London‐V. D. waals forces in colloidal systems

In a previous paper the London‐ v. d. Waals forces between spherical particles have been calculated as a function of the diameters and the distance separating the particles. On the basis of these data the probable magnitude and range of these forces in colloidal systems is discussed in this paper. The “range”, defined as that distance at which the energy of interaction equals the kinetic energy of Brownian motion, is found commonly to be of the order 0.06 to 0.2 times the diameter of the smallest particle: the size of the second particle is relatively of little importance. Comparing with experiment it is demonstrated that the adhesive forces observed under various circumstances are of the order of magnitude theoretically expected. Finally it is maintained that our concept of the range of a force will depend on the size of the particles considered. Different definitions of the range are compared with each other. The discussion illustrates that a correct comparison of observations of adhesive forces made by different methods or on different scales cannot be made without theoretical knowledge of the nature of the forces acting.