Full length articleEffect of organic and a Hofmeister series of inorganic counterions on the solvation dynamics and rotational relaxation in aqueous micelles of hexadecyltrimethylammonium surfactants

- The effect of an organic counterion and a Hofmeister series of inorganic counterions on solvation dynamics and rotational relaxation processes in micelles have been studied.
- These processes are controlled by the extent of release of water molecules during the formation of micelles which depends on the nature of counterions of surfactants.
- Counterions indirectly contribute to the slow solvation by the formation of clusters of water molecules.

The effect of an organic counterion, p-toluenesulfonate (p-TS−) and a Hofmeister series of inorganic counterions, NO3−, Br− and SO42- on the solvation dynamics and rotational relaxation of Coumarin 480 (C-480) in the Stern layer of aqueous micelles of hexadecyltrimethylammonium surfactants (C16TAX) has been studied. Studies have been carried out by means of UV-vis absorption, steady-state fluorescence and fluorescence anisotropy, time-resolved fluorescence and fluorescence anisotropy, and dynamic light scattering measurements. The rate of solvation increases in the order C16TABr < C16TANO3 < (C16TA)2SO4 < C16TAp-TS. Effectively, the solvation process is controlled by the extent of release of water molecules during the formation of micelles which depends on the nature of counterion. p-TS− counterions are more tightly bound to the headgroups because of added effect of its hydrophobic part. Counterions indirectly contribute to the slow solvation by the formation of clusters of water molecules. The decreasing order of the average rotational relaxation time of C-480 in the micelles of surfactants is C16TAp-TS >> C16TABr > C16TANO3 > (C16TA)2SO4, which is the same as the decreasing order of microviscosity of micelles. The rotational relaxation time is shorter in the micelles of (C16TA)2SO4 as compared to C16TAp-TS as the former micelles have less tightly packed structure than the latter. The slow rotational relaxation is mainly contributed by the lateral diffusion of C-480 along the surface of the micelle. The rotational motion is the slowest for the micelle of (C16TA)2SO4, and the same is the fastest for the micelle of C16TABr. There is an indication of different orientation of C-480 molecules in viscous micelles of C16TAp-TS as compared to other less viscous micelles. The fact of counterion dependent solvation processes might help us for various studies on physicochemical properties of surfactants in solutions.

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