Direct measurements of the interaction between adsorbed macromolecular layers

Abstract

Experimental methods have been developed for determining, as a function of their separation distance, the force of interaction between two macroscopic surfaces coated with adsorbed layers. The central element of the apparatus was two hemi-spherically capped silicone rubber cylinders. With the surfaces far apart, an aqueous solution of poly(vinyl alcohol) was placed between the caps to allow adsorption to occur. Under the influence of a normally applied pressure, the surfaces were forced together so that the liquid layer thinned and the adsorbed layers interacted. The rate of film thinning was measured by a reflectance technique and the distance of surface separation by multiple beam interferometry. The interaction pressure between the layers appeared to commence at a surface separation distance of 160 ± 20 nm and increased to about 1.5 × 104 N m–2 at a distance of 70 nm. At shorter distances, the pressure rose steeply with decrease in distance and suggested the formation of a film with a high concentration of PVA between the surfaces.