810 OIL SPILLAGE INTO WATER—TREATMENT
compatible portion to the oil compatible portion-sometimes
referred to as HLB (Hydrophilic–Lipophilic Balance).^24
This relationship between the molecular structure of the
surfactant and the emulsion type is also shown in Figure 4 and
the physical concept behind Bancroft’s Law may be appreci-
ated. For example, it can be visualized that for a more water
compatible surfactant, the physical location of the larger
hydrophilic group on the outside of the dispersed oil drop-
lets results in a more effective “fender” to parry droplet colli-
sions and prevent droplet coalescence. The converse, location
and the larger portion of the surfactant in the dispersed rather
than the continuous phase, would be geometrically awkward
and unstable.^25 The mechanism of oil slick dispersion by the
application of chemical dispersants has been covered in some
detail by Poliakoff^26 and Canevari, 27,28,29 among others. From
the above discussion, one can see that the chemical disper-
sant (surfactant) will locate at the oil–water interfaced reduce
interfacial tension. This will then act to increase the spreading
tendency of the oil fi lm as shown by Eq. (1). More important,
it promotes fi ne droplet formation which can be expressed as:WAko/wo/w g ,
, (2)where:W k mixing energy, ergs
A o/w interfacial area, cm^2γ (^) o/w interfacial tension, dynes/cm.
Thus, for the same amount of mixing energy, a reduction of
γ (^) o/w will result in a corresponding increase in A o/w.
It is important to emphasize that, as can be realized from
the above discussion, the chemically dispersed oil does not
sink. Rather, the surfactant merely enhances small droplet
formation for a given amount of mixing energy. Smaller
diameter oil droplets have a much lower rise velocity per
the familiar Stokes Law. Hence, once the oil is chemically
treated, and placed 3 to 5 feet below the surface of the water
by the mixing process, it does not rise to the surface as read-
ily, as illustrated by Figure 5.
There are many surfactants that will aid the formation
of fi ne droplets in the above manner. It has already been
noted that the surfactant structure (Hydrophilic–Lipophilic
Balance) infl uences the effi ciency of the emulsifi er.
However, a more subtle and less tractable requirement
for an effective dispersant is the prevention of droplet
coalescence once the fi ne oil droplets are formed. This is
illustrated by Figure 6 wherein a volume of oil has been
dispersed by a chemical surfactant and maintained in sus-
pension by gentle bubbling of air. After 24 hours, there has
been no coalescence or separation of these fi ne oil drop-
lets. In the control sample, with similar volume of oil and
mixing energy, the oil separated almost immediately and
reformed an intact, cohesive fi lm of oil.
In essence then, an effective dispersant must parry drop-
let collisions physically. For example, dispersed oil may
separate in a sample bottle but even though there may be
a “creaming” effect, i.e. oil droplets concentrate near the
surface, the droplets should not coalesce to reform an intact
slick. It is this same “fendering” action that reduces the ten-
dency of the droplets to stick to a solid surface.
The Physical and Environmental Incentives for
Dispersing Oil Slicks
Consideration of the previous summary of the potential
damaging aspects of an untreated and unrecoverable oil spill
indicates that the removal of the intact, cohesive mass of oil
from the surface of the water yields more than a cosmetic
effect as is often claimed. For this alternate approach when
conditions do not permit the recovery of the spilled oil, the
removal of oil from the surface by dispersing it into fi ne
droplets yields established benefi ts that can be summarized
by the following discussion:
- Oil properly dispersed with a chemical dispersant
will not stick to a solid surface. As previously out-
lined, the physical fending action of a properly
selected surface-active agent prevents the oil drop-
lets from coalescing after dispersion. This same
property also inhibits the oil from wetting out on
a solid surface. This has become a controversial
point and it has actually been claimed that the con-
verse is true. For example, in the First Report of the
President’s Panel on Oil Spills,^30 it has been stated
that such agents cause the oil to “spread into the
sand-surfaces which untreated oil would not wet.”
A laboratory experiment was conducted to evaluate
this aspect. A mixture of 256 cc of sea water, 95
cc of beach sand (New Jersey shore area), and 20
cc Kuwait Crude, were placed in a graduate. This
represented a vertical cross section of the marine
environment after an oil spill. The mixture was then
agitated to simulate the possible contact of sedi-
ment by the oil when turbulent conditions existed.
After mixing, the sample was settled to separate the
oil–sand water phases. In a body of water, either the
oil may be driven down into contact with the sandy
bottom or the sand may be suspended in the body
of water by wave action, such as deduced from the
previously cited Buzzards Bay spill. The graduate
was then purged with clean water to simulate the
return of the environment to a non-contaminated
condition.
The experiment was then repeated using 20 cc
of Kuwait Crude Oil and 4 cc of a chemical dis-
persant (5 parts oil/1 part dispersant).
Virtually no “treated” oil impregnated the sand.
For the experiment with the untreated crude oil, an
analysis of the oil content of the sand bed indicated
that 11.20 cc of oil remained of the initial 20 cc. - Oil removed from surface water prevents bird
damage. The aforementioned hazard to marine
fowl that is presented by the surface oil film is
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