Probing Surface Charge Potentials of Clay Basal Planes and Edges byDirect 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, andShell 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 andcomposite materials processing, paper making, cement production, and consumer product formulation. To fundamentallyunderstand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface chargeproperties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to studythe interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as arepresentative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edgesurface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropicsurface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonicallyrepulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planeshave permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces werefitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measuredforce profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measuredforces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, stronglysuggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not bereasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surfaceroughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces toaccount for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measuredforce profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surfacewas estimated to be pH 7-8.

Introduction

Clay minerals are extensively used in a wide range ofapplications, such as paper making, oil drilling, water pollutantremoval, oil sands industry, and consumer products.1-5 Due totheir distinctive surface chemical properties, the stability of claydispersions is of great importance in a number of industrialprocesses, as well as in soil chemistry and environmentalscience.5,6

The stability of clay dispersions is controlled largely by thecolloidal interaction forces between clay mineral particles. Mostclays in natural settings are of a phyllosilicate or sheet structure.The basic structural element of phyllosilicates can be simply

viewed as an octahedral sheet of aluminum or magnesiumhydroxides sandwiched between two sheets of tetrahedral silica,forming 2-D building blocks of so-called three-layer clayminerals.2 The 2-D building blocks are connected to each otherby interlayer cations or by van der Waals force, depending onthe type of clays. The cations are weakly bonded, often withwater molecules and/or other neutral atoms or molecules trappedbetween the sheets, which makes clay cleave easily. In dispersion,clays form laminar shape particles with distinguished basal planeand edge surface. For 2:1 phyllosilicates, the basal plane is theplane made of oxygen atoms being shared by other tetrahedrain the tetrahedral sheet. Usually, the basal plane has a permanentnegative charge (structural charge), resulting from variousisomorphic substitutions of lattice cations of a lower valancewithin the clay structure.3 At the edges of the platelike particles,on the other hand, the tetrahedral silica sheets and the octahedralalumina sheets are broken (disrupted), leading to broken primarybonds. Therefore, the electrical charge of the edge, arising fromhydrolysis reactions of broken Al-O and Si-O bonds, is pH-dependent. There is a strong evidence to indicate that the edgesof clay particles are positively charged in the neutral and acidpH ranges.7 The anisotropic surface charge of clay mineralsplays a critical role in determining the stability of clay mineralsuspensions.8 The importance of surface charge anisotropy waspointed out originally by van Olphen3 and then supported andelaborated further in many subsequent and recent investigations.

* To whom correspondence should be addressed. Phone: 1-780- 492-7667. Fax: 1-780-492-2881. E-mail: zhenghe.xu@ualberta.ca.

† Department of Chemical and Materials Engineering, University ofAlberta.

‡ 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.

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12899Langmuir 2008, 24, 12899-12910

10.1021/la802112h CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/17/2008

As a consequence of the different charge characteristics of claybasal and edge surfaces, three different modes of particleassociation 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 determinedby a rather delicate balance of the three potential curves ofinteraction for EF, EE, and FF association.3

An extensive and accurate knowledge of the surface propertiesof clays, especially phyllosilicates, is desirable in order to predictclay performance in a number of industrial applications. Whilenumerous studies focused on the basal plane of phyllosilicates,the surface chemistry of the edge surface is less well-known dueto the difficulty in experimentally isolating clay edge surfaces.Theoretically, many studies attempted to determine the edgesurface structure of phyllosilicates by ab initio molecular dynamicsimulations.10-12 The effective surface area, structure of the edgesites, site densities, and intrinsic acidity constants for the reactivesites were predicted on the basis of different models andcalculation methods by several researchers.10-12 Experimentally,potentiometric titration represents a main approach to study theacid-base properties of clay mineral surfaces.8,13-24 Despite avariety of the clay types used in the titration experiments, greatdifferences in the titration curves and subsequently in thethermodynamic constants were reported. One reason for suchdiscrepancy is the sensitivity of acid-base properties of claysto the protocols of sample preparation and titration measurementby different authors, as summarized by Duc et al.24 To date, anumber of models describing surface chemistry of clay edgesurfaces were used in literature. Bourg et al.22 summarized manyof these models available for the acid-base chemistry ofmontmorillonite surfaces.22 The diversity of models was attributedto the lack of data adequate to resolve the protonation chemistryof montmorillonite edge surfaces, compounded with the lack ofaccurate estimates of edge surface area and independentmeasurements of electric potential of edge surfaces.

Atomic force microscopy (AFM) has been widely used in thestudy of a wide range of surfaces since early 1990, includingclay surfaces.25-29 AFM imaging has been used for quantitative

analysis of clay platelet size, shape, and thickness;30-32 todetermine mean lateral surface area;33 study clay dissolution;34

and map cleavage basal planes at atomic resolution.32,35-38 TheAFM colloidal probe technique has been employed to measureface-to-face forces between synthetic smectites,39 colloidal forcesbetween mica and silica in aqueous solutions,26,40 and colloidaland adhesion forces between illite particles and cleaved illitesurfaces in aqueous solutions.41 However, there are only fewstudies that directly probe surface properties of clay edges inaqueous solutions. The challenge associated with using AFM toprobe the charge properties of clay edge surfaces is to preparea sufficiently smooth edge surface of clays, suitable for AFMmeasurements. To our knowledge, the first and only attempt toreveal the anisotropic character of clays by AFM colloidal probetechnique was performed by Nalaskowski et al.42 They attacheda 20 µm talc particle to the AFM cantilever and measured theforces between the edge of the talc particle and two differentcrystallographic planes of talc in aqueous solutions of variouspH values. Their measurements showed differences between theproperties of the basal plane and the edge of the talc. However,due to the ill-defined geometry of the interacting surfaces andthe roughness of the samples, the force curves could be onlyanalyzed semiquantitatively.42

In the present study, we investigated the capability of AFMcolloidal probe technique to measure anisotropic surface chargeproperties of muscovite mica. Muscovite mica was selected asa representative of 2:1 clays because mica occurs in large platycrystals and has an excellent cleavage property to producemolecularly smooth basal planes. For this reason, the basal planeof muscovite mica has been a common substrate in surfacechemical studies, particularly in those employing surface forcesapparatus (SFA)43-45 and AFM. A microtome cutting techniquewas developed in this study to make a smooth muscovite edgesurface suitable for AFM study. In the force measurements, acolloidal probe (a spherical silica particle) is attached to the endof an AFM cantilever. The interaction forces between the probeand a planar surface of clay basal planes or an edge surface aremeasured in aqueous solutions. Compared to the use of a sharpAFM tip as a probe, the advantage of the colloidal probe techniqueis the improved signal-to-noise ratio of force profiles, wherebyallowing results to be interpreted in terms of the energy per unitarea of known geometry. This approach uses a larger contact

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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.

(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.

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12900 Langmuir, Vol. 24, No. 22, 2008 Zhao et al.

area, which allows the measurements of average surface chargeproperties of the studied basal planes and edge surfaces.

The objectives of the present work are to find out how theanisotropic surface charge properties of the clay influence thesilica-clay colloidal interactions in aqueous solutions and todetermine the surface charge properties of both clay basal planesand edge surfaces. Such an approach might open doors for moreextensive studies of anisotropic clay surface properties for severalscientific and engineering applications. In order to obtain thesurface charge properties, the measured force profiles were fittedwith DLVO (Derjaguin-Landau-Verwey-Overbeek) theory.To account for the effect of surface roughness of clay edges onthe interaction forces, a surface element integration method wasadapted to calculate the DLVO forces, designated as SEI-DLVOmodel in this paper.

Experimental SectionMaterials. Muscovite mica supplied by S & J Trading (Glen

Oaks, NY) was used in this study. Natural muscovite mica layer isstructured in a series of three sheets (Figure 1a): an octahedral sheetof alumina (AlOOH) sandwiched between two identical tetrahedralsheets of silica. For the two silica sheets, about one-fourth of theSi4+ ions are replaced by Al3+ ions, so the 2:1 sheet has a negativecharge, balanced by an interlayer of K+ ions. The composition ofmuscovite is often given by the formula H2KAl3(SiO4)3.46 There isa diversity of muscovite micas in nature. The mica used in this studywas examined by X-ray diffraction (XRD) analysis. From the XRDpatterns shown in Figure 1b, the determined formula of the mica isKAl2(Si3Al)O10(OH,F)2, containing a trace amount of fluorine.

Silica microspheres of 8-µm diameter, purchased from DukeScientific Co. (Fremont, CA), were used as silica probes for colloidalforce measurements. Silica wafers (Silicon Valley MicroelectronicsInc., Santa Clara, CA) were used as substrate to support clay surfaces

for force measurements. Ultrahigh purity KCl (>99.999%, Aldrich)was used to prepare supporting electrolyte solutions. Reagent gradeHCl and NaOH were used as pH modifiers. Deionized water witha resistivity of 18.2 MΩ cm, prepared with an Elix 5 followed bya Millipore-UV Plus Ultra water purification system (Millipore Inc.),was used throughout this study where applicable.

Surface Preparation. The basal plane of muscovite mica wasfreshly cleaved using a sticky tape in a dust free, horizontal laminarflow hood (NuAire, Inc., Plymouth, MN), immediately prior to beingmounted in the AFM. Due to the fragile nature of the clay, thepreparation of an appropriate clay edge surface for AFM study isextremely difficult. In this study, a microtome cutting technique wasdeveloped to prepare suitable muscovite edge surfaces for AFMstudy. First, a very small and thin muscovite piece was embeddedin epoxy resin (LECO, Corp., St. Joseph, MI). Using a special AFMholder (Leica Microsystems Inc.) the muscovite block was mountedon a Reichert Jung UltraCut E Microtome instrument (LeicaMicrosystems Inc.). The epoxy block position was adjusted underthe microscope to make the muscovite sheets as perpendicular aspossible to the cutting edge of the knife in order to make a cuttingsurface with less slanting angles. The surface was cut following thesame routine for thin sectioning using the microtome. The muscoviteblock was trimmed with a glass knife followed by using a diamondknife to cut the edges. The final cut was performed with an ultraAFM diamond knife, supplied by Diatome AG (Biel, Switzerland).Instead of collecting thin sections, the remaining block with thecutting edge held in the AFM holder was collected for further AFMstudy. The obtained muscovite in epoxy block was taken off theAFM holder without touching the finished edge surface, and gluedon 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 wassubjected to high-pressure nitrogen gas to remove any possiblemuscovite flakes on the surface, originating from possible folding-over of muscovite edges. The edge surface was rinsed with deionizedwater and ethanol. It was cleaned by a plasma cleaner (HarrickPlasma, Ithaca, NY) to remove any organic contaminants from theedge surface immediately prior to its use.

SEM Imaging and Energy-DispersiVe X-ray (EDX) Analysis. Themorphology of the obtained muscovite basal and edge surfaces wereexamined with a Hitachi S-2700 scanning electron microscope (SEM)equipped with a PGT (Princeton Gamma-Tech) IMIX digital imagingsystem. After AFM force measurements, the muscovite edge surfaceswere coated with a very thin layer of carbon and then examined withSEM using an accelerating voltage of 20 kV. Figure 2 shows typicalSEM micrographs of the prepared muscovite basal and edge surfaces.The SEM micrographs of the muscovite basal plane, shown by Figure2a at lower magnification, clearly show that the muscovite wascharacterized by lamellar form and large flat layers. The basal planarsurface was found to be extremely smooth without any features atmicron-size level, as shown by a higher magnification micrographin Figure 2c. Figure 2b shows the large-scale image of the preparedmuscovite edge surface at lower magnification, on which the grayregion is the muscovite edge and the relatively dark region is epoxy.Although the whole cross section was not uniform, very smoothmuscovite edge area could be easily identified. When the smoothpart of the edge was scanned at a higher magnification, some tinypits and debris were observed on the edge surfaces, which werepossibly due to pull off and carryover of small muscovite piecesduring cutting. The presence of these pits and debris on the edgesurface 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 correctlyperformed on muscovite edge rather than on epoxy, energydispersive X-ray (EDX) analysis using a Princeton Gamma-TechPRISM IG (Intrinsic Germanium) detector was performed aftereach test, for those selected locations where the force measurementwere 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 ofmuscovite. On the other hand, the dominant elements of point “3”

Figure 1. Muscovite mica. (a) Profile of the molecular structure ofmuscovite; 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 micaused in this study.

Probing Surface Charge Potentials of Clay Langmuir, Vol. 24, No. 22, 2008 12901

(Figure 2b_3) were carbon and chloride, which are main compositionof epoxy.

AFM Imaging. For the purpose of quantitative analysis of thesurface roughness, the prepared muscovite basal plane and edgesurfaces were scanned by tapping mode using a Nanoscope IIIaatomic force microscope (Digital Instruments, Inc., Santa Barbara,CA) with a J scanner at ambient conditions. The tapping modesilicon probe (RTESP) with resonant frequency of ca. 280 kHz andspring constant of 20 N/m was used. Height and phase images wererecorded simultaneously at the resonant frequency of the cantileverwith a scan rate of 0.5-1 Hz. The tapping mode is the preferredmode for soft or brittle materials to minimize possible damage tothe sample. The atomic resolution image of muscovite basal planewas scanned with an E scanner and a commercial triangular siliconnitride AFM cantilever (Veeco, Inc., Santa Barbara, CA) under contactmode.

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-mean-square (rms or Rq) roughness of 0.32 nm over 2 µm2, which allowedfor the atomic resolution image shown by the 6 nm × 6 nm inset

image of Figure 3a. The large molecularly smooth area of muscovitemeets the requirement of AFM force measurements. The AFM imageof muscovite edge surface prepared by microtome cutting techniquein Figure 3b shows a rms of 3.9 nm over 2 µm2. As shown in Figure2, the features on the edge surface were not uniform. The surfacewas therefore scanned at many locations. The average rms value forthe obtained AFM edge images ranged from 3 to 12 nm over 2 µm2.

Atomic Force Microscope (AFM) Force Measurements. Theinteraction forces between a silica probe and muscovite surfaceswere measured using a multimode PicoForce atomic force microscope(Digital Instruments, Inc., Santa Barbara, CA) upgraded fromNanoscope IIIa AFM. The force measurements were carried out inaqueous solutions with a vendor-provided liquid cell. NP-OW tiplesscantilevers, purchased from Veeco Probes, were used. Cantileverswith a manufacturer’s nominal spring constant of 0.58 N/m werechosen for the force measurements. A colloidal silica sphere of 8µm in diameter was glued to the cantilever at the location close tothe apex using an extremely small amount of epoxy resin. The springconstant of the cantilever with a glued particle was determined bythe thermal tune method, a function available in the PicoForce AFM.The calibration was performed in testing liquids. The prepared probes

Figure 2. Typical SEM micrographs of the muscovite mica basal plane and edge surface prepared by microtome cutting technique: images a and b weretaken 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.

12902 Langmuir, Vol. 24, No. 22, 2008 Zhao et al.

were cleaned by a plasma cleaner, immediately prior to their use inthe force measurements. After force measurements, the silica probeswere examined under an optical microscope and the size of eachprobe was determined by analyzing the microscopic probe images.

Colloidal force measurements were carried out in 1 mM KClsolutions of various pH values. After the cantilever with silica probeand flat muscovite surface were assembled, the empty fluid cell wascautiously flushed with 3 mL of 1 mM KCl solutions. Prior tocollecting force curves, the system of both probe and clay surfacesimmersed in the aqueous solution was allowed to stabilize for 20-30min, after which the measured force profiles did not change withtime. For each solution, the forces were measured at various locations.For each location 20 approaching and retracting force curves wererecorded. After the measurement for a given pH value, the solutionwas sucked out gently from the fluid cell, which was flushed andfilled with the next solution of different pH. For each type of surfaces,more than three pairs of probe-clay were measured. All theexperiments were conducted at room temperature (22 ( 1 °C).

A detailed description of the use of the AFM in colloidal forcemeasurements is provided elsewhere.25 Briefly, the surface is broughttoward and away from the colloid probe, during which the interactionforces cause the cantilever to deflect. The deflection of the cantileveris measured by a laser beam reflecting off the cantilever onto aposition-sensitive photodiode. The force acting between probe andsubstrate is determined from the deflection of the cantilever and itsspring constant using Hooke’s law, F ) kD, where D represents thedeflection and k the spring constant of the cantilever. Zero separationis defined from the force profile as the onset of the “constantcompliance” region, where the deflection of the cantilever is linearwith respect to surface displacement. Surface separation is estimatedfrom the displacement of the lower surface relative to this constantcompliance region. When a sample surface approaches a probe, thelong-range interaction force between the two surfaces is measuredwhile the adhesion (or pull-off) force can be obtained during theretraction process after contact has been made between the probeand the planar surface.

In this study, the obtained force profiles were batch processedusing AFM software SPIP (Image Metrology), which allows theconversion of the raw AFM force data into force-separation profiles.The user-entered parameters are the spring constant and model sphereradius. For quantitative comparison, the measured long-rangeinteraction force (F) and adhesion force (Fadh) were normalized bythe probe radius (R). The maximum loading force used in the forcemeasurement was in the range of 5-8 mN/m.

To determine the dominant long-range forces between silica andmuscovite surfaces and to examine the surface charge properties ofmuscovite surfaces in an aqueous medium, the classical DLVO theory,which considers only electrostatic double layer and van der Waalsforces,47 were used to analyze the measured long-range interactionforces. The van der Waals forces were calculated by Hamaker’smicroscopic method in the form of a sphere of radius R interactingwith a flat surface at a distance D,47 shown by eq 1

Fv

R)- A

6D2(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. TheHamaker constant A131 for silica/water/silica system was taken as0.85 × 10-20 J.48 For silica/water/muscovite system, the value ofthe Hamaker constant A132 ) 1.2 × 10-20 J, where subscripts 1, 3,and 2 represent silica, water, and muscovite, respectively. It wasevaluated using the approximation for nonretarded Hamaker constant:48

A132 )34

kBT(ε1 - ε3

ε1 + ε3)(ε2 - ε3

ε2 + ε3)+

3hυe

8√2

(n12 - n3

2)(n22 - n3

2)

√n12 + n3

2√n22 + n3

2(√n12 + n3

2 + √n22 + n3

2)(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 andmuscovite, 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 absorptionfrequency, n1 ()1.46) and n2 ()1.58) are the refractive indexes ofsilica and muscovite, and n3 ()1.33) is the refractive index of theintervening medium.49 The calculated Hamaker constant A132 (or

(47) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; AcademicPress Ltd.: London, 1992.

(48) Hunter, R. J. Introduction to Modern Colloid Science, 1st ed.; OxfordUniversity Press: New York, 2002.

(49) Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces;Wiley-VCH GmbH & Co.: Weinheim, Germany, 2003.

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) over2 µm2. Inset: a 6 nm × nm molecular-scale image of the mica basal plane. (b) Mica edge surface prepared by microtome cutting technique witha mean roughness of 3.927 nm (Rq) and 2.978 nm (Ra) over 2 µm2.

Probing Surface Charge Potentials of Clay Langmuir, Vol. 24, No. 22, 2008 12903

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, werecalculated by numerically solving the nonlinear Poisson-Boltzmann(PB) equation, eq 2, at either constant surface potential or constantcharge density boundary conditions. The PB equation was solvedin the form of interaction force per unit area (F(x), eq 3), which isintegrated to get energy per area between two planar plates (UE(D),eq 4). Finally, Derjaguin’s approximation (eq 5) was used to calculatethe electrical double-layer forces (FE(D)) between a sphere and aflat plate at separation distance D. A MATLAB program wasdeveloped to theoretically calculate the DLVO forces. During theDLVO fitting, Debye length (κ) was obtained by fitting the slopeof the natural logarithm of AFM measured forces per unit length(log (F/R)) as a function of separation distance between interactingsurfaces. The fitted κ was also compared with the calculated valuethrough the Debye-Huckel theory. The electric surface potential(ψ) of each surface was set as an adjustable parameter.

εε0d2ψdx2

)-e∑i

zini∞ exp(-zieψkBT ) (2)

F(x)) kBT∑i

ni∞(exp(-zieψkBT )- 1)- εε0

2 (dψdx )2

(3)

UE )-∫∞DF(h) dh (4)

FE(D)

R) 2πUE(D) (5)

Surface Element Integration Method. Numerous calculationsfor the influence of surface roughness on DLVO interactions arereported in literature.51-57 Despite the diversity of theoretical

techniques for modeling surface roughness and the variety ofinteracting systems, nearly all studies on morphological heterogeneitydemonstrate the intuitive notion that the presence of asperitiessubstantially modifies the interaction energy between colloidalparticles and surfaces. Therefore, in order to quantitatively predictthe edge surface charge properties by fitting silica-muscoviteinteractions with DLVO theory, the effect of surface roughness shouldbe included, as the obtained muscovite edge surface was not ideallysmooth, especially compared with its basal plane. In our study, amethod of reconstructing the surface topology based on atomic forcemicroscopy in conjunction with surface element integration wasused. The procedure of this method is depicted in Figure 4 anddescribed below:

Since the silica probe was fairly smooth, as shown by the insetSEM image of Figure 5, only the roughness of the muscovite edgewas considered in the calculation. The rough edge surface used inthe computation was numerically recreated with height parametersexported 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 planeof the surface. Generated as such, the computing surface keptessentially the same feature of the scanned AFM image and hadapproximately the same root-mean-square roughness value.

The interaction energy between the probe and the computingsurface was calculated by the surface element integration method(SEI). In principle, the SEI method is a generalized form ofDerjaguin’s integration method applied to the exact geometry ofinteracting surfaces.53,54,58 SEI computes the total interaction energybetween two bodies by numerically integrating the interaction energy

(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.

(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.

Figure 4. Surface element integration (SEI) methods: (a) original AFM image of the muscovite edge surface; (b) computing AFM image used inSEI 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 theinteraction energy is on the plane of the highest point of the rough surface. All distances are measured along the positive z-direction in this coordinatesystem. 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 localseparation distance between the rough and the smooth plates at any position (x, y) is given by h.

12904 Langmuir, Vol. 24, No. 22, 2008 Zhao et al.

per unit area between opposing differential planar elements over theentire surfaces. Details of the SEI method and its application canbe found elsewhere.52-54,58 The interaction between the rough micasurface and the colloidal probe was calculated in two steps. In thefirst step, the interaction energy per unit cross-sectional area of therough surface and an infinite half-space comprising the probe material(silica) is evaluated (Figure 4c). Using the 2 µm × 2 µm area of therecreated surface (Figure 4b), the total energy between the half-space and the rough substrate is computed by integrating the DLVOenergy per unit area between silica and mica surfaces at each pointon the 511 × 511 mesh. This integral is then divided by the projectedarea (4 µm2) to obtain the interaction energy per unit area betweenthe silica surface and the rough substrate, Up-p. Finally Derjaguin’sapproximation (F/R ) 2πUp-p) was used to convert the interactionenergy per unit area to interaction forces between the silica probeand muscovite edge surface (Figure 4d). Here the separation D wasthe distance between the projected smooth plate and the plane of thehighest point, PQ, on the edge surface. The actual local separationdistance h for each patch of the computing surface was the sum ofthe separation distance D with the height difference between the

local patch and the highest point. Figure 4c depicts the parametersused in SEI calculations. In the present study, the minimum separationis defined as the first sphere-plate contact, i.e., the distance betweenthe smooth plate and the highest point on the edge surface, shownin Figure 4c. It is different from the distance based on the sphere-mean plane of the rough surface, as is considered in many otherstudies.57,59 Because we assume that the interacting surfaces areincompressible, and the probe is large compared with the normalcontact area; i.e. the probe will not reach other points lower thanthe highest one once it approaches the rough surface. In the SEImethod, the electrostatic interaction energy per unit area betweentwo flat plates was calculated using an analytical expression, theHHF-FP equation derived by Hogg et al.60 based on the linearizedPoisson-Boltzmann equation under constant potential (CP) condi-tions. The HHF-FP equation given below is quite accurate to predictthe interaction forces at larger separations or for the surfaces of lowsurface potentials.53 During the fitting, Debye length (κ) and electricsurface potential of the probe (ψa) and each mesh point of the roughsurface (ψb) were set as adjustable parameters. The van der Waalsforces were calculated by Hamaker’s microscopic approach.

U(D)area

)εε0κ

2 (ψa2 +ψb

2)[1- coth(κh)]+

2ψaψb cos ech(κh) (6)

Results and Discussion

In this section, the interaction forces between silica andmuscovite basal planes and edge surfaces were measured in KClsolutions of varying pH values. DLVO simulation was used tofit the obtained AFM force profiles for the purpose of evaluatingthe surface charge properties of clay basal and edge surfaces. Tocarry out this task, silica potentials in solutions of interest werefirst determined by fitting silica-silica interactions. Because theobtained muscovite edge surface was relatively rough, especiallycompared with the basal plane, the surface element integrationmethod was applied to calculation of the DLVO forces to accountfor the surface roughness. From the best fitting, the surface chargeproperty of muscovite edge was determined.

Silica Surface Potential Determination. In addition toelectrophoresis and streaming potential measurements, anothermethod available for the measurement of surface potentials ofparticle or planar plate is to fit the measured force profiles withDLVO theory.25,26,40,61 For a symmetric system, such assilica-silica system, the surface potential of silica can bedetermined by DLVO fitting when the Hamaker constant andelectrolyte concentrations are known. However, for an asymmetricsystem, for example, a silica-muscovite system, one cannotobtain unique surface potentials of the two interacting surfacesby fitting the force profile with DLVO theory.27 To obtain theunknown surface potential of muscovite through the fittingmethod, it is therefore necessary to know the surface potentialof the silica probe for each testing condition.

In this study, independent force measurements were carriedout between a silica probe and a planar silica substrate in 1 mMKCl solutions of varying pH values. The measured force profilesare shown in Figure 5. As expected, a repulsive force was observedas the silica surfaces are similarly charged. The value of therepulsive force increased with increasing solution pH. This is tobe 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 InterfaceSci. 1996, 184, 594–600.

Figure 5. Normalized interaction forces (F/R) between a silica probeand a silica flat surface in 1.0 mM KCl solutions at various pH values:the scattered force profiles correspond to experimental data, and thesolid curves represent calculated DLVO results obtained by numericalsolution of the nonlinear Poisson-Boltzmann equation calculated underconstant potential boundary conditions. Hamaker constant was used inthe fitting. The Debye lengths, obtained by fitting the slope of the naturallog(F/R) vs separation distance, are listed in Table 1. The fitted valuesof the silica surface potential, ψsi, are -51, -56, and -65 mV at pH5.6, 8.0, and 10, respectively.

Table 1. Comparison of the Fitted and Calculated DebyeLengths Used in Fitting Interaction Force Profiles between

Silica-Silica with DLVO Theory, and the Best Fitted Values ofthe Surface Potential (ψsi) with Literature Values of -Potentials

Obtained by Microelectrophoresis (MEP) and StreamingPotential (SP) Measurements, and AFM Fitting Method

electrical potential, ψsi (mV)

Debye length, κ-1 (nm) from the literature

pH fitted calculated fitted values method ref

5.6 9.6 9.6 -51 ( 3 -50 MEP 76-60 MEP 40a

-58 MEP 27a

-50 SP 40-25 to -60 AFM 40-48 to -62 AFM 27a

-54 AFM 26b

8.0 7.9 9.6 -53 ( 3 -60 MEP 76-80 MEP 40-80 MEP 27a

-60 SP 40∼-60 AFM 40-70 to -85 AFM 27

10 7.9 8.7 -56 ( 3 -65 MEP 76a Background electrolyte was 1 mM NaNO3. b Background electrolyte

was 2 mM NaCl.

Probing Surface Charge Potentials of Clay Langmuir, Vol. 24, No. 22, 2008 12905

dissociated with increasing solution pH, leading to more SiO-

on the surface.Double layer forces, calculated by numerically solving the

Poisson-Boltzmann equation at constant surface potentialboundary condition, and van der Waals forces were calculatedfor a given surface potential to fit the measured force profileswith the DLVO theory. It is implicitly assumed that there is nodifference between the potentials of silica probe and planar silicasubstrate. All the fitting parameters are listed in Table 1. In eachfitting, the value of Debye length was obtained by fitting thegradient value of the repulsive force on the natural log (F/R)versus separation distance. The fitted Debye length agreed wellwith the values calculated on the basis of the concentration ofions at pH 5.6, which is the natural pH of 1 mM KCl solution,equilibrated with atmospheric carbon dioxide. However, the fittedDebye lengths were slightly smaller than the calculated valuesat higher pHs, which might result from the additional ions addedto adjust the pH or more CO2 dissolved into the solutions of highpH. The experimental force-separation distance curves werefound to be in a reasonable agreement with theoretical predictionsof DLVO, as shown by the solid lines in Figure 5. At very shortseparation distances, a discrepancy between the experimentaldata and DLVO theory was observed for all the pH valuesinvestigated. This discrepancy could be attributed to the presenceof the hydration forces between the silica surfaces and/or surfaceroughness, as well described in literature.25,62-64 The fitted surfacepotentials of silica from AFM measurements at pH 5.6, 8.0, and10 were -51 ( 3, -56 ( 3, and -65 ( 3 mV, respectively. Asshown in Table 1, the fitted surface potential values agreed wellwith the measured -potential values using other methods.26,27

The derived surface potentials of silica from symmetricalsilica-silica system were taken as known values in the followingtheoretical fitting of the force profiles for silica-muscovitesystems.

Interactions between Silica and Muscovite Basal Plane.The forces measured between a silica probe and a muscovitebasal 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, theinteraction forces were monotonically repulsive on approach.More importantly, the measured repulsive force profiles werenearly the same. There was no adhesion when the probe waspulled away from the substrate for all the cases. A similarobservation was reported by Toikka and Hayes.26

The obtained AFM force profiles for the silica probe andmuscovite basal plane system were analyzed by DLVO theoryfor the purpose of quantitatively evaluating the surface potentialof the muscovite basal plane at various pHs. In the fitting, thesurface potential of silica derived above was used, while thesurface potential of muscovite basal plane, the only unknownparameter, was adjusted until the theoretical and experimentalforce profiles overlap. The theoretical force profiles werecalculated for both constant surface potential and constant surfacecharge density boundary conditions. The fitted surface potentialand Debye length are given in Table 2.

In general, the fitted Debye lengths were in reasonableagreement with the values calculated on the basis of theconcentration of ions. As in the case for silica-silica interactions,a slightly lower value than that calculated was observed at pH10. At a separation distance greater than 5 nm, the measured

long-range force profiles agreed reasonable well with DLVOtheory under either constant charge density or constant potentialconditions. At shorter separation distance, the measured forceprofiles were sandwiched between these two fitting curves. Thefitted 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 valuescould be considered within experimental error, indicating thatthe surface charge of muscovite was pH independent.

(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.; Galvez-

Ruiz, 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.

Figure 6. DLVO fitting of normalized long-range forces between asilica 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 dashedcurves represent fitting results of calculated DLVO forces obtained bynumerically solving the nonlinear Poisson-Boltzmann equation underconstant charge (upper, dashed) and constant potential (lower, solid)boundary conditions. Hamaker constant Aswm ) 1.2 × 10-20 was usedin the fitting. The fitted Debye length and surface potentials of bothsilica and mica basal plane are listed in Table 2.

Table 2. Comparison of the Fitted and Calculated DebyeLengths Used in Fitting Interaction Force Profiles Measured

between Silica-Muscovite Basal Planes with DLVO Theory, andthe Best Fitted Values of the Surface Potential (ψmb) with

Literature Values of -Potentials Obtained byMicroelectrophoresis (MEP), Streaming Potential (SP), Surface

Force Apparatus (SFA) and Electro-Osmotic (EO)Measurements, and AFM Fitting Method

surface potential of muscovite basalplane, 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 ∼-72 SP 65-68 SP 40a

∼-80 SP 77-77 EO 66-50 to -130 SFA 67b

-60 to -90 AFM 40-73 AFM 26c

-100 AFM 688.1 9.2 9.6 -78 ( 5 ∼-80 SP 65

-74 SP 40-50 to -130 SFA 67-65 to -70 AFM 40

10 7.7 8.7 -78 ( 5 ∼-80 SP 65-47 SP 69-100 ( 27.1 AFM 69d

-58 ( 2.3 AFM 69e

a Background electrolyte was NaNO3. b Mica surface potential variedwith different micas. Background electrolyte was NaNO3. c Backgroundelectrolyte was 2 mM NaCl. d Double layer force was calculated at constantpotential boundary conditions. e Double layer force was calculated at constantcharge boundary condition.

12906 Langmuir, Vol. 24, No. 22, 2008 Zhao et al.

The surface potentials derived from this study are comparedwith the results from literature26,40,65-69 in Table 2. Despite thevariety of micas and experimental methods, the obtained surfacepotential values of muscovite basal plane were close to the-potential values obtained using other methods, such as streamingpotential measurements, electro-osmotic potential measurements,and surface force measurements. Particularly, there was anexcellent agreement between our results and those obtained byScales et al.70 using a flat-plate streaming potential apparatus.Furthermore, the pH independence of surface potential or-potential over a pH range from 5.6 to 10 was also observedby other researchers.40,65,67 Scales et al.65 reported that from pH6 to 10, the -potential of mica in solutions of constant ionicstrength remained constant. From surface force measurements,Israelachvilli and Adams67 found that different micas exhibiteddifferent surface potentials (varying between -50 and -130mV), but those potentials were insensitive to pH change from5.5-7.0. Hartley et al.40 observed that, above pH 6.9, the-potential of mica remained at -75 mV. As mentioned earlier,the pH independence of the muscovite mica is attributed to thecharge mechanism of basal plane by isomorphic substitution oflattice elements, i.e. about one-fourth of Si4+ are replaced byAl3+ in the tetrahedral layer. The dominant siloxane group(Si-O-Si) on the basal plane of a 2:1 phyllosilicates is veryinert with a protonation constant of log KH

int)-16.9,15 indicatingthe absence of any significant protonation at pH values of ourstudy.

Interactions between Silica and Muscovite Edges. ForceMeasurements between Silica and MuscoVite Edge. Forcesbetween a silica probe and the muscovite edge surface in 1 mMKCl solutions of varying pH were measured in order to studysilica-muscovite edge interactions and to determine the surfacecharge of the edges. Due to the roughness of the prepared edgesurfaces, the locations on which force measurements wereperformed were carefully selected with the aid of an opticalmicroscope attached to the AFM. A set of typical force profilesrecorded from various locations on one edge sample at pH 5.6and 10 is shown in Figure 7. Each force curve in this figurerepresents the average trend of about 20 force profiles collectedon each location. Although highly variable, it is evident that themeasured long-range forces were repulsive at pH 10 but attractiveor zero at pH 5.6. The average adhesion measured at pH 10 ismuch smaller than that measured at pH 5.6.

Such a scatter indicates that the absolute magnitude of themeasured forces is sensitive to the contacting position where theforce measurement is taken. This level of scatter is attributed tothe random roughness of the muscovite edge surface.

To minimize the effect of surface roughness on the measuredforces, i.e., to reduce the scatter in force curves, a new procedurewas developed. For a given pH, the measurements were performedon 10 randomly selected locations over a “smooth” area underthe optical microscope. Then the location leading to the mostreproducible force profiles and the maximum force values withina separation distance 2-15 nm between two interacting surfaceswas targeted. After identifying this location, the force measure-ments were carried out for all other pH values at the target location

and two other locations about 100 nm away from the target usingAFM offset function. One representative set of results followingsuch a procedure is shown in Figure 8. Clearly, the measuredforce profiles became much less scattered, thereby allowingroughness correction for quantitative estimation of the surfacepotentials 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 atpH 5.6. The adhesion forces, on the other hand, increase withdecreasing pH. Both the measured long-range and adhesion forcesas a function of pH explicitly suggest that the surface charge ofmuscovite edges are pH-dependent, as anticipated. It is expectedthat for solutions of pH values above the point of zero charge(PZC) of the silica and muscovite edge surfaces, an electrostaticrepulsion between the unequally charged negative surfaces isexpected, which decreases in magnitude with decreasing solutionpH. For solutions of pH values between the PZC of the silicaand the muscovite, an electrostatic attraction between theoppositely charged surfaces is expected. The force profilesobtained with silica and muscovite edge surfaces in simpleelectrolyte solutions of pH below 6.5 agreed very well with suchexpectations. In this study, the force measurements were alwayscarried 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 5.6-10investigated. Therefore, the observed attractive force at pH 5.6strongly suggests a positively charged mica edge surface at thispH, though it might not be true, since an attractive electrostaticforce between colloids with the same sign of surface potentialwould possibly exist based on the Poisson-Boltzmann predic-

(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.

Figure 7. Distribution of the measured interaction forces between asilica probe and a muscovite mica edge surface at about pH 5.6 (emptysymbols) and pH 9.9 (solid symbols). The measurements were carriedout at randomly selected locations on the edge surface. (a) Long-rangeinteractions; each force curve is a representative profile selected from20 measured force profiles collected from one contact location. (b)Adhesion forces; each symbol represents the adhesion force of everyforce profile collected at one contact location.

Probing Surface Charge Potentials of Clay Langmuir, Vol. 24, No. 22, 2008 12907

tion.72 The PZC of the edge surface is a very important parameter,as it allows us to evaluate the critical role of anisotropiccharacteristics of clay surface charges in many interfacial science-based chemical processes, such as adsorption, stability of claysuspension in drilling fluids, rheology in ceramic and compositematerials processing, waste and portable water management,and so forth. In order to obtain quantitative surface potential ofmica edge so that we could explore anisotropic properties of claysurfaces for more extensive engineering and scientific interests,it is necessary to fit the measured force profiles to the DLVOtheory. However, the measured forces between the silica probeand mica edge surface could not be reasonably fitted by theDLVO theory, even with very small surface potential values.

Effect of Surface Roughness on Interactions. The magnitudeof measured long-range forces between silica and muscoviteedges in Figure 8a were found to be unexpectedly smaller thanthe repulsive forces between silica and muscovite basal plane.Although the significant reduction in the magnitude of themeasured forces could be attributed to the anisotropic surfacecharge properties of two different muscovite surfaces, also it isquite possible that the morphological heterogeneities of themuscovite edge surfaces play an important role. Surface roughnesswas well-documented to substantially reduce the interactionenergies between two interacting surfaces, the extent of whichdepends on the size of the asperities and the number of asperitieson the surface.51,52,55-59

To account for the morphological heterogeneities in estimatingsurface potential from the measured force profiles, surface elementintegration method was used to calculate DLVO forces betweenthe silica probe and mica edge surface recreated from its AFMimages. In this regard, several AFM images with their own distinct

roughness properties scanned on interested areas of the micaedge surfaces were reconstructed to match their roughness, asquantified by either the root-mean-square roughness (Rq) oraverage roughness (Ra), and then used in SEI-DLVO calculations.For instance, the AFM images A1-A4 were obtained by scanningthe edge surface at locations where the force measurements wereperformed. The effect of the surface roughness on van der Waalsforce and on long-range force is shown in Figure 9. In Figure9a, each dotted line represents the van der Waals forces calculatedbetween a silica probe and a reconstructed mica edge surfaceusing the SEI method. The solid line represents the van derWaals force between a probe and a smooth surface, also calculatedusing the SEI method. Figure 9a clearly shows that the van derWaals forces are dramatically decreased when surface roughnessis considered, and the extent of the reduction varies from locationto location of varying roughness. By quantitatively comparingthe forces at 10.3 nm, it was found that the van der Waals forcesfor rough edge surface were about 5-10 times less than theforces for a smooth surface. The similar reduction in theelectrostatic double layer forces between silica and rough micaedge surfaces, calculated by the SEI method using the HHFequation under constant surface potentials of -65 mV for bothinteracting surfaces, was observed.

The reduction was found to be sensitive to surface roughnessparameters of Ra or Rq (distributions of z-values). The highestpeak-to-valley height of asperities (the largest z-value) was alsofound to determine the extent of reduction, as the zero separationdistance between the silica probe and mica edge surface was

(72) Barouch, E.; Matuevic, E. J. Chem. Soc., Faraday Trans. 1 1985, 81,1797–1817.

Figure 8. Normalized interactions (F/R) of a silica probe with a muscovitemica edge surface in 1.0 mM KCl solutions at various pHs. (a) Long-range interactions; solid scatter profiles represent silica-muscovite edgeinteractions, while empty scatters represent silica-muscovite basal planeinteractions. (b) Adhesions measured at three locations.

Figure 9. Effect of surface roughness on the interaction forces betweensilica probe and muscovite mica edge surface in 1 mM electrolytesolutions: (a) van der Waals forces calculated by surface elementintegration method using Aswm ) 1.2 × 10-20 and (b) long-rangeinteractions by surface element integration method using the HHFequation under constant potential boundary conditions. Both silica andedge surface are similarly charged by -65 mV. In the legend, Rq, root-mean-square roughness, denotes the standard deviation of an entiredistribution of z-values of an AFM image; Ra, average roughness, is theaverage deviation of the measured z-values from the mean-plane of anAFM image; hmax denotes the largest z-values of an AFM image.

12908 Langmuir, Vol. 24, No. 22, 2008 Zhao et al.

defined in the SEI calculation by the first contact of probe withthe highest asperity of the surface rather than with the meanplane of the rough surface. It is important to note that once thesurface roughness is included in the force calculations, theresultant van der Waals forces have negligible contribution tothe 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 ofthe above analysis, the measured force profiles using silica probeand 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 lengthsfor silica-mica basal plane systems were used. By varying theonly free parameter of the surface potential of mica edge surface,the experimental force profiles could be fitted well with the SEI-DLVO model based on the AFM images of mica edge surfaces,as shown in Figure 10. From the fitting, the surface potentialvalues of -40, -5, and 8 mV were obtained for mica edges inaqueous solutions of pH 10, 8.0, and 5.6, respectively. On thebasis of the roughness corrected results, the true PZC of micaedges should be between pH 5.6 and 8.0.

Since the images used in the SEI calculation could not beconfirmed to be scanned exactly on the spot where the forcemeasurements were performed, the determined surface potentialsof mica edges cannot be considered to be the exact values, butthey are at least semiquantitative. The inaccuracy resulted fromthe fact that the colloidal forces were measured with a microsizedprobe, while an AFM cantilever with a sharp tip was used to

obtain a good AFM image of detailed morphological charac-teristics. Unfortunately, switching the cantilevers to the probeparticle precludes the alignment of the probe right back to thelocation where the AFM image is taken. One way to resolve thisuncertainty is to use several representative images to carry outthe calculations. The derived surface potentials for images A1-A4were plotted as a function of pH in Figure 11. It was clear that,except image A1, fittings using images A2-A4 resulted in verysimilar values of surface potentials. Image A1, giving rise to thelowest interaction energy, was excluded in calculating the averagesurface potential values, as we limited our measurements onselected locations of the largest interaction energies, as afore-mentioned. In such a manner, the average potentials of micaedges were determined to be -40 mV at pH 10, -5 mV at pH8.0, and 7 mV at pH 5.6. The standard deviations for these valueswere estimated to be about 8 mV. By plotting the potential vspH curves, the PZC of the mica edge is estimated to be withinpH 7-8 and can be narrowed to pH 7.3-7.6 on the basis of themost repeatable results from images A2-A3.

It should be noted that the obtained PZC value lies betweenthe respective isoelectric points of silica (pH 2-340,70,71) andgibbsite (pH 8-10), the major constitutes of edge surfaces.71,73

As in alumina, Al-OH is prone to protonation to becomepositively charged at pH <10, while deprotonation of silanolsin silica (Si-OH) contributes to the negative surface charge,with the balance determining the overall surface charge char-acteristics.15 The groups that populate edge surface in such 2:1phyllosilicate clays are Si-O-, Al-OH1/2-, Si-OH, andSiAl-Ox, whose protonation constants (∼pKH) are 11.9, 10,-1.9 and-16.9 to 12.3, respectively.15 Although the magnitudeof these constants from different sources spread over severalunits, the sign of surface charge predicted from site-bindingmodels is consistent with the value derived above.12,20,22,24,74

Therefore, the positive charges are believed to arise from theprotonation of Al-OH1/2- and SiAl-Ox.

The PZC values from the literature are summarized in Table3. Despite large variations in clay types and preparationprocedures, the PZC values of clay edge surfaces predicted bya number of studies using the acid-base potentiometric titrationtechnique are in the range of pH 3-8, among which pH 6-6.5and 8-8.5 are the most frequently reported values. The largescatter is attributed to the use of different thermodynamic constantsderived from dissociation and surface complexation models. ThePZC values determined in this study are within the reportedrange. The advantage of the current method is that there is noneed to build any model or rely on any derived parameters. Moreimportantly, the current approach of using a microtome to prepareclay edge surfaces will open the door for probing molecularinteractions with clays of anisotropic surface properties in a moredirect manner.

One of the major challenges to study anisotropic propertiesof clays is to prepare near molecularly smooth edge surfaces.Even with ultramicrotome available for us, the surface preparedremains too rough to be modeled by the traditional colloidaltheories. For this reason, we used the SEI method to calculatesurface electrical properties from the measured force profiles.Although effective, with the current capability of AFM, it remainschallenging to get exact images of surfaces where the force profilewas measured, as needed for calculation using SEI methods. Asolution for this would be using an AFM tip instead of probe tomeasure the force profiles so that a map of surface charges on

(73) Kosmulski, M. J. Colloid Interface Sci. 2006, 298, 730–741.(74) Bickmore, B. R.; Rosso, K. M.; Tadanier, C. J.; Bylaska, E. J.; Doud, D.

Geochim. Cosmochim. Acta 2006, 70, 4057–4071.

Figure 10. Normalized long-range forces between a silica probe and amuscovite edge surface in 1 mM KCl solutions at pH 5.6, 8.0, and 10(scattered profiles) with DLVO fitting results obtained by surface elementintegration method in corporation of roughness effect using the HHFequation under constant potential boundary conditions. In the fitting,Aswm ) 1.2 × 10-20 was used. The fitted surface potentials of both silicaand muscovite edge are listed in the figure.

Figure 11. Fitted muscovite surface potentials as a function of pH usingdifferent computing AFM images. The point of zero charge for themuscovite edge is determined to be within pH 7-8 (∼7.5).

Probing Surface Charge Potentials of Clay Langmuir, Vol. 24, No. 22, 2008 12909

clay edge surfaces could be measured. The problem with thisapproach is that the force between the AMF tip and surface istoo weak to be accurately probed for the change of solutionconditions. It should be noted that the microtome approachdescribed here is not limited to well-defined platy surfaces suchas mica or talc. It can be easily extended to real clay particlesthat could be imbedded in epoxy and cut by microtome for thesame purposes. The key is to establish a suitable microtometechnique so that the finished surface is sufficiently smooth forsuch measurements.

In the case of clay particles, it is also important to note theeffect of charges of basal plane on charge characteristics of edgeswhen immersed in aqueous solutions. It is likely that the permanentcharges on the basal plane will affect the charges on the edgesof a clay particle.75 However, we believe this effect is within arange of a double layer (<50 nm), which is negligible comparedwith the thickness (>50 µm) of edge surfaces in our experiments,where the probe interacts with it.

Conclusions

The AFM colloidal probe method was employed for the firsttime to quantitatively investigate anisotropic surface chargeproperties of mica, a 2:1 phyllosilicate. A microtome cuttingtechnique was developed to prepare edge surfaces suitable foratomic force microscopy (AFM) study. A surface elementintegration (SEI) method was used to extend DLVO calculationsby incorporating surface roughness.

Using the AFM colloidal probe technique, the interaction forcesof a silica sphere with mica basal plane and edge surfaces in 1mM KCl solutions of varying pHs were measured. Theexperimentally measured forces between silica and mica basalplane could be well-fitted with the classical DLVO theory. TheAFM-derived surface potentials of mica basal plane were pH-insensitive and agreed well with those reported in the literature.

The measured long-range interaction forces between silicaand mica edges were highly pH-dependent. The interaction forceswere repulsive at pH 10, decreased in the magnitude of therepulsive force at pH 8.0, and reversed to be attractive at pH 5.6.

To account for the random roughness of the prepared micaedge surfaces, the SEI method was used in calculation of DLVOforces. The measured forces could be well fitted with such DLVOcalculations. On the basis of fitting the experimentally measuredforce profiles with the SEI-DLVO model, the point of zero chargeof mica edges was estimated to be around pH 7.5 ( 0.5.

This study demonstrates the possibility of using the AFMcolloidal probe technique to study anisotropic surface chargeproperties of clays. However, further effort remains to improvethe quality of the prepared edge surfaces to minimize theuncertainties of the SEI calculations.

Acknowledgment. Financial support from NSERC IndustrialResearch Chair in Oil Sands Engineering (held by J.H.M.) isgratefully acknowledged. The CONRAD Bitumen ProductionGroup is also acknowledged for supporting the research. Weexpress our appreciation to Ms. Tina Barker and Ms. Diane Cairdfor their help with the scanning electron microscope and X-raydiffraction measurements.

LA802112H

(75) Tombacz, E.; Szekeres, M. Appl. Clay Sci. 2004, 27, 75–94.(76) Liu, J.; Xu, Z.; Masliyah, J. Langmuir 2003, 19, 3911–3920.(77) Zembala, M.; Adamczyk, Z. Langmuir 2000, 16, 1593–1601.(78) Missana, T.; Adell, A. J. Colloid Interface Sci. 2000, 230, 150–156.

Table 3. Published Experimental Studies of Point of Zero Charge (PZC) for Various Clays

claysa PZC values methods ref

Argentinean Na-montmorillonite ∼8.5 acid-base potentiometric titrations mass titrations 14montmorillonite 2.8 ( 0.2 potentiometric titration 19sodic montmorillonite <5 continuous potentiometric titration 23Na-montmorillonite <6.5 acid-base potentiometric titration 78Na-montmorillonite 8.11 potentiometric titrationmass titration 18Tunisian purified smectite 8.02 potentiometric titrationmass titration 18Na-montmorillonite ∼6.5b acid-base titration 8illite 6.4 ( 0.2 potentiometric titration 19kaolinite 4.8 ( 0.2 potentiometric titration 19kaolinite ∼6-6.5 potentiometric titrationmass titration 17

a Montmorillonite, which is the main component of bentonite, is a 2:1 layer (two tetrahedral and one octahedral clay. b Verified by characteristic changesin gel formation and in rheological properties.

12910 Langmuir, Vol. 24, No. 22, 2008 Zhao et al.