increasing LiCl (>0.5 mol·kg−1) and MgCl2 (>0.025 mol·kg−1) concentrations, the palygorskite surfaces became less negative. The electrostatic repulsion forces between rods decreased, and some association of rods was broken. So the decrease of yield stresses at higher LiCl and MgCl2 concentrations was attributed to the collapsed network structure of suspensions. For palygorskite dispersed in (0.01 and 0.025) mol·kg−1 AlCl3 solutions, although the zeta potentials of these two samples were less than −15 mV, an increase in yield stresses compared with pure palygorskite was observed. It might be attributed to the strong hydration interactions between palygorskite rods resulted from the hydration layers by the adsorbed and hydrated aluminum species. These hydration interactions could prevent the particle from close approach and serious collapse of network structure.35 The yield stresses of the suspensions decreased with further increasing AlCl3 concentration to 0.05 mol·kg−1 and higher. This was because the surfaces of palygorskite became more positively charged with increasing AlCl 3 concentration (Figure 3), resulting in the increase of repulsion between particles and the breakdown of network structure (Figure 5). The result was in line with the observations reported in the literature.10 It can be deduced from the above discussion that the phenomenon of colloidal properties of palygorskite suspension remaining constant or higher in the presence of electrolyte solutions depends on the charge density of rods and can be varied by changing the concentration and type of added electrolytes. 3.3. Colloidal Stability. The effects of the types (Figure 6) and the concentrations (Figure 7) of electrolytes on the colloidal stability of obtained palygorskite were discussed by the measurement of sedimentation volume of the suspension after allowing it to stand for given time. The higher the sedimentation volume, the more stable the suspension will be. Natural palygorskite existed as aggregates or bundles. It was difficult to get well-dispersed rods through mechanical stirring of the aqueous suspension. Consequently, the aggregates in palygorskite suspension were rapidly settled under the gravitational forces, and the colloidal stability of the suspensions was bad for systems without homogenization (Figure 6b).
shown in Figure 4 to compare the effects of homogenization process on the colloidal behavior of suspensions. The yield stresses of all of the unhomogenized samples are smaller than that of homogenized samples, indicating that the association of palygorskite rods was enhanced after homogenization. The variations of yield stresses of obtained palygorskite as a function of LiCl, MgCl2, and AlCl3 concentrations are shown in Figure 5. The yield stresses of suspensions increased after
Figure 5. Yield stresses τ0 of homogenized (■, ●, ▲) and unhomogenized (□, ○, △) palygorskite suspensions in the presence of LiCl, MgCl2, and AlCl3 with various concentrations C.
palygorskite homogenization at 30 MPa. Specifically, the yield stresses of the homogenized samples increase monotonically as LiCl concentration increased from (0.05 to 0.5) mol·kg−1 and then decrease with further increasing LiCl concentration to 1.0 mol·kg−1 (Figure 5), whereas a gradual decrease of the yield stresses occurred with increasing LiCl concentration for unhomogenized samples. The yield stresses increase with the increase in MgCl2 concentration to 0.025 mol·kg−1, after which the yield stresses decrease to (16 and 10) MPa for homogenized and unhomogenized palygorskite dispersed in 1590
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contributed by single rods, which led to the neighboring rods sticking to each other and forming large clusters.37 The variations of sedimentation volumes of palygorskite suspensions with LiCl, MgCl2, and AlCl3 concentrations are shown in Figure 7. A slight decrease in sedimentation volumes occurred when LiCl concentration increased from (0.05 to 0.1) mol·kg−1 (Figure 7a). The sedimentation volume increased to 92 cm−3 for palygorskite dispersed by a 0.5 mol·kg−1 LiCl solution. Further increasing LiCl concentration to 1.0 mol·kg−1 had no beneficial help in improving the colloidal stability of suspension. For the samples without homogenization, the sedimentation volumes of suspensions remain almost constant when LiCl concentration increased to 0.5 mol·kg−1 (Figure 7a′) but decrease with a further increase in the LiCl concentration. It indicated that the less negatively charged rods could agglomerate into large clusters with prolonging the settling time. No obvious change in sedimentation volumes is observed at the studied range of MgCl2 concentrations for the homogenized and unhomogenized palygorskite (Figure 7b,b′). The Mg2+ ion bridges among particles might be responsible for such high stablility of the suspensions. The sedimentation volume decreased as the AlCl3 concentration varied from (0.01 to 0.25) mol·kg−1 for the homogenized and unhomogenized palygorskite samples (Figure 7c,c′). This result further confirmed that well-dispersed palygorskite rods favored the formation of large clusters, resulting in poor colloidal stability of the suspension. Figure 6. Variation of sedimentation volumes V of homogenized (a) and unhomogenized (b) palygorskite suspensions in the presence of ○, LiCl; △, NaCl; ▽, KCl; ◆, CsCl; ◇, MgCl2; ☆, CaCl2; ●, AlCl3 and ■, FeCl3 as a function of time. □, pure palygorskite.
4. CONCLUSIONS In the current work, palygorskite modified with LiCl, NaCl, KCl, CsCl, MgCl2, CaCl2, AlCl3, and FeCl3 was prepared via mechanical stirring and high-pressure homogenization techniques. The electrokinetic and colloidal properties of obtained samples were studied. The following results are obtained: 1. The migration of a given ion from an aqueous solution onto the surface of palygorskite became easy during highpressure homogenization process. 2. The surface charge of the palygorskite was varied by changing the concentration, the charge, and the type of ions. 3. The magnitude of zeta potential of palygorskite gradually decreased with increasing LiCl and MgCl2 concentrations, but first decreased and then increased with increasing AlCl3 concentration. 4. The mechanism by which electrolytes affected the yield stress of suspension was through changing the surface charge of the palygorskite rods and then the flocculation state of the suspension. A slight change in surface charge of rods could promote interparticle interactions and enhance the yield stress of suspensions. On the other hand, the great decrease or increase of surface charge of rods resulted in serious aggregation or high dispersion of rods, respectively. Both of them could destroy the network structure and decrease the yield stress of suspension. 5. The colloidal stability of suspension was dependent on the relative contribution of zeta potential and viscosity. The higher zeta potential and suspension viscosity, the more stable the system was. The variations of colloidal properties of palygorskite suspensions in the presence of electrolytes have built a theoretical foundation for the practical application of this
The stability of suspension is due to the existence of potential energy barrier between the particles, which arises as a result of interactions of the electrical double layers and the van der Waals forces.23,24 As can be seen from Figure 6a, the sedimentation volumes increase in the order of Li+ > Na+ > Cs+ for homogenized samples containing monovalent cations, which follows an opposite trend to that of the change in zeta potential with electrolytes (Figure 1). It indicated that the reductions of charge density and potential energy barrier made the palygorskite rods easily agglomerate and settle. The sedimentation volume of KCl-modified suspension is higher than the suspensions containing LiCl, NaCl, and CsCl electrolytes (Figure 6a). It was likely that the higher viscosity of the suspension favored the slow sedimentation of particles.36 Even though the CaCl2-modified palygorskite has a lower zeta potential and yield stress as shown in Figures 1 and 4, the sedimentation volumes of suspension is 71 cm−3 at the settling time of 72 h (Figure 6a). It might be originated from stronger hydration forces produced by the adsorbed and hydrated Ca species, which was stronger than the hydration repulsion in NaCl- and CsCl-modified systems. Therefore, the coagulation induced by CaCl2 was weaker than NaCl and CsCl.35 The sedimentation volume of MgCl2-modified suspension is 93 cm−3 after incubation for 72 h. The FeCl3-modified suspension is stable until 10 h of incubation, after which the position of the interface rapidly fell to the equilibrium volume of 57 cm−3 (Figure 6a). The larger sedimentation volume before 10 h was due to good dispersion of palygorskite rods resulted from higher surface charge, and the rapid fall of solid−liquid interface above 24 h was attributed to the depletion attraction 1591
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Figure 7. Variation of sedimentation volumes V of homogenized (a, b, c) and unhomogenized (a′, b′, c′) palygorskite suspensions in the presence of LiCl, MgCl2, and AlCl3 with various concentrations C as a function of time.
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special clay used as drilling muds. The results indicate that the palygorskite suspension has good colloidal properties in the presence of Mg2+ species.
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REFERENCES
(1) Galan, E. Properties and applications of palygorskite-sepiolite clays. Clay Miner. 1996, 31, 443−453. (2) Change, S. H.; Ryan, M. E.; Gupta, R. K. The effect of pH, ionic strength, and temperature on the rheology and stability of aqueous clay suspensions. Rheol. Acta 1993, 32, 263−269. (3) Eriksson, R.; Pajari, H.; Rosenholm, J. B. Shear modulus of colloidal suspensions: Comparing experiments with theory. J. Colloid Interface Sci. 2009, 332, 104−112. (4) Teha, E.; Leong, Y. K.; Liu, Y.; Fourie, A. B.; Fahey, M. Differences in the rheology and surface chemistry of kaolin clay slurries: The source of the variations. Chem. Eng. Sci. 2009, 64, 3817− 3825. (5) Gustafsson, J.; Mikkola, P.; Jokinen, M.; Rosenholm, J. B. The influence of pH and NaCl on the zeta potential and rheology of anatase dispersions. Colloid Surf., A 2000, 175, 349−359. (6) Baltar, C. A. M.; da Luz, A. B.; Baltar, L. M.; de Oliveira, C. H.; Bezerra, F. J. Influence of morphology and surface charge on the suitability of palygorskite as drilling fluid. Appl. Clay Sci. 2009, 42, 597−600.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 931 4968118. Fax: +86 931 8277088. E-mail address:
[email protected] (A.W.). Funding
The authors would like to thank the Science and Technology Support Project of Jiangsu Provincial Sci. & Tech. Department (No. BY2010012) and Jiangsu Provincial Joint Innovation and Research Funding of Enterprises, Colleges and InstitutesProspective Cooperative Research Project (No. BY2011194) for financial support. Notes
The authors declare no competing financial interest. 1592
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(7) Nosrati, A.; Addai-Mensah, J.; Skinner, W. Influence of mineral chemistry on electrokinetic and rheological behavior of aqueous muscovite dispersions. Ind. Eng. Chem. Res. 2011, 50, 11087−11096. (8) Morariu, S.; Bercea, M. Effect of addition of polymer on the rheology and electrokinetic features of laponite RD aqueous dispersions. J. Chem. Eng. Data 2009, 54, 54−59. (9) Duman, O.; Tunç, S. Electrokinetic and rheological properties of Na-bentonite in some electrolyte solutions. Microporous Mesoporous Mater. 2009, 117, 331−338. (10) Penner, D.; Lagaly, G. Influence of anions on the rheological properties of clay mineral dispersions. Appl. Clay Sci. 2001, 19, 131− 142. (11) Saka, E. E.; Güler, C. The effects of electrolyte concentration, ion species and pH on the zeta potential and electrokinetic charge density of montmorillonite. Clay Miner. 2006, 41, 853−861. (12) Marouf, R.; Marouf-Khelifa, K.; Schott, J.; Khelifa, A. Zeta potential study of thermally treated dolomite samples in electrolyte solutions. Microporous Mesoporous Mater. 2009, 122, 99−104. (13) Alkan, M.; Demirbaş, Ö .; Doğan, M. Electrokinetic properties of sepiolite suspensions in different electrolyte media. J. Colloid Interface Sci. 2005, 281, 240−248. (14) Alkan, M.; Demirbaş, Ö .; Doğan, M. Electrokinetic properties of kaolinite in mono- and multivalent electrolyte solutions. Microporous Mesoporous Mater. 2005, 83, 51−59. (15) Johnson, S. B.; Franks, G. V.; Scales, P. J.; Healy, T. W. The binding of monovalent electrolyte ions on α-Alumina. II. The shear yield stress of concentrated suspensions. Langmuir 1999, 15, 2844− 2853. (16) Floury, J.; Desrumaux, A.; Legrand, J. Effect of ultra-highpressure homogenization on structure and on rheological properties of soy protein-stabilized emulsions. J. Food Sci. 2002, 67, 3388−3395. (17) Xu, J. X.; Zhang, J. P.; Wang, Q.; Wang, A. Q. Disaggregation of palygorskite crystal bundles via high-pressure homogenization. Appl. Clay Sci. 2011, 54, 118−123. (18) Kundu, S. K.; Yoshida, M.; Shibayama, M. Effect of salt content on the rheological properties of hydrogel based on oligomeric electrolyte. J. Phys. Chem. B 2010, 114, 1541−1547. (19) Neaman, A.; Singer, A. Rheological properties of aqueous suspensions of palygorskite. Soil Sci. Soc. Am. J. 2000, 64, 427−436. (20) Giustetto, R.; Wahyudi, O. Sorption of red dyes on palygorskite: Synthesis and stability of red/purple Mayan nanocomposites. Microporous Mesoporous Mater. 2011, 142, 221−235. (21) Melo, D. M. A.; Ruiz, J. A. C.; Melo, M. A. F.; Sobrinho, E. V.; Martinelli, A. E. Preparation and characterization of lanthanum palygorskite clays as acid catalysts. J. Alloys Compd. 2002, 344, 352− 355. (22) Marchuk, A.; Rengasamy, P. Clay behaviour in suspension is related to the ionicity of clay-cation bonds. Appl. Clay Sci. 2011, 53, 754−759. (23) Derjaguin, B. V.; Landau, L. Theory of molecular interaction. Acta Physicochem. 1941, 14, 633−668. (24) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the stability of lyophobic colloids; Elsevier: Amsterdam, 1948. (25) Johnson, S. B.; Franks, G. V.; Scales, P. J.; Boger, D. V.; Healy, T. W. Surface chemistry-rheology relationships in concentrated mineral suspensions. Int. J. Miner. Process 2000, 58, 267−304. (26) Herschel, W.; Bulkley, R. Konsistenzmessungen von GummiBenzollösungen. Colloid Polym. Sci. 1926, 39, 291−300. (27) Cao, E. H.; Bryant, R.; Williams, D. J. A. Electrochemical properties of Na−attapulgite. J. Colloid Interface Sci. 1996, 179, 143− 150. (28) Johnson, S. B.; Scales, P. J.; Healy, T. W. The binding of monovalent electrolyte ions on α-alumina: I. Electroacoustic studies at high electrolyte concentrations. Langmuir 1999, 15, 2836−2843. (29) Desai, H.; Biswal, N. R.; Paria, S. Rheological behavior of pyrophyllite-water slurry in the presence of anionic, cationic, and nonionic surfactants. Ind. Eng. Chem. Res. 2010, 49, 5400−5406.
(30) White, B.; Banerjee, S.; O'Brien, S.; Turro, N. J.; Herman, I. P. Zeta-potential measurements of surfactant-wrapped individual singlewalled carbon nanotubes. J. Phys. Chem. C 2007, 111, 13684−13690. (31) Ç ınar, M.; Can, M. F.; Sabah, E.; Karagüzel, C.; Ç elik, M. S. Rheological properties of sepiolite ground in acid and alkaline media. Appl. Clay Sci. 2009, 42, 422−426. (32) Prestidge, C. A. Rheological investigations of galena particle interactions. Colloid Surf., A 1997, 126, 75−83. (33) Zhang, Q. G.; Li, W.; Gu, M. Y.; Jin, Y. P. Dispersion and rheological properties of concentrated silicon aqueous suspension. Powder Technol. 2006, 161, 130−134. (34) Haden, W. L., Jr. Attapulgite: properties and uses. Clays Clay Miner. 1963, 284−290. (35) Rao, F.; Ramirez-Acosta, F. J.; Sanchez-Leija, R. J.; Song, S.; Lopez-Valdivieso, A. Stability of kaolinite dispersions in the presence of sodium and aluminum ions. Appl. Clay Sci. 2011, 51, 38−42. (36) Eriksson, R.; Kokko, A.; Rosenholm, J. B. Rheological characterization of the influence of PVOH on calcite dispersions. Langmuir 2010, 26, 7946−7952. (37) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. Effects of interparticle interactions on stability, aggregation and sedimentation in colloidal suspensions. Chem. Eng. Commun. 1996, 148−150, 123−142.
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NOTE ADDED AFTER ASAP PUBLICATION A reference citation was incorrect in section 3.1 in the version published on 4/5/2012. This was corrected in the version published on 4/13/2012.
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