EMULSIFICATION AND DISPERSION 



As the lighter fractions of petroleum in a spill are lost as a result 

 of evaporation and dissolution, some of the heavier fractions become viscous 

 as a result of the formation of a water-in-oil emulsion, sometimes referred 

 to as "chocolate mousse" because of its color and consistency. In open water 

 turbulence acts on an oil slick to form oil-in-water and water-in-oil emulsions, 

 as well as fine dispersed oil droplets. The production of surface-active agents 

 (surfactants) by bacteria can help to stabilize the dispersions and oil-in- 

 water emulsions, which can then remain suspended in the water for many days. 

 Some of the emulsions are stable for many months and can eventually be washed 

 ashore, where, after evaporation, a tarlike residue is left on the beach. 



DISSOLUTION 



The water solubility of most petroleum compounds is very low; hence, 

 the petroleum compounds in the underlying water of an oil slick are often 

 minor components in the whole oil. McAuliffe et al. (1975) found that, in 

 a crude oil spill in the Gulf of Mexico, less than 1 percent of the oil dis- 

 solved, while Gordon et al. (1976) found 5 percent of a spilled crude oil 

 in the water. The amount of turbulence and the concentration of dissolved 

 organic matter are important factors affecting the quantity of oil that is 

 dissolved in the underlying water (Boehm and Quinn 1973). Water extracts 

 of crude and fuel oil contain high concentrations of naphthalene-like 

 (two-ringed) and benzene-like (one-ringed) compounds (Anderson et al . 1974, 

 Guard et al . 1975). Polar compounds, such as phenols, quinolines, and 

 toluidines, which are very minor components in the whole oil, are in relatively 

 high concentrations in the water extract of fuel oils because of their solu- 

 bility in water (Winters et al . 1976). The products of photo-oxidation, such 

 as phenols and ketones, are more soluble than the parent compounds. Thus the 

 greater water solubility of aromatic hydrocarbons, especially those of lower 

 molecular weight, causes the oil slick to lose those compounds and retain 

 aliphatic hydrocarbons of similar molecular size. 



PHOTO-OXIDATION 



Under the influence of light, many of the hydrocarbons, particularly 

 the aromatics, can react with oxygen to produce polar compounds. Because 

 of their relatively high water solubility, these products of photo-oxidation, 

 including acids, alcohols, ketones, and phenols, are detected in water below 

 a slick (Hansen 1975). Photochemical oxidation of petroleum in oil is partic- 

 ularly important when there is a \/ery thin film of oil on the water and where 

 metal ions are available to act as a catalyst. Some high molecular weight 

 polycyclic aromatics absorb light in the visible region and are readily oxi- 

 dized by sunlight; thus, benzo(a)pyrene is completely decomposed in seawater 

 within a few days after exposure to sunlight (Andelman and Suess 1970). Ultra- 

 violet light is absent, and many of the short wavelengths present in the 

 visible region attenuate in water a few meters below the surface. One cal- 

 culation showed that a 100-ton slick, if spread over an area of 8 km^, would 

 be photo-oxidized at the rate of 1 ton per day (Dodd 1973). 



45 



