OIL SPILLAGE INTO WATER—TREATMENT 809
g (^) w is surface tension of water, dynes/cm
g (^) o is surface tension of oil, dynes/cm
g (^) o/w interfacial tension of oil and water, dynes/cm.
By an examination of the force balance shown in Figure 2
it can be seen that if S o/w —the resultant spreading force
is positive, the oil will spread on the water; if negative, it
will not spread but remain a “lens” of Liquid. For exam-
ple, spreading coeffi cient values for Kuwait Crude on sea
water, reported by Canevari^20 are positive and confi rm that
for this system the oil readily spreads on the water phase.
Garrett^21 has summarized spreading pressures of vari-
ous oils on sea water that vary from 25 to 33 dynes/cm.
Cochran^22 has also published values that generally agree
with these level on sea water. As one can see from Figure 3,
for positive spreading coeffi cients, the oil is capable of
fi lming out to very thin fi lms. A fi lm thickness of only
3.0 10 −6 inches representing a spill of 50 gallons of oil
distributed over a surface area of one mile will be quite
visible as a “fl at” silver sheen on the surface of the water.
However, the initial spreading rate of a large volume
of spilled oil is based on the volume and density of the oil
in essence, sort of static head that overcomes other factors
such as interfacial tension.
The Mechanism of Dispersing Surface Oil Slicks by
Chemical Dispersants
The dispersion of surface oil fi lms as fi ne oil droplets into
the water column is promoted by the use of a chemical dis-
persant. This oil spill dispersant consists primarily of a surface
active agent (surfactant) and a solvent. The solvent is added
as a diluent or vehicle for the surfactant. It also reduces the
viscosity and aids in the uniform distribution of the surfac-
tant to the oil fi lm.
A surfactant is a compound that actually contains both
water compatible (hydrophilic) and oil compatible (lipophilic)
groups. Due to this amphiphatic nature, a surfactant locates and
arranges itself at an oil–water interface as schematically shown
in Figure 4. The surfactant’s molecular structure, e.g. ratio of
hydrophilic to lipophilic portion, determines the type of disper-
sion (oil droplets dispersed in water phase or water droplets
dispersed in oil phase), as well as stability of the dispersion. In
essence, a surfactant that is principally water soluble disperses
oil-in-water and established water as the continuous phase; a
surfactant that is principally oil soluble, the converse. This is
Bancroft’s Law,^23 which has been tested and proven empiri-
cally true over the years. A convenient classifi cation for sur-
factants therefore, is based on the ratio or balance of the water
FIGURE 4 Influences of surfactant structure on type of dispersion.
HYDROPHILIC-LIPOPHILIC BALANCE (HLB)
SCHEMATIC OF
SURFACTANT
TYPE OF
EMULSION
FORMED
DISPERSE
WATER
DROPLETS
DISPERSE
OIL OIL DROPLETS WATER
1 5101520
Increase In
Oil Solubility
Increase In
Water Solubility
Hydrophilic Group
(Water Compatable)
Lipophilic Group
(Oil Compatable)
Oil Soluble Surfactant
Favors Water-In-Oil
Dispersion
Water Soluble Surfactant
Favors Oil-In-Water
Dispersion
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