Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 85

herbicides via energy transfer. Photodegradation of the surfactant proceeded via
central cleavage of the ether linkage to form the phenol and polyethyleneglycol, the
latter of which was further degraded via oxidation or cleavage of a C - C or C - O
bond at the terminal ethoxy moiety.
Detailed photoproduct distribution under UV irradiation of nonylphenol ethoxy-
lates has been investigated by liquid chromatography (LC)-MS. (Chen et al. 2007).
The branched dodecanol hexaethoxylate was indirectly photolyzed via random
ether cleavage with its E T value of 43-44 kcal/mol (Tanaka et al. 1986).
In a water body, surfactants are partly adsorbed to suspended particles such as
clay and biota and also associated with dissolved humic substances. Small-angle
X-ray scattering technique has shown the interactions between quaternary ammo-
nium surfactant and soil humic acids that modify the micelle structure (Shang and
Rice 2007). Either the association with various suspended and dissolved matters in
water or formation of micelles at concentration above cmc is considered to reduce
the bioavailability of surfactant. The monomer of surfactant is usually taken up by
fish via the gills, but its hydrophilic moiety may make it difficult to pass through
the membrane. The concentration ratio of LAS between aqueous and biota phases
is widely distributed from 8 to 4100 depending either on species or on alkyl chain
length (Tolls et al. 1994). The uptake rate seems to be controlled by hydrophobicity
of the surfactant. Alkylphenols metabolized from the corresponding surfactants are
also known to be bioaccumulated with factors of 20-1300 (Ying 2006).

III Effects on Physicochemical Properties

A Solubilization

A surfactant increases the apparent water solubility of a chemical, and its extent
depends not only on the polarity and size of a solubilized chemical but also on the
nature of the surfactant such as cmc, aggregation number, and shape of micelle.
There are many excellent reviews on enhanced solubilization by surfactants (Cordes
and Gitler 1972; Fendler and Fendler 1975; Grieser and Drummond 1988; Ta¸cio ˘ lu
1996; Thomas 1980; Turro et al. 1980). In general, the apparent water solubility
increases in proportion to the concentration of a coexisting surfactant slightly up to
cmc and markedly above cmc. The slope in Fig. 4a above cmc is referred to as the
molar solubilization ratio (MSR), defined next:

MSR = (S mic − S cmc ) / (C s − cmc)

where C s , S mic , and S cmc are concentration of surfactant (mol/L), and apparent solu-
bilities of substrate (mol/L) at C s > cmc and C s = cmc, respectively (Edwards et al.
1991). When the solubilization is considered as partition of a substrate molecule
into micelles, the following equation can also be defined: