Steric stabilization entropic

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). 

One of the first theoretical attempts to understand steric stabilization of dispersions was based on an entropic mechanism that resembles the elastic contribution to AGR. We consider this mechanism in Example 13.3. 

EXAMPLE 13.6 An Entropic Model for Steric Stabilization Due to Adsorbed Polymer Layers. Picture a flat surface to which rigid rods are attached by ball-and-socket-type joints. The free ends of the rods can lie anywhere on the surface of a hemisphere. The approach of a second surface blocks access to some of the sites on the cap of the hemisphere. Outline the qualitative argument that converts this physical picture to a theory for stabilization. What are some of the shortcomings of the model ... 

Discuss polymer-colloid interactions and steric stability from a thermodynamic perspective. What is enthalpic stabilization What is entropic stabilization What is the critical flocculation temperature (CFT) ... 

The second contribution to the steric interaction arises from the loss of configurational entropy of the chains on significant overlap. This effect is referred to as entropic, volume restriction, or elastic interaction, Gei. The latter increases very sharply with a decrease in h when the latter is less than 8. A schematic representation of the variation of Gmix, Gei, G, and Gj =G X + Gei + Ga) is given in Fig. 10. The total energy-distance curve shows only one minimum, at h 25, the depth of which depends on 5, R, and A. At a given R and A, G decreases with an increase in 5. With small particles and thick adsorbed layers (5 > 5 nm), G, becomes very small (approaches thermodynamic stability. This shows the importance of steric stabilization in controlling the flocculation of emulsions and suspensions. 

The other kind of systems largely studied, consists of polymethylmethacrylate (PMMA) or silica spherical particles, suspended in organic solvents [23,24]. In these solvents Q 0 and uy(r) 0. The particles are coated by a layer of polymer adsorbed on their surface. This layer of polymer, usually of the order of 10-50 A, provides an entropic bumper that keeps the particles far from the van der Waals minimum, and therefore, from aggregating. Thus, for practical purposes uw(r) can be ignored. In this case the systems are said to be sterically stabilized and they are properly considered as suspensions of colloidal particles with hard-sphere interaction [the pair potential is of the form given by Eq. (5)]. 

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... 

The third mechanism of imparting steric stabilization differs from the preceding two in that the enthalpy and entropy contributions to the free energy of flocculation act in concert instead of in opposition. Both now contribute to stability since AHp is positive and dSp is negative. The magnitudes of their respective contributions to AGp are no longer important This is termed combined enthalpic-entropic stabilization. 

With entropic stabilization, the reverse situation to that just described for enthalpic stabilization must pertain. Clearly the entropy term that promotes stability can be made less competitive by decreasing the temperature. Entropically stabilized dispersions can thus, in principle, be flocculated by cooling. A sterically stabilized dispersion that flocculates on cooling can be inferred to be entropically stabilized, at least just above the LCFT. 

In summary, for nonaqueous dispersions, the combinatorial free energy of interpenetration favours stabilization. Both of the corresponding free energies associated with contact dissimilarity and free volume dissimilarity favour flocculation. These conclusions are represented schematically in Fig. 7.2. Since the combinatorial free energy is purely entropic in origin, it is scarcely surprising that nonaqueous sterically stabilized systems are usually found to be entropically stabilized at room temperature and pressure for it is this term that imparts stability. Anticipating the results of the next section, we stress that this does not necessarily imply that all nonaqueous dispersions are entropically stabilized at room temperature. 

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. 

A characteristic feature of sterically stabilized systems is their different responses to temperature change. Those in (i) flocculate on cooling, while for (ii) flocculation occurs on heating, and for (iii) there is no accessible (critical) temperature for flocculation. Entropic stabilization seems to be more common in non-aqueous media, whereas enthalpic stabilization is more frequently encountered in aqueous media. Examples of the three types of stabilization were given by Napper. ... 

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... 

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