As the oil is discharged on ocean surface it starts spreading horizontally over the water surface to attain a constant level. As heavier oil drifts faster than thinner portion its accumulation forms the leading edge of an advancing oil slick. As the oil spreads the process of evaporation starts operating and as much as 50-60% is lost through this process.
Light, low boiling point fraction of crude oil, such as benzene, toluene, xylene etc is lost in this way. There is an appreciable reduction in the toxicity of crude petroleum as a consequence of removal of this fraction which is ultimately photo-oxidized or reacted upon by various constituents in the atmosphere.
It is very small fraction of petroleum crude which dissolves in water. Usually compounds having low molecular weight tend to be more soluble. A dilute oil-in water suspension is formed as the process of natural dispersion and turbulence of sea water breaks the sheet of oil into tiny droplets.
Nitrogen, sulphur and oxygen bearing compounds in the crude act as natural surfactants reducing oil-water interfacial tension. Finally the oil may be reduced to very small droplet and is so much diluted that its presence is barely detectable. Microbial degradation of crude also starts and a considerable portion of oil is thus eliminated from the system.
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Emulsification is also an important process which involves incorporation of water into the floating oil resulting in water-in-oil emulsion. Almost 20-80% water is contained in the frothy mass which often becomes so viscous that oil-slick cleanup operations run into difficulty. This emulsion is known as ‘mousse’ and once its formation starts it becomes almost impossible to pump out the oil mixture. Mechanical skimmers which skim out the upper oily layer are ineffective.
The mousse is ultimately broken by wind and wave action. The process of weathering and microbial degradation finally eliminates the easily decomposable part and reduces the crude petroleum to tar balls which keep floating for long durations. Tar balls are composed of rather harmless substances constituted by skeletal fragments, branched or unbranched chains and rings of the compounds, originally present or derived from their decomposition products in the crude oil.