Molecular orientation of monododecyl pentaethylene glycol at water/air and water/oil interfaces. A molecular dynamics computer simulation study

With a molecular dynamics computer simulation we investigated the dynamic properties of a monododecyl pentaethylene glycol (C₁₂E₅) molecule adsorbed at air/water and oil/water interfaces. In these simulations we investigated the molecular orientation of the surfactant molecules in detail. At the air/water interface the maximum of the C₁₂ chain tilt angle distribution measured with respect to the water surface is about 50°. This result is in fairly good agreement with neutron reflection experiments of monododecyl glycol ethers at the air/water interface. At the oil/water interface no significant changes were detected in the molecular orientation. We found that at equilibrium oil molecules penetrate into the hydrophobic monododecyl layer, this was also found by neutron reflection studies of the interactions between C₁₂E₅ and dodecane. The observed oil penetration results in an increase in the surface area per surfactant molecule. Received: 16 July 1999/Accepted in revised form: 28 August 1999