Charged polystyrene nanoparticles near a SiO2/water interface.

Quartz crystal microbalance with dissipation (QCM-D) monitoring is traditionally used to investigate adsorption processes at liquid-solid interfaces but has also been applied increasingly to characterize the viscoelastic properties of complex liquids. Here, we contribute new insights to the latter field by using QCM-D to investigate the structure near an interface and high-frequency viscoelastic properties of charge stabilized polystyrene particles (radius 37 nm) dispersed in pure water. The study reveals changes with increasing ionic-strength from the crystalline order at low salt concentration to that with a less-structured particle distribution at high ionic strength. Replacing pure water with an aqueous particle dispersion is due to an increased mass load expected to give rise to a decrease in frequency, f. In the present work increases in both f and dissipation, D, were observed on exchanging pure water for the particle dispersion at low ionic strength. However, the QCM-D data are still well-represented by a viscoelastic Voigt model, with the viscosity increasing from 1.0 to 1.3 mPa s as the particle volume fraction changed from 0.005 to 0.07. This increase is higher than predicted for dilute dispersions according to Einstein's equation for the viscosity of non-interacting hard spheres particles in liquids but can be explained by the charge repulsion between the particles giving rise to a higher effective volume fraction. It is also concluded that the polystyrene particles did not adhere to the solid surface but rather were separated by a layer of pure dispersion medium. The QCM-D response was successfully represented using a viscoelastic Kelvin-Voigt model, from which it was concluded that the thickness of the Newtonian dispersion medium layer was of the order of the particle-particle bulk separation, in the range 50 to 250 nm and was observed to decrease with both particle concentration and with addition of salt. Similar anomalous frequency and dissipation responses have been seen previously for colloidal systems containing weakly adherent colloidal particles and bacteria and in these cases interpreted in terms of coupled resonators. We here demonstrate that surface attachment is not required for such phenomena to occur, but that a viscoelastic liquid separated from the oscillating surface by a thin Newtonian layer can give rise to very similar responses.