The packing parameter of bare surfactant does not necessarily indicate morphological changes

Abstract

Morphological changes of surfactant in water are believed to rely on the packing parameter of individual molecules. However, its validity has not been well examined due to the difficulties in both experiments and simulations. Herein, we perform all-atom molecular dynamics simulations, combined with the heavy hydrogen atom method, for aqueous mixtures of non-ionic surfactant, pentaethylene glycol monododecyl ether. We demonstrate the spontaneous self-reorganization from hexagonal to lamellar phase and quantify the variation of the packing parameter, hydration manner, and thermodynamic components during the process. The results clearly show that the packing parameter of bare surfactant is almost identical between these phases; thus, it does not function as an indicator. Instead, the consideration of hydration shell is likely to be necessary to account for the effective head group. We also find that the water molecules disturbed by surfactant aggregates are depleted during the hexagonal-to-lamellar transformation and subsequently correlate the dehydration to an increase in surfactant-related entropy, which is the dominant thermodynamic factor for the morphological transformation.