Foam generation and stability: role of the surfactant structure and asphaltene aggregates

Understanding how the surfactant molecular structure affects foam stability is important in various applications, such as soil remediation, the food industry, enhanced oil recovery, and hydraulic fracturing. In this study, we conduct a systematic series of experiments in a Hele-Shaw cell to assess and explain the effect of surfactants on foamability and foam stability in the absence and presence of oil and asphaltene aggregates. Four surfactants with different molecular weights were studied, including three anionic surfactants, sodium 1,5-bis[(1H,1H,2H,2H-perfluorohexyl)oxy]-1,5-dioxopentane-2-sulfonate (FG4), C18H37SO4Na (LSES), and sodium dodecyl sulfate (SDS), and one cationic surfactant (CTAB, (C16H33NMe3)Br). We observed that the higher electrostatic repulsion between the foam film surfaces for the anionic surfactants (FG4 > LSES > SDS) strongly influences the process of foam generation and stability when compared with the cationic surfactant (CTAB). For example, the foam coarsening rate for CTAB was 700% higher than that for FG4 when we performed experiments in an empty Hele-Shaw cell. Furthermore, the foam experiments performed in the presence of oil revealed maximum long-term stability and minimum bubble coarsening for the partially fluorinated FG4 surfactant. The experiments in the presence of asphaltene-oil mixtures revealed that the latter has a detrimental effect on foam stability, except for FG4. The foam experiments clearly show the significance of the subtle interactions between the surfactant headgroup charge, chain length, and branching. The obtained results could be useful in designing appropriate surfactants for foam stabilization in various applications.