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 sol ution properties of the PEG are characterized for three molecular weights ( 1000, 6000 and 12 000) providing an opportunity for quantitative comparison s of measurements and theoretical predictions of B-2. We report non-monoton ic changes in B-2 as the concentration and molecular weight of PEG are incr eased. The observed minimum in B-2 cannot be predicted by the standard depl etion model of Asakura and Oosawa, and is closely associated with the proxi mity of the polymer solution lower critical solution temperature. The locat ion 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 tre ated as a function of temperature.