Dispersion Stability Evaluated by Experimental Design

The stability of paraffin and hydrocarbon oil dispersions stabilized by nonionic surfactants has been systematically evaluated. Using experimental design, the influence of the following parameters on dispersion stability was studied: surfactant concentration, shear rate, shear time and temperature of homogenisation. The experiments were evaluated with respect to particle size and particle migration velocity by a scanning optical analysis technique. This scanning technique monitors physical variations in a dispersion as a function of time and the technique is well suited for evaluation of dispersion stability. It was found that the only factor examined affecting particle migration velocity in a significant way was the surfactant concentration. A pronounced maximum in creaming rate was obtained at around 10 wt% surfactant both for the paraffin dispersions (suspensions at room temperature) and for the hydrocarbon oil emulsions. This surfactant-induced instability is explained as depletion flocculation caused by elongated surfactant micelles or by small oil-containing aggregates formed as microemulsion droplets during the emulsification process.