Influence of solvent quality and thermal fluctuations on polymer-mediated depletion interactions

Analytic integral equation theory for treating athermal mixtures of hard sp heres and polymers is extended to study thermal interaction and solvent dua lity effects in suspensions of infinitely dilute spherical particles in dil ute and semidilute polymer solutions. Our approach applies to all sphere/ p olymer size ratios, and polymer-polymer segmental attractive interactions a re included using both liquid state and Flory-Huggins theory. The effect of such thermal interactions on the polymer-induced depletion potential betwe en hard spheres is studied with emphasis on the sphere-sphere second virial coefficient and the free energy of particle insertion. Strong enhancement of the purely entropic attractive depletion interaction is found in the pro ximity of the polymer-solvent critical demixing point. The magnitude of thi s effect and the distance from the Theta temperature where it becomes impor tant are sensitive to system specific factors.