Hypothesis

When tetradecyltrimethylammonium bromide, TTAB, is added to aqueous solution of sodium salicylate, NaSal, the threading of the aromatic anion into the micellar palisade leads to the formation of wormlike micelles. Based on the calorimetric titration of NaSal with TTAB, and on the lifetime of fluorescence of salicylate, we propose that the aggregation of the two components directly leads to the formation of wormlike micelles, without any pre-aggregation.

Experiments

By using an isothermal titration calorimeter, aliquots of TTAB were added to a dilute solution of NaSal. The energy involved in each addition was then integrated and the variation of enthalpy was determined. In the same range of concentrations and molar ratios, the surface tensiometry and time-resolved emission spectroscopy experiments were performed.

Findings

A very characteristic calorimetric signal associated with wormlike micelle formation was obtained, being the enthalpy variation of this process, ${\mathrm{\Delta }}_{\mathrm{W}\mathrm{L}\mathrm{M}}{\mathrm{H}}_{298}^0$ < 0. When 1.2 mmol L⁻¹ of NaSal is titrated with 11.0 mmol L⁻¹ of TTAB at 298.15 K, ${\mathrm{\Delta }}_{\mathrm{f}}{\mathrm{H}}_{298}^0$ = −10.31 kJ per mol of injectant. By adding TTAB to NaSal solution, two fluorescence lifetimes of salicylate were observed solely after wormlike micelle being formed. The correspondent lifetime values of 4.0 ns and 7.2 ns are respectively associated with the free and associated species of salicylate. The new results demonstrated that wormlike micelles are the first aggregate formed when TTAB is added to salicylate. This aspect is relevant for understanding the mechanism of wormlike micelles formation.