Molecular dynamics study of the influence of calcium ions on foam stability

Molecular dynamic simulations have been performed with three common surfactants [linear alkylbenzene sulfonate (LAS), sodium dodecyl sulfate (SDS), and heptaethylene glycol monododecyl ether (C(12)E(7))] to investigate the influence of calcium ions on foam stability. The foam stability was examined by simulating film rupture during which critical thickness was measured. The simulation results showed that as Ca(2+) concentration increased, SDS showed a significant decrease in foam stability, but LAS and C(12)E(7) nearly maintained foam performance and remained free from calcium ions. This was validated by our experimental results. Microscopic interaction configurations between Ca(2+) and surfactants were analyzed through the interaction distance distribution and the binding number distribution. We found there are mainly two interaction distances between the calcium ion and the oxygen atom of surfactants, the short distance ( approximately 0.23 nm) and the longer distance ( approximately 0.45 nm). Results showed that LAS has many more interaction configurations at the short distance and SDS has fewer configurations at the short distance. This indicates that the foam stability of the surfactant less influenced by calcium ions relates to enhanced interaction configurations at the short distance.