Effect of bivalent cations on the interfacial tensions of extended anionic surfactant solutions

Extended surfactants have shown great potential in enhanced oil recovery (EOR). In this paper, the effects of extended surfactant concentration, polypropylene oxide (PO) number, and various counter ions (Na+, Ca2+, and Mg2+) on the oil-water interfacial tension (IFT) of extended surfactant solutions were investigated in detail. The selected extended surfactants were sodium dodecyl polyoxypropylene sulfate (L-C12POmS, m = 5, 10, 15). Based on our experimental results, under NaCl conditions, with an increase in the PO number, the space occupied by the surfactant molecules on the side of the oil phase increases, which matches the size of the anionic head, and the surfactant film becomes tight. An ultralow IFT value appears for L-C12PO15S at 0.5% NaCl against decane. The addition of bivalent cations can significantly reduce the IFT. As the Ca2+ concentration increases, IFTs of APS solutions against alkanes can achieve ultralow values at lower PO numbers and against alkanes with a wide ACN range. For the MgCl2 system, the smaller hydration radius of Mg2+ improves the tightness of the anionic head, resulting in a crowded arrangement of the PO chains on the oil phase side. Therefore, the ability of Mg2+ to reduce the IFT against alkanes is weaker than that of Ca2+. Moreover, the surface-active substances in the crude oil adsorb competitively on the interface, resulting in an increase in IFT. By adding Ca2+ or Mg2+, a tight adsorption film of surfactant molecules will be formed, and the IFT value reaches ultralow against crude oil. The studies in this paper have important implications for the application of extended surfactant solutions to enhance oil recovery under high salt reservoir conditions. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This paper investigates the effects of extended surfactant concentration, polypropylene oxide (PO) number, and various counter ions (Na+, Ca2+, and Mg2+) on the oil-water interfacial tension (IFT) of extended surfactant solutions. The results show that as the PO number increases, the space occupied by surfactant molecules on the oil phase side increases, resulting in a tighter surfactant film. Bivalent cations can significantly reduce the IFT, with calcium ions achieving ultralow values at lower PO numbers and a wide range of alkane concentrations. The ability of magnesium ions to reduce the IFT is weaker than that of calcium ions. Additionally, surface-active substances in crude oil competitively adsorb on the interface, leading to an increase in IFT. The addition of calcium or magnesium ions forms a tight adsorption film of surfactant molecules, resulting in an ultralow IFT against crude oil.