Kinetics of structure build-up in self bodied emulsions stabilized by mixed emulsifiers

Abstract

Ternary systems and emulsions prepared with anionic (sodium dodecyl sulfate), cationic (cetrimide), or nonionic (cetomacrogol) surfactants and cetostearyl alcohol were examined in creep and in continuous shear rheometry at the storage temperature of 25°C. Systems were tested frequently during the first 24 hr after preparation, and then regularly for 3 months. Creep tests were of short duration and the curves were analyzed to derive continuous spectra of retardation times. For each surfactant, the results obtained were correlated with the mechanisms by which gel networks formed. With age, anionic and cationic ternary systems and emulsions behaved similarly with respect to total compliances, retardation spectra, and continuous shear parameters. Additional structure, although not extensive, formed within the first 2 hr storage. This was indicated in creep by initial falls in total compliance values and decreases in the heights of retardation spectra, and in continuous shear by corresponding increases in apparent viscosities and hysteresis loop areas. Thereafter negligible new structure formed, and thus retardation spectra for systems tested after 2.5 and 24 hr storage were similar, and there was little variation in total compliance, apparent viscosity, and loop area values. The essentially constant position of maxima in the retardation spectra indicated that the high temperature interaction which occurred during the preparation of ionic systems was the major mechanism which operated to form gel networks. The low temperature interaction occurring during the first 2 hr storage was insignificant. Cetomacrogol ternary system and emulsion behaved differently to the ionic systems. Structure built up rapidly during the first 24 hr storage. This was shown in creep by marked reductions in total compliances and retardation spectral values and in continuous shear by changing complex flow properties. The movement of maxima in the retardation spectra to longer times indicated that the mechanisms which operated to form gel networks at the storage temperature produced structures which differed at a molecular level to those formed at high temperatures.