Effects of additional salts on the interfacial tension of crude oil/zwitterionic gemini surfactant solutions

Abstract

Zwitterionic gemini surfactants, which have the advantages of both zwitterionic and gemini surfactants, have been widely used in various disciplines. Sulfobetaine-type zwitterionic gemini surfactants consisting of 1,2-bis[N-methyl-N-(3-sulfopropyl)-alkylammonium]ethane (2C_nSb with 6, 8 and 10 carbon atoms) were evaluated for their interfacial activities at the water/crude oil interface. The 2C₁₀Sb molecules showed a remarkable ability to decrease the interface tension (IFT) of water/crude oil, and the degree of decrease was much greater than those in either zwitterionic or gemini surfactant systems by at least two orders of magnitude. Furthermore, the effects of salts (NaCl, CaCl₂, and MgCl₂) on the IFT of the 2C₁₀Sb system were thoroughly investigated. Interestingly, the delicate balance between the effects of additional cations and the intramolecular interactions of 2C₁₀Sb molecules played crucial roles in the interfacial arrangements of 2C₁₀Sb molecules, which were mainly dependent on the bonding abilities of the cations. Moreover, a zwitterionic surfactant and a cationic gemini surfactant were employed in control experiments to verify the proposed mechanisms.

Graphical Abstract

Keywords:

• Zwitterionic gemini surfactant
• salt
• electrostatic attraction
• interface tension
• water/crude oil interface

Acknowledgments

The authors are grateful for funding from the National Natural Science Foundation of China (No. 21403301), the Basic Fundamental Research Funds of Qingdao (16-5-1-41-jch), the Fundamental Research Funds for the Central Universities (16CX02058A), the foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education (201402) and National Training Program of Innovation and Entrepreneurship for Undergraduates (No. 201810425013).

Disclosure statement

No potential conflict of interest was reported by the authors.