Influence of fatty alcohol mixing ratios on the physicochemical properties of Stearyl--Cetyl Alcohol--Polysorbate 60--Water ternary System: Insights from Experiments and Computer Simulations

Abstract

The structure and stability of ternary systems prepared with polysorbate 60 and various combinations of cetyl (C16) and stearyl (C18) alcohols (fatty alcohol 16g, polysorbate 4g, water 180g) were examined as they aged over 3 months at 25oC. Rheological results showed that the consistency of these systems increased initially during roughly the first week of aging, which was succeeded by little changes in consistency (systems containing from 30% to 70% C18, with the 50% C18 system showing the highest consistencies in viscosity and elasticity) or significant breakdown of structure (remaining systems). The formation and/or disintegration of all ternary systems were also detected by microscopy and differential scanning calorimetry experiments. This study emphasizes the fact that the structure and consistency of ternary systems are dominantly controlled by the swelling capacity of the lamellar $α-$crystalline gel phase. When the conversion of this gel phase into non-swollen $β$- or $γ$-crystals occurs, systems change from semisolids to fluids. Molecular dynamics simulations were performed to provide important details on the molecular mechanism of our ternary systems. Computational results supported the hypothesis experimentally proposed for the stability of the mixed system being due to an increase in the flexibility, hence an increase in the configurational entropy of the chain tip of the alcohol with a longer hydrocarbon chain (with the highest flexibility observed in the 50:50 C18:C16 system). This finding is in excellent agreement with experimental conclusions. Additionally, simulation data show that in the mixed system, the alcohol with shorter hydrocarbon chain becomes more rigid. These molecular details could not be available in experimental measurements