Unusual Increasing Viscoelasticity of Wormlike Micelles Composed of Imidazolium Gemini Surfactants with Temperature

The viscoelasticity of wormlike micelles composed of ionic surfactants typically shows an exponential decrease with increasing temperature, which limits their application in relatively high-temperature (>90.0 degrees C) oilfields and the synthesis of functional materials as supramolecular templates at high temper-atures. In this work, a series of imidazolium gemini surfactants, 1,9-(ethane-1,2-diyl)bis(3-alkyl-1H-imidazol-3-ium) bromide ([Cn-2-Cnim]Br2, n = 12, 14, 16, 18, 20), were synthesized. Their surface activities and aggregation behaviors in water were studied by electrical conductivity, rheology, polarization optical microscopy, small -angle X-ray scattering, zeta potential, and hydrogen nuclear magnetic resonance measurements. [C12-2-C12im]Br2 and [C14-2-C14im]Br2 mainly precipitate in water. [Cn-2-Cnim]Br2 (n = 16, 18, 20) forms lamellar liquid crystals over a large range of concentrations at low temperatures. With the increase of temperature, the lamellar liquid crystals transit to wormlike micelles. Interestingly, the viscoelasticity of the three wormlike micelles first increases to the maximum and then decreases with increasing temperature. These wormlike micelles without additives retain high viscoelasticity up to 90.0 degrees C or above. With the increase of the alkyl chain length of the surfactants, the transition temperature of lamellar liquid crystal to wormlike micelles and the disintegration temperature of wormlike micelles increase. The unusual increase of the viscoelasticity of wormlike micelles was due to the desorption of weakly bound counterions and the extension of the long hydrophobic chains of surfactants at high temperatures.

Automated summary

A series of imidazolium gemini surfactants were synthesized and their surface activities and aggregation behaviors in water were investigated. These surfactants formed lamellar liquid crystals at low temperatures and transitioned into wormlike micelles as the temperature increased. The viscoelasticity of the wormlike micelles showed an unusual increase and decrease with temperature, retaining high viscoelasticity up to 90.0 degrees C or above.