10. Lipids in Water-Surface Microlayers and Foams
237
well-mixed air
stagnant air
surface I
microlayer
$urfa~ active
compounds
-O.OO1f;.m
-0.01 !lffi
- 30-1 00 !lffi
FIGURE 10.1. Schematic view ofthe water-surface microlayer and the air-water interface.
The dotted line represents the ideal contact between the two phases.
importance of turbulent processes is considered to be small (Schwarzenbach et aI.,
1993). This phenomenon is explained by the highly viscous behavior of fluids in
the surface micro layer and the concomitant reduction of wind effects at the
boundary layer, just above the surface water. Therefore, the transport of material
across this region is dominated by molecular diffusion. The fluid dynamic that
governs the transport of chemicals between the different layers of the air-water
interface led to the development of two conceptual frameworks, the stagnant
boundary layer model (Whitman, 1923) and the surface renewal model (Danckwerts, 1951; Higbie, 1935), which provide satisfactory descriptions of the microlayers of lentic and lotic environments, respectively.
A complete assessment of these models is beyond the scope of this chapter
(please see the review by Schwarzenbach et aI., 1993). Briefly, the stagnant
boundary layer model assumes that water and air turbulence are not strong enough
to stir the thin air and water layers adjacent to the interface. However, the surface
renewal model takes into account the continual turnover of air and water "parcels"
with their associated material load at the air-water interface (Danckwerts, 1951).
The rate of mass transfer of substances across the surface microlayer for the
stagnant and the renewal models are shown in Equations 1 and 2, respectively.
Stagnant boundary layer model:
F = I,OO~wX Zw (cwa - cw)
(I)
Surface renewal model:
F = J 10;::; r }cw - cwa)
(2)
where F (mol . cm - 2 . S - \) is the exchange flux of material per unit time and
area, D (cm2 . s -\) is the diffusion coefficient of the solute, z (cm) the thickness of
237
well-mixed air
stagnant air
surface I
microlayer
$urfa~ active
compounds
-O.OO1f;.m
-0.01 !lffi
- 30-1 00 !lffi
FIGURE 10.1. Schematic view ofthe water-surface microlayer and the air-water interface.
The dotted line represents the ideal contact between the two phases.
importance of turbulent processes is considered to be small (Schwarzenbach et aI.,
1993). This phenomenon is explained by the highly viscous behavior of fluids in
the surface micro layer and the concomitant reduction of wind effects at the
boundary layer, just above the surface water. Therefore, the transport of material
across this region is dominated by molecular diffusion. The fluid dynamic that
governs the transport of chemicals between the different layers of the air-water
interface led to the development of two conceptual frameworks, the stagnant
boundary layer model (Whitman, 1923) and the surface renewal model (Danckwerts, 1951; Higbie, 1935), which provide satisfactory descriptions of the microlayers of lentic and lotic environments, respectively.
A complete assessment of these models is beyond the scope of this chapter
(please see the review by Schwarzenbach et aI., 1993). Briefly, the stagnant
boundary layer model assumes that water and air turbulence are not strong enough
to stir the thin air and water layers adjacent to the interface. However, the surface
renewal model takes into account the continual turnover of air and water "parcels"
with their associated material load at the air-water interface (Danckwerts, 1951).
The rate of mass transfer of substances across the surface microlayer for the
stagnant and the renewal models are shown in Equations 1 and 2, respectively.
Stagnant boundary layer model:
F = I,OO~wX Zw (cwa - cw)
(I)
Surface renewal model:
F = J 10;::; r }cw - cwa)
(2)
where F (mol . cm - 2 . S - \) is the exchange flux of material per unit time and
area, D (cm2 . s -\) is the diffusion coefficient of the solute, z (cm) the thickness of