MOLECULAR-DYNAMICS INVESTIGATION OF A NEWTON BLACK FILM

Soap films consisting of amphiphilic molecules and water have been investigated by molecular dynamics simulations. The model systems studied contained a specified number of water molecules sandwiched between two monolayers of sodium dodecyl sulfate (SDS) molecules whose alkyl chains were confined to a fixed in-plane density through the imposition of periodic boundary conditions. The constituents interacted via potentials taken from a previous study of an aqueous SDS micelle. The calculated overall width and the variation of the density profile across the film are consistent with deductions based on recent X-ray reflectivity data. Equilibration from an ordered initial (far from equilibrium) structure involves extensive conformational changes in the alkyl chains, molecular displacements, and rearrangement of the water molecules around and behind the -O-SO3 headgroups. Films of different thickness (i.e. differing degrees of hydration) were stable over the time scale of our simulations which spanned hundreds of picoseconds. On the basis of energetic considerations, and comparison to experiment, the Newton black film studied in the X-ray work likely contained from four to six water molecules per surfactant.