Steric repulsive force

DLVO Theory. The overall stabiUty of a particle dispersion depends on the sum of the attractive and repulsive forces as a function of the distance separating the particles. DLVO theory, named for Derjaguin and Landau (11) and Verwey and Overbeek (12), encompasses van der Waals attraction and electrostatic repulsion between particles, but does not consider steric stabilization. The net energy, AGp between two particles at a given distance is the sum of the repulsive and attractive forces ... 

Two kinds of barriers are important for two-phase emulsions the electric double layer and steric repulsion from adsorbed polymers. An ionic surfactant adsorbed at the interface of an oil droplet in water orients the polar group toward the water. The counterions of the surfactant form a diffuse cloud reaching out into the continuous phase, the electric double layer. When the counterions start overlapping at the approach of two droplets, a repulsion force is experienced. The repulsion from the electric double layer is famous because it played a decisive role in the theory for colloidal stabiUty that is called DLVO, after its originators Derjaguin, Landau, Vervey, and Overbeek (14,15). The theory provided substantial progress in the understanding of colloidal stabihty, and its treatment dominated the colloid science Hterature for several decades. 

Steric hindrance (Sections 3.2, 6.3, and 8.6) An effect on structure or reactivity that depends on van der Waals repulsive forces. 

Since the elementary quantum-mechanical treatment does not seem to give a high enough barrier, various treatments of the problem have been proposed which use empirical data such as bond dipole moments and steric repulsive forces. These treatments do not introduce any new forces which would not be included in a proper quantum-mechanical analysis, but they attempt to short-circuit these difficult and uncertain calculations. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Small metal particles are unstable with respect to agglomeration to the bulk. At short interparticle distances, two particles would be attracted to each other by van der Waals forces and, in the absence of repulsive forces to counteract this attraction, an unprotected sol would coagulate. To counteract this, stabilization can be achieved in two ways electrostatic stabilization and steric stabilization. 

Schneider and co-workers (159) attempted to rescue the Grant-Cheney approach, at least for methyl substituents. They refined it by using a modified potential for the repulsive force (186) along with fully relaxed molecular structures as determined by force-field calculations. This indeed led to a satisfactory correlation of yg-SCS(CH3) with repulsive interactions in methylcyclohexane and some methylbicyclo[2.2.1]heptanes (159). Schneider s treatment, however, implies that a signal shift originates not only from steric H-H repulsion but includes effects from secondary carbon-framework distortions (85,159). 

Repulsive forces between Fe oxide particles can be established by adsorption of suitable polymers such as proteins (Johnson and Matijevic, 1992), starches, non-ionic detergents and polyelectrolytes. Adsorption of such polymers stabilizes the particles at electrolyte concentrations otherwise high enough for coagulation to occur. Such stabilization is termed protective action or steric stabilization. It arises when particles approach each other closely enough for repulsive forces to develop. This repulsion has two sources. 1) The volume restriction effect where the ends of the polymer chains interpenetrate as the particles approach and lose some of their available conformations. This leads to a decrease in the free energy of the system which may be sufficient to produce a large repulsive force between particles. 2) The osmotic effect where the polymer chains from two particles overlap and produce a repulsion which prevents closer approach of the particles. 

The dispersion polymerization system is composed of monomer, solvent, initiator, and stabilizer. The combination of monomer, solvent, and stabilizer is essential for particle preparation. That is to say, the stabilizer is chosen to meet the demand of the monomer and solvent. In any system, the stabilizer has affinity or cohesive strength for both the medium and the polymer particles. In a dispersion polymerization, the medium and polymer particles both are organic compounds. Therefore, it is not rational to rely on dispersion stabilization, which comes from the electrostatic repulsion force between particles. The stabilizer for dispersion polymerization that makes interfacial energy low must have affinity for particles due to the same quality and solvation at the surface of particles. It is desired that the stabilizer be a polymer that indicates a steric stabilization effect on the surface (5). 

On the basis of these comparable mechanisms, the observed regioselectivity with various 3-substituents summarized in Table I might be best interpreted in terms of the balance of three effects, namely attractive dispersion force, steric hindrance, and electrostatic repulsion which would all be operative between the 3-substituent and the ferricyanide ion in the rate-determining step. 

Formation of hydrophobic bonds between nonpolar hydrocarbon groups on the drug and those in the receptor site is also common. Although these bonds are not very specific, the interactions take place to exclude water molecules. Repulsive forces that decrease the stability of the drug-receptor interaction include repulsion of like charges and steric hindrance. 

In the eclipsed conformation of propane, we now have a larger CH3 close to H atom. This results in increased repulsive force or increased steric strain. The energy difference between the eclipsed and staggered forms of propane is greater than that of ethane. 

For particles covered by an adsorbed polymer layer, it is also necessary to take into account the effect on the steric repulsive forces of the structure of the adsorbed layer. For example, for the case of separations corresponding to 8 < H < 28, the values of mix associated with linear adsorbed layer profiles may be obtained from the equation (Vincent et al, 1986) ... 

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