used to stabilize the dispersion of vegetal oil droplets for example (Rosen and
Kunjappu 2012). Surfactants are amphiphilic molecules displaying at least one
hydrophilic head and one hydrophobic tail. Their structural characteristics vary a
lot, but they are categorized as charged (cationic or anionic) and neutral (nonionic or
zwiterrionic). They adsorb to interfaces and orient in order to lower the o/w
interfacial energy γ ow (Eq. 1.3) and they enhance NAPL mobility (Eq. 1.8). The
Gibbs equation links its variation to changes in chemical potential of any component
of the system, μ i (J mol
À1 ):
dγ ow ¼ À
X
i
Γ i dμ i
ð1:15Þ
where Γ i (mol m
À2 ) is the surface excess concentration of the ith component of the
system.
When equilibrium is reached:
dμ i ¼ RTd ln a i
ð1:16Þ
where a i is the activity of any component of the system. For the simplest case of
diluted solutions of a neutral surfactant (i.e., C S
10
À2 mol l
À1 ), Eq. (1.15)
becomes:
dγ ow ¼ ÀRTΓ S d ln C S ¼ À2:303RTΓ S d log C S
ð1:17Þ
where C S is the surfactant concentration in solution (mol l
À1 ). For the air/water
interface, the surface excess concentration is constant over a large range of concentration as shown in Fig. 1.9a. Above a fixed value of free surfactant concentration
called the CMC, surfactant monomers become no more soluble and associate as
aggregates called micelles. In absence of solid phase or NAPL, above the CMC
value, interfacial tensions become stable because the surfactant monomer
Log C S
Air
Air
Water
CMC
CMC
C s
S HOC,W
S
HOC
Water
a
b
Fig. 1.9 Variations for the air–water surface tension (a) and aqueous solubility of hydrophobic
organic contaminants (b) as a function of the surfactant concentration
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
23
Kunjappu 2012). Surfactants are amphiphilic molecules displaying at least one
hydrophilic head and one hydrophobic tail. Their structural characteristics vary a
lot, but they are categorized as charged (cationic or anionic) and neutral (nonionic or
zwiterrionic). They adsorb to interfaces and orient in order to lower the o/w
interfacial energy γ ow (Eq. 1.3) and they enhance NAPL mobility (Eq. 1.8). The
Gibbs equation links its variation to changes in chemical potential of any component
of the system, μ i (J mol
À1 ):
dγ ow ¼ À
X
i
Γ i dμ i
ð1:15Þ
where Γ i (mol m
À2 ) is the surface excess concentration of the ith component of the
system.
When equilibrium is reached:
dμ i ¼ RTd ln a i
ð1:16Þ
where a i is the activity of any component of the system. For the simplest case of
diluted solutions of a neutral surfactant (i.e., C S
10
À2 mol l
À1 ), Eq. (1.15)
becomes:
dγ ow ¼ ÀRTΓ S d ln C S ¼ À2:303RTΓ S d log C S
ð1:17Þ
where C S is the surfactant concentration in solution (mol l
À1 ). For the air/water
interface, the surface excess concentration is constant over a large range of concentration as shown in Fig. 1.9a. Above a fixed value of free surfactant concentration
called the CMC, surfactant monomers become no more soluble and associate as
aggregates called micelles. In absence of solid phase or NAPL, above the CMC
value, interfacial tensions become stable because the surfactant monomer
Log C S
Air
Air
Water
CMC
CMC
C s
S HOC,W
S
HOC
Water
a
b
Fig. 1.9 Variations for the air–water surface tension (a) and aqueous solubility of hydrophobic
organic contaminants (b) as a function of the surfactant concentration
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
23