Osmotic pressure and chemical potential of silica nanoparticles in aqueous poly(ethyleneoxide) solution

The applications of dispersed nanoparticles depend critically on accurate control and prediction of their stability and phase behavior. In many situations nanoparticles are used in conjunction with adsorbing polymers, oligomers or surfactants that can influence interparticle potentials and dispersion stability either positively or negatively. To advance understanding of these effects, this article presents osmotic pressure measurements of a model system: silica nanoparticles dispersed in an aqueous solution of poly(ethylene oxide) (PEO). Based on previous studies, PEO is known to adsorb to the silica surface. Osmotic pressure, itself a sensitive function of interparticle and particle–solution interactions, is used here to calculate nanoparticle chemical potentials. The presence of an adsorbing polymer changes the fundamental dependence of nanoparticle osmotic pressure and chemical potential on composition. In the absence of PEO, the silica chemical potential increases with silica concentration, as expected for a repulsive interparticle potential. In the presence of PEO, however, the nanoparticle chemical potential passes through a minimum as PEO or silica concentration increase. This dependence of chemical potential on concentration and temperature agreed well with Monte Carlo simulations using a coarse-grained Lennar–Jones potential model.