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

S0300-A6-MAN-060portions of the molecules are attracted toward oil, forming a layer of micelles on the oil surface.Figure 6-1 illustrates this mechanism.Figure 6-1. Mechanism of Chemical Dispersion.Dispersants reduce the interfacial tension between oil and water, weakening the cohesiveness ofthe oil slick, so that external energy, such as wave action, breaks up the oil slick into droplets.Once oil is in droplets, it disperses naturally throughout the water column. The layer of micellescovering the oil droplets, with hydrophilic portions outward, helps prevent the oil droplets fromcoalescing into another slick.Surfactants have varying degrees of hydrophilic and lipophilic behavior, depending upon theirchemistry. The range of hydrophilic-lipophilic balance (HLB) is measured on a scale of 1 to 20,with 1 being most lipophilic and 20 being most hydrophilic. A surfactant having an HLB of 1 to 4is very lipophilic and does not mix with water. A surfactant having an HLB of 17 to 20 is veryhydrophilic and mixes readily with water. Surfactants in oil spill dispersants are hydrophilic, withHLB numbers ranging from 8 to 18 and are formulated to stabilize oil-in-water emulsions.6.2.3 Considerations for Employing Dispersants. Dispersants are effective only under properconditions. Substantial portions of very light oils, such as Diesel Fuel, Marine (DFM), evaporate6-3