The influence of formulation variables on phase inversion temperatures of emulsions as determined by a programmed viscometric technique

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Abstract
A programmed temperature rise viscometric technique has been used to examine the influence of formulation variables on the phase inversion temperatures (P.I.T.) of oil-in-water emulsions stabilized with mixed polyoxyethylene alcohol surfactants. Using a heating rate of 0·5° min−1, the P.I.T. was defined either by a sharp change in apparent viscosity when recording shear stress-rate of shear rheograms for successive emulsion samples, or by the change in shear stress with time for a single emulsion sample. With an overall concentration of 6·00% w/w surfactant mixture of Brij 92 and Brij 96, the P.I.T. was found to rise in an approximately linear fashion as the HLB increased towards the optimum value. When the surfactant concentration was related to the aqueous or oil phase and these phase ratios varied, the P.I.T. changed in a complex manner. Water-soluble additives caused marked changes in the P.I.T. For example, 5·0% w/w sodium chloride depressed the P.I.T. by 17° and 5·0% w/w calcium chloride caused an 11° fall, whereas 2·0% w/w propylene glycol produced a 6° rise. These results followed the same trend as the cloud point of 1·0% w/w aqueous solutions of Brij 96 containing these additives. Linear correlations were obtained between the cloud point and P.I.T. data from which the effect of additives upon emulsion stability may be monitored.