THEORY OF POLYMER ADSORPTION AND COLLOID STABILIZATION IN SUPERCRITICAL FLUIDS - 1 - HOMOPOLYMER STABILIZERS

Homopolymer adsorption and stabilization of colloids in compressible s upercritical solvents are modeled with the lattice-fluid self-consiste nt field theory(LFSCF). Adsorption at a single surface is characterize d versus bulk solvent density for various polymer chain lengths, solub ilities, and concentrations. As solvent density is lowered, adsorbed c hains first collapse due to the loss in solvent quality, and then expa nd due to an increase in adsorption that raises the excluded volume. T his collapse-to-expansion transition in layer thickness occurs just ab ove the critical solution density for a bulk polymer solution. Colloid stability is examined by calculating the free energy of interaction o f two parallel surfaces coated with adsorbed polymer. Above a threshol d concentration and molecular weight, the surfaces are stabilized at h igh density. As density is lowered, the entropically favorable expansi on of solvent from the interface causes flocculation at the critical s olution density of the homopolymer in bulk solution, in agreement with recent light scattering experiments. At lower concentrations, bridgin g occurs and stabilization is not possible, even in a high density, go od solvent.