Res. Chem. Intermed., Vol. 31, No. 1–3, pp. 135–144 (2005)  VSP 2005. Also available online - www.vsppub.com Solvation dynamics in a worm-like CTAB micelle PRATIK SEN, SAPTARSHI MUKHERJEE, ARNAB HALDER, PARTHA DUTTA and KANKAN BHATTACHARYYA ∗ Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 30 January 2004; accepted 15 February 2004 Abstract—The solvation dynamics of 4-(dicyanomethylene)-2-methyl-(p-dimethyl-aminostyryl) 4H- pyran (DCM) is studied in a worm-like micelle consisting of cetyl trimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Solvation dynamics in the worm-like micelle is found to be about 4-times slower than that in ordinary CTAB micelle. This indicates that the fluorescent probe molecule (DCM) experience a different microenvironment inside the worm-like micellar aggregates compared to that in ordinary spherical CTAB micelle. The spectral width (Ŵ) of the time resolved emission spectra of DCM in the micellar aggregates is found to be time dependent. This is ascribed to the diffusion of the probe (DCM). Keywords: Solvation dynamics; 4-(dicyanomethylene)-2-methyl-(p-dimethyl-aminostyryl) 4H-pyran; cetyl trimethylammonium bromide; micelle; fluorescence; decay. INTRODUCTION In an aqueous solution, surfactant molecules form globular or spherical micelles above their critical micelle concentration (CMC). According to SANS studies, a micelle consists of a ‘dry’ hydrocarbon core surrounded by a hydrated layer containing the polar or ionic headgroups, counter ions and water molecules [1]. On addition of certain salts, e.g., potassium bromide or sodium salicylate (NaSal), the shape of these micelles changes from a globular to a worm-like form [2]. The principal reason for this structural change is binding of the counter ions which suppresses the micellar charge and decreases the surface area per surfactant molecule by reducing the electrostatic repulsion between the head groups. This results in a growth of the micelles from a spherical to an elongated rod like structure. As NaSal concentration increases the micelles grow in length and begin ∗ To whom correspondence should be addressed. Fax: (91-33) 2473-2805. E-mail: pckb@mahendra.iacs.res.in