Effective short-range Coulomb correction to model the aggregation behavior of ionic surfactants

Area of Science:

• Physical Chemistry
• Colloid and Surface Chemistry

Background:

• Amphiphilic molecules aggregate in aqueous solutions, forming micelles.
• Understanding counterion distribution and micelle ionization is crucial for predicting solution properties.

Purpose of the Study:

• To introduce a short-range correction to the Coulomb potential for modeling amphiphilic aggregation.
• To quantitatively predict counterion distribution and micelle degree of ionization (α).
• To investigate the behavior of a specific cationic surfactant (C24H49N2O2 (+) CH3SO4 (-)) relevant to the personal care industry.

Main Methods:

• Theoretical modeling with a modified Coulomb potential.
• Lattice Monte Carlo simulations using a coarse-grained model.
• Validation against experimental observations.

Main Results:

• The model accurately reproduces counterion distribution and micelle ionization (α) above the critical micelle concentration (CMC).
• The coarse-grained model predicts complex micelle morphology at equilibrium.
• Simulation results indicate a transition from monodisperse to bidisperse aggregate size distribution, suggesting a second CMC.

Conclusions:

• The short-range corrected Coulomb potential provides a robust framework for studying amphiphilic aggregation.
• Accurate estimation of α is vital for understanding micellar solution properties like viscoelasticity and transport.
• The discovery of a second CMC highlights complex aggregation behaviors and potential for novel formulations.