Charged Barite−Aqueous Solution Interface: Surface Potential and

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Charged Barite-Aqueous Solution Interface: Surface Potential and Atomically Resolved Visualization D. G. Bokern,† K. A. Hunter, and K. M. McGrath* Department of Chemistry, University of Otago, P. O. Box 56, Dunedin, New Zealand Received November 28, 2002. In Final Form: August 1, 2003 The charged barite-aqueous solution interface has been studied in water and electrolyte solution using laser Doppler electrophoresis to obtain ζ-potentials. High-resolution atomic force microscopy (AFM) was used to gain a local description of the distribution of the Ba2+ and SO42- ions that act as the potentialdetermining ions at the barite (001)-aqueous solution interface. The ζ-potential of natural barium sulfate particles in water, against its saturated solution, was measured to be ca. -20 mV. The addition of soluble barium and sulfate salts significantly influenced the surface potential through specific ion adsorption from solution. Over a small concentration range, for very low Ba2+ or SO42- concentrations, the ζ-potential of barite is a logarithmic function of the activity of the electrolyte solution and the slope roughly follows the Nernst relation for a thermodynamically reversible electrode. AFM imaging, using a low net attractive loading force, showed that in addition to the surface lattice, step edges and pointlike defects were visible with true lateral atomic resolution. In the Nernstian region, no surface rearrangement or evidence for adsorbed species was obtained. However, at higher concentrations of the potential-determining ions, islandlike agglomerations of ions (or counterions) were observed with lateral atomic resolution. This observation is explained in terms of changes in both the Debye length and electrophoretic charge densities.

1. Introduction Surface charge affects many properties of solids immersed in solution. Examples include the stability of colloidal systems such as milk and ink and processes such as mineral flotation or detergency. The basis of colloid stability is the existence of electrostatic repulsive forces between colloidal particles. Without this repulsive force, only attractive van der Waals forces would operate, leading to coagulation. The balance between these two types of forces is the essential framework for the understanding of colloidal interaction. Knowledge of surface charge is therefore vital in understanding the behavior of a solid in a liquid medium, especially in aqueous electrolyte solution. Placing barite in water leads to the generation of a net negative surface charge. This is in agreement with recent literature reporting on measurements of the ζ-potential of barite particles in water; however, there is some discrepancy with regard to the magnitude of the potential.1-3 Several mechanisms may be hypothesized for the generation of the observed surface charge: (1) an imbalance of cations and anions in the crystal surface lattice, (2) ion adsorption from solution, (3) surface defects, or (4) a diffuse space charge in the solid. The barium sulfate system, with its low solubility and good cleavage, was chosen in order to correlate the structural features of the barite surface with its surface charge. The barite-solution interface may be considered to be a model system similar to the classical AgI/Ag+, I- colloidal sol.4 Thus, we believed * To whom correspondence should be addressed. Phone: (+64 3 479 7932). Fax: (+64 3 479 7906). E-mail: [email protected]. † Present address: SONY International (Europe) GmbH, Advanced Technology Centre Stuttgart, Materials Science Laboratories, Stuttgart, Germany. (1) Taha, F.; Illyuvieva, G. V.; Megakhed, A. A. Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metall. 1984, 6, 15-18. (2) Sadowski, Z.; Smith, R. W. Miner. Metall. Process. 1987, 4, 114117. (3) Hu, Y.; Wang, D. Zhongnan Kuangye Xueyuan Xuebao 1990, 21, 31-38. (4) Ottewill, R. H.; Woodbridge, R. F. J. Colloid Sci. 1964, 19, 606620.

that its surface potential could be controlled simply by varying the composition of the solution with regard to [Ba2+] and [SO42-], thereby controlling adsorption processes on the substrate. This idea led us to consider and investigate the local nature of surface charge itself, that is, where the ions are located, which is made possible through the use of atomic force microscopy (AFM). We present here measurements of the bulk surface potential of barite in combination with direct visualization of the barite surface using AFM. This allowed us to explore the question first posed by Binnig in 1992 as to whether force microscopy is capable of producing images with true lateral atomic resolution.5 The intervening decade has seen many studies published in which true atomic resolution has been claimed. These studies have been under ultrahigh vacuum (UHV) conditions,6 at low temperatures,7 using both contact mode8 and noncontact mode.9-11 In 1993, Ohnesorge and Binnig investigated the calcite (CaCO3) {101 h 4} cleavage face in water.12 They obtained atomic-scale periodicities as well as the expected relative positions of atoms, corresponding to calcium and single oxygen sites within each calcite unit cell. They also reported the first pointlike defects resolved along monatomic steplines and concluded that they had achieved “true atomic resolution by AFM through repulsive and attractive forces”. Their data were acquired under water at room temperature using ultrasharp tips. These conditions reduce the usually strong (5) Binnig, G. Ultramicroscopy 1992, 42-44, 7-15. (6) Schwarz, A.; Allers, W.; Schwarz, U. D.; Wiesendanger, R. Phys. Rev. B 2000, 62, 13617-13622. (7) Allers, W.; Schwarz, A.; Schwarz, U. D.; Wiesendanger, R. Europhys. Lett. 1999, 48, 276-279. (8) Schimmel, T.; Koch, T.; Kueppers, J.; Lux-Steiner, M. Appl. Phys. A 1999, 68, 399-402. (9) Morita, S.; Fujisawa, S.; Kishi, E.; Ohta, M.; Ueyama, H.; Sugawara, Y. Thin Solid Films 1996, 273, 138-142. (10) Sugawara, Y.; Ueyama, H.; Uchihashi, T.; Ohta, M.; Morita, S.; Suzuki, M.; Mishima, S. Appl. Surf. Sci. 1997, 113, 364-370. (11) Loppacher, C.; Bammerlin, M.; Guggisberg, M.; Battiston, F.; Bennewitz, R.; Rast, S.; Baratoff, A.; Meyer, E.; Guntherodt, H. J. Appl. Surf. Sci. 1999, 140, 287-292. (12) Ohnesorge, F.; Binnig, G. Science 1993, 260, 1451-1456.

10.1021/la0269255 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003

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long-range attractive background interactions between the tip and the sample, such as van der Waals or capillary forces, and their associated “jump into contact” instability, by slightly retracting the sample so that images could be captured under a negative (attractive) overall loading force of about 10-11 N a few angstroms above the surface. This extremely difficult procedure can be understood as an unmodulated noncontact mode. The reported force curves do not show any significant instability or the typical hysteretic behavior of the retract trace. To understand the role of the tip in obtaining atomically resolved images, several models for the interaction between tips and ionic surfaces have been developed that account for distortions of both the tip and the surface.13-15 These models have, in particular, used static atomistic simulation and quantum-chemical techniques. Giessibl and Binnig16 suggested that inductive polarization of the tip in the field of the sample ions provides the major contribution to the imaging mechanism in noncontact mode. Giessibl17 presented an electrostatic imaging mechanism for the contact regime which allows atomic resolution of ionic crystal surfaces using a polarizable tip. The decay length of the electrostatic interaction in the z direction is sufficiently short for atomic resolution to be achieved, not only with a hypothetical tip consisting of only one atom but also by a more realistic tip of parabolic shape with a radius of 30 nm. Although his model was developed for vacuum situations, it appears to be partially applicable to imaging in electrolytic solution, as performed in the work reported here, if we assume the presence of additional force components resulting from hydration and the electrical double layer. These are important additional considerations since in liquids, van der Waals, repulsive electrostatic, solvation, and repulsive entropic forces may all be operating simultaneously.18 Therefore, it is the combination or balance of these forces between the tip and the substrate, along with the characteristics of the tip itself, that determine whether true atomic resolution is achievable in any given system. The simulations of an AFM tip at a hard wall in liquid19 indicate that even with a tip of molecular dimension, the forces listed above can still be interpreted as arising from the layering of solvent molecules at the surface of the tip and sample. Our system is more complicated in that there are divalent ions present due to the dissolution of the BaSO4 crystal. Of significant importance in this context is an experimental study by Cleveland and co-workers on surfaces of calcite and barite.20 They presented the first measurement of oscillatory hydration forces with an atomic force microscope, using thermally driven hopping of the cantilever between ordered layers of water molecules or hydrated ions. Generally, realistic models can provide some insight into the mechanisms of the tip-surface interactions and additional clues for the interpretation of AFM images. However, the central component of an atomic force (13) Perez, R.; Payne, M. C.; Stich, I.; Terakura, K. Phys. Rev. Lett. 1997, 78, 678-681. (14) Koga, K.; Zeng, X. C. Phys. Rev. B 1999, 60, 14328-14333. (15) Livshits, A. I.; Shluger, A. L.; Rohl, A. L.; Foster, A. S. Phys. Rev. B 1999, 59, 2436-2448. (16) Giessibl, F. J.; Binnig, G. Ultramicroscopy 1992, 42-44, 281289. (17) Giessibl, F. J. Phys. Rev. B 1992, 45, 13815-13818. (18) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-933. (19) Gelb, L. D.; Lynden-Bell, R. M. Chem. Phys. Lett. 1993, 211, 328-332. (20) Cleveland, J. P.; Schaeffer, T. E.; Hansma, P. K. Phys. Rev. B 1995, 52, R8692-R8695.

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microscope is the tip and its microstructure and composition at contact remain the greatest unknowns. The electrostatic forces caused by a charged surface are both strong and long-ranged and need to be understood for a full description of surface interactions. Since adsorption phenomena at solid-water interfaces are often controlled by the electrical double layer, it is important to understand the factors that are responsible for the charge on the solid (barite) surface and the behavior of ions that adsorb as counterions to maintain electroneutrality (of the whole system). Thus a complete description of the solid-liquid interface requires consideration of the composition, structure, and electrical properties of the interfacial region. Interpretation of this may be facilitated by the direct visualization of the surface made possible through the use of AFM. Hence, the main purpose of our study was to correlate electrophoretic data describing the overall interfacial charge distribution at the plane of shear of the microcrystals with high-resolution in situ AFM data obtained at tip-sample separations within the Stern layer. 2. Experimental Section Colorless (pure) and optically clear natural barite crystals (Geology Department, Otago University) were cut by guillotine yielding macroscopic cleavage along the {001} plane. Only one source of barite was used. Once cleaved, the sample was glued (Superfix, cyanoacrylate ester) onto a magnetic stainless steel sample puck and loaded onto the AFM scanner. AFM measurements were performed using a Digital Instruments Nanoscope III equipped with a standard A-scanner head and a glass liquid cell. The AFM equipment and operational procedure have been described elsewhere.21 High aspect ratio silicon Ultralevers (Park Scientific Instruments) were used throughout this study. Electrophoretic mobilities of crystalline barite particles suspended in aqueous solution and their size distribution were measured using a Malvern Zetasizer 3000 working on the principles of laser Doppler electrophoresis and photon correlation spectroscopy, respectively. The instrument was regularly calibrated against a carboxyl-modified polystyrene latex standard. The results were converted to ζ-potentials using the Smoluchowski equation (i.e., the ζ-potential in millivolts is 13.6 times the electrophoretic mobility at 25 °C). To characterize the morphology and size of the particles, scanning electron micrographs were taken from samples after grinding and after washing and separation (see sample preparation). After grinding, particles were placed on a sample holder with conducting glue. Centrifuging the solution and allowing the concentrated suspension to dry on a scanning electron microscopy (SEM) sample holder obtained particles from solution. The crystallites were coated with a film of a Au/Pd alloy to a thickness of ca. 80-90 Å in a Bio-Rad SEM coating system. WebLab Viewer Pro, from Molecular Simulations, Inc., was used to generate atomistic models of small blocks or layers from the 3-D barite crystal structure, providing a detailed visual model of the crystal’s surface structure. 2.1. Sample Preparation. A suspension of crushed and finely ground barite crystals was sonicated at high power for approximately 30 min (ultrasonic dispersion), washed with hot water, and left for sedimentation for ca. 1 h in order to separate larger crystallites. The overlying suspension was then pipetted into a 100 mL flask for further preparation through the addition of salts. AR grade barium nitrate (Ba(NO3)2, BDH Chemicals Ltd.), sodium sulfate (Na2SO4, Merck), and sodium nitrate (NaNO3, Riedel-de Haen) were used. Ba(NO3)2 and Na2SO4 were heated at 400 °C for 2 h before use. Distilled water, run through a Milli-Q system with a final resistivity of g18 MΩ cm, was used, and the experiments were carried out in neutral, unbuffered solution. All solutions for the electrophoretic measurements contained 10-3 mol L-1 NaNO3 as the background electrolyte. They were (21) Bokern, D. G.; Ducker, W. A. C.; Hunter, K. A.; McGrath, K. M. J. Cryst. Growth 2002, 246, 139-149.

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Langmuir, Vol. 19, No. 24, 2003 10021 Table 1. Electrophoretic Charge Densities σE (e nm-2) Calculated Following the Gouy-Chapman Treatment concentration (mol L-1)

ζ-potential (mV)

10-2

10-3

10-4

10-5

15 20 25 30

0.0897 0.1249 0.1647 0.2107

0.0283 0.0395 0.0520 0.0666

0.0089 0.0124 0.0164 0.0211

0.0028 0.0039 0.0052 0.0066

Table 2. Double Layer Thickness K-1 in Electrolytic Solution

Figure 1. ζ-Potentials for barium sulfate particles as a function of pBa (i.e., pBa > 5 corresponding to added SO42-). The pzc occurs around pBa ) 4.2. The ζ-potential of barium sulfate in water is -20.3 mV. The ideal behavior following the Nernst equation is shown as a dashed line. left overnight for equilibration, and solutions older than 2 days were not used.

3. Results and Discussion 3.1. Bulk Surface Characteristics. Crystals of natural barite, even after crushing and strong grinding, had regular faces as evidenced by SEM. Unfortunately, the size distribution of these particles was too broad for meaningful measurements. We found that the simplest way to obtain approximately monodisperse solutions was through sonication, washing, and sedimentation. The average size of particles prepared using this method was 479.8 nm with a half-height width of 144.8 nm, as measured by photon correlation spectroscopy. Measurements of particle size were made on the same solutions as those used for ζ-measurements. Each particle was a similarly shaped platelike crystal with a predominance of {001} faces. This predominance suggests that the results of the ζ-measurements should correlate with AFM measurements on a macroscopic (001) cleavage surface (the major structural feature at atomic scale of cleaved barite crystals21). Based on the known solubility of barite, the equilibrium concentrations of barium and sulfate ions in a saturated solution of barite in pure water at 25 °C are each 10-5 mol L-1. Under these conditions, the electrophoretic measurements in Figure 1 show that the crystalline barite particles carry a negative net charge. Varying the concentration of Ba2+ (or SO42-), by adding a common ion source (such as Ba(NO3)2), reverses the charge of the barite particles for [Ba2+] above ∼6 × 10-5 mol L-1, which corresponds to the point of zero charge (pzc). Since the pzc and the saturation concentration in water do not coincide, sulfate ions have a higher affinity for the surface or interface than barium ions and tend to be preferentially adsorbed. Over a narrow region of the graph, corresponding to low concentrations of added Ba2+ or SO42-, the potential on the barite particles varies approximately linearly as a function of the logarithm of added Ba2+ (pBa < 5) or SO42- (pBa > 5) ions. Hence the relative concentrations of barium or sulfate ions determine the potential close to the surface of the barite crystals (i.e., they are potentialdetermining ions). The charge on the crystal surface can be altered from highly negative through zero to highly positive by adding very small amounts of these two ions, while maintaining constant ionic strength and pH. On the macroscopic scale, this behavior can be thermodynamically rationalized in terms of a Gibbs adsorption

concn (mol L-1)

2:2 electrolyte κ-1 (m) ) 3.04 × 10-10|z|-1c-1/2

1:2 or 2:1 electrolyte κ-1 (m) ) ∑izi2ci)-1/2

10-5 10-4 10-3 10-2

4.81 × 10-8 1.52 × 10-8 4.81 × 10-9 1.52 × 10-9

5.56 × 10-8 1.76 × 10-8 5.56 × 10-9 1.76 × 10-9

isotherm. The chemical potentials of Ba2+ and SO42- ions at equilibrium in solution are equal to those on the crystalline barite particle, and both ions compete for adsorption sites on the solid surface. To relate ion concentration and ζ-potential, we assume the condition for equilibrium between the solution and the surface, where the free energy changes ∆GBa2+ and ∆GSO42- are both zero. Applying the Nernst equation to barite in water, where no common ion source is present, and using a total barium ion concentration of 10-5 mol L-1 and a concentration of barium ions at the pzc equal to 6 × 10-5 mol L-1, the surface potential is calculated to be -22.75 mV. The experimental value at this point is ca. -20.3 mV. It has been assumed that the charging process that establishes the surface potential ψ0 does not affect the activity of the potential-determining ions. This is a reasonable assumption for a solid surface for which the potential-determining ions are components of the crystal lattice, since only a small number of Ba2+ ions are required to establish the surface potentials, compared to a relatively large number of Ba2+ and an equal number of SO42- ions at the pzc. The chemical environment is therefore more or less constant over a range of surface potentials. Three regions can be identified from the graph shown in Figure 1, all of which differ from the ideal behavior described by the Nernst equation (given in the figure). Close to the pzc (pBa ) 4.2), a linear relation exists between the ζ-potential and pBa, with a slope comparable to that expected for a thermodynamically reversible electrode. At very high and low values of pBa, the ζ-potential saturates and then decreases. This corresponds to a breakdown in the assumptions related to reversible electrodes and corresponds to a shift in the plane at which the ζ-potential is measured (the plane of shear) due to a decrease in the electrophoretic mobility of the ions. In other words, the difference in the ζ- and Stern potentials increases. The nonsymmetric nature of the graph in Figure 1 emphasizes the different relative adsorption ability of the barium and sulfate ions onto the barite surface. In a simple model, one could assume that the excess barium or sulfate ions would be uniformly distributed over the surface and be indistinguishable from the lattice ions. This local nature of the charge distribution can be probed using AFM. To compare the electrophoretic data with the AFM results, we have calculated the electrophoretic charge densities for several bulk concentrations (Table 1) as well as the double layer thickness, κ-1, at the same concentrations (Table 2). This approach follows the theoretical treatment by Gouy-Chapman and assumes the Smoluchowski condition for large, smooth, and approximately

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spherical particles. Electrophoretic charge densities, σE, were calculated using the equation

zFζ (2RT )

σE ) (8RTr0cb)1/2 sinh

(1)

(where F is the Faraday constant, r and 0 are the relative and free space permittivities, and z is the charge on the ion) for several bulk concentrations cb and ζ-potentials related to typical experimental conditions. The calculated values of the charge densities in Table 1 are given in e nm-2, reflecting the scale of an atomic resolution AFM measurement. For example, for a 10 × 10 nm2 scan size the expected charge density at the Stern layer, ψδ, can be found by multiplying the value for a certain ζ-potential and bulk concentration by a factor of 100. For barite in water, this gives only ca. 0.3 point charges within this area. This may partly explain why such point charges are rarely seen in AFM experiments. The values obtained in Table 2 were calculated for a symmetrical 2:2 electrolyte and an asymmetric 1:2 or 2:1 electrolyte such as Ba(NO3)2 or Na2(SO4) at 25 °C using the standard equation:

κ-1 )

x

r0RT

1000zi2F2c

(2)

Hence, barite in water (∼10-5 mol L-1) exhibits a double layer thickness of 48.1 nm while in a 10-2 mol L-1 solution of Ba(NO3)2, it is only 1.76 nm. The most important factor determining the magnitude and sign of the ζ-potential of an ionic crystal such as barite is its composition and the structure, geometry, and nature of the field of force on the surface of the crystal. Buchanan and Heymann22 conducted a study in the 1940s of the electrokinetic properties of barite as a function of added electrolytes. Experiments performed used the streaming potential method, which involves a flow of fluid past a stationary charged surface, as compared to the more commonly used microelectrophoresis and laser Doppler electrophoresis. Typical particle size ranges are different for these methods: of the order of millimeters for the streaming potential method and nanometers-micrometers for microelectrophoresis. Surface contamination, crystal impurities, electrode differences, and different degrees of crystallinity strongly affect the measured electrokinetic mobility. A broad size distribution can be problematic in the generation of meaningful results for electrophoretic measurements. Buchanan and Heymann22 found a positive ζ-potential of BaSO4 in water of 26 mV and argued that SO42- is less strongly bound to its surface positions in the crystal than Ba2+. This is despite sulfate’s lower hydration energy, which leads to its more ready release from the surface than Ba2+. This relative ease of release of sulfate, giving rise to a positive potential, was found to increase with increasing imperfection of the surface structure. This may go some way to explaining the differences found in the present study, where the predominant feature of the crystals used was the perfect (001) surface. However, our results correlate more closely with recent work investigating the variation of the surface potential of barite with pH and in the presence of a variety of additives, which states that barite has a negative net surface charge.1-3 Although the change in ζ-potential on addition of barium or sulfate ions can be interpreted as being due to (22) Buchanan, A. S.; Heymann, E. Proc. R. Soc. London 1948, A195, 150-162.

preferential ion adsorption, the potential of barite in its own saturated solution cannot be wholly understood in terms of differential adsorption. The negative potential of barite may be due either to preferential release of Ba2+ from the surface lattice or to preferential adsorption of SO42- from the saturated solution of barite. The former alternative is preferred following recent publications discussing step structures, etch figure geometries and their stabilities for the surface of barite.21,23,24 The structure of a surface monostep favors the release of Ba2+ and the retention of SO42- due to the unbalanced electrostatic field generated at steps. We believe that surface steps, either due to cleavage or as a result of dissolution, play a crucial role in understanding the generation of surface charge. Experimentally, it has been shown here (see Figure 1) that the ideal Nernstian behavior of barite can be observed in a concentration range where a 5 × 10-4 mol L-1 Ba(NO3)2 solution is added to a barite suspension in pure water: increasing the potential by ca. 50 mV. Conversely, the addition of a 5 × 10-4 mol L-1 Na2SO4 solution decreases ζ by only ca. 8 mV. Hence, the relative tendencies of barium and sulfate to be released from the lattice or a step must be considered as well as the lattice energies. The hydration energy of Ba2+ is 13.7 eV, while that of SO42- is 9.8 eV.25 These values aid in the explanation of a negative surface potential for BaSO4 due to the greater affinity of Ba2+ as compared with SO42- for water. The local hydration energy of an ion placed in a step is likely to be much higher. The 15 × 15 nm2 AFM image of a vacancy in a row of barium ions along a [010] monostep shown in Figure 5 may be an instructive example: the point defect density seen in this image, one missing Ba2+ per (15 nm)2, can easily account for a surface potential of -20 mV for barite in water according to Table 1. 3.2. AFM Imaging of the Surface of Barite. Imaging the surface of barite under conditions similar to those used for the ζ-measurements opens up the possibility of obtaining direct information about the structure of the solid-liquid interface. This possibility takes advantage of the low solubility of barite and the fact that specifically adsorbed ions are a component of the solid as well as the solution. 3.2.1. True Atomic Resolution of the BaSO4 {001} Basal Plane. The experimental data in Figure 2 display rows of Ba2+ ions between a zigzag arrangement of the anions. The detailed fine structure indicates that detection of single oxygen sites of the sulfate tetrahedron is possible. In addition, atomic height data can be acquired by increasing the gain in feedback while still scanning at fast rates. Using this method and assuming a hard-sphere model, the radius of a single barium ion was measured to be 0.14 nm, which corresponds with a literature value of 0.134 nm for rBa with coordination number 6 in crystal structures.26 To account for the observed atomic resolution, the role of small asperities and that of the solvent and hydrated/ nonhydrated ions present in solution or concentrated at the solid-liquid interface both need to be considered. During imaging, when the silicon cantilever is brought to a distance of ∼3 nm from the surface of the barite, the van der Waals attraction exceeds the spring constant of the (23) Higgins, S. R.; Jordan, G.; Eggleston, C. M.; Knauss, K. G. Langmuir 1998, 14, 4967-4971. (24) Fenter, P.; McBride, M. T.; Srajer, G.; Sturchio, N. C.; Bosbach, D. J. Phys. Chem. B 2001, 105, 8112-8119. (25) Pina, C. M.; Becker, U.; Risthaus, P.; Bosbach, D.; Putnis, A. Nature 1998, 395, 483-486. (26) CRC Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1983-1984.

Charged Barite-Aqueous Solution Interface

Figure 2. A fine structure between the rows of barium is clearly visible and represents single oxygen sites of the sulfate tetrahedron. This deflection image was captured in electrolyte solution containing 10-4 mol L-1 SO42- (as Na2SO4). The solution composition created a repulsive electrical double layer and/or hydration layer at the barite-liquid interface that balanced the van der Waals forces and thereby provided low force imaging with atomic resolution.

cantilever and the tip jumps onto the sample surface. This corresponds to the transition from the noncontact imaging regime to the contact regime. This transition can to some extent be controlled by the solution composition. AFM image data reported here were acquired in the attractive force regime (negative force) very close to the point of mechanical instability. Under this condition, small fluctuations in applied force due to thermal drift, or small variations in the critical force, allow the tip to move onto the branch of the force curve which is at zero separation. To aid interpretation of our high-resolution AFM images and to obtain the most appropriate structural model, we used a molecular simulation package: WebLab Viewer Pro. This package allows visualization of the surface presented to the AFM tip during scanning. In addition, it is possible to simulate a solvent layer adsorbed onto the surface of the original model barite surface. Hence, we were able to simulate the image that would have been obtained by raster scanning the AFM cantilever across the barite surface. To achieve this, an imaginary atom was used whose radius could be freely adjusted. This imaginary atom acted as the AFM tip probing the modeled surface, with the atom being rastered over the model surface to generate a simulated AFM image. During the simulated scanning process, account was taken of the discrete charges of the surface ions. Initially, the radius of the imaginary atom (i.e., the AFM probe) was chosen to be 3 Å, on the basis of the CoreyPauling-Koltun (CPK) (closed-packed) model of barite.21 The data coincided only roughly with the experimental results when this value was used. However, when a solvent surface model was used, in combination with a probe radius of only 1.4 Å the “image” of the surface obtained was that shown in Figure 3. Note that a value of 1.4 Å corresponds approximately to the radius of a H2O molecule or a Ba2+ ion. This model correlates more strongly with the experimental data shown in Figure 2 and hence indicates that the effective tip radius is far below 1 nm (the nominal tip radius of an ultralever is 5-10 nm). We suggest therefore that a water/hydration layer, in combination with the presence of mobile surface ions, plays a crucial role in the contrast mechanism for this kind of imaging. Since images of this kind have been observed

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Figure 3. A solvent surface image of the barite (001) plane obtained by simulating a probe with a radius of 1.4 Å scanning the surface.

Figure 4. AFM deflection image of a monostep in the [010] direction on BaSO4 (001) captured in pure water. The nonbalanced long-range van der Waals forces between the tip and the sample lead to some kind of step erosion or abrasion of the tip over time. Equally possible are multitip effects preventing true resolution of the step structure and the fine structure of the surface unit cell.

with many different tips, we assume a generalizing aspect to this suggestion, in agreement with Cleveland and coworkers.20 Additional and more striking evidence for true lateral atomic resolution was achieved by imaging nonperiodicities such as step edge kinks. Figure 4 shows an AFM deflection image of a monostep on barite (001) in water with an eroded line defect (step edge kink) along the step revealing a somewhat weakly resolved, nonperiodic structure at atomic scale. The nonideal image behavior can be assigned to essentially three factors: (1) the microstructure and sharpness of the tip, (2) the scanning forces and additional lateral (friction) forces, and (3) solid-liquid interfacial properties such as adsorbed species, surface charges, and additional surface forces such as hydration. The weak resolution of the step is related to a nonideal tip (multitip), with a high repulsive or attractive interaction between the tip and the sample facilitating the degradation of the quality of the tip or, vice versa, abrading the step structure. It was found that the scanning force could be controlled through the solution composition, balancing the strong van der Waals attractive forces. A highlight in this respect is the image of a pointlike defect shown in Figure 5a. The deflection image of atomically resolved monosteps on barite (001) showing a pointlike defect (vacancy along the step, upper left corner)

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Figure 6. AFM deflection image at atomic scale displaying a pronounced change in image contrast between two terraces separated by a [210] monostep. The orientation of the rows of barium ions in the upper portion of the image, prior to the step, differs from those in the lower portion.

Figure 5. (a) AFM deflection image of barite (001) showing three monatomic steps including kink sites that can be viewed as point defects, marked as a vacancy in the upper left of the image. The data were obtained utilizing an electrical double layer repulsion to balance van der Waals forces. A slight friction force contribution may be seen in the herringbone arrangement of the lattice. (b) Sectional analysis of the line of barium ions between the two arrows shown in panel a. Each peak in the section taken corresponds to a barium ion; four atoms are evident followed by a gap where the fifth barium ion is expected, followed by a further four atoms. This correlates directly with the two-dimensional image, showing four bright circles, a dark region, and a further four bright circles.

demonstrates the true atomic resolution capability of the instrument. This is made all the more remarkable by the low occurrence of such defects as explained in section 3.1. A cross section along this row of barium ions is shown in Figure 5b. The cross section shows the presence of a total of eight barium ions, with one missing at position 5. The image was acquired under a net attractive force displaying monosteps that follow the crystallographic outline of the surface lattice, rows of Ba2+ ions at the termination of a [010] half unit cell (c/2) step.21 It is reasonable to find this configuration (a cationic vacancy (Ba2+)) in a single monatomic step line surrounded by aqueous solution since for this system, this type of defect is relatively stable. Another feature of interest with regard to both surface steps and atomic-scale imaging with AFM is an observation made while imaging at steps, shown in Figure 6. Deflection images of monosteps show a change of image contrast between the two terraces separated by the step. More specifically, from Figure 6 the orientation of the rows of barium ions appears to change before and after stepping down from the upper to the lower portions of the image. One possible explanation of this effect is a change in the interaction force from the upper to the lower terrace.

Figure 7. A structural model of a [210] monostep across barite (001), displayed as a solvent surface in order to explain the contrast change seen in Figure 6. The atomic arrangement of the upper (lower left of the figure) and lower (upper right of the figure) parts of the terraces is shifted. This shift occurs in such a way that the observed inversion of image contrast shown in Figure 6 could be explained structurally by the change in the solvent surface model of a [210] step running along the (001) plane.

This indicates a certain disadvantage of the deflection mode: the two parts of the image cannot be considered equiforce topographies, but the force applied to the upper terrace by the tip is different (higher) than the force on the lower terrace. A second equally likely reason for this type of image behavior is exemplified in Figure 7, which shows the solvent surface model of a [210] half layer step running along the (001) plane dividing the terraces in such a way that the atomic structure described by rows of cations is interrupted and shifted for one row. This model provides a possible structural explanation of the observed inversion of image contrast shown in Figure 6, requiring a [210] orientated monostep. The observed change in image contrast was observed frequently but only in association with [210] steps. Hence the proposed structural explanation for this phenomenon is preferred to that of a change in force. 3.2.2. In Situ High-Resolution AFM Mapping of the Stern Layer. In addition to imaging barite (001) under saturated solution conditions, AFM measurements were also performed for two different concentration ranges of added electrolyte: first, at low Ba2+ or SO42- concentrations around the Nernstian region (see Figure 1) and

Charged Barite-Aqueous Solution Interface

Figure 8. Height image of a monostep on BaSO4 (001) in 10-2 mol L-1 Ba(NO3)2 solution showing ion agglomeration at the step with atomic resolution.

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second, in the saturated region of concentrations above 10-3 mol L-1, where changes in concentration have little effect on the ζ-potential, controlling instead the position of the plane of shear. Of principal interest is the presence of the thin double layer, with respect to application of the force microscope, to see if it is possible to detect the immovably adsorbed species in the inner part of the Stern plane. All images acquired in the Nernstian region ([Ba2+] or [SO42-] ∼ 10-4 mol L-1) showed no significant difference as compared with data obtained in water, except for a slightly improved contrast at atomic scale. In contrast, for concentrations of 10-2 mol L-1 barium or sulfate ions, in which the electrical double layer thickness κ-1 is of the order of 2 nm (see Table 2), pronounced changes in the interfacial structure could be detected by AFM. The images displayed in Figures 8 and 9 show patches of adsorbed species seen as agglomerations on the barite (001) plane, rather than a uniformly distributed layer of specifically adsorbed ions, as might be expected. Such images were reasonably common and

Figure 9. (a) AFM deflection image at atomic scale of BaSO4 (001) immersed in 10-2 mol L-1 Na2SO4 solution showing patches of adsorbed species. The image was captured in the jump-in region (negative force) of the force curve; the image contrast of the underlying substrate lattice reveals only one type of surface ion (lower right). (b) Compression force curves of the Si tip on barite (001) and the adsorbate structure. The barite (001) curve does not show any significant repulsive behavior in the noncontact regime, indicating that the surfaces of the tip and sample are not equally charged. Note that the gap in the data collection on compression is due to the feedback loop being unable to respond quickly enough to trace the tip-surface interaction. The adsorbate curve shows a very small repulsive maximum at ∼1.8 nm just before the tip jumps into contact with the solid surface. This value agrees well with the calculated double layer thickness κ-1, as shown in Table 2.

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Figure 10. Height image of the barium surface sublattice captured in 10-2 mol L-1 Ba(NO3)2 solution; only one kind of surface ion is visible, here the barium ion. The image reveals the Ba2+ sublattice, suggesting a repulsive electrostatic contrast influence of Ba2+ ions between the tip and the sample. This contrast situation in the AFM image suggests a strong electrostatic contribution by mobile ions near the surface during data acquisition.

are indicative of the resolution of the data that can be obtained. Figure 8 shows a height image captured in 10-2 mol L-1 Ba(NO3)2 at a surface monostep with a concentration of species in the step region, indicating to some degree that the defect structure (the step) plays an important role as an active site in the adsorption and, consequently, charging process at the interface. Figure 9b shows a compression force curve of the Si tip on barite (001) in 10-2 mol L-1 Na2SO42- aqueous solution which is associated with the image in Figure 9a. The force data do not show any significant repulsive behavior in the noncontact regime, indicating that the tip and barite surfaces are not equally charged. The curve is typical and comparable to curves obtained in water prior to the injection of the electrolyte solution. Nonetheless, by carefully placing the tip above the adsorbate structure seen in Figure 9a, a force curve is obtained which shows a small repulsive maximum at ∼1.8 nm prior to the tip pushing through the Stern layer and jumping into contact with the solid surface. This value agrees very well with the calculated double layer thickness κ-1, as shown in Table 2. An important conclusion with regard to the electrical double layer is that the local structure of the interface is characterized by an inhomogeneous, patchlike distribution of ions. Images obtained at atomic scale for BaSO4 (001) in a potential-determining electrolyte solution (representing scan areas from 2 × 2 nm2 up to 80 × 80 nm2) were transformed into Fourier space using a 2D fast Fourier transform (FFT) algorithm, and a consistent pattern of periodicities could be observed, as described previously.21 Surface steps and other types of defect structures may play an important role in any surface interaction because the free energy of these sites is considerably higher than that of the unperturbed perfect cleavage face. We discuss here the specific ion adsorption to barite (001) with respect to both types of surface sites, the atomically smooth (001) terrace and defect sites such as surface steps. The electrophoretic potential of barite in pure water is moderately negative, which is associated with an electrophoretic charge density of only two point charges for an interfacial area of 100 × 100 nm2 (see Table 1). This

Figure 11. (a) FFT and (b) cross-section analysis of the image shown in Figure 10. The 0.88 nm periodicity of the barite surface lattice does not appear on the FFT plot; rather, the periodicity of 0.56 nm reflecting the Ba-Ba distances along [010] occurs with characteristically high intensity.

approximation readily explains why the initial charge has not been observed in AFM experiments described previously21 or for the experiments performed in the dilute region where the Nernst equation can be applied to describe the surface potential. On the other hand, this approach may illustrate the probable role of defects in the charging mechanism. The BaSO4 (001) surface displays a degree of heterogeneity and the presence of defect structures such as steps and, after partial dissolution, etch figures.21,23,24 The surface defect density of any part of the cleavage plane or of the surface of particles employed in the ζ-potential measurements easily allows two point charges within a 100 × 100 nm2 surface area. The accumulation of species at a surface step in a concentrated Ba(NO3)2 solution shown in Figure 8 underlines this view.

Charged Barite-Aqueous Solution Interface

While these considerations may explain the generation of a surface potential, they do not explain its sign. The AFM image displayed in Figure 10 was captured in 10-2 mol L-1 Ba(NO3)2 solution and reveals only the Ba2+ sublattice of the BaSO4 (001) surface. The image contrast may therefore be solely due to the repulsive interaction between surface lattice barite ions and adjacent mobile interface barite ions. The opposite situation of this scenario was also observed (not included here), where the SO42- sublattice was observed, indicating sulfate-sulfate repulsion as the dominating contrast element during data acquisition. Fourier analysis of Figure 10 (Figure 11a) is dominated by the 0.56 nm periodicity of Ba-Ba distances along [010] (the b-axis), despite the much larger periodicity of the (001) being observed in the cross-sectional analysis of the image (Figure 11b). This underlines the importance of short-range forces in imaging an ionic solid-solution interface at atomic resolution with the force microscope. The interpretation of AFM images for the BaSO4/Ba2+, SO42- system is limited by the dynamics of the features under investigation and differs substantially from concepts related to electrostatic force microscopy of surface charge on insulators in air or vacuum.27 The surface defect concentration of the barite particles prepared for the ζ-determination is also likely to be larger than the defect concentration on a macroscopic cleavage face of barite {001} used in an AFM measurement. Additionally, as shown in Table 1, the calculated electrophoretic charge density for the measured saturation area (ζ-potentials of ca. 30 mV, positive or negative, at ionic strengths of approximately 10-2 mol L-1) gives, for example, a value of 20 point charges (as Ba2+ or SO42-) per 10 × 10 nm2. Consequently, the classical Gouy-Chapman treatment (27) Wintle, H. J. Meas. Sci. Technol. 1997, 8, 508-513.

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appears to be a suitable approximation in order to explain certain features of the images. On the other hand, the force microscope results provide important supplementary information about the local charge distribution of preferentially adsorbed ions in which, for example, agglomerations of ions can be seen rather than a uniform layer. 4. Conclusions In addition to the surface lattice, step edges and pointlike defects were visible with true lateral atomic resolution for the barite (001) surface in solution. The presence of water or electrolyte solution of low ionic strength (Newtonian region) reduced the long-range attractive van der Waals forces between the tip and the sample by creating a repulsive hydration and/or electrical double layer force, providing force-balanced imaging with low net attractive force. Imaging at higher forces in water led to the inability to repeatedly image a monostep or the fine structure of the unit cell, due to strong repulsive contact and surface or tip abrasion. Barite in saturated solution carries a negative surface charge corresponding to 0.3 point charges per 100 nm2, due to the desorption of Ba2+ ions. The local structure of the interface in the presence of added electrolyte is characterized by an inhomogeneous, patchlike distribution of ions. The defect structure (and in particular surface steps) dominates the charging process at the interface. Under these conditions, atomic-scale sublattices of either barium or sulfate ions can be imaged via the repulsive interaction between surface lattice ions and adjacent mobile interface ions. Acknowledgment. D.G.B. thanks the University of Otago for a post-graduate scholarship. LA0269255