Isoelectric Point of Fluorite by Direct Force Measurements Using Atomic Force Microscopy Shoeleh Assemi,* ,² Jakub Nalaskowski, ‡ Jan D. Miller, ‡ and William P. Johnson ² Department of Geology and Geophysics and Department of Metallurgical Engineering, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed October 18, 2005. In Final Form: December 9, 2005 Interaction forces between a fluorite (CaF 2 ) surface and colloidal silica were measured by atomic force microscopy (AFM) in 1 × 10 -3 M NaNO 3 at different pH values. Forces between the silica colloid and fluorite flat were measured at a range of pH values above the isoelectric point (IEP) of silica so that the forces were mainly controlled by the fluorite surface charge. In this way, the IEP of the fluorite surface was deduced from AFM force curves at pH ∼9.2. Experimental force versus separation distance curves were in good agreement with theoretical predictions based on long-range electrostatic interactions, allowing the potential of the fluorite surface to be estimated from the experimental force curves. AFM-deduced surface potentials were generally lower than the published zeta potentials obtained from electrokinetic methods for powdered samples. Differences in methodology, orientation of the fluorite, surface carbonation, and equilibration time all could have contributed to this difference. Introduction The surface charge of ionic solids in water is determined by differential hydration of their lattice ions at the surface, which depends on the crystal structure and the cleavage plane of the crystal. Miller and Clara 1,2 demonstrated that the hydration energy of the surface ions can be calculated for fluorite by considering the lattice energy and surface Madelung constants. Microelectrophoresis of powdered fluorite samples and streaming potential measurements of fluorite crystals have yielded different results. Several studies have shown that in the absence of surface carbonation a high positive zeta potential for fluorite is obtained. 3-6 Surface carbonation results in the change of the character of the surface from fluorite (CaF 2 ) to calcite (CaCO 3 ), with a low, positive surface potential and thus a lower IEP. 7 A few studies report a lower IEP (∼pH 6.6) or a completely negative surface. 8,9 The advent of atomic force microscopy (AFM) 10 has made it possible to measure the interaction forces between a broad range of surfaces and thus allow for the estimation of their surface charge in different electrolyte solutions. 11 Interaction forces between two particles can be measured by AFM using the colloidal probe technique, where a sphere of the particle of choice can be glued to the AFM tip. Attachment of a sphere to the tip removes uncertainties in the interaction radius and allows a quantitative analysis of the force data by fitting the data to existing models. 11-14 AFM has been widely used to determine the isoelectric point of oxide surfaces such as silica and R-alumina. 11-13,15 In this letter, we report the application of the colloidal probe technique to estimate the surface potential and isoelectric point of CaF 2 in a dilute electrolyte (1 × 10 -3 M NaNO 3 ). This method can be particularly useful for estimating the IEP of small mineralogical samples at their different crystallographic planes. Materials and Methods Materials. Fluorite (CaF 2 ) optical windows (13 mm × 2 mm) were purchased from Harrick Scientific Corp. (Ossining, NY). The fluorite window was cleaned using UV/ozone for 15 min prior to AFM measurements. Characterization of the surface by X-ray diffraction (X’Pert Texture, Phillips Analytical, MA), revealed a (110) plane of orientation. Silica particles with a nominal diameter of 4.70 μm (Bangs Laboratories, Inc., IN) were cleaned by soaking in SC1 solution (5:1:1 H 2 O/NH 4 OH/H 2 O 2 ) and holding the suspension at about 80 °C for 15 min. The suspension was filtered through a 0.45 μm disposable filter and left to dry inside the filter. The filter was then cut, and the silica particles were spread on a precleaned glass slide using a clean tungsten wire. The AFM fluid cell, O-ring, and tubings were cleaned prior to the experiment by rinsing with acetone/methanol/acetone and several portions of deionized water, followed by blow drying with high- purity nitrogen. Deionized water was obtained from a Milli-Q system. The resistivity of the water was above 18 MΩ cm in all experiments. All of the glassware and plasticware were cleaned by overnight soaking in 10% HNO 3 and copious rinses with deionized water. Solutions were prepared using analytical-grade reagents. Atomic Force Microscopy Measurements. AFM force mea- surements were made using a Nanoscope IIIa (Veeco, Santa Barbara, CA) scanning probe microscope in a fluid cell (Veeco). V-shaped, gold-coated tipless silicon nitride cantilevers were obtained from Veeco. The spring constant of the cantilevers was reported to be 0.12 N m -1 by the manufacturer. The spring constant of the cantilevers was determined to be 0.10 ( 0.003 N m -1 using the Cleveland method, 16 which relies on monitoring the shifts in the resonance * Corresponding author. E-mail: sassemi@mines.utah.edu. Phone: (801) 585-1538. ² Department of Geology and Geophysics. ‡ Department of Metallurgical Engineering. (1) Miller, J. D.; Clara, J. V. In Froth Flotation; Fuerstenau, M. C., Ed.; AIME: New York, 1976. (2) Clara, J. V.; Miller J. D. J. Chem. Phys. 1976, 65, 843. (3) Hu, J. S., Ph.D. Thesis, University of Utah, Salt Lake City, UT, 1985. (4) Haas, S. R.; Nascimento, F. R.; Schneider I. A. H.; Gaylarde, C. ReV. Microbiol. 1999, 30, 225. (5) Yuehua, H.; Chi, R.; Xu, Z. Ind. Eng. Chem. Res. 2003, 42, 1641. (6) Miller, J. D.; Fa., K.; Clara, J. V.; Paruchuri, V. K. Colloids Surf., A 2004, 238, 91. (7) Miller, J. D.; Hiskey, J. B. J. Colloid Interface Sci. 1972, 41, 567. (8) Oberndorfer, J.; Dobias, B. Colloids Surf. 1989, 41, 69. (9) Fa, K.; Jiang, T.; Nalaskowski, J.; Miller, J. D. Langmuir 2003, 19, 10523. (10) Binnings, G.; Quate C. F.; Gerber, Ch. Phys. ReV. Lett. 1986, 56, 930. (11) Ducker, W. A.; Senden T. J.; Pashley R. M. Nature 1991, 353, 239. (12) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207. (13) Larson, I.; Drummond, C. J.; Chan D. Y. C.; Greiser F. Langmuir 1997, 13, 2109. (14) Yalamanchili, M. R.; Veeramsuneni, S.; Azevedo, M. A. D.; Miller, J. D., Colloids Surf., A 1998, 133, 77. (15) Veeramasuneni; S.; Yalamanchili, M. R.; Miller, J. D. J. Colloid Interface Sci. 1996, 184, 594. (16) Cleveland, J. P.; Manne, S.; Bocek, D. ReV. Sci. Instrum. 1993, 64, 403. 1403 Langmuir 2006, 22, 1403-1405 10.1021/la052806o CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006