Van der Waals/London-dispersion forces

Table A.4.1 Attractive Forces at Interfaces-surface Energy, y, and London-van der Waals Dispersion Force Component of Surface Energy, y(L) a>...

Curve P represents the physical interaction energy between M and X2. It inevitably includes a short-range negative (attractive) contribution arising from London-van der Waals dispersion forces and an even shorter-range positive contribution (Born repulsion) due to an overlapping of electron clouds. It will also include a further van der Waals attractive contribution if permanent dipoles are involved. The nature of van der Waals forces is discussed on page 215. 

Van der Waals postulated that neutral molecules exert forces of attraction on each other which are caused by electrical interactions between dipoles. The attraction results from the orientation of dipoles due to any of (1) Keesom forces between permanent dipoles, (2) Debye induction forces between dipoles and induced dipoles, or (3) London-van der Waals dispersion forces between fluctuating dipoles and induced dipoles. (The term dispersion forces arose because they are largely determined by outer electrons, which are also responsible for the dispersion of light [272].) Except for quite polar materials the London-van der Waals dispersion forces are the more significant of the three. For molecules the force varies inversely with the sixth power of the intermolecular distance. 

Figure S.3 Potential energies of interaction between two colloidal particles as a function of their distance of separation, for electrical double layers due to surface charge (VolK London-van der Waals dispersion forces (V ), and the total interaction (VT). From Schramm [426], Copyright 2003, Wiley.

Adsorption by dispersion forces, i.e., London-van der Waals dispersion forces acting between adsorbate and adsorbent... 

L) values for water and mercury have been determined by measuring the interfacial tension of these liquids with a number of liquid-saturated hydrocarbons. The inteimolecular attraction in the liquid hydrocarbons is entirely due to London-van der Waals dispersion forces for all practical purposes. Yjd was derived from contact angle measurements. 

Adsorption by Dispersion Forces. Occurs via London-van der Waals dispersion forces acting between adsorbent and adsorbate molecules (Figure 2-9). Adsorption by this mechanism generally increases with an increase in the molecular weight of the adsorbate and is important not only as an independent mechanism, but also as a supplementary mechanism in all other types. For example, it accounts in part for the pronounced ability of surfactant ions... 

The molecular component of the disjoining pressure, IIm(/i), is negative (repulsive). It is caused by the London-van der Waals dispersion forces. The ion-electrostatic component, IIe(/i), is positive (attractive). It arises from overlapping of double layers at the surface of charge-dipole interaction. At last, the structural component, IIs(/i), is also positive (attractive). It arises from the short-range elastic interaction of closed adsorption layers. 

Bowling [1988] describes van der Waals forces in the following way. At absolute zero temperature solids can exhibit local electric fields and above this temperature additional contributions come from the excitation of the atoms and molecules making up the solid material. Van der Waals forces include forces between molecules possessing dipoles and quadrapoles produced by the polarisation of the atoms and molecules in the material. These dipoles and quadrapoles may be present naturally or by induced polarity. Non-polar attractive forces may also be present. The non-polar van der Waals forces may also be referred to as London-van der Waals dispersion forces because London associated these forces with the cause of optical dispersion, i.e. spontaneous polarisation [Com 1966]. Such dispersion forces will make the major contribution to the intermolecular forces, except where the opportunity to polarise is small and the dipole moment is large. 

London-van der Waals dispersion forces V ) and the total interaction (Vf). (Adapted from Schramm [15]. Copyright (2003), reproduced with permission of John Wiley Sons, Inc)... 

In addition to electrostatic double-layer forces, London-van der Waals dispersion forces have long been recognized as being important in thin-liquid films. The calculation of these forces has been approached in two different ways, namely microscopically and macroscop-ically. 

Disjoining pressure is a macroscopic pressure correction accounting for long-range intermolecular interactions (of. Deryagin, 1955). For most common solid-liquid interactions, such as the London-van der Waals dispersion forces, the disjoining pressure has the following form ... 

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