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G.E. Napolitano and D.S. Cicerone
colloidal systems, foams are thermodynamically unstable because bubbles tend to
coalesce rapidly. However, the presence of surfactants at the air-water interface
reduces the surface tension and promotes stability between the neighboring bubbles, contributing to the persistence of the foam structure.
In carefully controlled environments, it is possible to produce surfactantstabilized static bubbles and foams with lifetimes of months or even years
(Schramm and Wassmuth, 1994). Natural foam formations, however, have a life
span of hours to a few days. Fig. 10.2 represents a slice of a typical foam
formation at the air-water interface and a magnified region showing its various
internal structures. Of interest for the purpose of this chapter is the thin, liquid film
dividing the gas phase, denominated lamella, the thin aqueous film that provides
the structure of the foam, and the plateau border, which is the connection of three
lamellae at an angle of 120 0 • These structures contains the materials that form the
water-surface microlayer of the body of water, where the bubbles emerge at the
surface and the foam is formed. The plateau border plays an important role in the
mechanism of film drainage and may collect different types of materials, such as
inorganic particles, oil droplets, and microorganisms (Wasan et aI., 1994; Adamson, 1990). Assuming that a typical foam consists of approximately 90% air
(Schramm and Wassmuth, 1994) and that the mean thickness of the surface
microlayer is 50 )..Lm (Hardy et aI., 1988), I L of foam water (destabilized or
collapsed foam) would represent 2 m 2 of surface microlayer.
10.4. Sampling Techniques
Sampling foam for lipid analysis is a rather simple manual operation that requires
nothing more than a clean jar and a piece of solvent-rinsed aluminum foil (Napolitano and Richmond, 1995). Nevertheless, the sampling procedure should be
performed carefully to avoid drainage of water or contamination of the foam with
subsurface water, practices that could bias the results of foam constituents.
In contrast to the simple procedures involved in the collection of foam samples,
the past few decades have witnessed an extensive development of tools for sampling water-surface microlayers. Despite these efforts, no standard methods have
yet been established for collecting the water-surface microlayer. Consequently,
considerable variations in the reported values of the thickness of the microlayers,
the enrichment factors of specific compounds, and even the definition of the
micro layer itself have arisen, partly as a reflection of the disparity between the
different sampling techniques. Table 10.3 presents a compilation of methods and
devices frequently used for sampling the microlayer. The design and materials
used for the construction of these sampling instruments affect not only the definition of the physical boundaries of the microlayer but may probably also affect the
results of chemical analysis, due to differential affinity and selective extraction of
certain compounds (Marty and Choiniere, 1979).
Garret (1965) developed the first tool for collecting slick-forming materials
from the sea surface. This simple device consisted of a metal screen (0.14-mm-
G.E. Napolitano and D.S. Cicerone
colloidal systems, foams are thermodynamically unstable because bubbles tend to
coalesce rapidly. However, the presence of surfactants at the air-water interface
reduces the surface tension and promotes stability between the neighboring bubbles, contributing to the persistence of the foam structure.
In carefully controlled environments, it is possible to produce surfactantstabilized static bubbles and foams with lifetimes of months or even years
(Schramm and Wassmuth, 1994). Natural foam formations, however, have a life
span of hours to a few days. Fig. 10.2 represents a slice of a typical foam
formation at the air-water interface and a magnified region showing its various
internal structures. Of interest for the purpose of this chapter is the thin, liquid film
dividing the gas phase, denominated lamella, the thin aqueous film that provides
the structure of the foam, and the plateau border, which is the connection of three
lamellae at an angle of 120 0 • These structures contains the materials that form the
water-surface microlayer of the body of water, where the bubbles emerge at the
surface and the foam is formed. The plateau border plays an important role in the
mechanism of film drainage and may collect different types of materials, such as
inorganic particles, oil droplets, and microorganisms (Wasan et aI., 1994; Adamson, 1990). Assuming that a typical foam consists of approximately 90% air
(Schramm and Wassmuth, 1994) and that the mean thickness of the surface
microlayer is 50 )..Lm (Hardy et aI., 1988), I L of foam water (destabilized or
collapsed foam) would represent 2 m 2 of surface microlayer.
10.4. Sampling Techniques
Sampling foam for lipid analysis is a rather simple manual operation that requires
nothing more than a clean jar and a piece of solvent-rinsed aluminum foil (Napolitano and Richmond, 1995). Nevertheless, the sampling procedure should be
performed carefully to avoid drainage of water or contamination of the foam with
subsurface water, practices that could bias the results of foam constituents.
In contrast to the simple procedures involved in the collection of foam samples,
the past few decades have witnessed an extensive development of tools for sampling water-surface microlayers. Despite these efforts, no standard methods have
yet been established for collecting the water-surface microlayer. Consequently,
considerable variations in the reported values of the thickness of the microlayers,
the enrichment factors of specific compounds, and even the definition of the
micro layer itself have arisen, partly as a reflection of the disparity between the
different sampling techniques. Table 10.3 presents a compilation of methods and
devices frequently used for sampling the microlayer. The design and materials
used for the construction of these sampling instruments affect not only the definition of the physical boundaries of the microlayer but may probably also affect the
results of chemical analysis, due to differential affinity and selective extraction of
certain compounds (Marty and Choiniere, 1979).
Garret (1965) developed the first tool for collecting slick-forming materials
from the sea surface. This simple device consisted of a metal screen (0.14-mm-