Competitive Adsorption Between PEO-Containing Block Copolymers and Homopolymers at Silica

The ability to manipulate polymer adsorption is useful for applications involving colloidal stabilization, for example, paints, cosmetics, lubricants, and mineral and waste-water treatment. We have an ongoing interest on the use of organic molecules for modulating the aqueous solution and adsorption properties of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers. In the present study, the influence of low molecular weight PEO homopolymer on the adsorption of a representative PEO–PPO–PEO block copolymer (Pluronic P105: EO₃₇-PO₅₆-EO₃₇) at the surface of protonated silica nanoparticles dispersed in water is investigated. Pluronic P105 forms hydrophobic domains on the surface of protonated silica at a critical surface micelle concentration, csmc, of 0.02 wt% in the presence of 0.1 wt% silica nanoparticles in water, well below the cmc of Pluronic P105 in water (0.6 wt%). Dye solubilization experiments reveal an increase in the PEO–PPO–PEO block copolymer csmc with increasing amounts of added PEO homopolymer. The resulting critical displacer concentration for PEO homopolymer of molecular weights 200 and 600 Da was measured to be 0.1 wt% and 0.07 wt%, respectively, in the presence of 0.1 wt% silica nanoparticles. Capillary viscometry measurements indicate a decrease in the adsorbed layer thickness at the protonated silica surface with increasing PEO homopolymer concentration. The data presented herein are consistent with a physical model which considers “patches” of PEO–PPO–PEO block copolymer and PEO homopolymer adsorbed at the silica surface. (Figure presented.)