Size Ratio Effects on Interparticle Interactions and Phase Behavior of Microsphere-Nanoparticle Mixtures

We investigate the interparticle interactions and phase behavior of microsphere-nanoparticle mixtures of high charge asymmetry and varying size ratio. In the absence of nanoparticles, negligibly charged microspheres flocculate as a result of van der Waals interactions. Upon addition of a lower critical nanoparticle volume fraction, the microspheres are stabilized by the formation of nanoparticle halos around each microsphere.(1,2) A weak attraction between the two species leads to a pronounced enhancement of the effective nanoparticle concentration near the microsphere surface relative to the bulk solution. Above an upper critical nanoparticle volume fraction, the microspheres undergo reentrant gelation. Binary mixtures, in which the effective nanoparticle size is reduced at a fixed microsphere diameter, exhibit a narrow window of stability that ultimately disappears with increasing ionic strength. By contrast, binary mixtures of varying microsphere diameter are stabilized at similar nanoparticle volume fractions and exhibit a broader window of stability with decreasing size ratio. This unexpected observation may arise from the reduced attraction between smaller microspheres because negligible differences in nanoparticle halo formation are observed in these mixtures.