Depletion Attraction between a Polystyrene Particle and a Hydrophilic Surface in a Pluronic Aqueous Solution

In practice, many colloidal suspensions also contain polymers where their presence has a major effect on the stability of colloidal particles. In this work, we use total internal reflection microscopy to directly measure the interactions between a ∼6.0 μm polystyrene spherical particle and a hydrophilic flat surface with the presence of an triblock copolymer, poly(ethylene oxide-block-propylene oxide-block-ethylene oxide) (Pluronic PE10500), in an aqueous solution with low ionic strength. A discernible attractive force between the particle and surface is observed with the measurable range of up to ∼100 nm. Dynamic laser light scattering study reveals that monomers, micelles, and larger nanobubbles (∼166 nm) coexist when PE10500 triblock copolymer is spontaneously dissolved in the low ionic strength aqueous solution. We attribute this measured long-range attractive force to the creation of a significant depletion force between the particle and surface as caused by the existence of relatively large nanobubbles free in solution. Replacement of the PE10500 copolymer solution with salt solution removes the nanobubbles, which is reflected in disappearing of the attractive force. Moreover, we find that the adsorbed PE10500 chains at both charged particle and flat surface may cause a redistribution of counterions and coions that make up the electric double layer of the surfaces, thus displacing the repulsive potential between the particle and surface.