Concentration and temperature induced dual-responsive wormlike micelle to hydrogel transition in ionic liquid-type surfactant [C16imC9]Br aqueous solution without additives

Abstract

A highly viscoelastic fluid formed by the ionic liquid-type surfactant 1-hexadecyl-3-nonyl imidazolium bromide ([C₁₆imC₉]Br) in water in the absence of any additive was studied. The phase behavior and morphology of aggregates were studied by a combination of rheological techniques, small-angle X-ray scattering (SAXS), cryo-etch-scanning electron microscopy (cryo-etch-SEM) and freeze-fractured transmission electron microscopy (FF-TEM). [C₁₆imC₉]Br aqueous solutions showed interesting rheological behavior as a function of both concentration and temperature, which invoked a transition between wormlike micelles and hydrogels. With the increase in [C₁₆imC₉]Br concentration, the aqueous solution could form viscoelastic wormlike micelles (50–80 mM), hydrogels (90–110 mM) and wormlike micelles (120–180 mM). As the temperature increased, the hydrogels (90–110 mM) could also transit to wormlike micelles. The unusual phase transition between wormlike micelles and elastic hydrogels was postulated to be the change of the average micellar length.