Toward a Hydrodynamic Description of Bimolecular Collisions in Micelles. An Experimental Test of the Effect of the Nature of the Quencher on the Fluorescence Quenching of Pyrene in SDS Micelles and in Bulk Liquids

The simple hydrodynamic description of bimolecular collision rate constants afforded by the Stokes-EinsteinSmoluchowski (SES) equation is tested in a bulk liquid as well as in sodium dodecyl sulfate micelles. Fluorescence quenching of pyrene by three different quenchers is used as a model to study collision rate constants. One quencher is itself a cationic surfactant, one an uncharged long alkyl chain labeled with a nitroxide, and the third an uncharged aromatic compound. The SES equation is found to give a remarkably good description of the fluorescence quenching rate constant not only in bulk but also in micellar media. The fluorescence quenching rate constant is independent of the type of quencher and varies with temperature, viscosity (microviscosity in the case of micelles), and quencher concentration in accordance with the SES equation. A zero-order model of diffusion in micelles is introduced in which pyrene and quencher diffuse through the polar shell. The microviscosity of the shell is deduced from electron spin resonance measurements of one of the quenchers, which is also a nitroxide spin probe. The probability of quenching upon collision between reacting pairs is on the order of order unity in both bulk and micellar media. The observed slight departure of this probability from unity in the case of micelles is discussed and arguments show it to be accommodated by minor adjustments to the zero-order model.