Depletion forces and flocculation with surfactants, polymers and particles — Synergistic effects

This paper provides an overview of recent work focused on investigating the depletion interaction in colloidal systems containing multiple depletant species (i.e., more than one system component that contributes to the depletion interaction between dispersed colloidal particles). Experimental studies in systems containing polymer/surfactant and polymer/nanoparticle mixtures clearly show that complexation of the two solutes into a larger depletant can produce a synergistic effect, such that the depletion force is significantly larger than the sum of the forces arising in systems containing only one of the components. For the polymer/nanoparticle system, this synergy leads to much lower polymer critical flocculation concentrations. In addition, computational studies in a system containing charged nanoparticle depletants of two vastly different sizes (e.g., nanoparticles and submicroparticles) predict the formation of a halo of the submicroparticles around the larger dispersed colloids (i.e., microparticles). These studies clearly show that attempting to predict the depletion interaction in mixed depletant systems using measurements or predictions obtained from systems containing only a single depletant can lead to quantitative, and even qualitative, error.

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