6 - Forces due to soluble polymer

Summary

Introduction

The preceding chapters address interactions between colloidal particles dispersed in pure liquid or electrolyte solution. The hydrodynamic and dispersion forces depend only on the bulk properties of the individual phases, i.e. the viscosity and the dielectric permittivities. Electrostatic forces arising from the surface charges, however, are accompanied by free electrolyte. The associated electric fields distribute these additional species non-uniformly in the surrounding fluid, thereby producing a spatially varying osmotic pressure. Electrostatic interactions between particles alter these ion distributions, affecting the electric and pressure fields and generating an interparticle force.

We now consider another component commonly present in colloidal systems, soluble polymer. In many ways, the phenomena and the theoretical treatment resemble those for electrostatics. The interactions between polymer and particle generate non-uniform distributions of polymer throughout the solution. Particle–particle interactions alter this equilibrium distribution, producing a force whose sign and magnitude depend on the nature of the particle–polymer interaction. The major difference from the ionic solutions lies in the internal degrees of freedom of the polymer, which necessitate detailed consideration of the solution thermodynamics.

The reasons for adding soluble polymer to colloidal dispersions are several. The earliest known role, as stabilizer, aids or preserves the dispersion through adsorption of the macromolecule onto the surfaces of the particles to produce a strongly repulsive interaction. Homopolymers achieve this by adsorbing to particles non-specifically at multiple points along their backbone, while block or graft copolymers adsorb irreversibly at one end with the other remaining in solution (Napper, 1983).