ABSTRACT
Consider a spherical solid particle 1 cm in diameter. Its surface S and volume V are, respectively, 3.14 x 10“4m2 and 5.24 x 10“7m3, and thus its surface/volume ratio will be S / V ~ 600 m-1. Assume that we divide the particle in a number N of spherical particles of radius lOOnm such that their total volume equals that of the original 1-cm sphere. However, their surface would be 15.7 m2, and S /V ~ 3 x 107m_1. This simple and wellknown example explains that an essential contribution to the properties of a system formed by dispersing the N particles in, say 1 L of water will be connected with the influence of the surfaces and interfaces of the particles. In particular, the electrical state of the surface of the particles may be determinant: if each of them bears a surface potential of 100 mV (about the order of magnitude typical of colloidal particles in aqueous media), the repulsive electrostatic force between two such particles dispersed in water and located at a surface-to-surface distance of 10 nm would be FEL ~ 2.12 x 10-12 N. This force has to be compared to the strength of other interactions that must or could exist between them. Thus, their gravitational attraction at the same distance would be F° ~ 6.3 x 10-15 N (if their density is 103 kg/m3); their van der Waals attraction FLW ~ 8 x 10-13 N (using typi cal values of the Hamaker’s constant, see Ref. 1). Again, these examples show that in most instances the electrostatic interactions are mainly responsible for the macroscopic proper ties of the suspensions.
