Res. Chem. Intermed.
, Vol. 28, No. 2,3, pp. 131–141 (2002)
Also available online - www.vsppub.com
The EPR study of dialkyl nitroxides as probes to investigate
the exchange of solutes between micellar and water phases
GIOVANNI BRIGATI, PAOLA FRANCHI, MARCO LUCARINI
GIAN FRANCO PEDULLI and LUCA VALGIMIGLI
Department of Organic Chemistry,‘ A. Mangini’ — University of Bologna, Via S. Donato 15,
I-40127 Bologna, Italy
Received 1 November 2001; accepted 16 November 2001
Abstract—An EPR investigation of the kinetics of the exit, k
, and entrance, k
, processes in
micelles of sodium dodecyl sulfate, hexadecyltrimethylammoniumbromide and polyoxyethylene(6)-
decanol of a family of
-butyl nitroxides and
hydroxyalkyl nitroxidesis reported. The inclusion of nitroxide probes in the hydrophobicenvironment
of the micelle gives rise to a reduction of the value of both nitrogen and ¯-proton splittings, with the
result that the resonance elds for the M
1 lines of the free and included species are
signi cantly different. The rate constants were obtained by analyzing the EPR line shape variations
as function of surfactant concentration and temperature. The experimental value of k
the study of benzyl
-butyl nitroxide indicates that the association reaction is very close to being
controlled by diffusion. The value of the exit rate, k
, instead, depends on the probe hydrocarbon
chain length. A comparison of our results with those obtained by luminescence quenching techniques
is also reported.
: Micelles; nitroxides; radicals; EPR.
For many years there has been a deep interest in the dynamics of micellar solutions
that have been examined by stopped ow, pressure jump, and ultrasonic relaxation
measurements . These measurements frequently provided evidence for two
distinct relaxation-times: a short one characteristic of a process occurring on a time
scale of microseconds and a longer one of the order of milliseconds to seconds .
The faster relaxation is attributed to an association-dissociation process involving
the exchange of individual surfactant molecules between the micelles and the water
To whom correspondence should be addressed.