Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 74

many microorganisms (Makkar and Rockne 2003) are known. The dirhamnolipid
has been demonstrated to form multilamellar vesicles in a heterogeneous size above
its cmc instead of spherical micelles (Sánchez et al. 2007). To grasp the
hydrophobicity or hydrophilicity of a surfactant, a hydrophilic-lipophilic balance
(HLB) shown below has been conveniently developed by Griffin (1949) and is
widely used:

HLB = 20(M h /M)

where M h and M are the molecular masses of a hydrophilic part and the whole
molecule, respectively. The lower and higher values, respectively, mean that
surfactant is hydrophobic and hydrophilic. According to this index, it is generally
considered that surfactants having the HLB values of 2-7 and 7-18 are adequate
for preparing emulsifiable concentrate (EC) and oil-in-water emulsion (EW)
formulations (Knowles 2001).
A surfactant molecule is dissolved in water as a monomer at a lower concen-
tration but tends to aggregate to form micelles at cmc (Fendler and Fendler 1975;
Bunton et al. 1991; Ying 2006). Micelles are dynamic colloidal aggregates and
optically transparent but scatter light. The size and shape are dependent on the
concentration of surfactants and the solution composition. At the concentration
slightly above cmc, the spherical micelles with a typical radius of 10-30 Å are
formed by specific numbers of surfactant molecules (aggregation number) (Fig.
3a), but either increase of surfactant concentration or addition of inorganic cati-
ons changes their shape to a rod-like or cylindrical configuration at a much larger
size. The model of a spherical micelle is known to be consistent with the results
of small-angle neutron scattering, but this structure is so dynamic that in a small

(b)

(c)

(d)

(a)

(h)

(e)

(f)

(g)

Fig. 3 Schematic representation of spherical micelle: (a) spherical micelle, (b) polar head group
of a surfactant, (c) hydrophobic moiety of a surfactant, (d) counterion, (e) hydrophobic core region,
(f) Stern layer, (g) Gouy-Chapman double layer, (h) substrate molecule solubilized in micelle