  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 results also show that the
shorter alcohol molecules become stiffer with higher values of the deuterium
order parameters and smaller area per molecule. 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.
