U.S. Navy Ship Salvage Manual Volume 6 - Oil Spill Response

S0300-A6-MAN-060As a result of the TORREY CANYON experience, dispersants are applied very conservatively tocombat oil spills, especially in the United States.Second-generation dispersants, also containing hydrocarbon solvents, were less toxic but also lesseffective. Third-generation dispersants consist mostly of surfactants formulated with relativelylittle alcohol or glycol solvent. The dispersants are supplied in concentrated form that is dilutedwith seawater as they are applied. Third-generation dispersants are less toxic to marine life thanearlier dispersants.The following terms are necessary to understanding dispersants:• Surfactant. Surface-acting agent, sometimes called a detergent. Contains moleculeswith both water-compatible and oil-compatible portions.• Hydrophilic. The water-compatible portion of a surfactant molecule.• Hydrophobic (also called lipophilic). The oil-compatible portion of a surfactant molecule.• Interfacial Tension. Surface tension existing at the oil-water interface. Surfactantsreduce interfacial tension.• Micelles. Micelles are ordered aggregates of surfactant molecules. Critical MicelleConcentration (CMC) occurs when so many surfactant molecules are added that theyno longer accumulate only at the oil-water interface.6.2.1 Dispersant Composition. Dispersants consist of surfactants dissolved in a solvent. Whendispersants are applied to an oil slick, the surfactant reduces the surface tension of the oil to breakup the oil into droplets. The solvent helps the surfactant to penetrate the oil. The oil droplets aredispersed throughout the water column, where natural degradation takes place. Dispersants do notmake oil go away; they only change it from a slick on the surface to tiny droplets suspendedthroughout the water column. Dispersal helps prevent emulsification and enhances oxidation, biodegradationand dissolution.Dispersants are classed as water-based, hydrocarbon-solvent-based or concentrate. Water-baseddispersants can be applied by eduction into a water stream. Hydrocarbon-based dispersants areformulated to enhance mixing and penetration of the surfactant into viscous oils. Concentrateddispersants disperse a higher volume of oil per volume of dispersant than conventional dispersantsand are suited for discharge from aircraft, where weight is an important factor.6.2.2 Surfactants. Surfactants have two roles: first, to penetrate the oil slick and break it intodroplets and second, to shield individual oil droplets to prevent emulsification and immediate coalescenceinto another oil slick.Surfactants become aligned to the oil-water interface when dispersants are applied to an oil slick.The hydrophilic portions of the surfactant molecules are attracted toward water and lipophilic6-2