Molecular Assembly of Surfactant Mixtures in Oil-Swollen Micelles: Implications for High Salinity Colloidal Stability

Alkylbenzene sulfonates are one of the most important synthetic surfactant families, considering their wide applicability, cost-effectiveness, and overall consumption levels. Nevertheless, their low salt tolerance (especially divalent ion contents) prevented wider applications such as enhanced oil recovery in high salinity reservoirs. Here, using experiments and atomistic molecular dynamics simulations, we demonstrated that the high salinity colloidal stability of alkylbenzene sulfonates can be dramatically increased by mixing with zwitterionic co-surfactants in oil-swollen micelles. By mixing with co-surfactants we had two important observations: (1) The polydispersity of surfactant mixture oil-swollen micelles were largely decreased due to the less rigid oil/water interfaces with mixed surfactants, which formed fewer but larger uniform micelles. (2) Strong dehydration of sulfonates due to the shielding from protruding more extended zwitterionic co-surfactants at the oil/water interfaces. Both observed molecular assembly characteristics triggered by the co-surfactants effectively reduced the total exposure of sulfonates to the water phase which may form divalent ion bridging and large aggregates, thus increasing the high salinity colloidal stability.