Langmuir 2008, 24, 12899-12910
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Probing Surface Charge Potentials of Clay Basal Planes and Edges by Direct Force Measurements Hongying Zhao,† Subir Bhattacharjee,‡ Ross Chow,§ Dean Wallace,| Jacob H. Masliyah,† and Zhenghe Xu*,† Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6, Department of Mechanical Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G8, Alberta Research Council, Edmonton, Alberta, Canada T6G 2G6, and Shell Canada Ltd. and Canadian Natural Resources Ltd., Calgary, Alberta, Canada T2L 1Y8 ReceiVed July 4, 2008. ReVised Manuscript ReceiVed August 27, 2008 The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.
Introduction Clay minerals are extensively used in a wide range of applications, such as paper making, oil drilling, water pollutant removal, oil sands industry, and consumer products.1-5 Due to their distinctive surface chemical properties, the stability of clay dispersions is of great importance in a number of industrial processes, as well as in soil chemistry and environmental science.5,6 The stability of clay dispersions is controlled largely by the colloidal interaction forces between clay mineral particles. Most clays in natural settings are of a phyllosilicate or sheet structure. The basic structural element of phyllosilicates can be simply * To whom correspondence should be addressed. Phone: 1-780- 4927667. Fax: 1-780-492-2881. E-mail:
[email protected]. † Department of Chemical and Materials Engineering, University of Alberta. ‡ Department of Mechanical Engineering, University of Alberta. § Alberta Research Council. | Shell Canada Ltd. and Canadian Natural Resources Ltd. (1) Wilson, M. J. Clay Mineralogy: Spectroscopic and Chemical DeterminatiVe Methods; Chapman & Hall: London, 1994. (2) Giese, R. F.; Van Oss, C. J. Colloid and Surface Properties of Clays and Related Minerals; Marcel Dekker: New York, 2002. (3) Olphen, H. v. An Introduction to Clay Colloid Chemistry: For Clay Technologists, Geologists, and Soil Scientists, 2nd ed.; Krieger Publishing Co.: Malabar, FL, 1991. (4) Sposito, G.; Skipper, N. T.; Sutton, R.; Park, S.-H.; Soper, A. K.; Greathouse, J. A. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3358–3364. (5) Li, L. Y.; Li, R. S. Can. Geotech. J. 2000, 37, 296–307. (6) Dixon, J. B.; Schulze, D. G. Soil Mineralogy with EnVironmental Applications; Soil Science Society of America, Inc.: Madision, WI, 2002.
viewed as an octahedral sheet of aluminum or magnesium hydroxides sandwiched between two sheets of tetrahedral silica, forming 2-D building blocks of so-called three-layer clay minerals.2 The 2-D building blocks are connected to each other by interlayer cations or by van der Waals force, depending on the type of clays. The cations are weakly bonded, often with water molecules and/or other neutral atoms or molecules trapped between the sheets, which makes clay cleave easily. In dispersion, clays form laminar shape particles with distinguished basal plane and edge surface. For 2:1 phyllosilicates, the basal plane is the plane made of oxygen atoms being shared by other tetrahedra in the tetrahedral sheet. Usually, the basal plane has a permanent negative charge (structural charge), resulting from various isomorphic substitutions of lattice cations of a lower valance within the clay structure.3 At the edges of the platelike particles, on the other hand, the tetrahedral silica sheets and the octahedral alumina sheets are broken (disrupted), leading to broken primary bonds. Therefore, the electrical charge of the edge, arising from hydrolysis reactions of broken Al-O and Si-O bonds, is pHdependent. There is a strong evidence to indicate that the edges of clay particles are positively charged in the neutral and acid pH ranges.7 The anisotropic surface charge of clay minerals plays a critical role in determining the stability of clay mineral suspensions.8 The importance of surface charge anisotropy was pointed out originally by van Olphen3 and then supported and elaborated further in many subsequent and recent investigations. (7) Swartzen-Allen, S. L.; Egon, M. Chem. ReV. 1974, 74, 385–400. (8) Tombacz, E.; Nyilas, T.; Libor, Z.; Csanaki, C. Prog. Colloid Polym. Sci. 2004, 125, 206–215.
10.1021/la802112h CCC: $40.75 2008 American Chemical Society Published on Web 10/17/2008
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As a consequence of the different charge characteristics of clay basal and edge surfaces, three different modes of particle association may occur in a suspension of platelike clay particles: face-to-face (FF), edge-to-face (EF), and edge-to-edge (EE).9 The rheological properties of clay-water systems are determined by a rather delicate balance of the three potential curves of interaction for EF, EE, and FF association.3 An extensive and accurate knowledge of the surface properties of clays, especially phyllosilicates, is desirable in order to predict clay performance in a number of industrial applications. While numerous studies focused on the basal plane of phyllosilicates, the surface chemistry of the edge surface is less well-known due to the difficulty in experimentally isolating clay edge surfaces. Theoretically, many studies attempted to determine the edge surface structure of phyllosilicates by ab initio molecular dynamic simulations.10-12 The effective surface area, structure of the edge sites, site densities, and intrinsic acidity constants for the reactive sites were predicted on the basis of different models and calculation methods by several researchers.10-12 Experimentally, potentiometric titration represents a main approach to study the acid-base properties of clay mineral surfaces.8,13-24 Despite a variety of the clay types used in the titration experiments, great differences in the titration curves and subsequently in the thermodynamic constants were reported. One reason for such discrepancy is the sensitivity of acid-base properties of clays to the protocols of sample preparation and titration measurement by different authors, as summarized by Duc et al.24 To date, a number of models describing surface chemistry of clay edge surfaces were used in literature. Bourg et al.22 summarized many of these models available for the acid-base chemistry of montmorillonite surfaces.22 The diversity of models was attributed to the lack of data adequate to resolve the protonation chemistry of montmorillonite edge surfaces, compounded with the lack of accurate estimates of edge surface area and independent measurements of electric potential of edge surfaces. Atomic force microscopy (AFM) has been widely used in the study of a wide range of surfaces since early 1990, including clay surfaces.25-29 AFM imaging has been used for quantitative (9) Lagaly, G. Appl. Clay Sci. 1989, 4, 105–123. (10) Churakov, S. V. J. Phys. Chem. B 2006, 110, 4135–4146. (11) Churakov, S. V. Geochim. Cosmochim. Acta 2007, 71, 1130–1144. (12) Bickmore, B. R.; Rosso, K. M.; Nagy, K. L.; Cygan, R. T.; Tadanier, C. J. Clays Clay Miner. 2003, 51, 359–371. (13) Duc, M.; Thomas, F.; Gaboriaud, F. J. Colloid Interface Sci. 2006, 300, 616–625. (14) Avena, M. J.; Pauli, C. P. D. J. Colloid Interface Sci. 1998, 202, 195–204. (15) Avena, M. J.; Mariscal, M. M.; Pauli, C. P. D. Appl. Clay Sci. 2003, 24, 3–9. (16) Tombacz, E.; Abraham, I.; Gilde, M.; Szanto, F. Colloids Surf. 1990, 49, 71–80. (17) Tombacz, E.; Szekeres, M. Appl. Clay Sci. 2006, 34, 105–124. (18) Kriaa, A.; Hamdi, N.; Srasra, E. Russ. J. Electrochem. 2007, 43, 167–177. (19) Taubaso, C.; Afonso, M. D. S.; Sanchez, R. M. T. Geoderma 2004, 121, 123–133. (20) Tertre, E.; Castet, S.; Berger, G.; Loubet, M.; Giffaut, E. Geochim. Cosmochim. Acta 2006, 70, 4579–4599. (21) Appel, C.; Ma, L. Q.; Rhue, R. D.; Kennelley, E. Geoderma 2003, 113, 77–93. (22) Bourg, I. C.; Sposito, G.; Bourg, A. C. M. J. Colloid Interface Sci. 2007, 312, 297–310. (23) Duc, M.; Gaboriaud, F.; Thomas, F. J. Colloid Interface Sci. 2005, 289, 148–156. (24) Duc, M.; Gaboriaud, F.; Thomas, F. J. Colloid Interface Sci. 2005, 289, 139–147. (25) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239–241. (26) Toikka, G.; Hayes, R. A. J. Colloid Interface Sci. 1997, 191, 102–109. (27) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Langmuir 1997, 13, 2109–2112. (28) Hillier, A. C.; Kim, S.; Bard, A. J. J. Phys. Chem. 1996, 100, 18808– 18817. (29) Butt, H.-J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1–152.
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analysis of clay platelet size, shape, and thickness;30-32 to determine mean lateral surface area;33 study clay dissolution;34 and map cleavage basal planes at atomic resolution.32,35-38 The AFM colloidal probe technique has been employed to measure face-to-face forces between synthetic smectites,39 colloidal forces between mica and silica in aqueous solutions,26,40 and colloidal and adhesion forces between illite particles and cleaved illite surfaces in aqueous solutions.41 However, there are only few studies that directly probe surface properties of clay edges in aqueous solutions. The challenge associated with using AFM to probe the charge properties of clay edge surfaces is to prepare a sufficiently smooth edge surface of clays, suitable for AFM measurements. To our knowledge, the first and only attempt to reveal the anisotropic character of clays by AFM colloidal probe technique was performed by Nalaskowski et al.42 They attached a 20 µm talc particle to the AFM cantilever and measured the forces between the edge of the talc particle and two different crystallographic planes of talc in aqueous solutions of various pH values. Their measurements showed differences between the properties of the basal plane and the edge of the talc. However, due to the ill-defined geometry of the interacting surfaces and the roughness of the samples, the force curves could be only analyzed semiquantitatively.42 In the present study, we investigated the capability of AFM colloidal probe technique to measure anisotropic surface charge properties of muscovite mica. Muscovite mica was selected as a representative of 2:1 clays because mica occurs in large platy crystals and has an excellent cleavage property to produce molecularly smooth basal planes. For this reason, the basal plane of muscovite mica has been a common substrate in surface chemical studies, particularly in those employing surface forces apparatus (SFA)43-45 and AFM. A microtome cutting technique was developed in this study to make a smooth muscovite edge surface suitable for AFM study. In the force measurements, a colloidal probe (a spherical silica particle) is attached to the end of an AFM cantilever. The interaction forces between the probe and a planar surface of clay basal planes or an edge surface are measured in aqueous solutions. Compared to the use of a sharp AFM tip as a probe, the advantage of the colloidal probe technique is the improved signal-to-noise ratio of force profiles, whereby allowing results to be interpreted in terms of the energy per unit area of known geometry. This approach uses a larger contact (30) Ploehn, H. J.; Liu, C. Ind. Eng. Chem. Res. 2006, 45, 7025–7035. (31) Garnaes, J.; Lindgreen, H.; Hansen, P. L.; Gould, S. A. C.; Hansma, P. K. Ultramicroscopy 1992, 42-44, 1428–1432. (32) Lindgreen, H.; Garnes, J.; Hansen, P. L.; Besenbacher, F.; Legsgaard, E.; Stensgaard, I.; Gould, S. A. C.; Hansma, P. K. Am. Mineral. 1991, 76, 1218–1222. (33) Tournassat, C.; Neaman, A.; Villieras, F.; Bosbach, D.; Charlet, L. Am. Mineral. 2003, 88, 1989–1995. (34) Bickmore, B. R.; Bosbach, D., Jr.; M. F. H.Charlet, L.; Rufe, E. Am. Mineral. 2001, 86, 411–423. (35) Baba, M.; Kakitani, S.; Ishii, H.; Okuno, T. Chem. Phys. 1997, 221, 23–31. (36) Kuwahara, K. Phys. Chem. Miner. 1999, 26, 198–205. (37) Nishimura, S.; Biggs, S.; Scales, P. J.; Healy, T. W.; Tsunematsu, K.; Tateyama, T. Langmuir 1994, 10, 4554–4559. (38) Sharp, T. G.; Oden, P. I.; Buseck, P. R. Surf. Sci. Lett. 1993, 284, L405-L410. (39) Nishimura, S.; Kodama, M.; Yao, K.; Imai, Y.; Tateyama, H. Langmuir 2002, 18, 4681–4688. (40) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207–2214. (41) Long, J.; Xu, Z.; Masliyah, J. H. Colloids Surf. A 2006, 281, 202–214. (42) Nalaskowski, J.; Abdul, B.; Du, H.; Miller, J. D. Proceedings of The Sixth UBC-McGill-UA International Symposium on Fundamental of Mineral Processing: Interfacial Phenomena in Fine Particle Technology, Montreal, Quebec, Canada; 2006; pp 73-87. (43) Israelachvili, J. N.; Adams, G. E. Nature 1976, 262, 774–777. (44) Claesson, P. M.; Christenson, H. K.; Berg, J. M.; Neuman, R. D. J. Colloid Interface Sci. 1995, 172, 415–424. (45) Dunstan, D. E. Langmuir 1992, 8, 740–743. (46) Zlobik, A. B. Mica; Bureau of Mines: Washington, DC, 1979.
Probing Surface Charge Potentials of Clay
Figure 1. Muscovite mica. (a) Profile of the molecular structure of muscovite; blue circles denote oxygen, red circles denote potassium, turquoise circles denote aluminum atoms, green circles denote hydroxyls, and pink circles are silicon atoms. (b) XRD graph of the muscovite mica used in this study.
area, which allows the measurements of average surface charge properties of the studied basal planes and edge surfaces. The objectives of the present work are to find out how the anisotropic surface charge properties of the clay influence the silica-clay colloidal interactions in aqueous solutions and to determine the surface charge properties of both clay basal planes and edge surfaces. Such an approach might open doors for more extensive studies of anisotropic clay surface properties for several scientific and engineering applications. In order to obtain the surface charge properties, the measured force profiles were fitted with DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. To account for the effect of surface roughness of clay edges on the interaction forces, a surface element integration method was adapted to calculate the DLVO forces, designated as SEI-DLVO model in this paper.
Experimental Section Materials. Muscovite mica supplied by S & J Trading (Glen Oaks, NY) was used in this study. Natural muscovite mica layer is structured in a series of three sheets (Figure 1a): an octahedral sheet of alumina (AlOOH) sandwiched between two identical tetrahedral sheets of silica. For the two silica sheets, about one-fourth of the Si4+ ions are replaced by Al3+ ions, so the 2:1 sheet has a negative charge, balanced by an interlayer of K+ ions. The composition of muscovite is often given by the formula H2KAl3(SiO4)3.46 There is a diversity of muscovite micas in nature. The mica used in this study was examined by X-ray diffraction (XRD) analysis. From the XRD patterns shown in Figure 1b, the determined formula of the mica is KAl2(Si3Al)O10(OH,F)2, containing a trace amount of fluorine. Silica microspheres of 8-µm diameter, purchased from Duke Scientific Co. (Fremont, CA), were used as silica probes for colloidal force measurements. Silica wafers (Silicon Valley Microelectronics Inc., Santa Clara, CA) were used as substrate to support clay surfaces
Langmuir, Vol. 24, No. 22, 2008 12901 for force measurements. Ultrahigh purity KCl (>99.999%, Aldrich) was used to prepare supporting electrolyte solutions. Reagent grade HCl and NaOH were used as pH modifiers. Deionized water with a resistivity of 18.2 MΩ cm, prepared with an Elix 5 followed by a Millipore-UV Plus Ultra water purification system (Millipore Inc.), was used throughout this study where applicable. Surface Preparation. The basal plane of muscovite mica was freshly cleaved using a sticky tape in a dust free, horizontal laminar flow hood (NuAire, Inc., Plymouth, MN), immediately prior to being mounted in the AFM. Due to the fragile nature of the clay, the preparation of an appropriate clay edge surface for AFM study is extremely difficult. In this study, a microtome cutting technique was developed to prepare suitable muscovite edge surfaces for AFM study. First, a very small and thin muscovite piece was embedded in epoxy resin (LECO, Corp., St. Joseph, MI). Using a special AFM holder (Leica Microsystems Inc.) the muscovite block was mounted on a Reichert Jung UltraCut E Microtome instrument (Leica Microsystems Inc.). The epoxy block position was adjusted under the microscope to make the muscovite sheets as perpendicular as possible to the cutting edge of the knife in order to make a cutting surface with less slanting angles. The surface was cut following the same routine for thin sectioning using the microtome. The muscovite block was trimmed with a glass knife followed by using a diamond knife to cut the edges. The final cut was performed with an ultra AFM diamond knife, supplied by Diatome AG (Biel, Switzerland). Instead of collecting thin sections, the remaining block with the cutting edge held in the AFM holder was collected for further AFM study. The obtained muscovite in epoxy block was taken off the AFM holder without touching the finished edge surface, and glued on a clean silica wafer. The glue was allowed to dry for at least 24 h. Before being used in the force measurements, the edge surface was subjected to high-pressure nitrogen gas to remove any possible muscovite flakes on the surface, originating from possible foldingover of muscovite edges. The edge surface was rinsed with deionized water and ethanol. It was cleaned by a plasma cleaner (Harrick Plasma, Ithaca, NY) to remove any organic contaminants from the edge surface immediately prior to its use. SEM Imaging and Energy-DispersiVe X-ray (EDX) Analysis. The morphology of the obtained muscovite basal and edge surfaces were examined with a Hitachi S-2700 scanning electron microscope (SEM) equipped with a PGT (Princeton Gamma-Tech) IMIX digital imaging system. After AFM force measurements, the muscovite edge surfaces were coated with a very thin layer of carbon and then examined with SEM using an accelerating voltage of 20 kV. Figure 2 shows typical SEM micrographs of the prepared muscovite basal and edge surfaces. The SEM micrographs of the muscovite basal plane, shown by Figure 2a at lower magnification, clearly show that the muscovite was characterized by lamellar form and large flat layers. The basal planar surface was found to be extremely smooth without any features at micron-size level, as shown by a higher magnification micrograph in Figure 2c. Figure 2b shows the large-scale image of the prepared muscovite edge surface at lower magnification, on which the gray region is the muscovite edge and the relatively dark region is epoxy. Although the whole cross section was not uniform, very smooth muscovite edge area could be easily identified. When the smooth part of the edge was scanned at a higher magnification, some tiny pits and debris were observed on the edge surfaces, which were possibly due to pull off and carryover of small muscovite pieces during cutting. The presence of these pits and debris on the edge surface is anticipated to affect the results of force measurements. Correction due to roughness will be discussed in the Results section. In order to confirm that the force measurements were correctly performed on muscovite edge rather than on epoxy, energy dispersive X-ray (EDX) analysis using a Princeton Gamma-Tech PRISM IG (Intrinsic Germanium) detector was performed after each test, for those selected locations where the force measurement were carried out. For instance, the elemental analysis of point “1” (Figure 2b_1) showed the dominant elements being aluminum, silicon, and potassium, which correspond to the main elements of muscovite. On the other hand, the dominant elements of point “3”
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Figure 2. Typical SEM micrographs of the muscovite mica basal plane and edge surface prepared by microtome cutting technique: images a and b were taken with lower resolution but larger scanning area; images c and d were taken on the corresponding smooth areas of surfaces a and b with higher resolution; images b_1 and b_3 are EDX element analysis results of corresponding points “1” (mica) and “3” (epoxy) labeled on surface b.
(Figure 2b_3) were carbon and chloride, which are main composition of epoxy. AFM Imaging. For the purpose of quantitative analysis of the surface roughness, the prepared muscovite basal plane and edge surfaces were scanned by tapping mode using a Nanoscope IIIa atomic force microscope (Digital Instruments, Inc., Santa Barbara, CA) with a J scanner at ambient conditions. The tapping mode silicon probe (RTESP) with resonant frequency of ca. 280 kHz and spring constant of 20 N/m was used. Height and phase images were recorded simultaneously at the resonant frequency of the cantilever with a scan rate of 0.5-1 Hz. The tapping mode is the preferred mode for soft or brittle materials to minimize possible damage to the sample. The atomic resolution image of muscovite basal plane was scanned with an E scanner and a commercial triangular silicon nitride AFM cantilever (Veeco, Inc., Santa Barbara, CA) under contact mode. Figure 3 shows representative AFM images of the two surfaces. For comparison, the same color scale and scanning size were used. The muscovite basal plane was very smooth with a root-meansquare (rms or Rq) roughness of 0.32 nm over 2 µm2, which allowed for the atomic resolution image shown by the 6 nm × 6 nm inset
image of Figure 3a. The large molecularly smooth area of muscovite meets the requirement of AFM force measurements. The AFM image of muscovite edge surface prepared by microtome cutting technique in Figure 3b shows a rms of 3.9 nm over 2 µm2. As shown in Figure 2, the features on the edge surface were not uniform. The surface was therefore scanned at many locations. The average rms value for the obtained AFM edge images ranged from 3 to 12 nm over 2 µm2. Atomic Force Microscope (AFM) Force Measurements. The interaction forces between a silica probe and muscovite surfaces were measured using a multimode PicoForce atomic force microscope (Digital Instruments, Inc., Santa Barbara, CA) upgraded from Nanoscope IIIa AFM. The force measurements were carried out in aqueous solutions with a vendor-provided liquid cell. NP-OW tipless cantilevers, purchased from Veeco Probes, were used. Cantilevers with a manufacturer’s nominal spring constant of 0.58 N/m were chosen for the force measurements. A colloidal silica sphere of 8 µm in diameter was glued to the cantilever at the location close to the apex using an extremely small amount of epoxy resin. The spring constant of the cantilever with a glued particle was determined by the thermal tune method, a function available in the PicoForce AFM. The calibration was performed in testing liquids. The prepared probes
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Langmuir, Vol. 24, No. 22, 2008 12903
Figure 3. AFM images of prepared muscovite mica surfaces. (a) Mica basal plane with a mean roughness of 0.322 nm (Rq) and 0.257 nm (Ra) over 2 µm2. Inset: a 6 nm × nm molecular-scale image of the mica basal plane. (b) Mica edge surface prepared by microtome cutting technique with a mean roughness of 3.927 nm (Rq) and 2.978 nm (Ra) over 2 µm2.
were cleaned by a plasma cleaner, immediately prior to their use in the force measurements. After force measurements, the silica probes were examined under an optical microscope and the size of each probe was determined by analyzing the microscopic probe images. Colloidal force measurements were carried out in 1 mM KCl solutions of various pH values. After the cantilever with silica probe and flat muscovite surface were assembled, the empty fluid cell was cautiously flushed with 3 mL of 1 mM KCl solutions. Prior to collecting force curves, the system of both probe and clay surfaces immersed in the aqueous solution was allowed to stabilize for 20-30 min, after which the measured force profiles did not change with time. For each solution, the forces were measured at various locations. For each location 20 approaching and retracting force curves were recorded. After the measurement for a given pH value, the solution was sucked out gently from the fluid cell, which was flushed and filled with the next solution of different pH. For each type of surfaces, more than three pairs of probe-clay were measured. All the experiments were conducted at room temperature (22 ( 1 °C). A detailed description of the use of the AFM in colloidal force measurements is provided elsewhere.25 Briefly, the surface is brought toward and away from the colloid probe, during which the interaction forces cause the cantilever to deflect. The deflection of the cantilever is measured by a laser beam reflecting off the cantilever onto a position-sensitive photodiode. The force acting between probe and substrate is determined from the deflection of the cantilever and its spring constant using Hooke’s law, F ) kD, where D represents the deflection and k the spring constant of the cantilever. Zero separation is defined from the force profile as the onset of the “constant compliance” region, where the deflection of the cantilever is linear with respect to surface displacement. Surface separation is estimated from the displacement of the lower surface relative to this constant compliance region. When a sample surface approaches a probe, the long-range interaction force between the two surfaces is measured while the adhesion (or pull-off) force can be obtained during the retraction process after contact has been made between the probe and the planar surface. In this study, the obtained force profiles were batch processed using AFM software SPIP (Image Metrology), which allows the conversion of the raw AFM force data into force-separation profiles. The user-entered parameters are the spring constant and model sphere radius. For quantitative comparison, the measured long-range interaction force (F) and adhesion force (Fadh) were normalized by the probe radius (R). The maximum loading force used in the force measurement was in the range of 5-8 mN/m.
To determine the dominant long-range forces between silica and muscovite surfaces and to examine the surface charge properties of muscovite surfaces in an aqueous medium, the classical DLVO theory, which considers only electrostatic double layer and van der Waals forces,47 were used to analyze the measured long-range interaction forces. The van der Waals forces were calculated by Hamaker’s microscopic method in the form of a sphere of radius R interacting with a flat surface at a distance D,47 shown by eq 1
Fv A )- 2 R 6D
(1)
where Fv is the van der Waals force; R, the radius of a sphere probe; A, the Hamaker constant; and D, the distance between surfaces. The Hamaker constant A131 for silica/water/silica system was taken as 0.85 × 10-20 J.48 For silica/water/muscovite system, the value of the Hamaker constant A132 ) 1.2 × 10-20 J, where subscripts 1, 3, and 2 represent silica, water, and muscovite, respectively. It was evaluated using the approximation for nonretarded Hamaker constant: 48
( )( )
ε1 - ε3 ε2 - ε3 3 A132 ) kBT + 4 ε1 + ε3 ε2 + ε3 3hυe
(n12 - n32)(n22 - n32)
8√2 √n 2 + n 2√n 2 + n 2 √n 2 + n 2 + √n2 + n2 1 3 2 3 1 3 2 3
(
)
(7)
where kB is the Boltzmann constant, T is the absolute temperature, ε1 ()3.8) and ε2 ()5.4) are the dielectric constants of silica and muscovite, ε3 ()78.5) is the dielectric constant of the water medium, h is Planck’s constant, υe (≈3.3 × 1015 Hz) is the mean UV absorption frequency, n1 ()1.46) and n2 ()1.58) are the refractive indexes of silica and muscovite, and n3 ()1.33) is the refractive index of the intervening medium.49 The calculated Hamaker constant A132 (or (47) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press Ltd.: London, 1992. (48) Hunter, R. J. Introduction to Modern Colloid Science, 1st ed.; Oxford University Press: New York, 2002. (49) Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces; Wiley-VCH GmbH & Co.: Weinheim, Germany, 2003.
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Figure 4. Surface element integration (SEI) methods: (a) original AFM image of the muscovite edge surface; (b) computing AFM image used in SEI calculation is recreated from the AFM image in panel a. In parts c and d, the origin of the coordinate system (O) used for computation of the interaction energy is on the plane of the highest point of the rough surface. All distances are measured along the positive z-direction in this coordinate system. The distance of the smooth plate from this PQ plane (the resulting position of the lower smooth plate) of the rough surface is D. The local separation distance between the rough and the smooth plates at any position (x, y) is given by h.
denoted as Aswm) is very close to that used by Vakarelski et al. (1.0 × 10-20 J)50 and Hartley et al. (1.2 × 10-20 J).40 The electrostatic double-layer forces, on the other hand, were calculated by numerically solving the nonlinear Poisson-Boltzmann (PB) equation, eq 2, at either constant surface potential or constant charge density boundary conditions. The PB equation was solved in the form of interaction force per unit area (F(x), eq 3), which is integrated to get energy per area between two planar plates (UE(D), eq 4). Finally, Derjaguin’s approximation (eq 5) was used to calculate the electrical double-layer forces (FE(D)) between a sphere and a flat plate at separation distance D. A MATLAB program was developed to theoretically calculate the DLVO forces. During the DLVO fitting, Debye length (κ) was obtained by fitting the slope of the natural logarithm of AFM measured forces per unit length (log (F/R)) as a function of separation distance between interacting surfaces. The fitted κ was also compared with the calculated value through the Debye-Huckel theory. The electric surface potential (ψ) of each surface was set as an adjustable parameter.
εε0
( ) ( ( ) ) (
d2ψ ) -e dx2
F(x) ) kBT
∑ zini∞ exp i
∑ ni∞ exp i
UE ) -
-zieψ kBT
εε0 dψ -zieψ -1 kBT 2 dx
∫∞ DF(h) dh
FE(D) ) 2πUE(D) R
(2) 2
)
(3) (4) (5)
Surface Element Integration Method. Numerous calculations for the influence of surface roughness on DLVO interactions are reported in literature.51-57 Despite the diversity of theoretical (50) Vakarelski, I. U.; Ishimura, K.; Higashitani, K. J. Colloid Interface Sci. 2000, 227, 111–118. (51) Elimelech, M.; O’Melia, C. R. Langmuir 1990, 6, 1153–1163. (52) Bhattacharjee, S.; Chen, J. Y.; Elimelech, M. Colloids Surf. A 2000, 165, 143–156.
techniques for modeling surface roughness and the variety of interacting systems, nearly all studies on morphological heterogeneity demonstrate the intuitive notion that the presence of asperities substantially modifies the interaction energy between colloidal particles and surfaces. Therefore, in order to quantitatively predict the edge surface charge properties by fitting silica-muscovite interactions with DLVO theory, the effect of surface roughness should be included, as the obtained muscovite edge surface was not ideally smooth, especially compared with its basal plane. In our study, a method of reconstructing the surface topology based on atomic force microscopy in conjunction with surface element integration was used. The procedure of this method is depicted in Figure 4 and described below: Since the silica probe was fairly smooth, as shown by the inset SEM image of Figure 5, only the roughness of the muscovite edge was considered in the calculation. The rough edge surface used in the computation was numerically recreated with height parameters exported from AFM image files. The AFM images, usually of 2 × 2 µm2, were several scans on the areas of interest on the edge surface. The reconstructed computing surface was characterized by 511 × 511 square meshes (based on the number of scanning lines, 512), and each mesh was taken as a tiny patch parallel to the mean plane of the surface. Generated as such, the computing surface kept essentially the same feature of the scanned AFM image and had approximately the same root-mean-square roughness value. The interaction energy between the probe and the computing surface was calculated by the surface element integration method (SEI). In principle, the SEI method is a generalized form of Derjaguin’s integration method applied to the exact geometry of interacting surfaces.53,54,58 SEI computes the total interaction energy between two bodies by numerically integrating the interaction energy (53) Das, P. K.; Bhattacharjee, S. Langmuir 2005, 21, 4755–4764. (54) Bhattacharjee, S.; Elimelech, M. J. Colloid Interface Sci. 1997, 193, 273– 285. (55) Suresh, L.; Walz, J. Y. J. Colloid Interface Sci. 1997, 196, 177–190. (56) Walz, J. Y. AdV. Colloid Interface Sci. 1998, 74, 119–168. (57) Suresh, L.; Walz, J. Y. J. Colloid Interface Sci. 1996, 183, 199–213. (58) Hoek, E. M. V.; Agarwal, G. K. J. Colloid Interface Sci. 2006, 298, 50–58.
Probing Surface Charge Potentials of Clay
Langmuir, Vol. 24, No. 22, 2008 12905
Figure 5. Normalized interaction forces (F/R) between a silica probe and a silica flat surface in 1.0 mM KCl solutions at various pH values: the scattered force profiles correspond to experimental data, and the solid curves represent calculated DLVO results obtained by numerical solution of the nonlinear Poisson-Boltzmann equation calculated under constant potential boundary conditions. Hamaker constant was used in the fitting. The Debye lengths, obtained by fitting the slope of the natural log(F/R) vs separation distance, are listed in Table 1. The fitted values of the silica surface potential, ψsi, are -51, -56, and -65 mV at pH 5.6, 8.0, and 10, respectively. Table 1. Comparison of the Fitted and Calculated Debye Lengths Used in Fitting Interaction Force Profiles between Silica-Silica with DLVO Theory, and the Best Fitted Values of the Surface Potential (ψsi) with Literature Values of ζ-Potentials Obtained by Microelectrophoresis (MEP) and Streaming Potential (SP) Measurements, and AFM Fitting Method electrical potential, ψsi (mV) Debye length, κ-1 (nm) pH
fitted
calculated
5.6
9.6
9.6
8.0
7.9
9.6
10
7.9
8.7
a
from the literature fitted
values
-51 ( 3 -50 -60 -58 -50 -25 to -60 -48 to -62 -54 -53 ( 3 -60 -80 -80 -60 ∼-60 -70 to -85 -56 ( 3 -65
Background electrolyte was 1 mM NaNO3. was 2 mM NaCl.
b
method ref MEP MEP MEP SP AFM AFM AFM MEP MEP MEP SP AFM AFM MEP
76 40a 27a 40 40 27a 26b 76 40 27a 40 40 27 76
Background electrolyte
per unit area between opposing differential planar elements over the entire surfaces. Details of the SEI method and its application can be found elsewhere.52-54,58 The interaction between the rough mica surface and the colloidal probe was calculated in two steps. In the first step, the interaction energy per unit cross-sectional area of the rough surface and an infinite half-space comprising the probe material (silica) is evaluated (Figure 4c). Using the 2 µm × 2 µm area of the recreated surface (Figure 4b), the total energy between the halfspace and the rough substrate is computed by integrating the DLVO energy per unit area between silica and mica surfaces at each point on the 511 × 511 mesh. This integral is then divided by the projected area (4 µm2) to obtain the interaction energy per unit area between the silica surface and the rough substrate, Up-p. Finally Derjaguin’s approximation (F/R ) 2πUp-p) was used to convert the interaction energy per unit area to interaction forces between the silica probe and muscovite edge surface (Figure 4d). Here the separation D was the distance between the projected smooth plate and the plane of the highest point, PQ, on the edge surface. The actual local separation distance h for each patch of the computing surface was the sum of the separation distance D with the height difference between the
local patch and the highest point. Figure 4c depicts the parameters used in SEI calculations. In the present study, the minimum separation is defined as the first sphere-plate contact, i.e., the distance between the smooth plate and the highest point on the edge surface, shown in Figure 4c. It is different from the distance based on the spheremean plane of the rough surface, as is considered in many other studies.57,59 Because we assume that the interacting surfaces are incompressible, and the probe is large compared with the normal contact area; i.e. the probe will not reach other points lower than the highest one once it approaches the rough surface. In the SEI method, the electrostatic interaction energy per unit area between two flat plates was calculated using an analytical expression, the HHF-FP equation derived by Hogg et al.60 based on the linearized Poisson-Boltzmann equation under constant potential (CP) conditions. The HHF-FP equation given below is quite accurate to predict the interaction forces at larger separations or for the surfaces of low surface potentials.53 During the fitting, Debye length (κ) and electric surface potential of the probe (ψa) and each mesh point of the rough surface (ψb) were set as adjustable parameters. The van der Waals forces were calculated by Hamaker’s microscopic approach.
U(D) εε0κ ) {(ψa2 + ψb2)[1 - coth(κh)] + area 2 2ψaψb cos ech(κh)} (6) Results and Discussion In this section, the interaction forces between silica and muscovite basal planes and edge surfaces were measured in KCl solutions of varying pH values. DLVO simulation was used to fit the obtained AFM force profiles for the purpose of evaluating the surface charge properties of clay basal and edge surfaces. To carry out this task, silica potentials in solutions of interest were first determined by fitting silica-silica interactions. Because the obtained muscovite edge surface was relatively rough, especially compared with the basal plane, the surface element integration method was applied to calculation of the DLVO forces to account for the surface roughness. From the best fitting, the surface charge property of muscovite edge was determined. Silica Surface Potential Determination. In addition to electrophoresis and streaming potential measurements, another method available for the measurement of surface potentials of particle or planar plate is to fit the measured force profiles with DLVO theory.25,26,40,61 For a symmetric system, such as silica-silica system, the surface potential of silica can be determined by DLVO fitting when the Hamaker constant and electrolyte concentrations are known. However, for an asymmetric system, for example, a silica-muscovite system, one cannot obtain unique surface potentials of the two interacting surfaces by fitting the force profile with DLVO theory.27 To obtain the unknown surface potential of muscovite through the fitting method, it is therefore necessary to know the surface potential of the silica probe for each testing condition. In this study, independent force measurements were carried out between a silica probe and a planar silica substrate in 1 mM KCl solutions of varying pH values. The measured force profiles are shown in Figure 5. As expected, a repulsive force was observed as the silica surfaces are similarly charged. The value of the repulsive force increased with increasing solution pH. This is to be expected as the silica surfaces are progressively more (59) Bhattacharjee, S.; Ko, C. H.; Elimelech, M. Langmuir 1998, 14, 3365– 3375. (60) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638–1651. (61) Veeramasuneni, S.; Yalamanchili, M. R.; Miller, J. D. J. Colloid Interface Sci. 1996, 184, 594–600.
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dissociated with increasing solution pH, leading to more SiOon the surface. Double layer forces, calculated by numerically solving the Poisson-Boltzmann equation at constant surface potential boundary condition, and van der Waals forces were calculated for a given surface potential to fit the measured force profiles with the DLVO theory. It is implicitly assumed that there is no difference between the potentials of silica probe and planar silica substrate. All the fitting parameters are listed in Table 1. In each fitting, the value of Debye length was obtained by fitting the gradient value of the repulsive force on the natural log (F/R) versus separation distance. The fitted Debye length agreed well with the values calculated on the basis of the concentration of ions at pH 5.6, which is the natural pH of 1 mM KCl solution, equilibrated with atmospheric carbon dioxide. However, the fitted Debye lengths were slightly smaller than the calculated values at higher pHs, which might result from the additional ions added to adjust the pH or more CO2 dissolved into the solutions of high pH. The experimental force-separation distance curves were found to be in a reasonable agreement with theoretical predictions of DLVO, as shown by the solid lines in Figure 5. At very short separation distances, a discrepancy between the experimental data and DLVO theory was observed for all the pH values investigated. This discrepancy could be attributed to the presence of the hydration forces between the silica surfaces and/or surface roughness, as well described in literature.25,62-64 The fitted surface potentials of silica from AFM measurements at pH 5.6, 8.0, and 10 were -51 ( 3, -56 ( 3, and -65 ( 3 mV, respectively. As shown in Table 1, the fitted surface potential values agreed well with the measured ζ-potential values using other methods.26,27 The derived surface potentials of silica from symmetrical silica-silica system were taken as known values in the following theoretical fitting of the force profiles for silica-muscovite systems. Interactions between Silica and Muscovite Basal Plane. The forces measured between a silica probe and a muscovite basal plane in 1 mM KCl solutions of varying pH values of 5.6, 8.0, and 10 are shown in Figure 6. For all the pHs tested, the interaction forces were monotonically repulsive on approach. More importantly, the measured repulsive force profiles were nearly the same. There was no adhesion when the probe was pulled away from the substrate for all the cases. A similar observation was reported by Toikka and Hayes.26 The obtained AFM force profiles for the silica probe and muscovite basal plane system were analyzed by DLVO theory for the purpose of quantitatively evaluating the surface potential of the muscovite basal plane at various pHs. In the fitting, the surface potential of silica derived above was used, while the surface potential of muscovite basal plane, the only unknown parameter, was adjusted until the theoretical and experimental force profiles overlap. The theoretical force profiles were calculated for both constant surface potential and constant surface charge density boundary conditions. The fitted surface potential and Debye length are given in Table 2. In general, the fitted Debye lengths were in reasonable agreement with the values calculated on the basis of the concentration of ions. As in the case for silica-silica interactions, a slightly lower value than that calculated was observed at pH 10. At a separation distance greater than 5 nm, the measured (62) Yoon, R.-H.; Vivek, S. J. Colloid Interface Sci. 1998, 204, 179–186. (63) Valle-Delgado, J. J.; Molina-Bolivar, J. A.; Galisteo-Gonzalez, F.; GalvezRuiz, M. J.; Feiler, A.; Rutland, M. W. J. Chem. Phys. 2005, 123, 034708/1–12. (64) Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375–383. (65) Scales, P. J.; Healy, T. W.; Evans, D. F. J. Colloid Interface Sci. 1988, 124, 391–395.
Zhao et al.
Figure 6. DLVO fitting of normalized long-range forces between a silica probe and a mica basal surface in 1 mM KCl solutions at pH 5.6, 8.1, and 9.9. Symbols correspond to experimental data. Solid and dashed curves represent fitting results of calculated DLVO forces obtained by numerically solving the nonlinear Poisson-Boltzmann equation under constant charge (upper, dashed) and constant potential (lower, solid) boundary conditions. Hamaker constant Aswm ) 1.2 × 10-20 was used in the fitting. The fitted Debye length and surface potentials of both silica and mica basal plane are listed in Table 2. Table 2. Comparison of the Fitted and Calculated Debye Lengths Used in Fitting Interaction Force Profiles Measured between Silica-Muscovite Basal Planes with DLVO Theory, and the Best Fitted Values of the Surface Potential (ψmb) with Literature Values of ζ-Potentials Obtained by Microelectrophoresis (MEP), Streaming Potential (SP), Surface Force Apparatus (SFA) and Electro-Osmotic (EO) Measurements, and AFM Fitting Method surface potential of muscovite basal plane, ψmb (mV) Debye length, κ-1 (nm)
from the literature
pH
fitted
calculated
fitted
value
method
ref
5.6
9.2
9.6
-80 ( 5
8.1
9.2
9.6
-78 ( 5
10
7.7
8.7
-78 ( 5
∼-72 -68 ∼-80 -77 -50 to -130 -60 to -90 -73 -100 ∼-80 -74 -50 to -130 -65 to -70 ∼-80 -47 -100 ( 27.1 -58 ( 2.3
SP SP SP EO SFA AFM AFM AFM SP SP SFA AFM SP SP AFM AFM
65 40a 77 66 67b 40 26c 68 65 40 67 40 65 69 69d 69e
a Background electrolyte was NaNO3. b Mica surface potential varied with different micas. Background electrolyte was NaNO3. c Background electrolyte was 2 mM NaCl. d Double layer force was calculated at constant potential boundary conditions. e Double layer force was calculated at constant charge boundary condition.
long-range force profiles agreed reasonable well with DLVO theory under either constant charge density or constant potential conditions. At shorter separation distance, the measured force profiles were sandwiched between these two fitting curves. The fitted surface potentials of muscovite basal plane were around -80 ( 3, -78 ( 3, and -78 ( 3 mV for pH 5.6, 8.0, and 10, respectively. The differences in these surface potential values could be considered within experimental error, indicating that the surface charge of muscovite was pH independent.
Probing Surface Charge Potentials of Clay
The surface potentials derived from this study are compared with the results from literature26,40,65-69 in Table 2. Despite the variety of micas and experimental methods, the obtained surface potential values of muscovite basal plane were close to the ζ-potential values obtained using other methods, such as streaming potential measurements, electro-osmotic potential measurements, and surface force measurements. Particularly, there was an excellent agreement between our results and those obtained by Scales et al.70 using a flat-plate streaming potential apparatus. Furthermore, the pH independence of surface potential or ζ-potential over a pH range from 5.6 to 10 was also observed by other researchers.40,65,67 Scales et al.65 reported that from pH 6 to 10, the ζ-potential of mica in solutions of constant ionic strength remained constant. From surface force measurements, Israelachvilli and Adams67 found that different micas exhibited different surface potentials (varying between -50 and -130 mV), but those potentials were insensitive to pH change from 5.5-7.0. Hartley et al.40 observed that, above pH 6.9, the ζ-potential of mica remained at -75 mV. As mentioned earlier, the pH independence of the muscovite mica is attributed to the charge mechanism of basal plane by isomorphic substitution of lattice elements, i.e. about one-fourth of Si4+ are replaced by Al3+ in the tetrahedral layer. The dominant siloxane group (Si-O-Si) on the basal plane of a 2:1 phyllosilicates is very 15 inert with a protonation constant of log Kint H ) -16.9, indicating the absence of any significant protonation at pH values of our study. Interactions between Silica and Muscovite Edges. Force Measurements between Silica and MuscoVite Edge. Forces between a silica probe and the muscovite edge surface in 1 mM KCl solutions of varying pH were measured in order to study silica-muscovite edge interactions and to determine the surface charge of the edges. Due to the roughness of the prepared edge surfaces, the locations on which force measurements were performed were carefully selected with the aid of an optical microscope attached to the AFM. A set of typical force profiles recorded from various locations on one edge sample at pH 5.6 and 10 is shown in Figure 7. Each force curve in this figure represents the average trend of about 20 force profiles collected on each location. Although highly variable, it is evident that the measured long-range forces were repulsive at pH 10 but attractive or zero at pH 5.6. The average adhesion measured at pH 10 is much smaller than that measured at pH 5.6. Such a scatter indicates that the absolute magnitude of the measured forces is sensitive to the contacting position where the force measurement is taken. This level of scatter is attributed to the random roughness of the muscovite edge surface. To minimize the effect of surface roughness on the measured forces, i.e., to reduce the scatter in force curves, a new procedure was developed. For a given pH, the measurements were performed on 10 randomly selected locations over a “smooth” area under the optical microscope. Then the location leading to the most reproducible force profiles and the maximum force values within a separation distance 2-15 nm between two interacting surfaces was targeted. After identifying this location, the force measurements were carried out for all other pH values at the target location (66) Debacher, N.; Ottewill, R. H. Colloids Surf. 1992, 65, 51–59. (67) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975–1001. (68) Atkins, D. T.; Pashley, R. M. Langmuir 1993, 9, 2232–36. (69) Brant, J. A.; Johnson, K. M.; Childress, A. E. J. Membr. Sci. 2006, 276, 286–294. (70) Scales, P. J.; Grieser, F.; Healy, T. W.; White, L. R.; Chan, D. Y. C. Langmuir 1992, 8, 965–74. (71) Stumm, W. Chemistry of the Solid-Water Interface Processes at the Mineral-Water and Particle-Water Interface in Natural Systems; John Wiley & Sons, Inc.: New York, 1992.
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Figure 7. Distribution of the measured interaction forces between a silica probe and a muscovite mica edge surface at about pH 5.6 (empty symbols) and pH 9.9 (solid symbols). The measurements were carried out at randomly selected locations on the edge surface. (a) Long-range interactions; each force curve is a representative profile selected from 20 measured force profiles collected from one contact location. (b) Adhesion forces; each symbol represents the adhesion force of every force profile collected at one contact location.
and two other locations about 100 nm away from the target using AFM offset function. One representative set of results following such a procedure is shown in Figure 8. Clearly, the measured force profiles became much less scattered, thereby allowing roughness correction for quantitative estimation of the surface potentials of the edge surfaces at different pHs. In Figure 8, the long-range interactions are repulsive at pH 10, reduce to almost zero at pH 8.0, and reverse to be attractive at pH 5.6. The adhesion forces, on the other hand, increase with decreasing pH. Both the measured long-range and adhesion forces as a function of pH explicitly suggest that the surface charge of muscovite edges are pH-dependent, as anticipated. It is expected that for solutions of pH values above the point of zero charge (PZC) of the silica and muscovite edge surfaces, an electrostatic repulsion between the unequally charged negative surfaces is expected, which decreases in magnitude with decreasing solution pH. For solutions of pH values between the PZC of the silica and the muscovite, an electrostatic attraction between the oppositely charged surfaces is expected. The force profiles obtained with silica and muscovite edge surfaces in simple electrolyte solutions of pH below 6.5 agreed very well with such expectations. In this study, the force measurements were always carried out at pHs above the PZC of silica (pH 2-340,70,71); i.e., silica was always negatively charged over the pH range of 5.6-10 investigated. Therefore, the observed attractive force at pH 5.6 strongly suggests a positively charged mica edge surface at this pH, though it might not be true, since an attractive electrostatic force between colloids with the same sign of surface potential would possibly exist based on the Poisson-Boltzmann predic-
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Figure 8. Normalized interactions (F/R) of a silica probe with a muscovite mica edge surface in 1.0 mM KCl solutions at various pHs. (a) Longrange interactions; solid scatter profiles represent silica-muscovite edge interactions, while empty scatters represent silica-muscovite basal plane interactions. (b) Adhesions measured at three locations.
tion.72 The PZC of the edge surface is a very important parameter, as it allows us to evaluate the critical role of anisotropic characteristics of clay surface charges in many interfacial sciencebased chemical processes, such as adsorption, stability of clay suspension in drilling fluids, rheology in ceramic and composite materials processing, waste and portable water management, and so forth. In order to obtain quantitative surface potential of mica edge so that we could explore anisotropic properties of clay surfaces for more extensive engineering and scientific interests, it is necessary to fit the measured force profiles to the DLVO theory. However, the measured forces between the silica probe and mica edge surface could not be reasonably fitted by the DLVO theory, even with very small surface potential values. Effect of Surface Roughness on Interactions. The magnitude of measured long-range forces between silica and muscovite edges in Figure 8a were found to be unexpectedly smaller than the repulsive forces between silica and muscovite basal plane. Although the significant reduction in the magnitude of the measured forces could be attributed to the anisotropic surface charge properties of two different muscovite surfaces, also it is quite possible that the morphological heterogeneities of the muscovite edge surfaces play an important role. Surface roughness was well-documented to substantially reduce the interaction energies between two interacting surfaces, the extent of which depends on the size of the asperities and the number of asperities on the surface.51,52,55-59 To account for the morphological heterogeneities in estimating surface potential from the measured force profiles, surface element integration method was used to calculate DLVO forces between the silica probe and mica edge surface recreated from its AFM images. In this regard, several AFM images with their own distinct (72) Barouch, E.; Matuevic, E. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1797–1817.
Zhao et al.
Figure 9. Effect of surface roughness on the interaction forces between silica probe and muscovite mica edge surface in 1 mM electrolyte solutions: (a) van der Waals forces calculated by surface element integration method using Aswm ) 1.2 × 10-20 and (b) long-range interactions by surface element integration method using the HHF equation under constant potential boundary conditions. Both silica and edge surface are similarly charged by -65 mV. In the legend, Rq, rootmean-square roughness, denotes the standard deviation of an entire distribution of z-values of an AFM image; Ra, average roughness, is the average deviation of the measured z-values from the mean-plane of an AFM image; hmax denotes the largest z-values of an AFM image.
roughness properties scanned on interested areas of the mica edge surfaces were reconstructed to match their roughness, as quantified by either the root-mean-square roughness (Rq) or average roughness (Ra), and then used in SEI-DLVO calculations. For instance, the AFM images A1-A4 were obtained by scanning the edge surface at locations where the force measurements were performed. The effect of the surface roughness on van der Waals force and on long-range force is shown in Figure 9. In Figure 9a, each dotted line represents the van der Waals forces calculated between a silica probe and a reconstructed mica edge surface using the SEI method. The solid line represents the van der Waals force between a probe and a smooth surface, also calculated using the SEI method. Figure 9a clearly shows that the van der Waals forces are dramatically decreased when surface roughness is considered, and the extent of the reduction varies from location to location of varying roughness. By quantitatively comparing the forces at 10.3 nm, it was found that the van der Waals forces for rough edge surface were about 5-10 times less than the forces for a smooth surface. The similar reduction in the electrostatic double layer forces between silica and rough mica edge surfaces, calculated by the SEI method using the HHF equation under constant surface potentials of -65 mV for both interacting surfaces, was observed. The reduction was found to be sensitive to surface roughness parameters of Ra or Rq (distributions of z-values). The highest peak-to-valley height of asperities (the largest z-value) was also found to determine the extent of reduction, as the zero separation distance between the silica probe and mica edge surface was
Probing Surface Charge Potentials of Clay
Figure 10. Normalized long-range forces between a silica probe and a muscovite edge surface in 1 mM KCl solutions at pH 5.6, 8.0, and 10 (scattered profiles) with DLVO fitting results obtained by surface element integration method in corporation of roughness effect using the HHF equation under constant potential boundary conditions. In the fitting, Aswm ) 1.2 × 10-20 was used. The fitted surface potentials of both silica and muscovite edge are listed in the figure.
Figure 11. Fitted muscovite surface potentials as a function of pH using different computing AFM images. The point of zero charge for the muscovite edge is determined to be within pH 7-8 (∼7.5).
defined in the SEI calculation by the first contact of probe with the highest asperity of the surface rather than with the mean plane of the rough surface. It is important to note that once the surface roughness is included in the force calculations, the resultant van der Waals forces have negligible contribution to the total long-range forces, even at very short separation distance, where it dominates for smooth surface as shown in Figure 9b. Determination of Mica Edge Surface Charge. On the basis of the above analysis, the measured force profiles using silica probe and mica edge surfaces were fitted with the SEI-DLVO model, which includes consideration of surface roughness. In the fitting, the silica surface potential derived in section 3 and Debye lengths for silica-mica basal plane systems were used. By varying the only free parameter of the surface potential of mica edge surface, the experimental force profiles could be fitted well with the SEIDLVO model based on the AFM images of mica edge surfaces, as shown in Figure 10. From the fitting, the surface potential values of -40, -5, and 8 mV were obtained for mica edges in aqueous solutions of pH 10, 8.0, and 5.6, respectively. On the basis of the roughness corrected results, the true PZC of mica edges should be between pH 5.6 and 8.0. Since the images used in the SEI calculation could not be confirmed to be scanned exactly on the spot where the force measurements were performed, the determined surface potentials of mica edges cannot be considered to be the exact values, but they are at least semiquantitative. The inaccuracy resulted from the fact that the colloidal forces were measured with a microsized probe, while an AFM cantilever with a sharp tip was used to
Langmuir, Vol. 24, No. 22, 2008 12909
obtain a good AFM image of detailed morphological characteristics. Unfortunately, switching the cantilevers to the probe particle precludes the alignment of the probe right back to the location where the AFM image is taken. One way to resolve this uncertainty is to use several representative images to carry out the calculations. The derived surface potentials for images A1-A4 were plotted as a function of pH in Figure 11. It was clear that, except image A1, fittings using images A2-A4 resulted in very similar values of surface potentials. Image A1, giving rise to the lowest interaction energy, was excluded in calculating the average surface potential values, as we limited our measurements on selected locations of the largest interaction energies, as aforementioned. In such a manner, the average potentials of mica edges were determined to be -40 mV at pH 10, -5 mV at pH 8.0, and 7 mV at pH 5.6. The standard deviations for these values were estimated to be about 8 mV. By plotting the potential vs pH curves, the PZC of the mica edge is estimated to be within pH 7-8 and can be narrowed to pH 7.3-7.6 on the basis of the most repeatable results from images A2-A3. It should be noted that the obtained PZC value lies between the respective isoelectric points of silica (pH 2-340,70,71) and gibbsite (pH 8-10), the major constitutes of edge surfaces.71,73 As in alumina, Al-OH is prone to protonation to become positively charged at pH
