Surface tension calculations of the cationic (CTAB) and the zwitterionic (SB3-12) surfactants using new force field models: a computational study

Actual CTAB and SB3-12 surfactant force field models fail to reproduce one of the most important thermodynamic property of those molecules, the surface tension. Molecular dynamics simulations were conducted to construct new force fields of the cationic surfactant, Cetyl Trimethyl Ammonium Bromide (CTAB), and the non-ionic, cocoamidopropyl betaine, surfactants using united atom models. By scaling the Lennard Jones parameters, the well depth potential (epsilon) and the intermolecular distance (sigma), we constructed an united atom model of the cationic and the betaine surfactants. The new models were tested with actual experiments reported in the literature. With the correct parameters, surface tensions of both surfactants were calculated at different temperatures and different areas per molecule. Electrostatic properties and micelle structures were also calculated with the new set of parameters and radius of gyrations, i.e. micelle radius, were evaluated showing good affinity with experimental data. The new force fields were proved with two different water models, TIP4P/epsilon and SPC/E, having good agreement with actual experiments