Van der Waals forces London attraction

According to Deijaguin-Landau-Verwey-Overbeek (DLVO) theory, a cornerstone of modem colloid science, two types of forces exist between colloidal particles suspended in a dielectric medium electrostatic forces, which result from an unscreened surface charge on the particle, and London-van der Waals attractive forces, which are universal in nature. The colloidal stability and rheology of oxide suspensions, in the absence of steric additives, can be largely understood by combining these two forces (assumption of additivity). 

The potential energy function, u(h), where h is the distance of separation, consists of London-van der Waals attractive forces, u and double-layer repulsion forces, u as given below ... 

The instability in the presence of London-van der Waals attractive forces that was studied in the preceding subsection is indicative of the fact that small perturbations in the film thickness... 

The question now is why a change in salinity or pH can trigger fines migration This point has been extensively studied over the last few decades and is discussed in Chapter 7 of this book. However, a brief explanation of this phenomenon will be given here. A fine particle attached to another larger particle (Figure 2) is subjected to several colloidal forces, including London-van der Waals attraction force, Born... 

London-van der Waals Attractive Forces. These forces depend on the dielectric constants of the particle materials and the smrounding solvent via the... 

Interparticle forces are a determinant factor for most properties of dispersions, including rheological behavior. They are produced by the molecular forces on the surfaces of the particles, due to their nature or to adsorbed molecules, that modify the interface. These are electrical forces arising from charges on the particles and London-van der Waals attraction forces. The role of these forces on suspension stability has been extensively study and is known as the DLVO theory. In addition, sterical forces encountered on dispersions stabilized with nonionic species also exert an important influence on rheological behavior. The nature of these forces will not be considered since they are matters of discussion in Chapters 1-4. However, from a rheological point of view it is impwtant to understand how these factors modify the flow characteristics of dispersions. 

Applying the same principles as used in Part II we now have to calculate the change in free energy accompanying the approach of two colloidal particles. To this end we shall first consider the free energy of the double layer system for spherical particles, and add to it the free energy of the London-Van Der Waals attraction forces. From the curves of total free energy so constructed we shall derive the criteria for the stability of colloids. 

Only small particles, less than 1 pm in diameter, would show this effect. Krupp explained this in terms of the equations for London-van der Waals attractive forces between rigid spheres, together with the Hertz equations of contacL Because the attraction is proportional to particle diameter, the force at the particle contact decreases with D. However, the elastic area of the contact spot decreases faster, from the Hertz Equation (9.1), with Thus, as the particle gets smaller, the contact pressure must rise to the point at which plastic deformation occurs. 

In the absence of adsorbed polymeric molecules, which are discussed below, colloid stability is governed in many cases by the combination of London-van der Waals attractive forces and the repulsive forces produced by double-layer overlap. This concept is the basis of the famous DLVO theory, developed independently in the late 1930s and early 1940s by Derjaguin and Landau in the Soviet Uruon and by Verwey and Overbeek in the Netherlands. 

Boiling Point When describing the effect of alkane structure on boiling point m Sec tion 2 17 we pointed out that van der Waals attractive forces between neutral molecules are of three types The first two involve induced dipoles and are often referred to as dis persion forces or London 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>...

Ionic species can induce a dipole in a nonpolar molecule over a short range. London forces exist between instantaneous and induced dipoles, and are operative between all bodies when they are close together. For molecular systems they are also commonly called van der Waals attractive forces after the Dutch physicist (J.D. van der Waals) who described these forces as being active in crystals [65]. The London/van der Waals force is also frequently referred to as the dispersion force and is important in the solution phase. 

Weak attractive forces between nonbonded atoms are called van der Waals attractive forces, London forces, or dispersion forces, and are of great importance in determining the properties of liquids. They also can be expected to play a role in determining conformational equilibria whenever the distances between the atoms in the conformations correspond to the so-called van der Waals minima. 

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. 

Also, in the 1930 s London (9) indicated the quantum mechanical origin of dispersion forces between apolar molecules and in subsequent work extended these ideas to interaction between particles (10). It was shown that whereas the force between molecules varied inversely as the seventh power of the separation distance, that between thick flat plates varied inversely as the third power of the distance of surface separation. These ideas lead directly to the concept of a "long range van der Waals attractive force. A similar relationship was found for interaction between spheres (10). 

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. 

In conventional latices, the colloidal stability of the particles arises from the predominance of the electrostatic forces of repulsion over the London-van der Waal s forces of attraction. These electrostatic forces of repulsion result from the electric double layer formed by the emulsifier ions adsorbed on the hydrophobic polymer particle surface and the counterions from the aqueous phase. The London-van der Waal s forces of attraction are strongest when the particle-particle distance is very small. Therefore, in most particle-particle collisions, the particles repel one another until the particle-particle distance is decreased to the point where the London-van der Waal s forces of attraction are predominant over the electrostatic forces of repulsion. Thus, many conventional latices remain stable indefinitely without significant stratification or flocculation of the particles. 

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. 

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. 

Hamaker Constant In the description of the London-van der Waals attractive energy between two dispersed bodies, such as particles. The Hamaker constant is a proportionality constant characteristic of the internal atomic packing and polarizability of the particles. Also termed the van der Waals-Hamaker constant. See also Dispersion Forces. 

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. 

As van der Waals postulated, the attractive forces between neutral molecules also originate from electrical interactions (Hiemenz 1986). Although there are several types of van der Waals attractive forces that originate from electrical interactions, the most important for colloids is that operating between nonpolar molecules. These forces are due to the polarization of one molecule by quantum fluctuations in the charge distribution in the second molecule, and vice versa. They are known as the London dispersion forces, their origin having first been explained by F. London in 1930. 

---