Molecular dynamics computer simulations of surfactant monolayers: Monododecyl pentaethylene glycol at the surface between air and water

In this publication we used molecular dynamics computer simulations in order to get a more detailed insight into the dynamic properties of monododecyl pentaethylene glycol (C12E5) monolayers at the water surface. We used a simulation box enclosing 36 surfactant molecules and the surfactant concentration was kept constant at an average value of 0.55 nm(2)/molecule. After reaching the thermodynamic equilibrium, it turned out that the dodecyl chains were approximately oriented diagonal to the surface with an average value of about 43 degrees measured in respect to the surface normal vector. This result is in fairly good agreement with a previously performed simulation containing only 25 surfactant molecules. The average orientation angle of the hydrophobic chains agrees very well with experimental data. In contrast to the unpolar part of the molecules the water-soluble glycol chains were aligned more perpendicular to the water surface with a tilt angle of only 11 degrees. It is interesting to note that these chains are rather stiff. Within the time scale of our simulation the surfactant molecules were firmly attached at the surface and did not enter into the water phase. All alkyl and glycol chain ends, however, were characterized by a striking flexibility. The calculation of pair distribution functions and detailed pattern analysis of the oxygen atoms within the glycol chains revealed a pentagonal arrangement in the highly ordered internal segments of the Eg chain.