Anisotropic Rotation of Salicylate Anions in Threadlike Micelles

Abstract

Microscopic dynamics of salicylate (Sal -) anions in long rodlike or threadlike micelles formed by cetyltrimethylammonium bromide (CTAB) and sodium salicylate in aqueous solution was examined by use of magnetic nuclear resonance (NMR) spin-lattice relaxation time (T1) measurement. Molecular motions of Salanions are remarkably restricted in the threadlike micelle. The rotational correlation times for the Salanion estimated from the T1, by taking into account only intramolecular dipole-dipole interaction, are in good agreement with those previously estimated from fluorescence anisotropy. Rotational motions of free Salanions in aqueous solution are slightly anisotropic presumably because their molecular shape is not spherical. Within the threadlike micelle, however, tumbling motions of the Salanion are highly restricted due to the fact that the Salanion is electrostatically restrained by CAT + cations. Molecular motions of the CTA + cation in the micelle are also highly restricted. The rotational correlation time for the CTA + cation in the micelle is not affected by the concentration of the free Salanion, although its concentration has a strong effect on rheological behavior. Thus, the microscopic motions and some related physical parameters, such as the persistence length of the micelle, are essentially independent of the concentration of the free Salanion once the threadlike micelle is formed.

Comparison of oleyl and elaidyl isomer surfactant–counterion systems in drag reduction, rheological properties and nanostructure

“…Thus, the low shear rate shear viscosities of the elaidyl solutions at n = 1.5 are much larger than those of the n = 1.5 oleyl solution. With further increase of counterion/surfactant concentration ratio, the oleyl (n = 2.5) solution becomes insoluble, and the shear viscosities of the branched elaidyl (n = 2.5) solution are low at high shear rates because the junctions provide a means to relieve stress [34,41]. Comparing Fig.…”

“…The formation of branches also reduces viscosity [25,32]. The decrease in viscosity associated with branched micelles occurs because the cross [36] in the multi-connected networks can slide along the micelles, serving as stress release points [34,41].…”

Action of hydrotropes and alkyl-hydrotropes

“…The repulsion decreased with increasing hydrotrope concentration, which was followed by an increase in the aggregation number. Shikata and Morishima 40 employed NMR and fluorescence spectroscopy to determine the relaxations of the molecular motions of sodium salicylate in aqueous solutions and in CTAB mixtures, concluding the salicylate ion adsorbed onto the micelle.…”

Structural and dielectric behaviour of non-aqueous lyotropic mixtures: influence of amphiphile chain lengths and counter ions

“…The total charge and displacement of the counter ions in the solvent pool resulted in the net dipole moment of the system. [27] Considering this Liquid Crystals 1093 mechanism, one can realise that −Na + − SO 4 − and CTA + −Br − form ionic pair in M S: EG and M C: EG , respectively. The tangential and rotational motion of the Na + and Br − in the solvent pool gives rise to the net dipole moment or the polarisation with the application of electric field.…”