Direct measurement of surface forces between sapphire crystals in aqueous solutions
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DR. Clarke Thomas J. Watson Research Center, IBM Research Division, Yorktown Heights, New York 10598
M.T. Clarkson Department of Applied Mathematics, Research School ofPhysical Sciences, Australian National University, GPOBox4, Canberra, Australia (Received 22 December 1987; accepted 8 February 1988)
Measurements are presented of the electrical double layer and van der Waals forces between the (0001) surfaces of two single-crystal sapphire platelets immersed in an aqueous solution of NaCl atpH values from 6.7 to 11. The resultsfitthe standard Deryaguin-Landau-VerweyOverbeek (DLVO) theory, with a Hamaker constant of 6.7 X 10 ~ 20 J. These are the first measurements made using the Israelachvili surface forces apparatus without mica as a substrate material, and they demonstrate the possibility of using this technique to explore the surface chemistry of a wider range of materials. It has long been recognized that surface forces can have a profound effect in many areas of materials science, for example, in the adhesion and bonding of small particles, the processing of ceramic powders, the fracture of fibrous composites, wetting phenomena, and biocompatibility. More recently, quantitative predictions of their role have been made in such diverse areas as surface adhesion, brittle fracture,' and the stability of intergranular phases in ceramics.2 However, despite their importance, direct measurements of surface forces in systems of interest to the materials community are lacking. Over the past decade the surface force apparatus developed by Israelachvili3 has proved to be an extremely precise and powerful instrument, particularly in measuring electrical double layer, van der Waals, and solvation forces between molecularly smooth surfaces immersed in liquids.3"8 The work of Pashley in particular5'9 has shown how force measurements can be used to explore surface chemistry. When immersed in aqueous solution, solids generally become charged due to the adsorption or dissociation of ions.4 This gives a repulsive force between two like surfaces, the so-called electrical double-layer force, which is an approximately exponential function of the surface separation. Its magnitude depends on the surface charge and its range depends on the ionic strength of the solution. At small separations this force is overpowered by the omnipresent van der Waals attraction, so the surfaces are expected to adhere when they are in contact. However, in many cases there are, in addition, solvation effects that modify the force at molecular distances.5"8 Force measurements can therefore tell us the magnitude of the surface charge or potential, how it varies with the concentration of different J. Mater. Res. 3 (3), May/Jun 1988
ions in solution, whether ions or other solutes are adsorbed to the surface, what the thickness of any adsorbed layer is, whether and how strongly the surfaces adhere, whether they are hydrophilic or hydrophobic, and in many cases, how the molecules of a liquid are arranged at the surface. Until now this highly accurate techniq
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