Orientation and structure of monododecyl pentaethylene glycol adsorbed at the air/water interface studied by molecular dynamics computer simulation

With a molecular dynamics computer simulation we have studied in detail the dynamic structural properties of a monododecyl pentaethylene glycol (C12E5) Langmuir monolayer on the water surface. We calculated the average orientation angles of the alkyl and the glycol ether chains in respect to the surface normal. For dodecyl chains we obtained an average value of about 47 degrees whereas the glycol chains are oriented nearly perpendicular to the water surface with a tilt angle of about 11 degrees. In order to compare the results with neutron reflection experiments we calculated the thickness of the alkyl layer. The average orientation angle and layer thickness of the dodecyl chains agree very well with experimental data. An analysis of the conformation distributions in the alkyl,I and glycol chains reveals that the terminal C-12 and E-5 chain sections are more flexible than the other chain parts. Measurements of tilt angles based on monolayer thickness can be influenced by this effect.