Effects of polymer density fluctuations on depletion interactions

The strength of depletion interactions has been characterized by measuring the second virial coefficient, B-2, of suspensions of lysozyme molecules in the presence of non-adsorbing polymer poly(ethylene glycol) (PEG). The solution properties of the PEG are characterized for three molecular weights (1000, 6000 and 12 000) providing an opportunity for quantitative comparisons of measurements and theoretical predictions of B-2. We report non-monotonic changes in B-2 as the concentration and molecular weight of PEG are increased. The observed minimum in B-2 cannot be predicted by the standard depletion model of Asakura and Oosawa, and is closely associated with the proximity of the polymer solution lower critical solution temperature. The location and depth of the minima in B-2 are well captured by the thermal polymer reference interaction site model theory where the polymer mesh size is treated as a function of temperature.