Determination of Reaction Rate Constants in Micelles. A Comparison of the Pseudophase and Microscopic Models

I.V. Soboleva, M.G. Kuzmin, J. van Stam, G. Bhaskar Dutt, F.C. De Schryver

XVIth IUPAC Symposium on Photochemistry, Helsinki, Finland, July 21-26, 1996, Book of Abstracts, p. 536.

ABSTRACT. We compared the possibilities of applying of two different kinetic models for the determination of reaction rate constants in micelles. The reaction kinetics in micellar solutions were described by analysis of both computer simulated and experimental data. The pseudophase model considers micellar solutions as two-phase systems, composed of a bulk phase and a microphase (pseudophase). It treats the reaction kinetics in these two phases as if they were two separate homogeneous solutions with particular concentrations of dissolved species. The microscopic model considers the compartmentalization of the dissolved molecules over the micelles which are viewed as individual entities.
Comparative analysis has indicated three different types of luminescence decay kinetics in micellar solutions in the presence of quenchers.
(a) When a non-exponential decay is observed, the rate constants for unimolecular quenching, reactant entry and exit, and intermicellar exchange can be determined from the experimentally obtained decay only when the quenching rate exceeds the exit rate.
(b) An essentially monoexponetial decay will be observed if the quenching rate is much slower than the reactant exit rate. In this situation, the rate constant for intermicellar quenching can be estimated from the luminescence decay rate.
(c) If the quencher occupation number is low and the quenching rate is faster than the reactant exit rate, an essentially monoexponetial decay will also be observed. Under these conditions, the luminescence decay rate will not yield information on the quenching rate constant. On the other hand, the rate constant for reactant entry can be estimated from the decay rate.
The pseudophase model can be used to estimate the reaction kinetics in micelles only in cases b and c. If the decay deviates insignificantly from an exponential decay, the accurate determination of the rate constants from decay data will be virtually impossible by any of the discussed kinetic models. The values of the intramicellar quenching rate constants estimated from experimental and simulated data as obtained by the use of the pseudophase model are close to those obtained by the microphase model.
This work was carried out with the financial support of the European Union (INTAS Grant No 93-0751), Russian Foundation of Basic Research (Grant No 95-03775a) and Grant RU.

Laboratory of Photochemistry