Investigation on the thermal stability of technical and cosmetic emulsions

One of the major problems, which cosmetic industry is confronted with, is the thermal stability of the products in emulsion form. This work proposes an evaluation of the thermal stability of selected emulsion formulations – fabric softeners, as simple emulsion systems, and cosmetic creams and lotions, as complex multicomponent systems – by interpreting thermal analysis data, obtained from differential scanning calorimetry (DSC) experiments with the tool of nonisothermal reaction kinetics. This approach is supplemented with structural information about the emulsion samples from optical microscopy observations in bright and polarised light and thermo-microscopy examination under non-isothermal conditions. For a selected group of fabric softener bases, chosen for the stability studies, the influence of fragrance addition, as well as of storage at high and low temperatures and of time of storage on unperfumed samples is investigated. Cyclic temperature programmes with different heating rates and constant cooling rate were performed and the kinetic parameters were calculated from the endothermic processes occurring during heating. Complex curve profiles demand the separation of the overall endothermic effect into individual processes by means of linear deconvolution procedure using Gaussian curves. Imposing additional restrictions to arrive at a better agreement with the physical reality, lead to improved fits. The kinetic parameters values (in the compact form of the rate constant) were derived using model-free (isoconversional) and model-based approaches. Nonisothermal reaction kinetical methods applied to fabric softener samples proved to be able to differentiate the thermal stability of the different groups of bases. Practical experience confirmed the results of DSC profile deconvolution and of the kinetic evaluation. Extending the scope of the investigation, selected commercial cosmetic creams and lotions were investigated by DSC and the information obtained from the DSC profiles and their kinetic analysis were correlated with thermo-microscopy examinations. The most stable and the least stable cosmetic emulsion in the chosen set were identified