Effect of Ca2+/Mg2+ on the stability of the foam system stabilized by an anionic surfactant: A molecular dynamics study

• The foam stability of anionic–nonionic surfactant (A12E2SO3) was studied by molecular dynamics simulations.
• Two forms of hydrogen bond bridging structures (O1⋯H2O⋯O2 and O2⋯H2O⋯Os) were observed for the first time in anionic surfactants.
• The favorable structure of A12E2SO3 monolayer for the increased stability was obtained in the foam system with the addition of Mg2+.

Molecular dynamics simulations were carried out to investigate the influence of cations (Ca2+/Mg2+) on the stability of the foam system stabilized by sodium dodecyl polyoxyethylene sulfonate (A12E2SO3). First, the structure of A12E2SO3 monolayer at the air/water interface was studied through analyzing the distribution of A12E2SO3 headgroups and the orientation of surfactant molecules. Results show that the addition of Mg2+ can increase the distance between A12E2SO3 headgroups, broaden the distribution thickness of A12E2SO3 headgroups along z axis, and improve the orientation of A12E2SO3 molecules, which favored the foam stability. On the contrary, the addition of Ca2+ in the foam system made surfactant molecules more aggregated, which was detrimental to the foam stability. Second, the interaction of surfactant molecules and cations was also studied through calculating their binding/dissociation energy and mobility. Results reveal that the binding tendency of the head-Ca2+ ion-pair was stronger than that of the head-Mg2+ ion-pair, which disturbed the foam stability. Third, we studied the hydration of surfactant headgroups and cations through calculating the number and the mobility of hydrated water molecules around headgroups. Results show that due to the extra hydration of cations in the foam system, especially for that with the addition of Mg2+, the number of water molecules around headgroups was increased and their mobility was restricted, which was beneficial to the foam stability. In addition, two forms of hydrogen bond bridging structures (O1⋯H2O⋯O2 and O2⋯H2O⋯Os) were observed for the first time in anionic surfactants. The location and the formation probability of these structures were found to be related to the gauche conformation of A12E2SO3 headgroup backbones. It was also found that hydrogen bond bridging structures can affect the foam stability.

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