London-van der Waals interactions

A surface charge may be established by adsorption of a hydrophobic species or a surfactant ion. Preferential adsorption of a surface-active ion can arise from so-called hydrophobic bonding or from bonding via hydrogen bonds or from London-van der Waals interactions. The mechanism of sorption of some ions (e.g., fulvates or humates) is not certain. Ionic species carrying a hydrophobic moiety may bind inner-spherically or outer-spherically depending on whether the surface-coordinative or the hydrophobic interaction prevails. 

As in part II, we shall assume also in this part that the attraction between coUoidal particles is entirely based upon the London-Van der Waals forces. Hamaker showed how the London-Van der Waals interaction between two spherical particles may be found from the interaction b etween the elements of these spheres. His expression for the energy of attraction Va) runs, using our symbols,... 

Calculation of the molecular contribution to the disjoining pressure , has been approached in two ways from the approximatiOTi of interactions as pairwise additive and from a field theory of many-body interactions in condensed matter. The simpler and historically earher approach is a theory based on summing up of individual London-van der Waals interactions between molecules pair-by-pair xmdertaken by Hamaker [1]. 

Due to the fact that interactions between surfaces fall off much more slowly with distance than those for individual atoms or molecules, a significant comphcating factor enters into the quantum mechanical evaluation of the attractive forces. The quantum-mechanical effects leading to the London-van der Waals interactions occur close to the speed of light, yet even at the short distances involved in colloids, relativistic effects can be significant. 

Close proximity between atoms leads to synchronization of their orbiting electrons. This causes the induction of dipoles that attract each other (London-van der Waals interaction). At closer approach, the electron clouds overlap, giving rise to repulsion (Born repulsion). The variation of the Gibbs energy of dispersion interactions Gji p between two atoms with their separation distance r is given by... 

In the case of a loosely structured, highly solvated train-loop-tail-like conformation of the adsorbed layer, as is the case with flexible polymers, the density of the adsorbed layer approaches that of the bulk solution and, hence, the contribution from dispersion (London-van der Waals) interactions may be negligibly small. However, for the formation of a compact adsorbed protein layer, dispersion interactions have to be taken into account. 

Adhesion requires attractive interparticle forces between two particles in contact. Usually, such an attraction inevitably exists as aresult of the London-van-der-Waals interaction (cf. Sect. 3.2.1). However, short-range repulsive forces, e.g. originating from adsorbed molecule layers (cf. Sect. 3.2.4), may outweigh the van-der-Waals forces and impede the adhesion even in the case of small surface distances. 

An increase in the number of cis double bonds from oleic (9) to linoleic (9, 12) and a-linolenic (9, 12, 15) acid increases the desaturation (Brenner and Peluffo, 1966) and the same happens with 20 3 (9, 12) and 20 3 (9, 12, 15). This increase corresponds to an increase of the curvature of the molecule. Therefore it is possible to explain this effect if we assume that the enzyme also possesses a curve structure where the acyl-CoA will be located. An increase of curvature of the substrate would increase the fitting and bonding by cumulative London - Van der Waals interactions. An addition of 2 carbons in the tail of the acid from 18 2 (9, 12) to... 

The attractive forces between suspension particles are considered to be exclusively London-van der Waals interactions (except where interparticle bridging by long polymeric chains occurs). The repulsive forces, as discussed in Chapter 8, comprise both electrostatic repulsion and entropic and enthalpic forces. In aqueous systems the hydrophobic dispersed phase is coated with hydrophilic surfactant or polymer. As adsorption of surfactant or polymer (or, of course, both) at the solid-liquid interface alters the negative charge on the suspension particles, the adsorbed layer may not necessarily confer a repulsive effect. Ionic surfactants may neutralize the charge of the particles and result in their flocculation. The addition of electrolyte such as aluminium chloride can further complicate interpretation of results electrolyte can alter the charge on the suspension particles by specific adsorption, and can affect the solution properties of the surfactants and polymers in the formulation. Some aspects of the application of DLVO theory to pharmaceutical suspensions and the use of computer programmes to calculate interaction curves are discussed by Schneider et al. [4]. 

The second term on the right hand side is negative consistent with the original RKK theory. The first tern is positive and differs from binary system to binary system. It represents a theoretical estimate of the contribution to the mixing enthalpy which arises from the London-van der Waals interaction between second nearest neighbour cations. 

Spontaneous aggregation of nonpolar moieties is readily understandable from the appreciable entropy decrease resulting from London-van der Waals interactions only. However, hydrocarbons do not form micellar systems when dispersed in water. Hence polar heads of amphiphiles play a determinant role in micelle formation this, as shown by nonionic amphiphiles is not dependent on the presence of ionizable groups. Thus the ability to form hydrogen bonds with water molecules, and electrical dipoles appear to be determinant contributions from polar heads to the formation of micellar structures. 

Forces within subunits are of two kinds protein-lipid interactions (I) in polar layers and London-van der Waals forces (II) in the hydrocarbon phase. Intersubunit cohesion also arises from two kinds of forces London-van der Waals interactions (II) between hydrocarbon phases, and forces (III) between polar layers of adjacent subunits. Since forces (II) are the same in both cases, a difference, if any, must exist between total forces (I) and (III). 

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