Enthalpy measurements for the formation of salt-induced wormlike micelles using isothermal titration microcalorimetry

A simple yet novel method, isothermal titration microcalorimetry (ITC), was employed to determine the enthalpy change of salt-induced wormlike micelle formation. The observed changes in molar enthalpy of 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), sodium tosylate (NaTos) and sodium salicylate (NaSal) in aqueous solutions have been measured. The transformation enthalpy change from spherical micelle to wormlike micelle (ΔHtra) has been obtained from the simulation method based on the microcalorimetric data. Then the enthalpy change of wormlike micelle formation (ΔHw) was obtained using a suitable cycle which was closed by the enthalpy change of micelle formation (ΔHm) and the transformation enthalpy change (ΔHtra). The value of ΔHw indicated that the wormlike micelle formation in C16mimBr/NaTos system was exothermic while it was endothermic in C16mimBr/NaSal system. The planar structure of the salicylate anion and the π–π interaction between the salicylate anion and the aromatic headgroup of C16mimBr may attribute to this difference. Moreover, the technique presented here to obtain ΔHw could be extended to other salt-induced wormlike micelle systems.