Nature of the water molecules in the palisade layer of a triton X-100 micelle in the presence of added salts: A solvation dynamics study

The effect of added electrolytes on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation dynamics studies of C153 dye in the presence of different concentrations of NaCl, KCl, and CsCl salts. In all of the cases, the solvation dynamics is found to be biexponential in nature. It is seen that in the presence of added salts the solvation dynamics becomes slower. As previously reported (Charlton et al. J. Phys. Chem. B 2000, 104, 8327; Molina-Bolivar et al. J. Phys. Chem. B 2002, 106, 870), the presence of salt increases micellar hydration (and also size) for TX-100, mainly due to enhancement in the mechanically trapped water content in the palisade layer. Under normal circumstances, increased micellar hydration was expected to cause faster solvation dynamics (Kumbhakar et al. J. Phys. Chem. B 2004, 108, 19246), though in the present work, a reverse trend is in fact observed with the added salts. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microviscosity for the palisade layer of the TX-100 micelle with the added salts. The present results have been rationalized assuming that the ions reside in the palisade layer, and due to the hydration of the ions, especially the cations, the water molecules in the palisade layer undergo a kind of clustering, causing the microviscosity to in fact increase rather than decrease as expected due to increased micellar hydration. A partial collapse of the surfactant chains due to their dehydration as caused by the hydration of the ions in the palisade layer may also add to the increase in microviscosity and the consequent retardation in relaxation dynamics in the presence of salts.