Oscillatory testing of o/w emulsions containing mixed emulsifiers of the surfactant/long chain alcohol type: Influence of surfactant chain length

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Abstract
A Weissenberg Rheogoniometer with parallel plates coupled to a Solartron Digital Transfer Function Analyser was used at 25° in oscillatory mode to investigate liquid paraffin-in-water emulsions stabilized by alkyltrimethylammonium bromides (of C12-C18 chain lengths) and cetostearyl alcohol. Ternary systems, formed by dispersing the quaternary surfactants and cetostearyl alcohol in water, were similarly examined. Each preparation was tested in the linear viscoelastic region and fundamental rheological parameters, the storage modulus (G') and the dynamic viscosity (η′), were derived as functions of frequency over the range 7.91 × 10−4 to 25 Hz. The rheological properties of the emulsions were related to viscoelastic gel networks present in the continuous phases. The corresponding ternary system parameters implied that these networks were formed by the interaction of cetostearyl alcohol and aqueous surfactant solution. Ternary systems and emulsions behaved similarly with respect to frequency. As this increased, G' rose and η′ fell, the latter through several logarithmic decades. At low frequencies, G' and η′ rose as the quaternary alkyl chain length increased. Formulations containing quaternary surfactants of medium chain lengths (C14 and C16) formed strong viscoelastic networks. The C12 systems contained tenuous networks, and networks formed from the C18 surfactant were not as rigid as predicted by the trend of the lower homologues. The oscillatory data were compared and correlated with creep data described previously.