The energy of a van der Waals attraction is proportional to 1/r6
VdW attractions are much stronger at short range – they rapidly diminish as the separation, r, between the two interacting atoms or molecules increases
If the difference between the dielectric constant of molecules and of the medium is large, vdW attractions between molecules are strong
If the dielectric constants of the molecules and the medium are similar, vdW attractions between molecules are weak
The energy of vdW attractions in a medium is proportional to (ε2−ε1)2 and to 1/r6
Such energy is usually expressed in terms of the Hamaker constant, A21
VdW forces between identical molecules are always attractive
The Hamaker constant for vdW attraction (A21) between molecules in a medium depends on the Hamaker constants for the molecules (A2) and the medium (A1) in a vacuum:
In the limit of large separation h between two colloid particles each of radius R,