Adhesion with Water Present at Surfaces

Pashley and Israelachvili carried out a detailed study of mica surfaces approaching each other through water and dilute electrolyte solutions in an attempt to find hydrate crystal layers. These layers had been inferred from the structure of damp clay, which is known to swell in water and to have a distinct lubricious surface, quite different from normal oxides. In the 1930s such clay had been investigated by the X-ray diffraction method, which showed that the clay plates moved apart in water to distances of 0.25 and 0.55 nm, about the diameter of one or two layers of water molecules, as shown schematically in Fig. 6.15. 

Figuro 6.15. Diagrammatic picture of clay platelets under dry, damp, and wet conditions showing the water molecules between the layers. 

The measurements of force could only be conducted in the regions shown by the black lines in Fig. 6.16. These measurements showed steep repulsions. If too great a force of compression was applied, then the mica jumped closer by removing one layer of water molecules, onto the next black line, which was again steeply repulsive. Eventually, the mica surfaces made contact to give a work of adhesion of 11 ml m . In addition to this molecular contact state, toe were two other stable points of adhesion corresponding to one and two molecular layers of water between the mica, respectively. In Fig. 6.16, these stable slates are pointed 

When the mica plates were well separated, i.e. more than 2 nm apart, the results fitted the DLVO theory of double-layer repulsions corresponding to a potential of - 78 mV, which amounts to 40% coverage of the mica with positively charged potassium ions. These long-range forces were those previously measured by Adams and Israelachvili.  

The presence of water at the contact between probe and substrate in the atomic force microscope (AFM) makes life easier by reducing adhesive force and preventing the vicious jumping together of the surfaces. This allows simple measurement of adhesion, together with the prospect of atomic resolution of the surfaces. 

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