Effect of crude oil fractions on the interfacial tensions of alkali-betaine mixed solutions

•Crude oil fractions show synergistic effect with both two betaines.•The adding of alkali will improve the synergistic effect.•The betaine with smaller hydrophilic part shows synergistic effect with fractions.•The betaine with larger hydrophilic part shows synergistic effects with both fractions and soaps.•The size dependence of synergistic effect becomes unimportant for crude oil.

The effects of alkalis on the dynamic interfacial tensions (IFTs) of two betaine solutions with different hydrophilic parts, erucyl dimethyl amidopropyl carboxybetaine (EDAC) and erucyl dimethyl amidopropyl sulfobetaine (EDAS), against crude oil and model oils containing crude oil fractions (such as saturate, aromatic, resin, asphaltene and acidic fractions) have been investigated by spinning drop tensiometer. The experimental results show that the IFTs of EDAC solutions against decane are slightly lower than those of EDAS solutions because the hydrophilic part of EDAS is larger than that of EDAC. The crude oil fractions can directly adsorb at the interface to form mixed film with betaine molecules and drastically reduce IFT values. This synergistic effect becomes better with the enhancement of surface activity of fractions. The adding of alkali will improve the synergistic effects between betaines and fractions in general by enhancing the adsorption of fractions and producing soaps. EDAS molecules have synergistic effects with both fractions and soaps, while EDAC molecules show synergistic effect mainly with fractions, because of the difference of hydrophilic parts. Therefore, after adding different alkalis, the IFTs of EDAS solutions are obviously lower than those of EDAC solutions for almost all model oils. The dependence of size-compatibility becomes unimportant for the synergistic effect between betaines and crude oil in reducing IFT and pH value plays the most crucial role in controlling synergistic effect. Ultralow IFT values can be reached for both two betaines during a wide range of alkali concentrations.