Molecular dynamics study of the foam stability of a mixed surfactant/water system with and without calcium ions

The foam stability performance of a mixture surfactant system with and without calcium ions, including linear alkylbenzenesulfonate (LAS) and sodium dodecyl sulfate (SDS), has been studied by molecular dynamics. Microscopic interaction analysis reveals that the average binding number (ABN) indicates the population of interaction configurations at short distance (∼0.23 nm), ABN = 0.88 for the mixture system that is larger than that of SDS (0.08) and smaller than that of LAS (1.00). ABN of the mixture 0.88 in between is consistent with experimental observations; the performance of the mixture becomes worse than that of LAS and better than that of SDS on the foam stability influenced by calcium ions. In the present anionic system, the fraction of free calcium ions, X(f), in the film system was obtained. If X(f) is in the high calcium ion zone (X(f) > 0.5), the foam stability performance appears to be considerably impacted by calcium ions. In the present study, we validated the variable of tail mass out of water film, W, is an indicator of foam stability in a system without Ca(2+). LAS+SDS mixture system has lower W value than the LAS single system, corresponding to its experimental observation on more stable foam film. Results show that a maximum of W value occurs before reaching the critical thickness for each investigated system. This was probably due to structural transition of the surfactant film system.