Necessity of Aromatic Carboxylate Anions To Be Planar To Induce Growth of Cationic Micelles

Abstract

It is well-known that some aromatic anions have the ability to induce viscoelastic transformation in aqueous solutions of cationic surfactants even at added salt concentrations as low as 10-20 mM. This behavior is associated with the formation of an entangled network of elongated micelles. However, the effect of aromatic ring substituents on the anion's ability to promote rapid micelle growth is not well-understood. We have performed ab initio calculations of the carbonyl group rotation barriers in a series of substituted benzoate and naphthoate anions at the MP2/STO-3G level of theory. It was found that aromatic carboxylates, known to be particularly effective in causing sphere-rod transition in cationic micelles, preferably adopt conformations with the COO(-) group in the same plane as the ring(s). This structural preference can be attributed to either intramolecular hydrogen bonding (o-hydroxyl derivatives) or pi-conjugation effects (m- and p-halogenated derivatives). In the former case the barrier to rotation is 40-50 kcal/mol, whereas in the latter case the threshold value is around 3.0 kcal/mol. Propensity for the planar conformation correlates with a greater depth of counterion penetration into the micelle surface, as inferred from NMR experiments, compared to the anions with less hindered carbonyl rotation. This points to favorable hydrophobic interactions between the surfactant methylene groups and the aromatic ring(s) of the anion as a possible explanation for the rapid growth of cationic micelles observed upon addition of certain aromatic carboxylates.

Shape Transitions in Supercritical CO2 Microemulsions Induced by Hydrotropes

“…This has built on previous work showing that hydrotropes can cause micellar growth in both aqueous systems 32,34 and in w/o microemulsions with AOT 33 . For water-in-oil microemulsions, w value and hydrotrope mole fraction were shown to affect the degree of elongation and enhanced anisotropy of the microemulsion nano-domains.…”

“…Another approach to form rod-like micelles in alkane solvents is using surfactant/hydrotrope mixtures [32][33][34][35] . Hydrotropes are small amphiphilic molecules which have hydrophilic character, and the ability to increase solubility of organic compounds in water [36][37][38] .…”

“…Hydrotropes are small amphiphilic molecules which have hydrophilic character, and the ability to increase solubility of organic compounds in water [36][37][38] . They have been shown to induce both aqueous micellar and w/o microemulsion growth 32,34 . Recently, Hatzopoulos et al found that when AOT was mixed with a series of simple carboxylic acid salt hydrotropes (Table 1b-e), depending on the structure of the additive and the amount of water present, either spherical or rod-like nanodroplet microemulsions could be formed 33 .…”

Effects of small ionic amphiphilic additives on reverse microemulsion morphology

“…The presence of a bulky hydrotrope is recognised as being critical for micellar elongation, with these systems shown to produce substantial increases in solution viscosities [10]. Close investigation by NMR and computer simulations revealed that the hydrotrope counterions adsorb at the micellar interfacial region [11]. In mixed solutions of surfactants and hydrotropes of the same charge only ellipsoidal structures have been observed [12].…”

Action of hydrotropes in water-in-CO2 microemulsions

“…AOT). 11 And even for those specifically designed surfactants that effectively stabilise 13 water-in-CO2 (w/c) microemulsions, modification of the self-assembled structures, which 14 has been well-established as an effective method to enhance the viscosity in alkane solvent, 15 is generally inhibited in scCO2. 16 In one of the very few reports on anisotropic self-assembled structures in scCO2, it was 17 found that by exchanging the surfactant counter ion Na + with divalent species such as Co 2+ 18 and Ni 2+ , a sphere-rod shape transition can be obtained for the micelles formed not only in 19 alkane solvents with hydrocarbon surfactants, 12 but also in scCO2.…”