Micromechanism of impact of hydrocarbon gases on stability of foam films

Key words:  film drainage  ripening  molecular diffusion  aqueous foam  molecular dynamics

Abstract:

The sandwich simulation system of gas phase-surfactant/polymer phase-aqueous phase containing inorganic salt ions was established by the molecular dynamics method. The micromechanism of the impact of hydrocarbon gases on the film drainage and Ostwald ripening of aqueous foam was revealed. The results indicate that the film stability of foams produced by hydrocarbon gases is lower than that of N₂. With the increase of carbon number of hydrocarbon gas molecules, the surface tension increases, the absolute value of interface formation energy decreases, and the film drainage capability strengthens. The diffusion coefficient of water molecules in the first hydration layer is increased from 1.73×10-5 cm²/s in the CH₄ foam system to 2.40×10-5 cm²/s in the C₃H₈ foam system. The thickness of interfacial water layer and the coordination number of water molecules decrease from 10.93  and 2.11 in the CH₄ foam system to 7.72  and 1.96 in the C₃H₈ foam system, respectively. Moreover, the Ostwald ripening activated by gas molecule aggregation at the interface is critical to the foam film 's stability. Furthermore, the hydrocarbon gas molecules with higher carbon numbers are more likely to penetrate the produced foam film and result in rupture of foams, which reflects that the adsorption configuration of surfactant molecules directly affects the Ostwald ripening.