Molecular Dynamics Simulations of Film Rupture in Water/Surfactant Systems

Molecular dynamics simulation has been performed on water/surfactant film rupture in order to investigate foam stability. A periodic boundary film model which was simulated in a lateral dimension of 8 x 8 nm(2) for 4 ns was established to stand for a part of a foam bubble. On the basis of critical film thickness, which is the lowest thickness before film rupture, a stability index was calculated to describe the capabilities of surfactants to stabilize water films. We investigated the influence of film size and simulation duration on the critical thickness and proved that our model is reasonable. The stability index versus surfactant concentration curve suggests that the capabilities of three surfactants-linear alkylbenzene sulfonate (LAS), sodium dodecyl sulfate (SDS), and heptaethylene glycol monododecyl ether (C12E7)-in stabilization of water film decrease in the order of SDS > LAS > C12E7. In the present study, the simulated results have been validated by the foam generation and decay experiment results, thus indicating that this method of predicting the stability of water/surfactant film is feasible.