Numerical analysis of nonionic surfactant monolayers at water/air interfaces

The time evolution of a morrododecyl pentaethylene glycol monolayer at the water-air interface is investigated using velocity resealed NVT molecular dynamics. The model we consider consists of 40 surfactant molecules and 2350 water molecules enclosed in a periodic box. The time evolution of this system is ruled by the CFF91 force field that includes intra- and intermolecular degrees of freedom for both surfactant and water molecules. The interface we consider herein is hence described in all atomic detail. We discuss the initial condition problem and study the relaxation properties to stationarity. Transient regimes of self-assembly of the surfactant chains in entangled structures in both air and water are described. From the stationary configuration, we define the interface location, determine the mass distribution across the interface, and discuss the validity of the tilt angles notion when structural roughness is considered. Using pair distribution functions, we show that, besides typical tilt angles, monododecyl pentaethylene glycol molecules might also develop domains on the interface that suggest an in-plane orientational order.