Electroacoustic estimates of non-ionic surfactant layer thickness on emulsion drops

Abstract

The effects of an adsorbed non-ionic surfactant [nonyl phenol ethoxylate (C₉ϕE_N) (N = 100)] on the electroacoustic signals of hexadecane-in-water emulsion droplets were studied. The emulsions were prepared in the presence of two levels of sodium dodecyl sulfate (SDS), both well below the c.m.c. The dynamic mobility of the oil droplets was measured using a flow-through version of the AcoustoSizer. Adsorption of the non-ionic surfactant produced significant changes in the dynamic mobility of the particles and these were interpreted in terms of O'Brien's gel layer model. The model allows an estimate to be made of the elasticity and the approximate thickness of the adsorbed polymer layer and the influence which it has in moving the plane of shear out from the droplet surface. The SDS concentration had little effect on the polymer layer thickness which was also stable with time.