The background to surface forces

Two themes emerge in dealing with the force between two such surfaces -(see Appendix 3)  

Once the forces between rigid surfaces are imderstood, we can proceed to the more complex question of how an object reorganises its shape in response to the change in chemical potential, of force induced by a neighbouring object. 

The classical intuition on molecular forces is embodied in the famous Derjaguin-Landau-Verwey-Overbeek theory of colloid stability. It blends themes (i) and (ii) above in a contradictory way that has taken imtil the last few years to sort out. 

The best elementary account of the theory is contained in the book by Israelachvili [4]. But there have been very many developments since, as outlined below. These developments are very complicated, but cannot be ignored, and the subtleties have to be recognised to make ser se of many phenomena in colloid science and biology. 

Imagine a suspension of colloidal particles in water. What causes stability, and what, imder changing solution conditions like addition of salt causes flocculation (precipitation of the suspension) Two opposing forces were considered to operate between two such particles. The one, attractive, is the quantum mechanical van der Waals force and treats an intervening liquid as if it has bulk liquid properties up to the interfaces of the particles (theme (i)). The other, repulsive, due to charges formed by dissociation of ionisable surface groups, is electrostatic in origin, and depends on salt concentration. 

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