Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 112

cycloaddition or dimerization of alkenyl species such as cyclopentenone was found
to be highly dependent on the orientation of the solubilized molecule in a micelle
with the promoted reaction because of the increase of local concentrations of reac-
tants (Ramamurthy 1986; Ta¸cio˘lu 1996). As an electrostatic effect, the enhance-
ment of photo induced nucleophilic substitution of nitro aromatics by a cyanide ion
was reported for the cationic micelles (Hautala and Letsinger 1971).
B PhotolysisofPesticides
Simple Organic Chemicals
Photoinduced isomerization is one of the basic photoreactions, and that of stilbene
derivatives has been examined extensively in the various aggregates. The photosta-
tionary isomerism of 4-hydroxystilbene in the HDTMA Br micelles under UV irra-
diation shifted more to the cis -isomer than that in methanol because of an increase
of microviscosity (Wolfe and von Bünau 1986). No formation of the corresponding
dimer and phenanthrene derivative may imply its association with HDTMA Br in
the Stern layer. Photoinduced regioselective dimerization in micelles has been
reported for 3- n -butylcyclopentenone (Lee and de Mayo 1980). The isomeric ratio
of dimers, head-to-head/head-to-tail, markedly increased above cmc, indicating a
preorientational effect so that the molecule is solubilized in the Stern layer with its
butyl chain inserted toward the micelle core. A similar effect on regioselectivity in
dimer formation in micelles was reported for uracil (Fendler and Bogan 1974) and
cinnamic acid (Nakamura 1988). Photoinduced polyene cyclization is known to
proceed more favorably in micelles, presumably as a result of stabilization of inter-
mediate radicals (Hoffmann et al. 1993).
Photoinduced dechlorination is a common reaction in pesticide photochemistry.
Photodegradation of trichloroethane in the Brij 35 micelles at 253.7nm was
enhanced by a factor of 3 more than that in water (Choy and Chu 2001). Much
higher quantum yield of photodechlorination via electron transfer mechanism in
ionic micelles than water was reported for many polychlorinated benzenes (Chu
and Jafvert 1994). The lack of formation of biphenyls shows the compartmentaliza-
tion effect of the micelle that results from isolation of the aryl radical from other
aryl molecules. This technique was applied to efficiently decompose polychlorobi-
phenyls in soil (Chu et al. 1998). Product analysis showed that the dechlorination
proceeded stepwise by using the surfactant as a hydrogen source finally to the corre-
sponding nonchlorine aromatics or phenol (Chu and Jafvert 1994; Shi et al. 2000).
The photoinduced dechlorination of 2-chlorphenol to form phenol proceeded in
higher quantum yields with the additional products when dissolved in micelles than
that in water (Shi et al. 1997). The nucleophilic displacement of Cl in the undissociated
species with water under UV irradiation gave catechol, while the anionic species
underwent Wolff rearrangement to form cyclopentadienecarboxylic acid. The pho-
toreduction of nitroaromatics in the alkaline borohydride solution was promoted by