Stabilization osmotic repulsion

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... 

Ideally, it would be desirable to be able to develop quantitative expressions for the interaction energies so that we can deal with coagulation or flocculation, at least in the case of fairly dilute dispersions, the way we did in Sections 13.3-13.4 for electrostatic stabilization. It is possible to develop approximate expressions for interaction energy due to various individual effects such as osmotic repulsion, attraction or repulsion due to the overlap of the tails of the adsorbed (or grafted) polymer layers, interaction of the loops in the layers, and so on (see Fig. 13.15). However, the complicated nature of polymer-induced interactions makes these tasks very difficult. In this section, we merely illustrate some of the issues that need to be considered in developing a fundamental quantitative understanding of polymer-induced forces. In Section... 

This chapter described the basis principles involved in stabilization of dispersions by polymeric surfactants. The first part described polymeric surfactants and their solution properties. The second part described the adsorption of polymeric surfactants and their conformation at the interface. The methods that can be applied to determine the adsorption and conformation of polymeric surfactants were briefly described. The third part dealt with the stabilization mechanism produced using polymeric surfactants. Two main repulsive forces were considered. The first arises from the unfavorable mixing of the chains on close approach of the particles or droplets, when these chains are in good solvent conditions. This is referred to as mixing or osmotic repulsion. The second force of repulsions... 

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. 

Figure 10.18 is a schematic representation of depletion stabilization in which the polymer is prevented from the zone of close approach between two particles. As a result of this low polymer concentration between the particles due to size exclusion, there is a lower osmotic pressure, which results in (1) an attractive force for greater than theta solvents and (2) a repulsive force for less than theta solvents. Theta solvents will be discussed in the section on the thermodynamics of polymer solutions, but first a discussion of pol3naaer properties. 

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