Colloidal brushes in complex solutions: existence of a weak midrange attraction due to excluded-volume effects

Towards the prediction of thermodynamic properties for complex systems and the design of self-assembly techniques for the production of nanocomposite materials, we employ Monte Carlo simulations to calculate the effective interactions between colloidal brushes dissolved in nonadsorbing polymer solutions. For simplicity, all interactions are reduced to excluded-volume potentials. Our results indicate that (1) due to excluded-volume effects and the absence of attractive van der Waals forces in our calculations, the effective interactions between colloidal brushes in a continuum medium are always repulsive; and (2) that the short-ranged repulsion between the colloidal brushes may be coupled to a midrange attraction when nonadsorbing polymers are in solution. We prove that a depletion mechanism is responsible for the midranged attraction and we observe that the strength of the induced attraction is weaker compared to the depletion attraction computed for hard-sphere colloids in nonadsorbing polymer solutions. More interestingly, however, our results indicate that, even for the simple interaction potentials employed here, it is not possible to superimpose depletion attraction and steric repulsion to predict the correct pair potential between colloidal brushes in nonadsorbing polymer solutions.