Steric stabilization entropic repulsion

In non-aqueous media of low dielectric constant ionic charge stabilization is unlikely to be very important. In such cases stabilization depends on steric or entropic repulsion and polymeric agents are preferred... 

Pigment dispersions are stabilized by charge repulsion and entropic, ie, steric or osmotic, repulsion. Although both types of stabilization force may be present in most cases, for pigment dispersions in solvent-bome coatings entropic repulsion is usually the most important mechanism for stabilization. 

Two mechanisms of steric stabilization can be distinguished entropic stabilization and osmotic repulsion. Entropic stabilization arises when two opposing adsorbed polymer layers of adjacent particles overlap, resulting in compression and interpenetration of their... 

Another way to interpret the above observations would be in terms of the general principle that effective steric stabilization of polymer-coated droplets requires that the continuous phase be a good quality solvent for the polymeric stabilizer. Under poor quality solvent conditions (asi-casein at high ionic strength), the required entropic stabilizing repulsion of the adsorbed protein layer is converted into a destabilizing polymer-mediated attraction (Dickinson and Stainsby, 1982 Dickinson, 2006). 

The effects of improved wettability, entropic repulsion, and sterical hindrance undoubtedly play a role in stabilizing dispersed solid particles by block or graft copolymers. However, since the dispersions of titanium dioxide in toluene stabilized by carboxylated styrene-butadiene block copolymers are so much more stable than dispersions stabilized by carboxylated homopolymers under otherwise identical conditions, we must assume that an additional factor comes into play when block copolymers are used. The model in Figure 1 is an attempt to explain this additional... 

Two results of the Mackor analysis, which is now merely of historic interest, still linger on today. The first is the misconception that the overall repulsion in steric stabilization is always the consequence of the loss of configurational entropy of the stabilizing moieties. If this were really true, no sterically stabilized dispersion could be flocculated by heating, which perforce favours entropic effects. Yet almost all sterically stabilized dispersions can be so flocculated. The second misconception is that the potential energy diagrams for sterically stabilized particles always resemble those of an electrostatically stabilized system in that they exhibit a primary maximum, which is what Mackor found. As we shall see, this is not generally correct. 

In summary, electrostatic repulsion stabilizes lamellar phases in ionic systems, whereas entropy reduction stabilizes lamellar phases in nonionic systems or in ionic systems in apolar solvents or in high ionic strength water. Also, the presence of suitable cosurfactants (generally alcohols), which increase the flexibility of the membranes, leads to the formation of dilute lamellar phases, for example, in the system brine-SDS-pentanol [133] or brine-SDS-pentanol-dodecane [134]. Recently, it was shown [135] that two distinct lamellar phases coexisted in the dilute region of the system cetylpyridinium chloride-hexanol-brine. The two phases differ in turbidity, viscosity, density, and some other physical properties. One of these lamellar phases is classically stabilized by the competition between van der Waals, hydration, and electrostatic forces. The other phase is entropically stabilized. The difference between electrostatically and sterically stabilized lamellar phases was demonstrated by transmission electron microscopy on thin vitrified... 

Electrically stabihzed colloidal dispersions are very sensitive to the addition of electrolytes. If the concentration of ions in the solution increases, decreases as a result of both entropic and electrical screening effects, leading to a reduction in the repulsive potential. On the other hand, colloid particles dispersed in organic media (low dielectric constant) cannot be effectively stabilized by charges because is extremely short. In these cases, steric stabilization is recommended. Steric stabihzation is imparted by nonionic amphiphilic molecules (usually polymeric molecules). The lyophobic moiety will adsorb onto the surface of the colloidal particles, while its lyophilic moiety will be extended in the continuous phase. When two sterically stabihzed particles approach each other, the concentration of the lyophilic segments in the portion of the continuous phase between the particles is increased. This higher local concentration results in an osmotic pressure that... 

The overall stability of a colloid will depend on the net form of the interaction energy curve for the system—the sum of the attractive and repulsive energy terms as a function of the distance of separation of the particles. For the moment, we will consider only two contributing factors the attractive van der Waals term and the repulsive double-layer term, leaving aside any consideration of entropic or steric stabilization. 

Interactions among atoms and molecules, as we have seen, are a result of various forces stemming from their atomic or molecular structure, including electrostatic or charge interactions, steric or entropic phenomena, and the ever-present van der Waals forces. Of these, electrostatic and steric interactions may be repulsive in that they act to force the interacting units apart or at least reduce the net attraction between units. The van der Waals forces, on the other hand, are usually (but not always) attractive. When one discusses the use of a surfactant as an emulsion stabilizer, as in the above sections, the concept of the function of the surfactant is that it have a strong tendency to... 

Steric stabilization is based on attaching low molecular weight, water soluble polymers to the particle surface. This layer of soluble polymers on the parHcle surface provides an entropically based repulsive interacHon between parHcles, thus conferring addiHonal colloidal stability. Both coulombic and steric stabiHzaHon inhibit un-... 

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