Calorimetric and Light Scattering Investigations of the Transition from Spherical to Wormlike Micelles of C₁₄TAB Triggered by Salicylate

Abstract

Although wormlike micelles (WLM) were first described more than 30 years ago, many aspects of their formation process are still unclear. Herein, a systematic experimental investigation of the process for wormlike micelle (WLM) formation in mixtures of tetradecyltrimethylammonium bromide (C14TAB) and salicylate (2-hydroxybenzoate) was carried out. This system was used as a model to investigate the conditions for the formation of the giant aggregate. For comparison, the other two isomers of salicylate (3- and 4-hydroxybenzoate) were also investigated, once in these cases wormlike micelles are not formed. The studies were based on calorimetric titration, static light scattering, and rheological measurements. Enthalpy changes upon titration of C14TAB into 2-hydroxybenzoate solutions revealed a highly cooperative and exothermic process that was associated with micelle growth. The size of the aggregates, obtained by static light scattering measurements, confirms the shape transition. In addition, the correlation of these two sets of results with measurements of micelle charge surface indicates that this transition occurs around the point of charge neutralization.

Micellization of alkyltrimethylammonium bromide surfactants in choline chloride:glycerol deep eutectic solvent

“…The unusual behaviour of SDS in such a solvent was suggested to arise from specific interactions in the headgroup region, which we do not see with the cationic surfactants in the present DES. Although a similar type of transition was found for alkyltrimethylammonium bromides and salts in aqueous solutions, 36,38,57,58 Table 4 Fitting parameters for SANS data of different concentrations of C 12 TAB in choline chloride:glycerol shown in Fig. 3a Conc./mM r eq,core /Å the ammonium group does not show affinity for the positively charged choline ion, meaning that such a mechanism for screening and curvature modification is not present in these cationic surfactant -DES systems.…”

“…[33][34][35] However the transition to elongated micelles can be achieved by specific ion interactions between salt in solution and the surfactant headgroup. [36][37][38] These surfactants were found to have a very low solubility in the choline chloride:urea DES used in our previous work. In the work presented here we have used choline chloride:glycerol DES (eutectic composition: 1 : 2 molar ratio; melting point = À40 1C) which represents one of the most studied DES and showed high solubility for these surfactants.…”

Counterion binding alters surfactant self-assembly in deep eutectic solvents

“…[56][57][58] Furthermore, the co-surfactancy of organic counterions with lyophobic moieties has been investigated, showing that not only does the charge screening produced by the condensation of the salts affect the micellization, but there is also a contribution from the counterion acting as a co-surfactant which changes the monomer packing by penetrating into the micelle core. 52,[59][60][61][62] Varying the native counterion of the surfactant changes the micellisation of the surfactant, forming shorter aggregates for lithium than for caesium. This fact is suggestive of caesium counterions showing a more significant condensation at the micelle surface (tightly bound), therefore more strongly modifying the headgroup effective area and allowing the formation of more elongated aggregates than in the case of lithium (strongly solvated).…”

Probing the Formation of Wormlike Micelles Formed by Cationic Surfactant with Chlorobenzoate Derivatives

“…For instance, in the case of the hydroxybenzoate isomers, if the OH group is at position 2 of the aromatic molecule, the micelles are promptly formed when combined with C 16 TAB. However, this is not observed for the isomer with the OH group at position 3 or 4 . Another interesting case is the combination of C 16 TAB with 3-phenylpropionate or cinnamate, in which the long aggregates are only formed when combined with the latter .…”