David J. Hibberd

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

Abstract 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.

Creaming and rheology of oil-in-water emulsions

The creaming and rheology of fine n-tetradecane oil-in-water emulsions at pH 6.8 containing the commercial protein sodium caseinate and the ionic surfactant sodium dodecyl sulfate (SDS) have been studied, and an overview diagram relating surfactant composition and creaming stability has been constructed. The presence of both SDS and sodium caseinate in an emulsion system increases the overall stability with respect to creaming. Excess SDS promotes destabilization through fast creaming; this can be attributed to depletion flocculation brought about by unadsorbed surfactant micelles. Addition of sodium caseinate was found to reduce this effect, even at relatively high SDS concentrations. The behavior of the caseinate + SDS emulsions is thus different from the behavior of the previously reported caseinate + Tween 20 systems, where the combination of the two surface-active agents was found to reduce the emulsion stability, as indicated by fast creaming and shear-thinning rheology. Addition of sodium chloride was found to increase the extent of non-Newtonian behavior and to enhance the degree of creaming for SDS-containing emulsions. Increased caseinate levels in these systems seem to offer some stabilization through reduction of the shear-thinning character and improvement in creaming stability. These phenomena can be explained in terms of a considerable amount of SDS binding to the protein, which reduces the amount of SDS available to promote protein displacement and depletion flocculation. In contrast to the SDS systems, the properties of equivalent emulsions containing caseinate + nonionic surfactant Tween 20 are relatively insensitive to salt content. Copyright 2000 Academic Press.