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

Area of Science:

• Physical Chemistry
• Materials Science
• Soft Matter Physics

Background:

• Surfactant self-assembly into various phases (e.g., hexagonal, lamellar) is crucial for applications.
• The packing parameter is traditionally used to predict these morphological changes.
• Experimental and simulation challenges have limited the validation of the packing parameter's predictive power.

Purpose of the Study:

• To investigate the validity of the packing parameter in predicting surfactant morphological transitions.
• To elucidate the role of hydration and thermodynamics in the hexagonal-to-lamellar phase transformation.
• To identify key factors governing surfactant self-organization in aqueous solutions.

Main Methods:

• All-atom molecular dynamics simulations.
• Heavy hydrogen atom method for enhanced sampling.
• Analysis of packing parameter, hydration, and thermodynamic components during phase transitions.

Main Results:

• The packing parameter of the bare surfactant molecule remained consistent between hexagonal and lamellar phases.
• The hydration shell significantly influences the effective head group size and packing.
• Dehydration of water molecules surrounding surfactant aggregates accompanied the hexagonal-to-lamellar transition.

Conclusions:

• The packing parameter is insufficient to predict surfactant phase behavior; hydration effects are critical.
• Dehydration and the resulting increase in surfactant entropy are the primary drivers of the hexagonal-to-lamellar transformation.
• Understanding hydration and entropy is key to controlling surfactant self-assembly.