Molecular dynamics simulations of ammonium surfactant monolayers at the heptane/water interface.

Molecular dynamics simulations have been performed on the monolayers of dodecyltrimethylammonium bromide and gemini surfactants 12-S-12 with S=3, 6, and 12 at the n-heptane/water interfaces. The normal density profiles of the interface show that the distributions of surfactants at the liquid/liquid interface are significantly broader than those at air/water interfaces from comparisons with neutron reflection experiments and previous simulations. The spacers of 12-3-12 and 12-6-12 do not migrate much from the interface, while that of 12-12-12 tends to bend into the oil phase. The conformation of the surfactants shows that the spacers are more flexible than the tails. The characteristic angles of the surfactant well depict the geometry of the surfactants at the interface. The connected N+s of 12-3-12 and 12-6-12 have a prominent peak in the radial distribution functions, while those of 12-12-12 have nearly the same peak with those not connected. It is also found by three-dimensional spatial distribution functions that water molecules and bromide ions prefer to be shared between the positively charged methyl or methylene groups.