Adsorption and stabilization

Abstract Dispersions of any size, neutrally buoyant particles will be thermodynamically stable if the attraction between particles is kept adequately low. Nonneutrally buoyant particles in a gravitational field will set up concentration gradients which put an upper limit onto the size of particle which can be stably dispersed for a given close-range, interparticle attraction through the surface. Particles which have a saturated layer of macromolecules adsorbed to their surfaces, “see” each other as do these macromolecules in solution. If the macromolecules give stable solutions so do the coated particles. By the same token coated particles and macromolecules of the same nature as the coat, interact as though all the solute particles were the same kind of macromolecules. Since adsorbed macromolecules are, as a rule, strongly (“irreversibly”) adherent, the stability of a coated particle suspension and the effect of adding other macromolecules to a coated particle suspension in solution can be treated as though the coated particles were giant macromolecules of the kind that constitute the coat. A repulsive segment/segment interaction between two kinds of polymers in solution, very severely restricts stability. It follows that for coated particle systems with repulsive surface characteristic a similar criterion exists. Aggregation occurs when more than a single strong binding site exists per particle. First dimers are formed but an increase in particle concentration leads to coagulation. Aggregation by bridging by macromolecules between surfaces coated with polymer, occurs to an important extent only when particles with partially coated surfaces interact. Not only linear synthetic polymers, but also suitable proteins, can be used to modify particle surfaces for all these purposes.