Surfactant stability and motion: Structural and dynamic properties of a sodium dodecyl sulfate micelle, its counterions, and surrounding water molecules

An all-atom five nanosecond molecular dynamics simulation of a water-solvated micelle containing 60 sodium dodecyl sulfate monomers was performed. Structural properties of the micelle, counterions, and solvent were evaluated. The results indicate a stable micellar aggregate over the duration of the simulation with penetration of water molecules into the micelle restricted to the headgroup region, leaving a 12 Å water-free hydrocarbon core. Evaluation of the structure and motion of the sodium counterions show (1) a long equilibration time (1 nanosecond) is required to achieve a stable distribution of counterions and (2) approximately 25% of the sodium ions are located in the first shell and 50% are located in the first two shells of the micelle during the course of the simulation. Contact-ion pairs were observed between sodium ions and sulfate headgroup oxygen atoms.; Evaluation of the dynamic properties of the sodium counterions and water molecules revealed a micelle distance dependence. Values of the diffusion coefficient for the sodium ions followed a decreasing trend for ions in the third shell of the micelle, ions in the second shell of the micelle, and those in the first shell of the micelle. Definitions of shells are based on the water oxygen to micelle oxygen radial distribution function. Water molecules near the headgroup exhibit a distortion of the water-water hydrogen bonding network due to headgroup oxygen-water hydrogen bond formation. The dynamic implications of this distortion are manifested in the decay of the dipole autocorrelation function, Φ(t) and translational diffusion coefficient which followed the same decreasing trend as the sodium counterions. True bulk-like behavior of the water molecules was not observed when compared to a simulation of 512 water molecules which implies long range effects of the micelle on solvent properties.