OSMOTIC-PRESSURE, CREAMING, AND RHEOLOGY OF EMULSIONS CONTAINING NONIONIC POLYSACCHARIDE

We report measurements of the osmotic pressure, theology, and creaming of concentrated oil-in-water emulsions (30 wt% mineral oil, 1 wt% surfactant, pH 7, ionic strength 0.05 M) as a function of the concentration of added nonionic polysaccharide dextran (5 X 10(5) Da). Experiments were carried out at 25 degrees C on emulsions prepared with two different low-molecular-weight surfactants-nonionic Tween 20 (polyoxyethylenesorbitan monolaurate) and anionic SDS (sodium dodecyl sulfate). For both types of emulsifier, the addition of dextran was found to lead to a reduction in osmotic pressure, an increase in the limiting zero-shear-rate viscosity, and an enhancement in the rate of serum separation. These observations are consistent with the theory of depletion flocculation of the droplets by the nonadsorbing polymer, The effects of the surfactant type on emulsion properties are most evident at low or high dextran contents. The results are compared with those reported earlier for the same emulsions containing the anionic polysaccharide rhamsan, The combined data indicate a strong but complex relationship between creaming stability and low-stress creep theological behavior. Substantial differences in properties between emulsions made with nonionic and charged surfactants indicate the importance of surfactant-biopolymer interactions in controlling the flocculation of emulsion droplets. (C) 1995 Academic Press, Inc.