ACADEMIA ROMÂNĂ Revue Roumaine de Chimie http://web.icf.ro/rrch/ Rev. Roum. Chim., 2012, 57(9-10), 863-869 PROBING THE MICELLE/WATER INTERFACE BY NON-RADIATIVE ENERGY TRANSFER Monica Elisabeta MAXIM, Gabriela STÎNGĂ, Adriana BĂRAN, Alina IOVESCU and Dan F. ANGHEL * Laboratory of Colloids, “Ilie Murgulescu” Institute of Physical Chemistry, Roumanian Academy, 202 Spl. Independenţei, 060021, Bucharest, Roumania Received June 14, 2011 (Accepted April 10, 2012) The non-radiative energy transfer (NRET) between the pyrene labeled poly(acrylic acid) (PAA-Py) dissolved in bulk solution and naphthalene (Np) solubilized in surfactant micelles is studied at various pH values and salt concentrations. Additional information was obtained from DLS measurements. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT), sodium dodecyl sulfate (SDS) and decaethylene glycol mono-tetradecyl ether (C 14 EO 10 ) are the surfactants considered. NRET is estimated as the ratio between the emission intensity of pyrene and that of naphthalene (I Py /I Np ) and depends on surfactant, pH, and salt concentration. For SDS, I Py /I Np is not affected by changing the pH from 2 to 11. AOT shows an initial decrease of I Py /I Np ratio with increasing pH followed by a leveling off above a pH value exceeding the pK a of PAA-Py. The nonionic surfactant has a peculiar behavior with a minimum of I Py /I Np around the pK a of PAA-Py and a plateau at pH 7. A maximum of NRET appears in the SDS micellar system at a NaCl concentration equal to 0.01 M. These behaviors are explained taking into account the obtained values of Förster radius, the distance between acceptor and donor, the quantum yield of donor and the efficiency of energy transfer. INTRODUCTION * A matter of major interest for scientists working on polymer-surfactant systems is the driving force behind the interaction. In mixtures of oppositely charged surfactants and polymers, a decisive role is played by the electrostatic attraction. 1,2 When the solution contains a nonionic surfactant and a polyelectrolyte, the interaction involves hydrophobic attraction between the species. 3,4 If the surfactant and polyelectrolyte have the same charge, the electrostatic repulsion hinders the formation of intermolecular aggregates. 5 Non- radiative energy transfer (NRET) is a powerful tool to investigate the phenomena appearing at nanometric level in polymer, surfactant, polymer- surfactant mixtures, and in life sciences. NRET is able to shed light on the interpenetration of polymer chains, 6 and on the interactions between proteins, 7,8 proteins and biomaterials, 9,10 and proteins and drugs. 11,12 It is also used in bioanalysis, 13 and to monitor the distribution of proteins and lipids in membranes. 14 The method * Corresponding author: adan@icf.ro describes the transfer of energy between an excited donor and a ground-state acceptor through non- radiative dipole-dipole coupling. NRET depends on the distance between donor and acceptor, the spectral overlap of donor emission spectrum and acceptor excitation spectrum, and the relative orientation of the dipole moments of donor and acceptor. Naphthalene (Np) and pyrene (Py) with a “Förster radius” of about 29 Å, 15,16 are often used as donor-acceptor pair in NRET experiments. 17,18 Previously we used this technique to study the conformational changes and interactions between poly(acrylic acid) separately labeled with naphthalene and pyrene. 19 In this paper we use NRET as a means to probe the micelle-water interface. Toward this end, we confined the Np donor in surfactant micelle and dissolved the pyrene-labeled poly(acrylic acid) (PAA-Py) acceptor in bulk solution. The obtained results show that NRET depends on surfactant, pH and ionic strength. Additional information is obtained by dynamic light scattering (DLS).