Charging Behavior of the Gibbsite Basal (001) Surface in NaCl

The charging behavior of the gibbsite γ-Al(OH)3 basal (001) surface in aqueous solution is important for correctly modeling the overall charging prop...
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Langmuir 2006, 22, 6087-6092

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Charging Behavior of the Gibbsite Basal (001) Surface in NaCl Solution Investigated by AFM Colloidal Probe Technique Yang Gan*,† and George V. Franks‡ Department of Chemical Engineering, The UniVersity of Newcastle, Callaghan, NSW 2308 Australia, and Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, VIC 3010 Australia ReceiVed December 14, 2005. In Final Form: March 27, 2006 The charging behavior of the gibbsite γ-Al(OH)3 basal (001) surface in aqueous solution is important for correctly modeling the overall charging properties of gibbsite particles which controls surface phenomena such as adsorption and crystal growth. However, the question of whether the hydroxyl groups on the basal plane are proton active has been raised recently both from experimental and theoretical points of view. Using gibbsite crystals prepared from industrial Bayer process, the surface potentials of cleaved (001) surfaces were calculated from forces measured by the colloidal probe technique in 1 mM NaCl solution with differing pH. It was surprisingly found that the basal plane is proton active in pH less than 7 and protonation seems to level off at about pH 5. The potential-pH data was accurately fitted with a single pKa surface protonation model with pKa ) 5.9 ( 0.2.

Introduction Gibbsite (γ-Al(OH)3) is important as the key phase to nucleate from solution during Bayer processing of bauxite for alumina and aluminum production.1,2 It is also abundant in soil as silicate minerals’ stably weathering product.3 Recently, gibbsite has been used as a model system to study the phase behavior of platelets and platelet/sphere mixtures.4-7 Like mica, gibbsite consists of weakly bonded sheets, where the basal (001) plane is the cleavage plane. The surface properties of gibbsite control dissolution, crystal growth, and surface reaction. However, regardless of the increased interest in the surface chemistry of gibbsite,8-15 the surface charging behavior of gibbsite particles in aqueous solution is still not well understood. The surface charge of crystalline particles is controlled by the ensemble average of the surface charging behavior of all of the crystallographic faces of the particle. One outstanding issue concerns the role that the basal plane plays in the particle’s overall proton and charge balance. The traditional view, backed by the multisite model,8-11 prevailed for quite a long time. It was believed that the surface hydroxyl groups bound to two aluminum atoms (doubly coordinated aluminol sites, ≡Al2OH) on the basal plane were proton inactive * To whom correspondence should be addressed. Tel: +61-2-4921-6335. Fax: +61-2-4921-6920. E-mail: [email protected]. † The University of Newcastle. ‡ The University of Melbourne. (1) Freij, S. J.; Parkinson, G. M.; Reyhani, M. M. J. Cryst. Growth 2004, 260, 232. (2) Sweegersa, C.; et al. J. Cryst. Growth 2001, 233, 567. (3) Essington, M. E. Soil and Water Chemistry: An IntegratiVe Approach; CRC Press: Boca Raton, FL, 2004; Chapter 2. (4) Wijnhoven, J. E. G. Chem. Mater. 2004, 16, 3821. (5) van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582. (6) Voorn, D. J.; et al. Langmuir 2005, 21, 6950. (7) Wierenga, A. M. T.; Lenstra, A. J.; Philipse, A. P. Colloids Surf. A 1998, 134, 359. (8) Hiemstra, T.; van Riemsdijk, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989, 133, 91. (9) Hiemstra, T.; de Wit, J. C. M.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (10) Hiemstra, T.; Venema, P.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 184, 680. (11) Hiemstra, T.; Yong, H.; van Riemsdijk, W. H. Langmuir 1999, 15, 5942. (12) Phambu, N.; Humbert, B.; Burneau, A. Langmuir 2000, 16, 6200. (13) Rosenqvist, J.; Persson, P.; Sjo¨berg, S. Langmuir 2002, 18, 4598. (14) Bickmore, B. R.; et al. Geochim. Cosmochim. Acta. 2004, 68, 2025. (15) Jodin, M.-C.; Gaboriaud, F.; Humbert, B. J. Colloids Interface Sci. 2005, 287, 581.

and could not contribute to charging over the pH range 0-11.9. Only the surface hydroxyl groups bound to one aluminum atom (singly coordinated sites, ≡AlOH) along the sheet edges were thought to be active with a proton association constant pKa of 10, resulting in a point of zero charge (PZC) of pH 10 for each gibbsite particle composed of basal and edge sites. Recently, this view was challenged by Rosenqvist et al.,13 who assumed that ≡Al2OH sites on the basal plane should be proton active in forming surface complexes to account for their potentiometric results. However, they still believe that the ≡Al2OH sites do not contribute to the net surface charge. Therefore, the question remains: can the basal plane give rise to a net surface charge? In the past, morphological heterogeneity of gibbsite samples varied greatly in different investigations;9,11-13,15 thus, all of the potentiometric titration results are different since the technique is sensitive to the specific surface area ratio between basal and edge surfaces. To resolve this question we decided to study the individual surface charge contribution from a homogeneous basal plane sample where the surface is populated only with ≡Al2OH. Technically it’s very difficult to prepare such samples for macroscopic measurements. The atomic force microscope (AFM) colloidal probe technique16 enables researchers to study local surface chemistry with high accuracy. Using this technique, we have measured the forces between a silica sphere and an atomically smooth cleaved gibbsite basal plane in NaCl solution with varying pH. The results presented in the current paper are the first unambiguous evidence that the basal plane of gibbsite can be charged in the acidic pH range lower than 7, and the value of pKa of the proton dissociation reaction is found to be 5.9 using a single pKa surface protonation model. Experimental Section Gibbsite Crystals and Cleaving Procedure. Monocrystalline gibbsite particles (Aluminum Pechiney, France) with nearly hexagonal prism-shape of 50-150 µm in width and height were used. These particles are an industrial sample prepared through the Bayer process. The impurity level of crystals is Na2O 1600 ppm, SiO2 50 ppm, CaO 100 ppm, Fe2O3 65 ppm. The surface morphology of the as-received particles has been characterized by AFM. As shown in Figure 1a, the RMS roughness over an area of 1 µm2 is 4 nm, with (16) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239.

10.1021/la053391+ CCC: $33.50 © 2006 American Chemical Society Published on Web 06/09/2006

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Figure 2. Surface potential as a function of pH for the silica sphere used.

Figure 1. Surface morphology of the pseudohexagonal upper surface of an as received (uncleaved) gibbsite particle; (a) left: 1 × 1 µm2; (b) right: 50 × 50 nm2. a maximum peak to valley height of 27 nm. Smaller area imaging (Figure 1b) revealed that the as received (uncleaved) particle has a very high surface step density, the distance between neighboring steps is only about 2-3 nm. Selected prisms were first attached to a freshly cleaved mica substrate with an extremely small amount of two component epoxy resin (Selleys, Australia). Care was taken to rest the hexagonal surface flatly on the substrate and ensure the particle was not covered by resin. After the resin set, the substrate was blown with high velocity pure nitrogen gas to remove any loosely attached dust and particles, then put into a UV/ozone cleaner for 10 min, then rinsed with copious water, and blown dry with nitrogen gas. Then inside a laminar flow cabinet, under optical microscopy, a pair of clean stainless steel tweezers with sharp tips was used to peel off a slab of the prism to expose a fresh basal surface. Only those particles showing a reasonably smooth cleaved surface with mirrorlike terraces were examined by AFM and used immediately for force measurements. AFM and Colloidal Probe Preparation. A Nanoscope III Atomic AFM (Digital Instruments Inc., Santa Babara, CA) with a liquid cell was used. Standard silicon nitride probes were used for both large scale imaging and force measurements. The long wide cantilever was used for force measurements, the spring constant was calibrated to be 0.14 N/m by the added mass method;17 the other parallel short wide cantilever on the same side was used for imaging. An S-shaped thin wall O-ring was used to facilitate millimeter scale movement of the sample during imaging and force measurements. A hydrophilic silica sphere (Bangs Laboratories, Inc., Fishers, IN) of 5.08 µm in diameter was glued to the tip of the long wide cantilever with epoxy resin. Before force measurements, the attached sphere was inspected (17) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. ReV. Sci. Instrum. 1993, 64, 403.

by scanning over an ultra sharp silicon grating (ND-MDT Co., Moscow) to ensure that the sphere’s surface was not covered with glue and was free of any debris. The RMS roughness of the spherical surface was 1.5 nm over an area of 1 µm2. The probe was then put into the UV/ozone cleaner for 10 min to remove any remaining surface organics immediately before force measurements. The surface potential of the silica sphere was determined by force measurements conducted between a silica sphere and a smooth hydrophilic silica substrate in 1 mM NaCl solution of differing pH. The standard DLVO interaction model18 was used to fit the surface potential of the silica sphere from the measured forces, assuming that the sphere and substrate have the same surface potential. The results are shown in Figure 2. These results are consistent with the well accepted charging behavior of silica.19 It should be noted that throughout this article surface potentials obtained by fitting a DLVO model are diffuse layer potentials. Solution and Other Accessories. The NaCl solution was prepared with reagent grade salt. The solution pH was adjusted by adding 10 mM HCl and NaOH solutions. Deionized water filtered with a Milli-Q water system was used throughout. A clean glass syringe was used to inject solution into the liquid cell; a 0.1 µm pore-sized nylon syringe filter (GE Osmonics, Inc., Chicago, IL) was put between the syringe and the inlet of the AFM liquid cell to filter out particles from air or chemicals. Force Measurements. Before a solution was injected into the liquid cell, the short wide cantilever was used to image the cleaved surface to find a suitable area. A solution of about 10 mL volume was then injected. After waiting for 10-15 min to allow the system to become stable, the long wide cantilever was moved to the selected area to capture a number of force curves. Force measurements usually took 20 min to finish for a single pH condition. Force measurements proceeded from lower pH to higher pH, thus avoiding both gibbsite dissolution and deposition of silicate ions from the sphere or liquid cell onto the gibbsite surface. All experiments were performed at room temperature 25 (1 °C.

Results and Discussions Surface Morphology Characterization of Gibbsite Cleavage Surface. After cleaving a crystal, areas of the basal surface over 100 µm2 were found to contain a low density of steps as revealed by AFM imaging (see Figure 3a,b). The RMS roughness on the terraces is only 0.2 nm. We also obtained atomic scale topographical images (Figure 3c-e) of the cleaved basal plane in air using supersharp tips with a nominal tip radius of 7 nm. Close examination of the images reveals that the upper half is a hexagonal periodical structure with periodicity 0.47 nm corresponding to the size of the unit cell of the basal plane; however, the lower half shows finer features. Each high point (18) Reerink, H.; Overbeek, J. Th. G. Discuss. Faraday Soc. 1954, 18, 74. (19) Hartley, P. G.; Larson, I.; Scales, P. Langmuir 1997, 13, 2207.

Charging BehaVior of the Gibbsite Basal (001) Surface

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Figure 4. Force vs separation distance data for a silica sphere and the gibbsite basal surface in 1 mM NaCl at various pH. The solid lines are the fitting result of the HHF model.28 However, it should be noted that the best fit curves in Figure 4 require surface potentials of silica to be set up to 50% lower than the measured values (Figure 2), especially when the double layer repulsion dominates at pH higher than 6.

Figure 5. Force vs separation distance data for a silica sphere and the gibbsite basal plane in water.

Figure 3. Surface morphology characterization of gibbsite cleavage surface. (a) Optical image of a cleavage surface; (b) AFM topographical image (4 × 4 µm2); (c) atomic scale image of cleaved gibbsite basal plane obtained with a super sharp tip (5 × 5 nm2), a topographical transition occurs between the top and bottom of the image; (d) and (e) after processing with a Fourier transformation, the upper half (d) shows a hexagonal structure with periodity 0.47 nm; on the lower half (e) every bright spot in upper half is split into multiple smaller spots; the average distance between these spots is 0.27 nm based on the FFT analysis.

(bright spots) in the upper half appears to be split into multiple smaller peaks (smaller bright spots) in the lower half of the image. FFT analysis of the image indicates that the average distance between these smaller spots is 0.27 nm corresponding to the distance between neighboring OH groups.20 We do not claim here that we have achieved true atomic resolution; however, STM imaging of the cleaved hematite (001) surface by Eggleston et al.21 suggested that a topographical transition similar to that we have observed is very likely caused by a change in the tip’s resolution power, i.e., the lower half corresponds to true atomic resolution (a single atom apex at the end of tip) and the upper half to unit cell resolution imaged by a blunted apex. However, (20) Gan, Y.; Franks, G. V. J. Phys. Chem. B 2005, 109, 12474. (21) Eggleston, C. M.; et al. Geochim. Cosmochim. Acta 2003, 67, 985.

we can at least confirm that the cleaved basal surface is atomically smooth and has a well-defined periodical surface structure. Force Measurements. The force verses separation distance curves for a silica sphere approaching a cleaved gibbsite basal plane in 1 mM NaCl at various pH were obtained by the AFM colloidal probe technique as shown in Figure 4. These curves clearly demonstrate that the interaction between the two surfaces is responding to the solution pH change. It can be found that attraction dominates at more acidic solution, below pH 5.58. With increasing pH, attraction gradually gives way to repulsion. Some especially interesting features can be observed at pH 5.58 and 5.87, i.e., at separation greater than 5 and 10 nm respectively. Net attraction still dominates; however, upon close approach, a repulsion appears to prevent the sphere from touching the surface, finally at about 1 or 2 nm, as at other pH values, a jump-in occurs due to the attractive van der Waals force. At pH higher than 6.5, the force curves have only slight differences and only net repulsion was observed. In pure water, a long-range double layer attraction with a jump-in was observed (Figure 5). The pH of pure water equilibrated with air is around 5.7. Unlike the silica/silica system,22-24 no short-range hydration force was found in water and under any of these pH conditions. (22) Grabbe, A.; Horn, R. G. J. Colloids Interface Sci. 1993, 157, 375. (23) Chapel, J.-P. J. Colloids Interface Sci. 1994, 162, 517. (24) Donose, B. C.; Vakarelski, I. U.; Higashitani, K. Langmuir 2005, 21, 1834.

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to derive the surface potential of the basal plane of gibbsite to match the experimentally measured force data at each pH. To find the best fit gibbsite potentials, we used the surface potential of the silica (Figure 2) and a nonretarded Hamaker constant of 1.2 × 10-20 J for the silica/water/gibbsite system. This value is based on the observation that the refractive index26 of gibbsite (1.57) is close to that of mica (1.56) and the Hamaker constant of silica/water/mica system is 1.2 × 10-20 J.19 The force curves (solid lines in Figure 4) were obtained using the Wiese-Healy model27,28 for the interaction of unequal double layers at constant charge boundary condition

Figure 6. Five force curves captured at pH 5.87 showing the variation in the strength of attraction.

Figure 7. Best fit surface potentials of the gibbsite basal plane in 1 mM NaCl at various pH.28 The red solid line is the fitting result based on a single-pKa surface protonation reaction eq 1.

Reproducibility. The transition from attraction at low pH to repulsion at high pH was repeatable using other silica spheres and gibbsite samples. It should be noted that at pH lower than 6.5 it has been found that force curves captured at a certain pH condition shows a certain degree of variation in the magnitude of attraction. Figure 6 shows five force curves captured at pH 5.87. The common feature of these curves is the same: at large separation (3-15 nm for specific curves) attraction dominates, whereas upon further approaching repulsion appears, and the strength of this repulsion is weaker if the attraction at large separation is stronger. It should be emphasized that all curves were captured at 10-15 min after the solution had been injected into the liquid cell and system had reached stability. Currently, we can only postulate that charge regulation that occurred at the silica and gibbsite surfaces is responsible for this phenomenon. It should be noted that all five curves as shown in Figure 6 have been used to fit the gibbsite surface potential assuming the silica surface remained unchanged as outlined in the next section. The average gibbsite surface potential and standard deviation of the fits from the five different force curves are shown in Figure 7. The same procedure has also been employed for all other pH conditions. Surface Potential Fitting. According to the DerjaguinLandau-Verwey-Overbeek (DLVO) theory,25 the net interaction energy between a sphere and a smooth plate in an electrolyte are the sum of the electrical double layer energy plus the van der Waals attractive energy. By fitting the experimental force curves with an interaction model for dissimilar materials, we were able (25) Derjaguin, B. V.; Landau, L. Acta Physiochem. 1941, 14, 633. Verwey, E. G. W.; Overbeek, J. T. G. The Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.

2ψsψg + exp(-κd)(ψs2 + ψg2) FDLVO ) 2πr0 exp(-κd) R 1 - exp(-2κd) H (1) 6d2 where R is the radius of the silica sphere, r is the relative dielectric constant of the solution, 0 the dielectric constant of a vaccum, κ is the inverse Debye length, d is the separation between two surfaces, ψs and ψg are the surface potentials of silica and gibbsite, and H is the Hamaker constant of the silica/water/gibbsite system. The Wiese-Healy model was used to fit the full range of data to show that interaction between silica and gibbsite can be well reproduced assuming the constant charge interaction. However, it should be noted that the best fit curves in Figure 4 require surface potentials of silica to be set up to 50% lower than the measured values (Figure 2), especially when the double layer repulsion dominates at pH higher than 6. Therefore, the fitted potentials (absolute value) of gibbsite will be overestimated (about 3-5 mV) than the potentials obtained by fitting curves at large separation. The surface potentials of gibbsite in Figure 7 were obtained by fitting curves at large separation (κd > 2, κ-1 is the Debye length and d is the separation between two the surfaces) using the Wiese-Healy model with measured silica potentials. The average value and the standard deviation in Figure 7 are the statistical results for at least five force curves at each pH condition. Potentials have also been fitted (results not shown here) using the Reerink-Overbeek model at 25 °C 18

FDLVO ) 2π(1.47 × 10-11) × κ × R tanh (ψs/103) tanh(ψg/103) exp(-κd) -

H (2) 6d2

where symbols have the same meaning as in eq 1. This model uses the linear superstition approximation, and obtained potentials are only slightly (about 2 mV) lower than the potentials fitted using the Wiese-Healy model at large separation. The subtle differences between the various double layer interaction models are discussed in the literature.29,30 We believe that these results are the first observation of the complex features of interactions between unequal double layers at constant charge to be revealed by AFM force measurements. It is noteworthy that at basic solution conditions even an uncharged gibbsite surface interacting with the charged silica sphere can give rise to repulsion as predicted by the Wiese-Healy model. (26) Nesse, W. D. Introduction to Optical Mineralogy; Oxford University Press: Oxford, 1991; Chapter 9 and 10. (27) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638. (28) Wiese, G. R.; Healy, T. W. Trans. Faraday Soc. 1970, 66, 490. (29) Scales, P. J.; Grieser, F.; Healy, T. W. Langmuir 1992, 8, 965. (30) Gregory, J. J. Colloids Interface Sci. 1975, 51, 44.

Charging BehaVior of the Gibbsite Basal (001) Surface

The resulting surface potential verses pH results are shown in Figure 7. Two significant features of the charging behavior of the basal plane can be observed. First when at the solution pH 7 or higher, the surface remains totally uncharged within the experimental error and the surface potential reaches zero (its minimum); therefore, the point of PZC of the basal plane lies around 7 or even higher. Second the basal plane does become charged at pH 6.5 and below. Protonation seems to begin to level off at around pH 5. The surface potential at pH 4.58 reaches ca. 12 mV. Considerations on Gibbsite Dissolution, CO2 Contamination, and Surface Heterogeneities. Before we proceed to introduce a surface protonation model, other possible mechanisms giving rise to a positively charged surface in acidic solution should be considered. The first possibility is the surface dissolution of gibbsite. According to the dissolution theory of crystalline gibbsite in equilibrium with Al3+ ions, at pH 4, 5, 6, and 7 the activities of Al3+ ions are approximately 10-4, 10-7, 10-10, and 10-13 M, respectively.3 Therefore, at acidic conditions (for pH lower than 5), dissolved Al3+ ions may stay on the surface and give rise to a positive charge. However, though we have not measured Al3+ ion concentration in our experiments, we can safely exclude this possibility based on the considerations as follows. First, the concentration of Al3+ ions is the equilibrium value; that is, it can only be reached after the crystal remains contact with solution for a very long time for samples of large surface area. In contrast, our force measurements were finished in hours using a crystal sample with a surface area of approximately 4 × 10-8 m2. Second, it should be noted that a long range double layer attraction (Figure 5) was observed between the silica sphere and (001) basal surface in water even though the crystal only contacted with water for about 15 min before any salt solution was added. The other problem is the possible contamination of CO2 in the solution since our solution was not degassed. CO2 in the solution may give rise to two complications. The first one is the drift of pH with time due to the buffering capacity of dissolved CO2 in the solution. The pH values reported in this paper were all recorded before the solution was injected into the liquid cell. We anticipate there should be a small drift (decrease) of pH during force measurements. Second, hydrolysis of CO2 produces CO32- and HCO3- in the solution with HCO3- as the major product in acidic solution. However, the equilibrium concentration of HCO3at pH 5 is only as low as about 10-7 M in saturated solution.3 And more importantly, HCO3- cannot contribute a positive charge to the gibbsite surface. Therefore, the effect of CO2 in the solution can also be safely excluded. The contribution of aluminum surface hydroxyls other than the doubly coordinated aluminol sites (≡Al2OH) is vanishingly small since the width of the flat terraces of the cleaved basal plane is greater than the interaction zone of the colloid probe. It can be found that there are several steps across the surface (see Figure 3b), where the distance between them is generally larger than 800 nm. As the silica sphere begin to feel the double layer repulsion from a separation of around 30 nm as demonstrated in 1mM solution in Figure 4, a simple calculation reveals that the “effecting diameter” of the sphere (diameter 5 µm) is around 750 nm, which is smaller than the distance between two steps. Thus, the influence of the step edges (which contain singly coordinated aluminols) is negligible. We have repeated some of the results on cleaved crystals with steps farther apart and on various locations, and no difference has been found. Therefore, we can confidently exclude the interference of steps. Also, we

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do not anticipate that many aluminium vacancies will be created during the cleavage process since the basal plane is the cleavage plane. Single pKa Surface Protonation Model. On the gibbsite (001) basal plane, the only aluminol species presented is ≡Al2OH. It is thus reasonable to assume that the only possible surface protonation reaction that occurs on basal plane of gibbsite is31

≡Al2OH2+(surface) S ≡Al2OH(surface) + H+(solution)

(3)

where (surface) means the corresponding species is a surface species and (solution) means the speciesis immediately adjacent to the surface on the solution side. The dissociation constant is

Ka )

1-f + [H ]s f

(4)

where f is the fraction of protonated doubly coordinated sites

pKa ) -log Ka ) pHs - log

(1 -f f)

(5)

The surface H+ concentration obeys a Boltzmann distribution

[H+]s ) [H+]0e-eψ/kBT ) [H+]0e-Fψ/RT

(6)

where [H+]0 is the bulk H+ concentration, ψ is the surface electrostatic potential, F is the Faraday constant, kB is the Boltzmann constant, and R is the molar gas constant. Thus, we have

pHs ) pH0 +

ψ 2.303(RT/F)

(7)

where pH0 is the solution pH. Combining eqs 5 and 7 gives

log

ψ (1 -f f) ) pH - pK + 2.303(RT/F) 0

a

(8)

Surface charge density, σ, can be calculated by the GouyChapman theory (no specific adsorption assumed, further discussion see below) as

σ ) 0sκ(2kT/e) sinh

eψ (2kT )

(9)

where κ-1 is the Debye length of the diffuse layer. Since f is the fractional degree of site dissociation, it is directly related to the proportion of surface charge

f)

sinh(eψ/2kT) σ ) σm sinh(eψm/2kT)

(10)

σm is the maximum surface charge density when the surface potential reaches its highest value (12 mV here). In this equation, the only parameter needed to fit the surface potential-pH curve is the pKa value. It is found that the best fit value of pKa is 5.9 ( 0.2 taking the errors into account. In fact, this pKa is the solution pH at which the surface potential of gibbsite is midway between its plateau at high and low pH. Implication of Our Results. These findings are quite contrary to the traditional view of surface chemistry of gibbsite,8-11 that doubly coordinated sites ≡Al2OH on the basal plane are proton inactive and cannot contribute to charging over the pH range 0 (31) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114. The following derivation is essentially same as in this paper, though the two experimental systems are not same.

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Figure 8. Schematic representation of potential-pH relationship for gibbsite particles assuming approximately equal contribution from both basal sites and edge sites. The solid line represents the total particle potential; the dashed line represents the contribution from the basal (≡Al2OH) sites and the dotted line from the edge (≡AlOH) sites.

to 11.9, only singly coordinated edge sites ≡AlOH were thought to be active with a proton association constant pKa of 10, resulting in a PZC of pH 10 for each gibbsite particle composed of basal and edge sites. On the other hand, our results provide strong support to the recent work of Bickmore et al.14 They developed a refined surface charging model taking into account heterogeneous charging behavior even for atomically smooth planar surfaces of oxides. For gibbsite, this model predicts that some ≡Al2OH sites have a pKa of 5.2 and become charged at a pH higher or lower than 5.2. Although this value is slightly lower than the PZC we found and we did not observe any negative charge at high pH, we believe that their model is still qualitatively correct. Using Our Findings to Explain the Results of Rosenqvist et al. For particle gibbsite samples, assuming that the ≡Al2OH sites on basal surfaces have the potential-pH relationship as presented in Figure 7, the ≡AlOH sites still have iep between 9 and 10 as used by others,8,9 and then the overall potential-pH curve will like the solid line schematized in Figure 8. This scenario can explain the rise in the potentiometric titration curve and zeta potential results of Rosenqvist et al.13 Gibbsite particles were synthesized with a plate like shape having a larger area on the basal plane (occupied by ≡Al2OH sites) than edge area (occupied by both ≡Al2OH and ≡AlOH sites). They found a bump in the titration curve of the aged sample observed between pH 5 and 6. Their zeta potential curve in 100 mM NaCl also shows a steeper potential rise at a pH around 6 to 7. If the dissolution of gibbsite at acidic solution can be excluded, we can ascertain that these features are caused by the protonation of ≡Al2OH sites that contribute to the net surface charge of the whole particles. However, the maximum diffuse layer charge density calculated with the present potential-pH data using the Gouy-Chapman theory is only 0.0008 C/m2. If no inner layer specific adsorption occurs (see discussion below), this value corresponds to a site density of 0.005 site/nm2 which is more than 3 orders of magnitude lower than the crystallographic density (13.7 site/nm2) of basal ≡Al2OH sites. When specific adsorption occurs in the inner layer, the diffuse layer charge density calculated from the diffuse

Gan and Franks

layer potential is expected to be much lower than the surface charge density which is somewhat lower than the crystallographic site density as we have observed. The ion pair mechanism or specific adsorption mechanism described by Rosenqvist et al.13 could be invoked to account for this apparent discrepancy; that is, most of ≡Al2OH sites are associated with Na+ or Cl- as ≡Al2OH2+Cl- or Al2O-Na+ and thus do not contribute to the net charge. In other words, only a very small fraction of ≡Al2OH sites contribute to the net charge, these particular ≡Al2OH site may give the unknown OH stretching band in IR spectra at pH 5.2 observed by Rosenqvist et al.13 Comparison with Surface Charging Behavior of R-Al2O3 Single Crystals. Recently there has been progress in the understanding of the surface structure and chemistry of singlecrystal alpha alumina (R-Al2O3). Synchrotron radiation X-ray crystal truncation rods (CTR) measurements together with simulations32 suggest that the surface hydroxyl arrangements of a hydrated sapphire (0001) surface should resemble the basal plane of gibbsite. The iep of sapphire (0001) is unambiguously around pH 4-6 determined by streaming potential, AFM force measurements, and optical second-harmonic generation techniques,33-36 whereas the iep of alpha alumina particles is much higher at around pH 9. In theory, a (0001) sapphire surface should be covered with ≡Al2OH sites. However, even for the singlecrystal alumina (0001) surface, the iep and surface potential results have not been consistent; one can find quite different surface potential values even under equal electrolyte concentration.33-35 This variation is believed to be caused by a number of factors including surface polishing quality, misorientation angle, and very importantly the surface cleaning procedures. It thus will be very interesting to compare results from cleaved gibbsite and high quality (0001) sapphire with different width terraces to shed light on the understanding of surface chemistry of both materials.

Conclusions The diffuse layer potentials of cleaved (001) surfaces were calculated from forces measured by the colloidal probe technique in 1 mM NaCl solution with differing pH. It was found that the basal plane is proton active in pH less than 7, and protonation seems to level off at about pH 5. The potential-pH data was accurately fitted with a single pKa surface protonation model with pKa ) 5.9 ( 0.2. Acknowledgment. We are grateful to Dr. J. Addai-Mensah of University of South Australia for providing the gibbsite crystals, Nanosensors for donating supersharp AFM tips, and Dr. E. J. Wanless for use of the AFM facilities. Thanks to the Australian Research Council for financial support (DP0343326). Y.G. acknowledges the financial support from the University of Newcastle through the research fellowship scheme. LA053391+ (32) Eng, P. J.; et al. Science 2000, 288, 1029. (33) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Langmuir 1997, 13, 2109. (34) Franks, G. V.; Meagher, L. Colloids Surf. A 2003, 214, 99 and references therein. (35) Kershner, R. J.; Bullard, J. W.; Cima, M. J. Langmuir 2004, 20, 4101. (36) Fitts, J. P.; et al. J. Phys. Chem. B 2005, 109, 7981.