Ultrasonic characterisation of colloidal dispersions: detection of flocculation and adsorbed layers

A series of experiments to detect flocculation and the presence of adsorbed layers on polystyrene latices is presented. The system consisted of 504.1 nm diameter polystyrene latex at 4.39% w/w in the absence or presence of non-ionic surfactant, Triton (TX100) and hydroxyethylcellulose (HEC). Ultrasonic scattering calculations were performed on each system using the theory of Allegra and Hawley. Ultrasonic spectroscopy was carried out using the Institute of Food Research (IFR) discrete frequency spectrometer, accurate to 0.01% and 4% in velocity and attenuation, respectively. The instrumental precision was up to an order of magnitude better. There were marked increases in ultrasonic attenuation upon flocculation, whether by a bridging or a depletion mechanism. Small but significant increases in attenuation were observed in the presence of adsorbed layers of surfactant or HEC, compared with a model that assumed the solutes to be uniformly dispersed in the continuous phase.