Interaction forces and their combination

Three main interaction forces can be distinguished (i) van der Waals attraction (ii) double layer repulsion (iii) steric interaction. These interaction forces and their combination were described in detail in Vol. 1 and only a brief description is given here [9]. 

The van der Waals attraction between two spherical particles or droplets each of radius R separated by a surface-to-surface distance of separation h, is given by the following expression (when h R) [10], 

The magnitude of repulsion increases with increasing zeta potential and decreasing electrolyte concentration and decreasing valency of the counter- and co-ions. 

The combination of the van der Waals attraction with double layer and steric repulsion is schematically illustrated in Fig. 1.3 (c) and this is sometimes referred to as electrosteric stabilization as for example produced by use of polyelectrolytes. This V h curve has a minimum at long distance of separation, a shallow maximum at intermediate distance (due to double layer repulsion) and a steep rise in repulsion at smaller h values (due to steric repulsion). 

States (a)-(c) in Fig. 1.4 represent the case for colloidally stable suspensions. In other words the net interaction in the suspension is repulsive. Only state (a) with very small particles is physically stable. In this case the Brownian diffusion can overcome the gravity force and no sedimentation occurs. This is the case with nanosuspensions 

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Interaction Energies (Forces] between Emulsion Droplets and Their Combinations... 

This section, which is by no means exhaustive, will deal with the following topics (i) Surfactants used in cosmetic formulations, (il) Interaction forces between particles or droplets in a dispersion and their combination, (iil) Description of stability in terms of the interaction forces, (iv) Self-assembly structures and their role in stabilization, skin feel, moisturization and delivery of actives, (v) Use of polymeric surfactants for stabilization of nanoemulsions, multiple emulsions, liposomes and vesicles. 

Equations (l)-(3) in combination are a potential energy function that is representative of those commonly used in biomolecular simulations. As discussed above, the fonn of this equation is adequate to treat the physical interactions that occur in biological systems. The accuracy of that treatment, however, is dictated by the parameters used in the potential energy function, and it is the combination of the potential energy function and the parameters that comprises a force field. In the remainder of this chapter we describe various aspects of force fields including their derivation (i.e., optimization of the parameters), those widely available, and their applicability. 

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

As we have just seen, the initial encounter complex between an enzyme and its substrate is characterized by a reversible equilibrium between the binary complex and the free forms of enzyme and substrate. Hence the binary complex is stabilized through a variety of noncovalent interactions between the substrate and enzyme molecules. Likewise the majority of pharmacologically relevant enzyme inhibitors, which we will encounter in subsequent chapters, bind to their enzyme targets through a combination of noncovalent interactions. Some of the more important of these noncovalent forces for interactions between proteins (e.g., enzymes) and ligands (e.g., substrates, cofactors, and reversible inhibitors) include electrostatic interactions, hydrogen bonds, hydrophobic forces, and van der Waals forces (Copeland, 2000). 

The most frequently used protein assay is based on a method after Bradford (Bradford, 1976), which combines a fast and easily performed procedure with reliable results. However, the Bradford assay has sensitivity limitations and its accuracy depends on comparison of the protein to be analyzed with a standard curve using a protein of known concentration, commonly bovine serum albumin (BSA). Many commercially available protein assays such as those from Pierce or BioRad rely on the Bradford method. The assay is based on the immediate absorbance shift from 465 nm (brownish-green) to 595 nm (blue) that occurs when the dye Coomassie Brilliant Blue G-250 binds to proteins in an acidic solution. Coomassie dye-based assays are known for their non-linear response over a wide range of protein concentrations, requiring comparison with a standard. The dye is assumed to bind to protein via an electrostatic attraction of the dye s sulfonic groups, principally to arginine, histidine, and lysine residues. It also binds weakly to the aromatic amino acids, tyrosine, tryptophan, and phenylalanine via van der Waals forces and hydrophobic interactions. 

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