Polymer polydispersity effect on depletion interaction between colloidal particles

The effect of polymer polydispersity on the polymer-induced interaction between colloidal particles due to non-adsorbing ideal chains is investigated. An analytical theory is developed for the polymer segment density between two plates and in the space surrounding two spheres by extending a recently proposed superposition approximation to include polymer polydispersity. Monte Carlo computer simulations were made to test the validity of the analytical theory. The polymer densities predicted by the superposition approximation are in reasonable agreement with simulation results for the polydisperse case. The simulations show that depletion leads to a size fractionation of the polymers. It is shown that size polydispersity has a small effect on the interaction between two parallel plates but a more significant effect on the interactions between two spheres. The range of the potential increases and the contact potential drops with increasing polydispersity. Polymer-segment density as a function of y for three values of x, as indicated, in the space surrounding two colloidal spheres with radius R = R(go) and h = 0.48R(go). Symbol are the MC results: polydisperse polymer (O; Z = 1) and monodisperse polymer(O) samples. Curves are the predictions of the product-function, approximation for monodisperse polymer (solid lines) and polydisperse polymer(z = 1, dash lines).