Three pseudo-gemini surfactants(2C18N3N/XA,XA=SA,GA and AA)were formed by non-covalently connecting dibasic acid(HOOC(CH2)n COOH,n=2,3 and 4,abbreviated as SA,GA and AA,respectively)with long-chain tertiary amine(C17H35CONHCH2CH2CH2N(CH3)2)in the molar ratio of 1∶2 through electrostatic interactions.The whole process had avoided any complicated synthetic procedures.The surface activity and rheological properties of the surfactants were investigated.The critical micelle concentration(cmc)of 2C18N3N/GA was 4.60×10-5 mmol/L,which was significantly lower than that of conventional gemini surfactants,indicative of strong aggregation ability.2C18N3N/SA and 2C18N3N/AA also had low cmc values.These surfactants could form wormlike micelles when the concentrations were above 50 mmol/L,exhibiting viscoelastic behavior,and the viscoelastic solutions thereof had good temperature and shear resistance.Due to the tertiary amine group in the pseudo-gemini surfactant,it exhibited pH-responsive properties.It was found that when the pH was 6.1,the viscosity of the solution was very high,whereas when the pH was 4.0 or 9.3,the viscosity rapidly decreased.Furthermore,the pseudo-gemini surfactant was also responsive to temperature.With the increase of temperature,the viscosity of the system rapidly increased.At 60℃,the zero-shear viscosity of 2C18N3N/GA at 300 mmol/L could be as high as 11 967.73 Pa·s.These surfactants were simple to be prepared and showed excellent performance,which expanded the range of preparation and application of novel surfactants.

Abstract A Gemini-like surfactant is formed by connecting short chain dibasic acid and long chain tertiary amine in the molar ratio of 1:2 through the non-covalent action of electrostatic attraction, and does not need complicated synthesis. Due to the tertiary amine group in the Gemini-like surfactant, it exhibits pH-responsive properties. However, in the research, we were pleasantly surprised to find that the Gemini-like surfactants also showed temperature response characteristics. With the increase of temperature, the viscosity of the system increases rapidly, and it is resistant to high temperatures. At 60°C, the zero-shear viscosity of the system at the concertation of 300 mM can reach an extremely high value of 11967.73 Pa·s, which was higher than most of wormlike micelles formed by the conventional covalently linked oligomeric counterparts. The reason for this phenomenon may be that as the temperature increases, the hydrogen bonds in the system are destroyed, resulting in the reduction of the repulsive force between molecules and the shortening of the intermolecular distance, which makes the micelles more entangled closely.

Taking stearic acid as the raw material,N-(2-(dimethylamino)ethyl)stearamide(C18N2N)was synthesized.A new surfactant was prepared by mixing C18N2N and cinnamic acid(TA)with fixed ratio.This surfactant was formed by non-covalent electrostatic interactions,which avoided complicated synthetic procedures.The surface activity and rheological properties of the surfactant were investigated with a surface tensiometer and a rheometer.The critical micelle concentration(cmc)and the surface tension at cmc(γcmc)of the surfactant were 0.11 mmol/L and 32.4 mN/m,indicative of strong aggregation and adsorption ability.The maximum amount adsorbed(Γmax)and the minimum molecular area occupied(Amin)were 2.77 μmol/m2 and 0.6 nm2.Wormlike micelles were formed at the concentration of 25 mmol/L.When the concentration was over 100 mmol/L,the zero-shear viscosity(η0)abruptly increased and then reached the maximum value for the surfactant of C18N2N/TA containing 150 mmol/L TA.The solution viscosity was as high as 1 761.38 Pa·s.The effects of temperature on the rheological behavior were also considered.With the increase of temperature,the viscosity gradually increased.And when the temperature reached 40℃,η0 of the solution with the concentration of 100 mmol/L achieved the maximum value of 1 370.386 Pa·s.When the temperature continued to rise,η0 began to decrease with further increasing temperature.A remarkable viscosity of 305.55 Pa·s was still remained at 50℃.The viscoelastic solutions showed good temperature resistance and shear resistance.The surfactant solutions applied as the fracturing fluid were also investigated.This surfactant also showed excellent sand-carrying performance,and the settling rate of the sand was merely 0.26 cm/min.This surfactant could be simply prepared and showed excellent performance,which expanded the preparation and application field of novel surfactants.

A novel Bola surfactant FA–AA was formed via dynamic covalent bonding of AA and FA. In acidic pH the H + AA released co-stabilizes different emulsions with nanoparticles, but in alkaline pH FA–AA returns to the aqueous phase being recycled and re-used.

Stimulus-responsive surfactants (N+-C-n-N,n = 14 or 16) can stabilize conventional emulsions, Pickering emulsions and oil-in-dispersion emulsions in the presence of oppositely and similarly charged nanoparticles. Rapid demulsification can be successfully achieved by bubbling CO2. In the presence of CO2, N+-C-n-N becomes a hydrophilic Bola-type surfactant, N+-C-n-NH+, and is an inferior emulsifier either when used alone or together with charged nanoparticles, resulting in demulsification. In addition, nearly all N+-C-n-NH+ molecules are present in the aqueous phase after demulsification without contaminating the oil phase. The aqueous phase can be recycled and used again for further emulsification of oils. Compared with the pH responsive systems, there is no accumulation of salt after the gas cycles, and more cycles can be performed. This protocol is a green process and leads to preparation of various temporarily stable emulsions used in oil emulsion transportation, emulsion polymerization, industrial catalysis, nanomaterial synthesis and other fields. (C) 2022 Elsevier B.V. All rights reserved.