5
8
M. Bettoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 53–59
could relate the different surfactant effect to the maximum num-
ber of substrate molecules that each kind of surfactant micelle can
surfactant concentration leads to an increase of the number of sub-
strate molecules solubilized in micelles (through co-micellization),
transport close to TiO . This depends on the solubilization ability of
which can be transported close to the TiO particle surface where
2
2
the micelle vs substrate 1, and it also depends on the micelle num-
ber that approaches the TiO2 particle surface. In fact, association
of substrate 1 to surfactant aggregates was measured to elucidate
if a different association extent of it in different surfactants can
influence the product yield. In our experimental conditions the
determination of the binding constants solute–micelles seemed
not adequate. In fact reliable values of binding constants are gen-
erally obtained provided that the solute is not very hydrophobic,
that the amphiphile is in big excess over substrate (this is referred
as “kinetic conditions”), and that amphiphile concentration is well
above the cmc [22]. Two conditions are not fulfilled in our case,
because we have a hydrophobic substrate and because the ratio
surfactant/substrate is rather low, ranging from 1:1 to ca. 20:1.
Therefore we carried out experimental measures of substrate solu-
bilization in different surfactant solutions, in conditions similar to
those used for reaction. In surfactant solutions 0.05 M the solubi-
lization of substrate 1 changes with surfactant nature: it is 7.5 mM,
1 reacts. After a certain concentration, which varies with the
nature of the surfactant, the presence of the competitive partition
between the co-micellized substrate 1 adsorbed on TiO2 (where
reaction occurs) and the co-micellized substrate in the bulk tends
to limit more and more the beneficial kinetic effect of the surfac-
tant. The surfactant SB3-14 0.015 M gives the best performance
in terms of product (aldehyde) yield (42%). A better yield (55%) is
obtained with CTABr 0.05 M + HCl 0.1 M.
Acknowledgements
This work was funded by MIUR (Ministero dell’Istruzione,
dell’Università e della Ricerca Scientifica), Italy (PRIN 2008, grant
number 2008AZT7RK 002) and by Fondazione della Cassa di
Risparmio di Perugia, Italy (2009.010.0355).
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