Adsorbed polymer layers, interaction with

Absolute porosity, definition, 220 Absolute viscosity, definition, 386 Acid number (total acid number, TAN), definition, 403 Adhesion, definition, 26-27 Adsorbed polymer layers, interaction with droplets, 62 Adsorption definition, 386... 

When two particles, each with a radius R and containing an adsorbed surfactant or polymer layer with a hydrodynamic thickness 5, approach each other to a surface-surface separation distance h that is smaller than 25, the surfactant or polymer layers interact with each other, with two main outcomes [1] (i) the surfactant or polymer chains may overlap with each other or (ii) the surfactant or polymer layer may undergo some compression. In both cases, there will be an increase in the local segment density of the surfactant or polymer chains in the interaction region this is shown schematically in Figure 8.1. The real-hfe situation perhaps lies between the above two cases, however, with the surfactant or polymer chains undergoing some interpenetration and some compression. 

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... 

Two Spheres. The interaction of spheres which are covered with adsorbed polymer layers is expected to follow that of macroscopic surfaces. If the surfaces of the spheres are saturated, the force between them will be repulsive, and if they are unsaturated there will be an attraction caused by bridging (2.3). However special effects are expected to arise due to the finite sizes of spheres and macromolecules. Firstly, when the spheres are small, their radii may be comparable with the range of the bridging attraction. Secondly, when the macromolecules are large, one of them may saturate two spheres then it may keep them bound to each other even though they are both saturated. 

In an adsorbed polymer layer, the Interaction of the chains with the solvent remains very important. Therefore, before discussing the adsorption behaviour of polymers and polyelectrolytes, we review briefly some properties of these molecules in solution. In the following sections, we follow roughly the outline of a recent monograph on polymers at interfaces... 

Interaction of Droplets with Adsorbed Polymer Layers. In the... 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

Finally, we briefly mention interactions due to adsorbing polymers. Block copolymers, with one block strongly adsorbing to the particles, have already been mentioned above. Here, we focus on homopolymers that adsorb moderately strongly to the particles. If this can be done such that a high smface coverage is achieved, the adsorbed polymer layer may again produce a steric stabilization between the particles. 

When two sterically stabilized particles with adsorbed polymer layers approach one another sufiBciently closely for the adsorbed layers to interact, two extreme cases can occur (19). 

The total interaction energy of two colloidal particles with adsorbed polymer layers is described by the equation,... 

Figure 3 The interaction energy of the particles with their surface-adsorbed polymer layers.

When the colloidal particles are completely covered with adsorbing polymer, adding more polymer gives rise to excess polymer in the bulk solution, which is thus not adsorbed. This non-adsorbing polymer may lead to depletion interaction as well. In such a case depletion effects are weaker for two reasons. Firstly, more polymer is required before depletion-induced instability of the dispersion occurs because polymer is first consumed in order to cover the particles [1]. Secondly, the depletion interaction is weak due to the soft repulsion between the adsorbed polymer layers. It is known that depletion effects between such soft surfaces are rather small [1, 5]. 

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