Environmental Engineering Reference
In-Depth Information
Through the registration process of a new pesticide, many physicochemical
properties and environmental fate profiles should be clarified from laboratory
studies basically using an a.i., followed by field trials using a typical formulation
product to confirm the actual residues of a pesticide and its relevant metabolites.
Because surfactants above the characteristic critical micelle concentration (cmc)
aggregate to micelles that result in a different reaction environment from an aque-
ous phase, basic physical and chemical processes for a pesticide are most likely to
be modified in formulation (Bunton and Savelli 1986; Edwards et al. 1992; Fendler
and Fendler 1975; Grieser and Drummond 1988; Ta¸cio ˘lu 1996; Thomas 1980).
Furthermore, some interactions with surfactant monomers are known to result in
different behavior of a substrate from those in an aqueous phase. From this point of
view, the physicochemical properties should be measured for an a.i., and the usage
of a formulated product is not recommended in the Organization for Economic
Development (OECD) testing guidelines for hydrolysis (OECD 2004), soil adsorp-
tion-desorption (OECD 2000), and soil metabolism (OECD 2002a,b) studies.
Although some of the underlying mechanisms of the surfactant effects on each
process in Fig. 1 have been investigated for small organic molecules and polycyclic
aromatic hydrocarbons (PAH), their effects on environmental behavior of pesticides
have not been systematically examined. Based on these considerations, this review
first deals with basic properties of surfactants used in pesticide formulation together
with their environmental behavior. The effects on the behavior of pesticide are then
discussed for the typical formulations, including surfactants, after the basic investi-
gations on small organic molecules together with its mechanism.
II
Surfactants in Pesticide Formulations
A
Type and Properties of Surfactant
The surfactant molecule basically possesses a hydrophilic polar head group and a
hydrophobic part such as a long alkyl chain, thus showing an amphiphilic character.
Surfactants mostly lower the surface tension of an air-water interface from 72 mN/
m to around 30-40 mN/m and are generally classified from their chemical struc-
tures mainly in three categories: cationic, anionic, and nonionic. The chemical
structures of surfactants typically used for pesticide formulation are listed in Fig. 2.
Nonionic surfactants with a polyethoxy chain as a hydrophilic part are the most
popular. The commercially available nonionic alcohol and alkylphenoxy ethoxy-
lates are usually polydisperse with the carbon numbers of an alkyl chain and
CH 2 CH 2 O segment being distributed around their averages. The most familiar
cationic surfactant is a quaternary ammonium salt such as hexadecyltrimethylam-
monium bromide (HDTMA Br) having the (CH 3 ) 3 N + CH 2 moiety as a polar head
group and used as a thickener for suspension and emulsion formulations. In various
types of anionic surfactants, either a sulfate or sulfonate group exhibits a hydrophilic
character and the hydrophobic part is usually a long saturated alkyl chain or
Search WWH ::




Custom Search