On the Origin of Seemingly Non-Surface Active Particles Partitioning between Phase Separated Solutions of Incompatible Non-Adsorbing Polymers and Their Adsorption at the Phase Boundary.

We have computed the free energy per unit area (i.e., interfacial tension) between a solid surface and two co-existing polymer solutions, where there is no specific interaction between the particles and either polymer, via self-consistent field calculations. Several different systems have been studied, including those where the two polymers differ in molecular weight (Mw) by a factor of ~ 2, or where the polymers have the same Mw but one set of chains is branched with the other being linear. In the absence of any enthalpic contribution resulting from adsorption on the particle, the differences in free energy per unit area resulting from the polymer depleted regions around the particles in the two co-exiting phases are found to be ~ 1 μN m-1. Although this value may seem rather small, this difference is more than capable of inducing the partitioning of particles of 100 nm in size (or larger) into the phase with the lower interfacial free energy at a solid surface. By examining the density profile variation of the polymers close to the solid surface, we can also infer information about the wettability and contact angle (Θ) of the solid particle at the interface between the two co-existing phases. This leads to the conclusion that for all systems of this type, when the incompatibility between the two polymers is sufficiently large, will be close to 90°.