Direct Force Measurements between Silica and Alumina § I. Larson, † C. J. Drummond, ‡ D. Y. C. Chan, † and F. Grieser* ,† Advanced Mineral Products Special Research Centre, University of Melbourne, Parkville, Victoria 3052, Australia, and CSIRO Division of Chemicals and Polymers, Clayton, Victoria 3169, Australia Received July 11, 1996. In Final Form: January 2, 1997 X AFM force measurements were carried out between a silica colloid sphere and an alumina flat crystal over a wide pH range and in both 1 × 10 -4 and 1 × 10 -3 M KNO3 aqueous solutions. Microelectrophoresis and streaming potential experiments were performed on the silica colloid sample and the alumina plate, respectively. The potentials measured by the different techniques were in very good agreement. The results clearly indicate that AFM force measurements can be used to accurately determine diffuse layer potentials of metal oxide materials under these solution conditions. Introduction Over the last few years there has been a rapid increase in the number of investigations in which the atomic force microscope (AFM) 1 has been used to measure DLVO 2 interaction forces between a single colloid particle and, usually, a flat surface in aqueous solution. 3 There has been little systematic effort made, however, to compare these results from AFM force measurements to results on the same materials obtained by traditional electrokinetic techniques such as microelectrophoresis and streaming potential. 4,5 To establish the ability of the AFM to make accurate force measurements, it is imperative to match AFM data with other techniques on identical surfaces. In this study both surfaces used in an AFM experiment have been independently characterized using traditional electrokinetic techniques. Silica colloids and R-alumina flat single crystals were chosen because of the large quantity of information available for these materials. Also, these surfaces have different isoelectric points (iep’s), so that at certain pH values the surfaces can be oppositely charged, which enabled us to study both attractive and repulsive electrostatic interactions between the surfaces. Electrophoretic mobility measurements on the colloid sample and streaming potential experiments on the flat single crystals have been performed to allow a direct comparison with the results obtained from the AFM technique. Theory AFM force measurements were made between a spherical colloid and a flat plate. The force (F) measured between the sphere of radius (R) and the flat plate can be related to the interaction free energy between flat parallel plates by the Derjaguin approximation. 6 In this approximation the total force, scaled by the radius of the sphere, has the form where V A is the van der Waals interaction free energy per unit area and V R is the electric double-layer interaction free energy per unit area between two flat parallel plates. Here the total interaction free energy between the charged surfaces is separated into the sum of two separate contributions: the van der Waals component and the electrical double-layer term. The van der Waals term will generally be attractive for two metal oxide surfaces separated by an aqueous solution. The method described by Hough and White, 7 that employs the Ninham-Parsegian 8 representation of dielectric data, was used to calculate the value of the Hamaker constant using Lifshitz theory. 9 Dielectric data are used to construct the function ǫ(i)sthe dielectric constant at imaginary frequency i. At  ) 0, ǫ(0) is the static dielectric constant ǫ DC. As  increases ǫ(i) is real and decreases toward 1 as  f ∞. We used the representation where C i is the oscillator strength and ωi is the absorption frequency in the infrared, IR, and ultraviolet, UV, regions. In this approach for calculating the Hamaker constant the function ǫ(i) is sampled at frequency steps of ≈ 2.4 × 10 14 rad s -1 . Therefore there are many more sampling points in the UV region of ǫ(i), and consequently this region has a greater importance in calculating Hamaker constants than the microwave and infrared, IR, regions. The more important parameters in calculating the Hamaker constant are the oscillator strength, C UV, and the relaxation frequency, ωUV. Effective CUV and ωUV parameters can be obtained from a Cauchy plot of refractive index data in the visible region. 7 The oscillator strength in the IR region can be worked out from For simplicity we have ignored the effect of retardation 10 on the van der Waals force, so we have * Author to whom correspondence should be addressed. § Presented at the 69th Colloid and Surface Science Symposium, Salt Lake City, Utah, June 11-14, 1995. † University of Melbourne. ‡ CSIRO Division of Chemicals and Polymers. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Binnig, G.; Quate, C.; Gerber, G. Phys. Rev. Lett. 1986, 56, 930. (2) Derjaguin, B. V.; Landau, L. D. Acta Physiochem. 1941, 14, 633. Verwey, E. J. W.; Overbeek, J. Th. G. The Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (3) A recent review of AFM force measurements in aqueous solution is given in: Butt, H.-J.; Jaschke, M.; Ducker, W. Biochem. Bioenerg. 1995, 38, 191. (4) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885. (5) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Langmuir 1995, 11, 2367; Colloids Surf. A 1995, 103, 195. (6) Derjaguin, B. V. Kolloid Z. 1934, 69, 155. (7) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3. (8) Ninham, B. W.; Parsegian, V. A. J. Chem. Phys. 1970, 52, 4578. (9) Lifshitz, E. M. Sov. Phys. JETP 1956, 2, 73. (10) A very good account of the retardation of vdW forces is given in: Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1989; Vol. I, p 188. F/R ) 2π(V A + V R ) ǫ(i) ) ǫ DC when  ) 0 ǫ(i) ) 1 + C IR 1 + (/ω IR ) 2 + C UV 1 + (/ω UV ) 2 when  > 0 C IR ) ǫ(0) - C UV - 1 V A ) -A H 12πH 2 2109 Langmuir 1997, 13, 2109-2112 S0743-7463(96)00684-1 CCC: $14.00 © 1997 American Chemical Society