Concentration-dependent effective attractions between PEGylated nanoparticles

Effective attractions between colloidal particles bearing a grafted poly(ethylene glycol) (PEG) layer in water have been studied and quantified by measurements of the collective diffusion coefficient and by quantitative analysis of small-angle neutron scattering (SANS) data. Results for the collective diffusion coefficient in the dilute limit indicate that effective attractions develop gradually as carbonate anions are added to the dispersions. Analysis of SANS data within a square-well interaction model at a constant salt concentration allows for quantitative analysis of scattering patterns of samples prior to crossing into an aggregation regime, where particles form large clusters, reached either through increasing the temperature or the particle concentration. Aggregation is observed visually and is also evident in the scattering as a lowering of the intensity at intermediate wavevectors while leaving enhanced scattering in the forward direction, suggesting a nearby fluid-fluid phase transition. In addition, at low and moderate particle concentrations the attraction strength is shown to depend mainly on temperature but at high particle concentrations a much stronger temperature dependence is observed, which shows that the attraction acquires a dependence on particle concentration at sufficiently high concentrations. The concentration dependence is attributed to a decreased solvation of PEG chains due to an increased ratio of ethylene oxide segments to water.