We have measured the force of adhesion between two curved surfaces of molecularly smooth mica in the presence of water vapour and various organic vapours at relative vapour pressures in the range 0-0.99. We find that the annulus of capillary-condensed liquid around the contact zone does not affect the adhesion force unless the surface tension of the liquidγLVis greater than the (experimentally determined) surface free energy of the solidγSV. Under such conditions (whenγLV>γSV) the meniscus force arising from the Laplace pressure dominates the adhesion, and for cyclohexane and other inert organic liquids the adhesion force is within about 10% of that calculated from microscopic thermodynamics via the Young-Laplace equation for relative vapour pressures as low as 0.1. The Kelvin equation for capillary condensation, which is itself derived from the Young-Laplace equation, predicts that at these relative vapour pressures meniscus radii are of the order of one or two molecular diameters. The results therefore suggest that macroscopic thermodynamics is in principle applicable to such small menisci and - by extension - to liquids only a few molecules away from a solid-liquid interface. The results for water are quite different, and show that the adhesion force due to the Laplace pressure is within 10% of that expected from bulk thermodynamics only for relative vapour pressure above 0.9, corresponding to Kelvin meniscus radii greater than 5 nm. This result may reflect the previously suspected long-range cooperative nature of the hydrogen bonding interaction in water, though the equally long-range effects of double-layer forces may also play a role in reducing the effective surface tension of water. The results are relevant to the problem of the applicability of macroscopic thermodynamics to microscopic liquid systems as well as on the effects of capillary condensed liquids on adhesion forces. © 1981.
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