Osmotic steric stabilisation

Standard theories of steric stabilisation assume that the free energy of the system can be expressed as a sum of two terms, volume restriction and osmotic, which are mutually independent. 

At this point we see qualitatively that the mechanisms of steric stabilisation and depletion flocculation are closely related. In the former instance the concentration of polymer segments in the space between the particles increases as the particles come together, leading to a repulsion caused by the osmotic flow of solvent into this space in the latter case the concentration between the particles is lower than that in the bulk, and diffusion of solvent out of the interparticle space results in an attraction. 

Figure 1.2 Schematic representation of the repulsive forces giving rise to steric stabilisation. Top shows interpenetration of polymer layers giving rise to an increase in osmotic pressure in the overlap region and bottom shows compression of the polymer layers on close approach leading to a loss of configurational entropy.

The second mechanism by which flocculation may be prevented is that of steric stabilisation. This is produced using nonionic surfactants or polymers that adsorb at the liquid/liquid interface with their hydrophobic portion, leaving a thick layer of hydrophilic chains in bulk solution, e.g. poly(ethylene oxide) (PEO) or poly( vinyl alcohol). These thick hydrophilic chains produce repulsion as a result of two main effects. The first, usually referred to as mixing interaction (osmotic repulsion), re-... 

Fig. 3.14. Stabilisation of particles by steric repulsion. (a) Osmotic effect, (b) volume restriction effect...

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