Isothermal titration calorimetry (ITC) is a technique based on the controlled titration of a solution into a cell, and it can be used to monitor self-assembly processes. Previously, the titration of tetradecyltrimethylammonium bromide (TTAB) in sodium salicylate (NaSal) was studied, and the resulting enthalpogram was separated into three different parts, corresponding to different processes. These three parts were associated with the formation of mixed micelles, followed by the fusion of these micelles into wormlike micelles, at a specific [TTAB]/[NaSal] ratio, and then the decrease in length of such type of aggregates as the concentration of TTAB increased. Herein, we present a comparative study that was carried out with solutions containing TTAB and NaSal in different molar ratios in order to investigate the structural variations throughout the enthalpogram. The techniques used in the investigations were ITC, time-resolved emission spectroscopy and surface tensiometry. It is possible to observe considerable variation in the properties studied at a specific surfactant concentration of 0.4 mmol L-1, which corresponds to a molar ratio [TTAB/NaSal] of 0.35, lower than in previous studies. We propose that the formation and growth of wormlike micelles occurs by a very cooperative process in this concentration range, lower than what was previously proposed, without a preliminary mixed micelle state, once no evidence of mixed micelles was seen. In addition, we propose a simple model which allow us to estimate the proportion of the species TTAB and salicylate that are present in the structure of the aggregate based on the preexponential factors obtained through spectroscopy. Moreover, thermodynamic parameters were investigated using the ITC experiments in different temperatures to study the spontaneous formation and growth of wormlike micelles.

New finds about the mechanism of wormlike micelle formation involving a cationic surfactant and salicylate

2019 Fall National Meeting

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