Chemistry Reference
In-Depth Information
vious field work comparing small-scale wave spectra in slicked and un-
slicked regions (Bock and Frew 1993) has suggested that sea-surface films
exhibit a complex surface response to compressional-dilational straining,
dependent on chemical composition. In this work we used static elasticity
as representative of dynamic elasticity, recognising that
in situ
measure-
ments of dynamic elasticity, although preferable, were not feasible. Recent
laboratory work on the dynamic behaviour of sea surface films (Mass and
Milgram 1998) suggests that static elasticities predict dynamic elasticities
accurately, since the time scales for interfacial exchange processes are long
relative to the time scales for surface straining by capillary waves. The
strong influence of chemical composition on interfacial elasticity is illu-
strated by comparing mass spectra of selected samples with their respec-
tive H
o
-3 characteristics. The mass spectra of three surfactant mixtures iso-
lated by bubble adsorption and
n
-hexane/dichloromethane extraction
(fraction F1) from (1) a surface bulk water sample, (2) an unslicked micro-
layer, and (3) a heavily slicked microlayer are presented in Figure 3. The
mass spectrum of the bulk seawater sample is dominated by intense ions
from only a few compounds, whose structures we have shown to be similar
to poloxamers (mixed ethylene oxide-propylene oxide copolymers), most
likely of anthropogenic origin. In the spectrum of the visibly unslicked mi-
crolayer, additional low intensity ions are observed in the
m/z
80-150, 200-
300, and
m/z
350-470 regions, representing the presence of small amounts
of adsorbed lipid materials, primarily free and bound fatty acids, steroids,
and terpenoids, along with the poloxamers. The spectrum of the third sam-
ple indicates that these lipid components, seen in minor amounts in the un-
slicked microlayer, are heavily enriched in the slicked microlayer relative
to the poloxamers. The effect of this changing composition on interfacial
elasticity is shown in Figure 4, in which measured static surface elasticities
are plotted as a function of film surface pressure for the same three sam-
ples. Film balance measurements of surface elasticities of films composed
of poloxamers isolated from coastal seawater and purified by gas chroma-
tography have shown that these compounds have intrinsically low elastic-
ity at a given surface pressure. This is reflected in Figure 4 by the H
o
vs
. 3
curve for the bulk water sample, which exhibited the lowest elasticity as a
function of film pressure. Increasing amounts of the lipid components in
the microlayer samples had a measurable effect on surface elasticity, in-
creasing it up to a factor of four in the most concentrated slicks.
