Journal of Photochemistry and Photobiology, A: Chemistry, 47 zyxwvutsrqponmlkjihgfedcbaZY (1989) 97 - 102 97 zyxwvuts FLUORESCENCE ENHANCEMENT dF p-TOLUIDINO NAPHTHALENESULPHONATE IN A MICELLAR ENVIRONMENT ASHIS NAG and KANKAN BHATTACHARYYA Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032 (India) (Received September 2, 1988) Summary The relative fluorescence yield of p-toluidino naphthalenesulphonate increases markedly above the critical micellar concentration of Triton- X-100 and sodium dodecylsulphate. This is attributed to the reduced po- larity of the hydrophobic interior of the micelles and is shown to be a simple method for following micellization. 1. Introduction Owing to their importance as model systems mimicking biomembranes micelles have been subject to several investigations [I - 91. Several different strategies have been employed to study the process of micellization or, more precisely, to determine the critical micellar concentration (CMC) above which micellar aggregates are formed. These include fluorescence [ 1, 2, 4 - 61, polymerization 171, cage effects [3] and proton-transfer processes [lo] among others [8]. For some time a search has been underway to find a good fluorescence probe to follow micellization; the most popular found so far is pyrene [ 1, 2, 5, 61. The choice of a fluorescent probe depends on the fact that the viscosity and polarity of the interior of the micellar aggregates are quite different from those in the bulk aqueous phase. Of these two pro- perties the polarity aspect has received less attention [9]. The main, dif- ficulty in probing the reduced polarity of micelles arises from the fact that polarity is not expected to be uniform everywhere. While the interior of the micelle resembles a pure hydrocarbon solvent, the polarity at the boundaries of the micelle must be much higher because of the presence of polar head groups and water molecules. If a probe resides both at the less polar interior and at the polar boundaries the superposition of the diverse spectral pro- perties of the probes in the different environments produces a very blurred picture. However, this difficulty can be overcome if a fluorescent probe with an ionic head group is used. The ionic group restricts the probe to the boundary region with the ionic part projected into the aqueous phase. Apart from having an ionic group, the fluorescence properties of the probe must be lOlO-6030/89/$3.50 @ Elsevier Sequoia/Printed in The Netherlands