Investigating molecular mechanism for the stability of ternary systems containing cetrimide, fatty alcohol and water...

Computer simulations using atomistic model are carried out to investigate the stability of ternary systems of pure or mixed fatty alcohols, cetrimide, and water. These semi-solid oil-in-water systems are used as the main component of pharmaceutical creams. Experiments show that the mixed alcohol systems are more stable than pure ones. The current experimental hypothesis is that this is the result of the length mismatch of the alkyl chains. This leads to higher configurational entropy of the chain tip of the longer alcohol molecules. Our simulation results support this hypothesis. The magnitude in fluctuations in the area per molecule also increases in mixed systems, indicating a higher configurational entropy. Analysis of the molecular structure of simulated systems also shows good agreements with experimental data. Additionally, the results also show that the shorter alcohol molecules become stiffer with higher values of the deuterium order parameters and smaller area per molecule. This leads to more configurational space for the longer alcohol to maximize overall the free energy of the system. This is not known so far in available experimental studies of these systems.