Osmotic force dispersions

The quantity of water is two to three times the weight of the hides. The salt from the cure dissolves in the water and the reverse of the curing takes place. The water is drawn into the hides by osmotic forces. The concentration of the salt solution is about 3-5 g/lOO mL. At this concentration some of the soluble proteins disperse. The soak water removes the salt, some proteins, some loose fat, blood, dirt, and manure. 

The base resin contains a styrene-divinylbenzene polymer, DVB. If styrene alone were used, the long chains it formed would disperse in organic solvents. The divinylbenzene provides cross-linking between the chains. When the cross-linked structure is immersed in an organic solvent, dispersion takes place only to the point at which the osmotic force of solvation is balanced by the restraining force of the stretched polymer structure. 

It has been seen in Chapter 7 that the use of macromolecules as dispersion stabilisers depends in part on the osmotic forces arising from the interaction of solvated polymer chains as neighbouring particles approach (see Fig. 7.7). It is thus important to know how factors such as temperature and additive affect this interaction. Flory has given the free energy of dilution (the opposite process to the concentration effect discussed in section 7.2)... 

The range over which depletion attraction operates equals 2R. In particular, for highly swollen polymers, R may reach values of some tens of nanometer and, hence, the depletion forces may be effective over separation distances between particles that exceed the range of dispersion and double layer forces (cf Section 16.1). On the other hand, the osmotic forces are relatively weak. Depletion flocculation occurs when the molar polymer concentration is sufficiently high, which is more readily achieved by using polymers of a relatively low degree of polymerization. 

The structure of the EDL is neither simple nor universal it depends to a great extent on the physico-chemical properties of particles and dispersion medium. In general, it is assumed that some ions from the solvent adhere on the particle surface and partially neutralise the surface charge. This layer of immobile ions is called Stem layer. The other ions spread in the solvent by thermal motion yet are subject to the electric field generated from the charged surface. With growing surface distance the concentrations of the ionic species tend to their equilibrium values of the free solvent. The region adjacent to the Stem layer with excess of counter-ions is called the diffuse layer. In this part of the EDL, the ion distribution results from the balance of electrostatic and osmotic forces. 

The distance between two droplets or colloidal dispersion determines the depletion force as well. A gradient of polymer segmental concentration exists in the surface area. Depletion attraction arises when the depletion layers, each of thickness A (or Rg), associated with two surfaces, overlap. Theoretically, at large separations (D > 2A), the segment concentration increases from zero at the surface to that of the bulk solution in the middle of the gap between the surfaces. However, at smaller separations (D < 2A), the concentration at the midplane falls below that of the bulk. As a result the pressure in the bulk solution is greater than that in the gap, and there is an attractive osmotic force between the surfaces (Kuhl et al, 1998). 

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. 

This forms one of a hierarchy of equations we can write, representing the properties of the dispersion in terms of the microstructure. We can apply this idea to the osmotic pressure by considering the force acting on a pair of particles. The first derivative of energy with respect to distance provides us with force ... 

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... 

In order to understand the source of this force, consider two particles separated by a distance d as shown in Figure 13.17. The dispersed polymer molecules exert an osmotic pressure force on all sides of the particles when the particles are far apart, that is, when d > Rg. Then, there is no net force between the two particles. However, when d < Rg, there is a depletion of polymer molecules in the region between the particles since otherwise the polymer coils in that region lose configurational entropy. As a consequence, the osmotic pressure forces exerted by the molecules on the external sides of the particles exceed those on the interior (see Fig. 13.17), and there is a net force of attraction between the two particles. The range of this attraction is equal to Rg in our highly simplified model. 

When a liquid dispersion contains non-adsorbing polymers there will be a layer of liquid surrounding each dispersed species that is depleted in polymer, compared with the concentration in bulk, solution. This causes an increase in osmotic pressure in the system compared with what it would be were the dispersed species absent. If the dispersed species move dose to each other then the volume of solvent depleted is reduced, reducing the overall osmotic pressure, which provides a driving force for flocculation. Xanthan gum, added in low concentrations, can cause depletion flocculation [291]. 

The flocculation of dispersed species induced by nonadsorbing polymer molecules due to depletion forces. When solutes such as polymer molecules do not, for some reason, enter the gap between adjacent surfaces an attractive force is created between the surfaces. This depletion force arises out of the solute s ability to influence osmotic pressure in bulk but not in the gap between the surfaces. 

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