London dispersive force definition

Although the fact that the cycloamyloses include a variety of substrates is now universally accepted, the definition of the binding forces remains controversial. Van der Waals-London dispersion forces, hydrogen bonding, and hydrophobic interactions have been frequently proposed to explain the inclusion phenomenon. Although no definitive criteria exist to distinguish among these forces, several qualitative observations can be made. 

The term van der Waals forces includes three types of intermolecular forces London (dispersion) forces, permanent dipole-dipole forces (sometimes referred to as Keesom forces) and permanent-induced dipole interactions (Debye forces). In 1910, van der Waals was awarded the Nohel Prize for his work on the equation of state for gases and liquids concerned with the reasons for non-ideal behaviour in real gases. His equation introduced compensatory terms to account for the non-zero size of the particles and the inter-particle forces between them. This broader definition of van der Waals forces runs contrary to the use of the term in many current textbooks, but is consistent with its use in the IB syllabus. 

The universal van der Waals attraction which occurs in all disperse systems is described in Vol. 1. The dipole-dipole, dipole-induced dipole and London dispersion forces for atoms and molecules are described. This is followed by the microscopic theory of Hamaker for colloidal particles and definition of the Hamaker constant. This microscopic theory is based on the assumption of additivity of all atom or molecular attractions in each particle or droplet. The variation of van der Waals attraction with separation distance h between the particles is schematically represented. This shows a sharp increase in attraction at small separation distances (of the order of a few nanometers). In the absence of any repulsion, this strong attraction causes particle or droplet coagulation which is irreversible. The effect of the medium on the overall van der Waals attraction is described in terms of the effective Hamaker constant which is now determined by the difference in Hamaker constant between the particles and the medium. The macroscopic theory of van der Waals attraction is briefly described, with reference to the retardation effect at long sepeiration distances. The methods that can be applied for determination of the van der Waals attraction between macroscopic bodies are briefly described. 

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