Tilt transitions in Langmuir monolayers of long-chain molecules

Molecular dynamics simulations have been used to investigate tilt transitions in a monolayer model of amphiphilic molecules at an air–water interface. Eight simulations were performed at 300 K on monolayers in the density range of 18.5–25 Å2/molecule. The model amphiphilic molecules contained 19 pseudoatoms, each representing a methyl or a methylene group, and a head group representing a carboxylate group. Amphiphile–amphiphile interactions were modeled using a new anisotropic united atom model that accounts implicitly for the presence of hydrogen atoms in alkanes; water–amphiphile interactions were modeled using two external potentials that do not constrain the head groups to the interface, allow methylene segments to enter the water, and provide a finite size interface of the same order of magnitude as the size of the experimental water–air interface. The tilt behavior of the monolayer was monitored as a function of molecular area. Tilt angle results and structure factor analysis point to the occurrence of a transition between 20 and 21 Å2/molecule from an almost upright to a tilted monolayer. At 21 and 22 Å2/molecule, the monolayers do not have a well-defined tilt order. Upon monolayer expansion to 23 Å2/molecule, the molecules become tilted over their nearest neighbors.