Energy transfer */a tool for probing micellar media Swati De a,1, *, Agnishwar Girigoswami a , Anil Kumar Mandal b a Department of Chemistry, University of Kalyani, Kalyani 741 235, Nadia, India b Women’s Polytechnic, Government of West Bengal, Chandannagore, West Bengal, India Received 11 September 2002; accepted 8 November 2002 Abstract The non-ionic polyoxyethylene chain-containing surfactant Triton X-100 (TX-100) forms well-defined micelles and reverse micelles in aqueous and hydrocarbon media, respectively. Nonradiative energy transfer between two charged fluorescent dyes, fluorescein (FL) and acridine orange (AO) has been used to probe the micelles and reverse micelles of TX-100. In the energy transfer system employed, FL acts as the donor and AO as the acceptor. This is borne out by the fluorescence spectral data. Time-resolved studies further corroborate the steady-state results. As the fluorescence emission spectra of the two dyes show a considerable amount of overlap, they are resolved into individual donor and acceptor components using the principal component analysis (PCA) method. This study also focuses on the more important role played by hydrophobic forces (compared with electrostatic interactions) in promoting energy transfer between charged species in micellar media. # 2003 Elsevier B.V. All rights reserved. Keywords: Energy transfer; Micelles; Reverse micelles; Hydrophobic forces; Encounter probability 1. Introduction Study of the transfer of electronic excitation energy between molecules can be used to obtain information about the distance of separation of these molecules. Knowledge of this separation can in turn be used to study the interactions between the molecules themselves and those with their immediate environment. The photophysics of dyes can be effectively utilised to study energy transfer. Interactions of dyes with their environ- ment are reflected in their UV-visible absorption and fluorescence spectra. Interesting features of such interactions may appear in organised media, e.g. micelles and reverse micelles formed by some surfactants. Energy transfer between dyes is an example of such phenomena. The aim of this work is to study the energy transfer between a chosen pair of dyes and also to use this as a monitor for investigating the micellar and reverse micellar systems formed by a non-ionic surfactant. This study shall concentrate on the phenomenon of nonradiative transfer of excited-state energy from an initially excited donor dye (D) to an acceptor dye (A) [1  /4]. The phenomenon is experimentally manifested in the simultaneous * Corresponding author. E-mail address: swati@klyuniv.ernet.in (S. De). 1 Email: swati_de1@rediffmail.com. Spectrochimica Acta Part A 59 (2003) 2487  /2496 www.elsevier.com/locate/saa 1386-1425/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00043-X