Emulsified oil phase induced internal instability of ionic and nonionic foams revealed by coarse-grained molecular dynamics simulation

Oil phase has strong hydrophobicity and weak interface tension in the foam system, which will destroy the internal structure of foam and reduce the intrinsic stability of the foam film. Coarse-grained molecular simulation techniques were employed to investigate the effect of oil phases on the structural instabilities of two typical foam systems, i.e., ionic sodium dodecyl sulfate (SDS) and nonionic pentaethylene glycol monododecyl ether (C12E5). In both systems, the pseudo-emulsion film ruptures at a critical thickness, subsequently the oil phase enters the air-water interface subsequently. Instability mechanisms are obtained by analyzing dynamic processes of the foam systems. For the SDS foam system, an oil-bridge is formed in the foam film, which is attributed to the electrostatic repulsion between the SDS at the oil-water interface and that at the air-water interface, as well as the discontinuous distribution of SDS at the air-water interface caused by the structural reversal of SDS molecules. However, for the C12E5 foam system, oil phase spreads at the air-water interfaces, which is caused by the continuous distribution of the C12E5 molecules at the air-water interface and the attractive interaction between the neutral C12E5 molecules at the air-water interface and that at the oil-water interface. The outcomes of this work shed light on the destructive mechanism of foam films induced by oil phase and provide guidelines for the design of novel oil-resistant foam system.