areas: it does not account for evaporation, the topography of the ocean floor or the geography of the spill zone. Johansen's model is more complex than Mackay's model. It considers particles to be in one of three states: at the surface, entrained in the water column or evaporated. The empirically based model uses probabilistic variables to determine where the dispersant will move and where it will go after it breaks up oil slicks. The drift of each particle is determined by the state of that particle; this means that a particle in the vapor state will travel much further than a particle on the surface (or under the surface) of the ocean. This model improves on Mackay's model in several key areas, including terms for: Probability of entrainment – depends on wind Probability of resurfacing – depends on density, droplet size, time submerged and wind Probability of evaporation – matched with empirical data Oil dispersants are modeled by Johansen using a different set of entrainment and resurfacing parameters for treated versus untreated oil. This allows areas of the oil slick to be modeled differently, to better understand how oil spreads along the water's surface. Surfactants Surfactants are classified into four main types, each with different properties and applications: anionic, cationic, nonionic and zwitterionic (or amphoteric). Anionic surfactants are compounds that contain an anionic polar group. Examples of anionic surfactants include sodium dodecyl sulfate and dioctyl sodium sulfosuccinate. Included in this class of surfactants are sodium alkylcarboxylates (soaps). Cationic surfactats are similar in nature to anionic surfactants, except the surfactant molecules carry a positive charge at the hydrophilic portion. Many of these compounds are quaternary ammonium salts, as well as cetrimonium bromide (CTAB). Non-ionic surfactants are non-charged and together with anionic surfactants make up the majority of oil-dispersant formulations. The hydrophilic portion of the surfactant contains polar functional groups, such as -OH or -NH. Zwitterionic surfactants are the most expensive, and are used for specific applications. These compounds have both positively and negatively charged components. An example of a zwitterionic compound is phosphatidylcholine, which as a lipid is largely insoluble in water. HLB values Surfactant behavior is highly dependent on the hydrophilic-lipophilic balance (HLB) value. The HLB is a coding scale from 0 to 20 for non-ionic surfactants, and takes into account the chemical structure of the surfactant molecule. A zero value corresponds to the most lipophilic and a value of