Waals forces between particles and surfaces - basics

2 van der Waals forces between particles and surfaces - basics 

The vdW forces are typically attractive but can be repulsive in special cases (see later). They depend on the size and shape (geometry) of particles/surfaces and their proximity but above all they depend on the nature (chemistry) of the materials and the medium via the so-called Hamaker constant, A. 

For example the van der Waals force between equal-sized spherical colloidal particles of radius R (see also Chapter 2) is expressed in potential energy-distance form (V-H) as  

It has been shown (IsraelachvUi, 2011) that for diameters beyond 0.5 nm a molecule must be considered a small particle and the equations of Table 2.2 should be used for the van der Waals forces. The effective Hamaker constant, which depends on the particles (chemical composition) and the medium, determines the effective strength of the vdW interactions. 

When we have a colloid system in a medium othffl than vacuum or air, the Hamako constant is decreased and an effective value must be used. We thoeforc use the symbol A in the relevant equatiems. Thus, in the more realistic situation where spherical particles or surfaces are imm ed in anoth liquid or sohd medium, th i the van der Waals forces can be greatly reduced. As most industrial processes are carried out in a liquid, thCTC arc typically two effects (i) we may have adsorption phmomraia on the particle surface and thus reduction of the surface raiogy and (ii) we have reduced van der Waals forces due to the smaller value of the effective Hamaker constant in aliquid compared to vacuum or air. 

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