STABILITY OF HIGHLY ASYMMETRIC BINARY COLLOIDAL DISPERSIONS AND THEIR APPLICATION TO FILM FORMATION

Abstract

Binary waterborne suspensions of hard and soft particles can yield tough, impervious, crack-free films when hard-particle percolation is suppressed in the drying process. Small soft colloids can be distributed around large hard colloids by coupling particle size asymmetry with surface potential asymmetry in the blend, according to recent experiments where highly charged zirconia nanoparticles formed adsorbed layers around nearly-neutral silica microspheres. Yet, the phase behavior of highly asymmetric silica-zirconia blends is rather complex, and the underlying mechanism has remained a subject of debate. We contribute detailed microstructure calculations and explore the driving forces affecting stability of these blends.

Our model of the silica-zirconia blends at experimental conditions suggests that colloid-nanoparticle attraction of electrostatic origin assembles an adsorbed layer, a halo, of nanoparticles around each colloid. When nanoparticle halos are sufficiently dense, electrostatic stabilization of the silica colloids against van der Waals flocculation is predicted. Meanwhile, further densification of the charged halos leads to well-defined nanoparticle depletion zones adjacent to the halos. These depletion zones provide an entropic incentive for the haloed colloids to aggregate, destabilizing the blend. Detailed calculations of effective colloid-colloid interactions, as well as estimates of rheological behavior, are in qualitative agreement with existing experiments.

Our computational findings motivate an aqueous formulation that is less size-asymmetric and consists of silica microspheres and deformable acrylic latex spheres with the size ratio of 5:1, respectively. Composite films, cast from these blends at various pH, are studied via scanning electron microscopy and thermogravimetric analysis. The suspension pH, which sets the surface potential of silica colloids, is correlated with radial and crosssectional uniformity of dried films. Low-pH precursor suspensions yield uniform composite films with solids loading up to 71% by volume that remain crack-free.