Aggregation of charge colloidal particles in the binary solvent: not all sizes are equal*

It has long been established that when moving the temperature of a binary solvent close to its demixing temperature, an effective attraction can be introduced between colloidal particles, and drive the formation of large particle aggregates. Combining with the results measured by AFM and small angle neutron scattering (SANS), we show that for small charged particles (radius between 5 and 10 nm) in the mixture of water/2,6-lutidine, the particle aggregation is highly sensitive to the particle size. Larger sized particles tend to aggregate first. This size sensitive aggregation is thus potentially a generic particle purification method to separate particles by sizes. Further, this size sensitive aggregation is also observed in samples with much larger particles (average particle size of about 50 nm) with a large size polydispersity. Comparing to the diameter of the particles, the relative range of the attraction of the investigated samples ranges from about 6% to about 60%. Thus, we expect that the observed size sensitive aggregation may work for a wide range of charge particle systems, and may be a generic phenomenon of a system with both an attraction and charge repulsion.