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VAN DER WAALS FORCE Van der Waals force is the term used to describe a general attractive force among neutral atoms or molecules. Since the first proposal of an attractive force among neutral atoms by van der Waals in 1873, to account for certain properties of non-ideal gases and liquids, three different types of interaction have been found to contribute to van der Waals force: (I) dipole-dipole interaction (Keesom interaction), (2) dipoleinduced dipole interaction (Debye interaction), and (3) instantaneous dipole-induced dipole interaction (London interaction) (van Oss et al., 1988). Of these three, the first two are found with molecules that have permanent dipoles. Only London interaction is universal, being present in atom-atom interactions as well, and predominantly important between the macroscopic bodies in condensed systems. Thus, the London force is responsible for bonding in solid state rare gases and organic molecules. It is also responsible for bonding between non-charged carbon layers in graphite, and electrically neutral silicate and oxide layers in clay minerals and micas. Van der Waals-London force is an attractive force caused by the dipole moments between an instantaneous dipole on one atom or molecule and the induced dipole on the neighboring ones by the instantaneous dipole. The energy of London interaction, also called London (dispersion) energy, is generally given by: (Kittel, 1991 ),
(VI)
where A is a constant uniquely determined by the bonding pair and r is the distance between the pair. For London interactions between the same species (atoms or molecules), A in eqn (VI) may become: 381IX 2 A=-4-
(Huheey, 1978),
(V2)
where 8 1 is the ionization energy and IX is the polarizability of the species. For the interactions between different species, for example between a cationic and an anionic species, eqn (V2)
31X +IX- 818A A=---2(el + 8A)
(Sebera, 1964),
(V3)
where IX+ and IX- are the polarizabilities and 8 1 and sA are the ionization energy and electron affinity of the cationic and the anionic species, respectively. More detailed discussion and references on London interactions are in van Oss et al. (1988); see Davies (1965) and Huheey (1978) for the discussion on the energies of the van der Waals interactions other than London interaction. Many researchers have studied the mineral properties in terms of binding energy calculation. However their binding energy calculations have rarely included van der Waals energy, because it is much weaker than the other principal bonding energies, usually contributing less than 15% of the total binding energy of a mineral. Ward and Phillips ( 1971) suggested that van der Waals energy may have an important role in holding the layers of pyrophyllite and talc. Yu (1990) theoretically calculated the London energy of 2: 1 dioctahedral phyllosilicate and indicated that the role of London energy in determining the c-dimensions is increasingly significant as the interlayer charges decrease. Jae-Young Yu
Bibliography Davies, M. (1965) Some Electrical and Opti
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