Molecular Interpretations

Van der Waals recognized the existence of intermolecular force in 1879, and introduced an attractive energy term and an excluded volume term into the ideal gas law. Subsequent workers have found that there are various types of intermolecular forces, including dispersion (nonpolar) force, dipole (dipole-dipole) force, induction (dipole-induced dipole) force, and hydrogen bonding. These intermolecular forces are commonly known as the van der Waals forces. The van der Waals force between two molecules is a short-range force, varying with r⁻⁷, where r is the intermolecular distance. On the other hand, the van der Waals force between two macroscopic bodies is a long-range force, varying with z⁻³, where z is the distance between two flat plates. It should be noted here that the variation of van der Waals force between two macroscopic bodies depends on the shapes of the bodies. For instance, it varies with z⁻² between two spheres (see Section 2.5.5). Various molecular forces are discussed and used to analyze the interfacial energies in this chapter. Several reviews of intermolecular forces are available elsewhere [1–8].