Structure-activity relationship of anionic-nonionic surfactant for reducing interfacial tension of crude oil

In this article, we have investigated interfacial tensions (IFTs) of anionic-nonionic surfactant fatty alcohol polyoxyethanol carboxylate (CnEOmC) and their mixed solutions against n-alkanes, crude oil and model oils containing crude oil fractions, such as saturates, aromatics, resins and asphaltenes. The experimental results show that EO group plays a crucial role in reducing IFT by adjusting the area occupied for CnEOmC molecules at the interface and only surfactant with optimum EO number produces lower IFTs against n-decane, named size compatibility.The hydrophilic-lipophilic balance is another important factor for reducing IFT. Ultralow IFT value can only be achieved when both hydrophilic-lipophilic balance and size compatibility are met simultaneously. Fortunately, one can adjust the hydrophilic-lipophilic balance and keep size compatibility simultaneously by mixing CnEOmC with different EO numbers to produce ultralow IFTs for both hydrocarbons and crude oil. Moreover, the spaces with different sizes occur at the mixed adsorption film. As a result, we can observe synergism between model oils containing resins, aromatics or asphaltenes during wide concentrations and mixed solution of C14EO3C and C14EO7C with mass fraction ratio of 1:1. This may be another reason for the ultralow IFT values of mixed solutions against crude oil. The mechanism provided in this paper is very important for the design of chemical flooding at high salt reservoir conditions. (C) 2020 Published by Elsevier B.V.