Steric or Enthalpic Stabilization

The attractive interaction is given by Equation (10.11) so that the total interaction will be 

In summary, then, if the total potential-energy maximum is large compared to the average thermal energy of the particles, say 10 kT, then the system should be stable if not, the system will flocculate or coagulate. The height of the barrier will be determined by the surface potential at the Stern layer and the thickness of the double layer, 1/k. 

The mechanism of stabilization discussed above refers only to those systems in which the dispersed particles carry a surface electrical charge, so that the interaction of their respective double layers provides the necessary energy barrier for kinetic stability. Another stabilizing mechanism has been known for centuries but has until recently been much less thoroughly studied and understood. That mechanism involves the presence of a lyophilic colloid, which adsorbs onto the particle surface and provides what is termed steric or 

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In the case of nonionic surfactants, the steric or enthalpic barrier to droplet coalescence produced by the hydrated hydrophilic chain is a major factor in emulsion stabihty. The addition of additives that increase the heat of hydration should result in an increase in the enthalpic contribution to stability. Experimentally, unambiguous evidence to support such a hypothesis has been difficult to obtain. 

As conformational anomeric effects represent only a fraction (—1 to —3 kcal/mol) of the global enthalpic anomeric effect or gem-dioxy stabilizing effect (—6 to —17 kcal/mol), additional factors have to be taken into account. Depending on substitution, steric factors can affect the relative stability of acetal conformers. Dubois et al.2S have demonstrated that in furanose pyranose derivatives a bulky substituent at the furanose... 

Proteins adsorbed at an oil-water interface may stabilize the oil droplets by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) interactions and/or the steric stabilization mechanism. The proteins may possess or be capable of adopting extended structures, which protrude into a solution for a considerable distance from the interface. This extended hydrated layer may form the basis for steric stabilization of the emulsion. Interactions between the adsorbed protein layers can involve a reduction in conhgurational entropy as molecular chains overlap (Darling and Birkett, 1987). In addition, hydration of adsorbed hydrophilic components can lead to an enthalpic repulsion when two particles are in close proximity. This tends to force the oil droplets apart (Darling and Birkett, 1987). 

When going from ethene as the simplest possible monomer for radical polymerization to monomers with higher reactivity, one or more hydrogen atoms are formally substituted by groups activating the double bond and stabilizing the macroradical. Of course, steric hindrance is added at the same time, so a complete separation of entropic and enthalpic effects on the propagation rate seems impossible. 

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