238
G.E. Napolitano and D.S. Cicerone
the surface microlayer, c (mol' dm - 3) is the concentration of the solute, and r the
surface renewal rate due to microscopic eddies above and below the interface. The
subscripts wand wa indicate water and the water-air interfaces, respectively.
Although Equations I and 2 have the same mathematical form, they have
different adjustable parameters (2 and r) and different dependence on the empirical diffusion coefficient, proportional to D in the former, proportional to >1[5 in the
latter. It has been found that F depends on Da, where a varies between 0.5-1. The
stagnant boundary layer model assumes that the concentration gradient across the
layers of thickness 2 (the driving force of the flux) must be constant. This assumption implies that the concentration profile has time to reach a steady-state condition and is not significantly modified by chemical reactions. The surface renewal
model assumes that the concentration gradient across the water-air interface is
time dependent. It couples the rate of delivery of new bulk media "packets" to the
interface, a consequence of the level of turbulence in the air and water reservoirs,
with the local molecular diffusive exchange out of, or into, the interfacial region.
This dependence is incorporated in the model by the surface renewal rate parameter (r), which represents the mean frequency of creating contact surfaces.
The surface renewal rate r\l is an idealized model parameter determined experimentally. This parameter is a "fitting" coefficient used to adjust for wind and
water turbulence local effects. O'Connor and Dobbins (1958) proposed an empirical relation to determine r ll • which is shown in Equation 3.
(3)
where u", and d w are the water velocity and water depth of a turbulent aquifer,
respectively.
A great variety of natural lipidic surfactants is produced by many different
organisms for a variety of functions (Table 10.1). These compounds profoundly
affect the physicochemical properties of the water-surface microlayer (Adamson,
1990). The top layer of surfactants (Fig. 10.1) influences the solubility and the
flux of hydrophobic compounds across the region. They also modify the diffusion
characteristics due to hydrophobic and electrostatic interactions with other compounds present in the microlayer. When a surfactant is dissolved in water, the
hydrophobic tail of the molecule produces a reordering of water molecules in its
surroundings. This results in the development of repulsive forces and an increase
in the free energy of the system. Minimization of the repulsion forces is achieved
by the orientation of the surfactant molecules at the interface: the hydrophilic head
of the surfactant readily dissolves in water, whereas the hydrophobic tail goes to
the air layer. The net result is a reduction in the free energy per unit area of the
interface and an increase of the interfacial viscosity.
A different way in which a surfactant can reduce the free energy of the system is
by forming micelles, bilayers, and vesicles (Fig. 10.2). Surfactant micelles form
assemblages in which the hydrophobic portion of the surfactant orients inward
and the hydrophilic portion remains in contact with the water. The forces that hold
these structures together include hydrophobic, van der Waals, electrostatic, and
G.E. Napolitano and D.S. Cicerone
the surface microlayer, c (mol' dm - 3) is the concentration of the solute, and r the
surface renewal rate due to microscopic eddies above and below the interface. The
subscripts wand wa indicate water and the water-air interfaces, respectively.
Although Equations I and 2 have the same mathematical form, they have
different adjustable parameters (2 and r) and different dependence on the empirical diffusion coefficient, proportional to D in the former, proportional to >1[5 in the
latter. It has been found that F depends on Da, where a varies between 0.5-1. The
stagnant boundary layer model assumes that the concentration gradient across the
layers of thickness 2 (the driving force of the flux) must be constant. This assumption implies that the concentration profile has time to reach a steady-state condition and is not significantly modified by chemical reactions. The surface renewal
model assumes that the concentration gradient across the water-air interface is
time dependent. It couples the rate of delivery of new bulk media "packets" to the
interface, a consequence of the level of turbulence in the air and water reservoirs,
with the local molecular diffusive exchange out of, or into, the interfacial region.
This dependence is incorporated in the model by the surface renewal rate parameter (r), which represents the mean frequency of creating contact surfaces.
The surface renewal rate r\l is an idealized model parameter determined experimentally. This parameter is a "fitting" coefficient used to adjust for wind and
water turbulence local effects. O'Connor and Dobbins (1958) proposed an empirical relation to determine r ll • which is shown in Equation 3.
(3)
where u", and d w are the water velocity and water depth of a turbulent aquifer,
respectively.
A great variety of natural lipidic surfactants is produced by many different
organisms for a variety of functions (Table 10.1). These compounds profoundly
affect the physicochemical properties of the water-surface microlayer (Adamson,
1990). The top layer of surfactants (Fig. 10.1) influences the solubility and the
flux of hydrophobic compounds across the region. They also modify the diffusion
characteristics due to hydrophobic and electrostatic interactions with other compounds present in the microlayer. When a surfactant is dissolved in water, the
hydrophobic tail of the molecule produces a reordering of water molecules in its
surroundings. This results in the development of repulsive forces and an increase
in the free energy of the system. Minimization of the repulsion forces is achieved
by the orientation of the surfactant molecules at the interface: the hydrophilic head
of the surfactant readily dissolves in water, whereas the hydrophobic tail goes to
the air layer. The net result is a reduction in the free energy per unit area of the
interface and an increase of the interfacial viscosity.
A different way in which a surfactant can reduce the free energy of the system is
by forming micelles, bilayers, and vesicles (Fig. 10.2). Surfactant micelles form
assemblages in which the hydrophobic portion of the surfactant orients inward
and the hydrophilic portion remains in contact with the water. The forces that hold
these structures together include hydrophobic, van der Waals, electrostatic, and