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May25-10, 03:26 PM
While the physical and chemical properties of vegetable
oils and animal fats are highly variable, most fall within a range that
is similar to the physical parameters for petroleum oils.
properties--such as solubility, specific gravity, and viscosity--are
responsible for the similar environmental effects of petroleum oils,
vegetable oils, and animal fats.
In one respect, however, many petroleum oils differ from most
vegetable oils and animal fats. Unlike most vegetable oils and animal
fats, many petroleum oils have a high vapor pressure.
The high vapor
pressure of petroleum oils can lead to significant evaporation from
It may also produce exposure of nearby populations through the
We describe some important properties of oil below.
. Solubility refers to the ability of a chemical to dissolve in water or solvents. Like petroleum oils, vegetable oils and animal fats have limited water solubility and high solubility in organic solvents.
. Specific gravity is the ratio of the density of a material to the density of fresh water. Specific gravity determines whether an oil floats on the surface of a water body or sinks below the surface and how long oil droplets reside in the water. It can also give a general indication of other properties of the oil. For example, oils
with a low specific gravity tend to be rich in volatile components and are highly fluid International Tanker Owners Pollution Federation, 1987). The specific gravity of vegetable oils and animal fats whose properties we examined is within the range of specific gravity values for petroleum oils.
Viscosity refers to the resistance to flow. It controls the rate at which oil spreads on water and how deeply it penetrates the shore. Viscosity also determines how much energy organisms need to overcome resistance to their movement. At similar temperatures, the dynamic viscosity (shear stress/rate of shear) and kinematic viscosity (dynamic viscosity/density) of vegetable oils and animal fats are
somewhat greater than those for light petroleum oils but less than those for heavy petroleum oils.
The viscosity of canola oil represents a medium weight oil and is comparable to that of a lightly weathered Prudhoe Bay crude oil after it has evaporated by 10 percent (Allen and Nelson, 1983).
. Vapor pressure is the pressure that a solid or liquid exerts in equilibrium with its own vapor depending on temperature. It controls the evaporation rate of an oil spill and air concentrations. The higher the vapor pressure of an oil, the faster it evaporates. Vapor pressure varies over a wide range for petroleum oils, from moderately volatile diesel-like products to slightly volatile heavy crude oils and residual products. The vapor pressure of animal fats and vegetable oils is generally much lower than that of many petroleum oils.
Evaporation is significant for many petroleum oil spills, some of which completely evaporate in one to two days, but it is rarely an important factor in spills of vegetable oils and animal fats.
In some vegetable oils, however, there is a small volatile fraction that can evaporate. Thermal decomposition can also cause the formation of many volatile degradation products.
The spreading of oil relates to surface tension (interfacial tension) in a complex manner. When the sum of the oil-water and oil-air interfacial tensions is less than the water-air interfacial tension, spreading is promoted.
At 25 deg.C, the oil-water interfacial tension for canola oil is far less than that of Prudhoe Bay crude oil, suggesting that canola oil could spread more (Allen and Nelson, 1983).
Surface tension measurements in the laboratory, however, are not necessarily predictive of the behavior of oil that is being transformed by many processes in the environment.
Emulsions are fine droplets of liquid dispersed in a second, immiscible liquid. When oil and water mix vigorously, they form a dispersion of water droplets in oil and oil droplets in water (Hui, 1996c). When mixing stops, the phases separate. Small water drops fall toward the interface between the phases, and the oil drops rise. The emulsion breaks. When an emulsifier is present, one phase becomes continuous, while the other remains dispersed. The continuous phase is usually the one in which the emulsifier is soluble.
The tendency of petroleum and non-petroleum oils to form emulsions of water-in-oil or oil-in-water depends on the unique chemical composition of the oil as well as temperature, the presence of stabilizing compounds, and other factors. When an emulsion is formed in the environment, the oil changes appearance and its viscosity can increase by many orders of magnitude. Removal of the oil becomes harder because of the increased difficulty in pumping viscous fluids with up to fivefold increases in volume.
Although the ability to form adhesions is difficult to measure and predict, adhesions influence the ease with which spilled oil can be physically removed from surfaces. When water is colder than the oil pour point, oils become viscous and tar-like or form semi-solid, spherical particles that are difficult to recover. Weathering and evaporation are slowed, and oils may become entrapped or encapsulated in ice and later may float on the surface when ice breaks up.
In ice adhesion tests, canola oil and Prudhoe Bay crude oil had the same tendency to coat the surface of sea ice drawn up through an oil/water interface
(Allen and Nelson, 1983). Neither oil adhered to submerged sea ice even after surface coating. This study suggests that some vegetable oils and petroleum oils have a similar ability to form adhesions under certain environmental conditions.