Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 110

effect by zwitterionic dodecylcarnitine chloride surfactant was also reported. The
effect of micelles on acidic hydrolysis of pesticide has been less investigated. Van
Eden et al. (2004) examined the hydrolysis of amitraz (19) in SDS, HDTMA Br,
and Tween 80 micelles. The hydrolysis rates in the latter two micelles were found
to be less than that in water, while SDS micelles enhanced hydrolysis by a factor of
5-10. The observed enhancement of acid hydrolysis can be accounted for by an
attracting proton on the surface of micelle where (19) is solubilized. A similar
profile was expected for the acid-catalyzed hydrolysis of atrazine (13) in the ani-
onic SDBS micelles, but its effect was found to be minimal (Lei et al. 2001).

V Effects on Photolysis

Photolysis is considered to be another important route of pesticide degradation in
formulation. During some period of water evaporation from the applied formulation
on the surface of foliage and soil, adjuvants in the formulation may affect the pho-
todegradation profiles of pesticide. Furthermore, because sunlight irradiance is
known to change only slightly in shallow water (Barron et al. 2000) and surfactants
in formulation tend to gather in the surface microlayer of the water (Katagi 2006),
the effect of the surfactant on aqueous photolysis may become important in some
cases. There are many excellent reviews on the effect of surfactant and micelle on
photophysical and photochemical processes (Fendler and Fendler 1975; Ramamurthy
1986; Ta¸cio ˘lu 1996; Thomas 1977, 1980; Turro et al. 1980).

A Mechanism

The basic investigation on the effect of surfactants and micelles has focused mostly
on photolysis in the solution phase (Fig. 8). The excited singlet or triplet state of a
pesticide (S * ) by absorption of light undergoes either chemical reactions or deacti-
vation to the ground state (S) via internal conversion or quenching, emission of flu-
orescence, or phosphorescence via intersystem crossing and energy transfer. Each
process can be significantly affected by not only the polarity and viscosity of the
medium where a pesticide molecule is solubilized but also the locus and orientation
of the molecule in the micelle. The fine structure of pyrene fluorescence sensitive
to solvent polarity showed the hydrophilicity of the solubilized locus in the
HDTMA Br micelle, and its enhanced excimer formation implied the higher micro-
viscosity in the interior of the micelle (Thomas 1977, 1980; Turro et al. 1980). The
quenching of the excited state in a micelle was markedly controlled by the partition
of nonionic quencher (Q) into the micelle as well as the charge of a quencher (Q + ,
Q − ) that determined its accessibility to the ionic micellar surface. For a chemical
solubilized in the Stern layer, the heavy atom effect by a counterion such as Br − in
HDTMA Br results in a decrease of its fluorescence lifetime by a heavy atom effect