From homogeneous dispersion to micelles - A molecular dynamics simulation on the compromise of the hydrophilic and hydrophobic effects of sodium dodecyl sulfate in aqueous solution

The structural and functional diversity of surfactant systems has attracted simulation works in atomistic, coarse grain, and mesoscopic models (Bandyopadhyay, S.; et al. Langmuir 2:000, 16, 942; Senapati, S.; et al. J. Phys. Chem. B 2003, 107, 12906; Maiti, P. K.; et al. Langmuir 2002, 18, 1908; Srinivas, G.; et al. J. Phys. Chem. B 2004, 108, 8153; Groot, R. D.; et al. J. Chem. Phys. 1999, 110, 9739; Rekvig, L.; et al. Langmuir 2003,19, 8195). However, atomistic models have suffered from their tremendous computational cost and are, so far, not able to simulate the structural behaviors in sufficient spatio-temporal scales (Shelley, J. C.; Shelley, M. Y. Curr. Opin. Colloid Interface Sci. 2000, 5, 10 1). The other two approaches are not microscopic enough to describe the configurations of the surfactants that determine their behaviors (Shelley and Shelley). In this study, we propose to simplify atomistic models based on the observation that the compromise of the hydrophilic and hydrophobic effects (Li, J.; Kwauk, M. Chem. Eng. Sci. 2003, 58, 521-535) and molecular structures of surfactants are the dominant factors shaping their structures in the systems. With this simplification, we are able to simulate with moderate computing cost the whole process of micelle formation from an initially uniform dispersion of sodium dodecyl sulfate (SDS) in aqueous solution. The resulting micelle structures are different from those predicted by atomistic simulations that started with a predefined micelle configuration at the same surfactant concentrations. However, if we use their initial micelle configuration, micelle structures the same as theirs are obtained. Analyses show that our results are more realistic and that the results of the atomistic simulations suffer from artificial initial conditions. Therefore, our model may serve as a reasonable simplification of atomistic models in terms of the general structure of micelles.