Accelerated stability for bar soaps by differential scanning calorimetry

Abstract: Using Differential Scanning Calorimetry (DSC) to determine the thermal and kinetics parameters of the oxidation's reactions of fats and oils from bar soaps (BS), we investigate the activation energy (Ea) of fatty acid sodium salts from commercial BS, in order to conduct accelerated stability studies to ensure that the product remains safe and effective over the shelf life duration. Placement on the market of bar soaps is usually made based on accelerated stability studies using protocols that have been proven to be predictive for long term stability. Colgate Palmolive makes no exception to those principles. Several external references are providing useful recommendations regarding the stability studies for cosmetic products. CHPA and ICH are some of those associations providing guidelines for voluntary use by the industry. The common ground of those guidelines are that any tentative expiry period should be made based on scientific justification, historical data on same/similar formulas/products, and calculations using stability equations through activation energy. The activation energy of Bar soaps could be determined through oxidation of saturated and unsaturated fatty acids from bar soap fats and oils. This research provides a scientific justification for the accelerated aging conditions used by Colgate-Palmolive for bar soap products and complex systems through the application of Differential Scanning Calorimetry technique coupled to the Arrhenius equation & Eyring kinetic equation. The Arrhenius model & Eyring kinetic results indicate that 4 weeks stability at 40 °C is predictive for 2 years (104 weeks) stability at 25 °C. The above justify the expiry period of bar soaps at specific accelerated temperatures, time and humidity conditions.