Entropic interactions in mixtures of colloids and non-ionic polymers

Abstract

In binary colloidal mixtures, depletion forces of purely entropie origin play an important role besides interaction like Coulomb or van der Waals interactions. We have measured the potential energy between a single, negatively charged polystyrene sphere (R = 1.5 μm) close to an equally charged wall in the presence of uncharged polymer coils (r = 150 nm) by means of total internal reflection microscopy. Our results show a strong dependence on the polymer concentration n and can be explained by the occurrence of depletion forces. Comparison of our data with a theory which is only correct in first-order n shows good agreement. We also present first observations in a system with an increased particle/polymer size ratio (R = 5 µm, r = 0.15 μm) where higher-order terms in the depletion potential become important.