Prediction of the physical stability and quality of O/W cosmetic emulsions using full factorial design

Context: Full factorial design is effective in predicting the properties optimization and processing conditions of cosmetic emulsions. However, in most factorial designs, the technological quality and accelerated stability tests of emulsions are not included together as dependent variables. Aims: To predict the technological quality and physical stability of model cosmetic emulsions from the combination of formulation and processing factors using a 23 full factorial design. Methods: A 23 full factorial design with 95 % of confidence was generated in order to evaluate the influence of critical factors during the manufacture of cosmetic emulsions on their quality and physical stability. Emulsifier, concentration of stearic acid (formulation factors) and type of cooling (processing factor) were the independent variables, whereas spreadability, bulk density and pH were dependent variables for technological quality and centrifugation, heating-cooling and freeze-thaw cycles were dependent variables for physical stability of the emulsions. All cosmetic emulsions were prepared with a low stirring speed of 100 rpm. Results: The formulation containing 1% w/w sodium lauryl sulfate, 1.05% w/w stearic acid and prepared with continuous cooling, did not show indicative changes of physical instability by visual inspection after accelerated physical stability tests. These results were confirmed with emulsions stored for 12 and 24 months using real storage conditions. However, formulations containing an emulsifier blend composition (Tween 80: Sodium lauryl sulfate, 2:1 w/w), showed signs of creaming after these tests. Conclusions: Sodium lauryl sulfate at 1 % w/w, increased the physical stability of the emulsions by increasing their consistency and by other possible mechanisms. Real storage conditions confirmed the stability prediction performed using the combination of accelerated stability tests (centrifugation/cooling-heating/freeze-thaw cycles), aided by the purposed full factorial design.