Electrostatic and Dispersion Forces

In this discussion of colloid stability we will explore the reasons why colloidal dispersions can have different degrees of kinetic stability and how these are influenced, and can therefore be modified, by solution and surface properties. Encounters between species in a dispersion can occur frequently due to any of Brownian motion, sedimentation, or stirring. The stability of the dispersion depends upon how the species interact when this happens. The main cause of repulsive forces is the electrostatic repulsion between like charged objects. The main cause of attractive forces is the van der Waals forces between objects. 

There also exist dispersion, or London-van der Waals forces that molecules exert towards each other. These forces are usually attractive in nature and result from the orientation of dipoles, and may be dipole-dipole (Keesom dispersion forces), dipole-induced dipole (Debye dispersion forces), or induced dipole-induced dipole 

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According to Fig. 9, the signals of the complexes with secondary alcohols and amines display A v values which are less negative than expected on the grounds of the linear correlation obtained for the primary alcohols. These deviations suggest that the relative contributions of electrostatic and dispersive forces in these systems are substantially different from those operating in the corresponding complexes with primary alcohols. 

Comparison of the Av values of diastereomeric complexes with 2-butanols, 2-pentanols, and 2-butylamines indicates that relative extent of electrostatic and dispersive forces depends upon the namre, the bulkyness, and the configuration of M. The A(Ar> ) = ((Ar )ijonjo — (Ai )i,etero) = +13 cm difference between the red shifts of the homochiral and the heterochiral complexes with 2-butanols finds a close analogy with LIF red-shift difference of the corresponding adducts with F/ = (/ )-(+)-2-naphthyl-l-ethanol (A(Ai ) = ((Aji)homo (AF)hetero)= +H cm Table 1). However, while LIF results from the diastereomeric [F/j M] (M = 2-pentanol) complexes are qualitatively similar to those with the other secondary... 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

When toner particles are transferred from the photoreceptor to the receiver, some particles invariably remain and must be removed before the subsequent image-forming cycle. Toner to photoreceptor adhesion results from electrostatic and dispersion forces which must be overcome to separate the toner particles from the photoreceptor (Nebenzahl et al.. 1980 Mastrangelo, 1982). Many... 

DLVO theory predicts one maximum separating two minima for the disjoining pressure-distance isotherm (30), which suggests the possible existence of two types of films 1) Common thick film achieved by the balance of electrostatic and dispersion forces and 2) Newton film (thin film) with approximately a bilayer structure. Increasing the electrolyte concentration depresses the electrostatic repulsive forces and causes transition from the thick to the thin film. The electrostatic effect will of course vary with difference in the structure of the double layer. 

Stoddart and co-workers developed the efficient template-directed one-pot synthesis of the [2]catenane from a bipyridinium salt, bis(bromomethyl)benzene, and a paracyclophane crown ether [63], The dominant, noncovalent interactions involved are based on electrostatic and dispersive forces brought about by the reci-... 

By dimensional analysis the following dimensionless groups can be identified [25] for electrocratic (only electrostatic and dispersion forces) systems ... 

Wasan and co-workers (48, 50, 51) have found pseudoemulsion film stability to be influenced by micelle structuring effects, Marangoni surface effects, and the presence of oil droplets, (see the discussion in Chapter 2) They have also found that in some systems, the emulsification and imbibition of oil can actually stabilize foams. Manlowe and Radke s results (25) were interpreted in terms of pseudoemulsion film stabilities depending mainly on electrostatic and dispersion forces. Undoubtedly, interfadal viscosity could also be important. 

In the different adsorption processes, both in gas and liquid phase, the molecules or atoms (adsorbable) are fixed (adsorbed) on the carbon (adsorbent) surface by physical interactions (electrostatic and dispersive forces) and/or chemical bonds. Therefore, a relatively large specific surface area is one of the most important properties that characterize carbon adsorbents. The surface of tlie activated carbons consists mainly of basal planes and the edges of the planes that form the edges of miciociystallites. 

The origin of the solvatochromic shifts in the electronic spectra is related to the change in the electrostatic and dispersion forces between the solvent and the chromophoric solute molecule in the ground and in the excited state, respectively. The semiclassical approach to the treatment of the respective effects is based on the assumption that the solute and the solvent molecules are sufficiently separated to neglect the overlap between the electron distribution of these two molecular systems. The wave function for the whole system can then be approximated as the product of the wavefunctions of each individual system, i.e., the solute and individual solvent molecules ... 

It is expected that similar considerations of both electrostatic and dispersive forces will be necessary for developing more reliable interaction potentials in the case of the adsorption of gases and vapors on chemically heterogeneous activated carbon surfaces. 

In MM, a molecule is modeled as a collection of masses (atoms) and springs (bonds), with additional forces added to describe other interactions such as hydrogen bonding, electrostatics, and dispersion forces. Although such simulations have been done using carefully constructed mechanical models [22], MM has been most successfully implemented computationally. The present discussion will focus on the MM3 method [13], since it is popular and is implemented in a number of software packages. Beware that not all implementations of MM3 provide thermochemical information. 

Electrostatic and Dispersion Forces. Several repulsive and attractive forces operate between colloidal species and determine their stability (7,8,10,25, 31,54). In the simplest example of colloid stability, particles would be stabilized entirely by the repulsive forces created when two charged surfaces approach each other and their electric double layers overlap. The overlap causes a Coulombic repulsive force acting against each surface, and that will act in opposition to any attempt to decrease the separation distance. One can thus express the Coulombic repulsive force between plates as a potential energy of repulsion. There is another important repulsive force causing a strong repulsion at very small separation distances where the atomic electron clouds overlap, called the Bom repulsion. 

Scheme 14.10), can form a complex stabilized by a combination of electrostatic and dispersive forces, as well as hydrogen bonds. 

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