Structural properties and dynamics of C12E5 molecules adsorbed at water/air interfaces: A molecular dynamic study

We investigate structural and dynamical properties of a pentaethylene glycol monododecyl ether monolayer (C12E5) adsorbed at water/air interface for varying surface densities. The model we consider relies on an atomic description and consists of a water sample of few thousands of molecules on the top of which surfactant molecules are adsorbed with a molecular surface area A ranging between 0.64 nm2 and 0.51 nm2. The time evolution of this model is ruled by the CFF91 force field that includes intra- and inter-molecular degrees of freedom for both surfactant and water molecules. The interface model we consider herein is hence described in all atomic details. The variation of surface area A presents the interest to allow the description of the influence of the molar area on the mass density profile of the studied monolayers. The surfactant molecule tilt angles to the interface are also studied and show a weak A-dependence. The narrowness of the mass density distributions and the large values of the tilt angles observed in the simulations indicate a strongly entangled behaviour of the surfactant molecules in the monolayers. Transport properties are also studied and the diffusion coefficient of the surfactant molecules is estimated.