Depletion potential between two attractive plates mediated by polymers

Depletion interactions in polymer-colloid mixtures in the presence of the attraction between polymers and particle surfaces are computed from the simulation data on polymer partitioning. The gap between two large colloid particles is approximated by the slit plates immersed in a dilute polymer solution. The difference between the intra-slit and bulk pressures of polymers Δp (net pressure) and the effective pair potential of plates W is calculated as a function of the plate separation D and the attraction strength ε of plates to polymer segments. Weak attraction strengths ε are considered, including the region around the compensation point εc (the depletion/adsorption threshold) where the partition coefficient of polymers K=1. A fit of the simulation data for the free energy confinement ΔA=−kBT ln K served as a base of computations of the above depletion characteristics. The analytical functions deduced for Δp(D,ε) and W(D,ε) predict a reduction of the range and depth of depletion interaction by an increase in the attraction strength |ε|. Variation of the thickness δ of depletion layers near the plates with attraction ε was also determined. Consequences of reduced depletion interaction for the effective second virial coefficient B2 and for the stability of colloidal suspensions were discussed. At the compensation point εc the depletion effect disappears and the stability of polymer-colloid mixtures in dilute solution is not affected by the polymer concentration.