Notes Direct Measurement of Repulsive and Attractive van der Waals Forces between Inorganic Materials Anders Meurk,* ,² Paul F. Luckham, ‡ and Lennart Bergstro¨m ² Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BY, United Kingdom, and Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden Received November 12, 1996. In Final Form: April 14, 1997 Introduction Among the many contributions to the interaction between surfaces, e.g. double layer, structural, steric, depletion, hydration, and hydrophobic forces, there is one type of interaction that is always present, the van der Waals (vdW) interaction. 1 The vdW interaction has an electrodynamic origin, as it arises from the interactions between oscillating or rotating electrical dipoles within the interacting media. It was early recognized that a repulsive interaction may arise when electric fields created by the fluctuating dipoles in different materials across a medium interact destructively and not constructively, as in the normal, attractive case. 2 On the basis of Lifshitz theory, 2,3 the vdW interaction energy can be estimated from the frequency-dependent dielectric spectra of the materials and media and the body geometry. Several material combinations, typically involving interactions with air as one material, have been considered where repulsive vdW forces should occur. 4-9 There has also been some indirect evidence to support the concept of repulsive vdW forces based on wetting properties of liquid helium 1,2 and particle rejection by solidification fronts. 5,6 Direct measurements of repulsive vdW forces, however, have been sparse and the interpretation has been com- plicated by the possible existence of other types of surface forces, also resulting in a repulsive interaction. Previous studies have invariably used the atomic force microscope (AFM) 10 to probe the repulsive vdW forces. Hutter and Bechhoefer 11,12 measured attractive, close-to-zero, and repulsive interactions between a silicon nitride tip and a mica surface in three different media. On the basis of the dielectric properties of the different systems, they dis- cussed different possible interpretations, including re- pulsive vdW forces. Recently, Milling et al. 13 presented direct AFM measurements of repulsive van der Waals dispersion forces where they compared measurements with theoretical Lifshitz calculations. Using a gold-coated tungsten sphere against a PTFE surface in a range of liquids, the experimental results corresponded well with theory in apolar liquids but a large discrepancy, even in sign (attraction instead of repulsion), was obtained in polar liquids. In this study, the objective was to establish an experi- mental procedure and show direct AFM measurements that unequivocally can be assigned as repulsive vdW forces. The versatility of the AFM and the possibility of using different material combinations have proved to be indispensable in these measurements. Working with insulating, inorganic systems of high stiffness, we mini- mize the contributions and complications caused by conduction or surface deformation. From theoretical Lifshitz calculations we designed an experimental system where a symmetric material combination, 131 (material 1 interacting with a similar material across medium 3), results in attraction and an asymmetric combination, 132 (material 1 interacting with material 2 across medium 3), displays a repulsive interaction. Hence, by measurement of the force between two identical materials in a liquid followed by replacement of one material, the substrate, with another material, the sign, magnitude, and distance scaling of the force curves enable a detailed analysis of the physical origin of the interaction and identification of the possible existence of other, additional interactions. Experimental Section The AFM experiments were conducted in diiodomethane and 1-bromonaphthalene (Aldrich Chemicals) using a noncrystalline Si3N4 tip (Topometrix) against two different flat substrates: a polished, polycrystalline -Si3N4 ceramic (produced by AC Cerama), and an amorphous SiO2 glass surface. The polished Si3N4 substrate was etched in 8% HF prior to every experiment to remove a possible silica layer, stemming from oxidation of the silicon nitride material. The etched substrate was rinsed in distilled 18 MΩ deionized water, dried in compressed air, and then stored immersed in the same liquid that was to be used as the medium in the AFM measurement. The SiO2 glass was treated in a similar manner, but the etching step was exchanged for sonication in toluene to remove any organic contaminations. Similarly, the cantilever tip was rinsed in ethanol, dried, and kept in either diiodomethane or 1-bromonaphthalene prior to measurement. Both liquids, diiodomethane and 1-bromonaph- thalene, were treated with molecular sieves (4 Å) to remove any water. The scanning force microscope used in this study was a commercial Topometrix Explorer (Topometrix). The shape of the cantilever tips, pyramidal with spherical caps, complicates the modeling and evaluation of the force curves. At short separations, D , a, where the tip radius, a, is much larger than the separation distance, D, we have made the approximation of a sphere against a flat surface for the AFM tip-substrate interaction geometry (see Figure 1). Under these conditions, the vdW force, F vdW, can be expressed as * To whom correspondence should be addressed. ² Institute for Surface Chemistry. ‡ Imperial College of Science, Technology and Medicine. (1) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992; Chapter 11. (2) Dzyaloshinskii, I. E.; Lifshitz, E. M.; Pitaevskii, L. P. Adv. Phys. 1961, 10, 165-209. (3) Lifshitz, E. M. Sov. Phys. JETP (Engl. Transl.) 1956, 2, 73-83. (4) Visser, J. Adv. Colloid Interface Sci. 1981, 15, 157-169. (5) Neumann, A. W.; Omenyi, S. N.; van Oss, C. J. Colloid Polym. Sci. 1979, 257, 413-419. (6) van Oss, C. J.; Omenyi, S. N.; Neumann, A. W. Colloid Polym. Sci. 1979, 257, 737-744. (7) Chappuis, J.; Hoeppner, D. W.; Neumann, A. W. Tribol. Ser. 1982, 71-80. (8) Chaudhury, M. K.; Good, R. J. Langmuir 1985, 1, 673-678. (9) Frenzl, W. Ber. Bunsen-Ges. Phys. Chem. 1994, 3, 389-391. (10) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930-933. (11) Hutter, J. L.; Bechhoefer, J. J. Appl. Phys. 1993, 73, 4123- 4129. (12) Hutter, J. L.; Bechhoefer, J. J. Vac. Sci. Technol. B 1994, 12, 2251-2253. (13) Milling, A.; Mulvaney, P.; Larsson, I. J. Colloid Interface Sci. 1996, 180, 460-465. 3896 Langmuir 1997, 13, 3896-3899 S0743-7463(96)01096-7 CCC: $14.00 © 1997 American Chemical Society