MolecularDynamics Simulations of Sodium Dodecyl SulfateMicelles in WaterThe Effect of the Force Field.

Moleculardynamic (MD) simulations of preassembled sodium dodecylsulfate (SDS) micelles are carried out using three versions of GROMOS,as well as CHARMM36, OPLS-AA, and OPLS-UA force fields at differentaggregation numbers and box sizes. The differences among force fieldshave little effect on the overall micelle structure of small aggregatesof size 60 or 100, but for micelles of an aggregation number of 300or higher, bicelle structures with ordered tails, rather than themore realistic rodlike or cylindrical micelles with disordered tails,occur when using versions of GROMOS45A3 or the OPLS-AA force fieldsthat are adapted to model the sulfate head group atoms using methodsgiven in the literature. We find that the Lennard-Jones (L-J) parametersfor the sodium ions and the ionic oxygens of the SDS head group, aswell as the water model, control the transition to bicelles, regardlessof other L-J parameters. A closer binding of the sodium ions to thehead group ionic oxygens screens the electrostatic repulsions morestrongly, resulting in condensation of SDS head groups, leading tounphysical bicelles for GROMOS45A3 or the OPLS-AA force fields, whenthe aggregation number is large. A telltale sign that the sodium–oxygeninteraction is too strong shows up in high nearest neighbor peaks(height >8 and height >20 for micelles with 60 and 100 surfactants,respectively) in the radial distribution functions (RDFs) of sodiumions to ionic oxygens. In the 100-surfactant micelles, the high RDFpeak is accompanied by “crystal-like” layering of sodiumions onto the surface of the micelle. The distance between the sodiumions and micelle also depends on the number of waters binding to sodiumions in the presence of surfactant head groups, which depends on boththe sodium ion and water models, and for the same sodium model increasesas the water model is changed in the order: TIP4P, SPC/E, SPC, andTIP3P. [ABSTRACT FROM AUTHOR]