Effective interactions between solid particles mediated by free polymer in solution.

Effective potentials of interaction between solid non-adsorbing surfaces in semidilute or concentrated polymer solutions are calculated using two approaches: the numerical self-consistent-field theory (SCFT) and the analytical theory generalizing the ground-state dominance approximation by taking into account the finite chain-length (chain-end) effects (GSDE). A good agreement between the two approaches is demonstrated within the natural region of validity of the asymptotically exact GSDE theory. It is shown that in most cases the interaction potentials involve the short-range depletion attraction which is replaced by the polymer-mediated repulsion at longer separations ranging from roughly 10ξ to 3Rg (ξ is the concentration correlation length and Rg is the polymer coil gyration radius). For a given polymer concentration and molecular weight the highest repulsion energy barrier, Um, is predicted for ξ∕Rg ~ 0.2 and in the theta-solvent conditions. The fluctuation-induced (anti-Casimir) polymer-mediated repulsion forces are analyzed as well. It is shown that the overall polymer-mediated repulsion energy between naked spherical particles of radius Rc = 200 nm is typically not enough for their kinetic stabilization. There are, however, a few special cases where colloidal stability can be imparted by free polymers only.