Page 188 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 179

surfactant sorbed to the soil does not strongly enhance the contaminant
uptake.
A general equation similar to Eq. (7.24) has been formulted to account for
the solubility enhancement of a solute by a surfactant solution at a given
temperature (Kile and Chiou, 1989):

(7.27)
S* S w = 1 + X mnK mn + X mcK mc
w
where S* and S w are as defined before; X mn is the concentration of the
w
surfactant as monomers in water (dimensionless); X mc is the concentration of
the surfactant in micellar form in water (dimensionless); K mn is the partition
(enhancement) coefficient of the solute between surfactant monomers and
water; and K mc is the partition coefficient of the solute between micelles and
water. At a given temperature, S w, K mn, and K mc are constants. The X mn and X mc
values are assigned as follows. If the surfactant concentration X £ CMC, then
X mn = X; if X > CMC, X mn = CMC and X mc = X - CMC.
By Eq. (7.27), a plot of the apparent solute solubility (S* ) against the sur-
w
factant concentration (X), extended over the CMC, will be bilinear, yielding a
straight line with a slope of S wK mn from X = 0 to X = CMC, followed by another
straight line of a much higher slope, S wK mc, at X ≥ CMC, provided that the sur-
factant is molecularly homogeneous and has a single monomer–micelle tran-
sition point (i.e., a single CMC). If the surfactant is molecularly heterogeneous,
a succession of micelle formation may take place for different molecular frac-
tions of the surfactant and the resulting plot of S* versus X will not exhibit a
w
single sharp transition. [In this case, the measurement of surface tension versus
X, as commonly used to detect CMC, gives a breadth of monomer–micelle
transition rather than a single sharp break.]
Kile and Chiou (1989) applied Eq. (7.27) to analyze the apparent water
solubilities of extremely water-insoluble DDT and relatively soluble 1,2,3-
trichlorobenzene (TCB) as influenced by several nonionic surfactants (Triton
series and Brij 35), an anionic surfactant [sodium dodecyl sulfate (SDS)], and
a cationic surfactant [cetyltrimethylammonium bromide (CTAB)]. The struc-
tures and properties of these surfactants are given in Table 7.16. Triton series
and Brij 35 surfactants are molecularly heterogeneous in that the lengths of
polar head groups [i.e., the number of ethylene oxides (EO) units] are average
rather than single fixed values, whereas SDS and CTAB are virtually molecu-
larly homogeneous. In related studies, Edwards et al. (1991) measured the
apparent water solubilities of nathphalene, phenanthrene, and pyrene in water
solutions of TX100, Brij 30, and other nonionic surfactants, and Jafvert et al.
(1994) measured the apparent solubilities of hexachlorbenzene with similar
surfactant types.
The solute solubility enhancement by a surfactant was treated by Edwards
et al. (1991) in terms of molar solubilization ratio (MSR) (i.e., the number of
moles of solute solubilized per mole of surfactant in micellar form) and by
Jafvert et al. (1994) as a dimensional equilibrium constant of solute and micelle