A dispersant is a chemical agent designed to reduce the surface tension between two immiscible liquids, most commonly oil and water. They enable oil to break apart and mix into the surrounding water rather than remaining as a cohesive slick on the surface. Dispersants are primarily used as a response tool during large-scale environmental emergencies, such as marine oil spills. Their application aims to move the oil away from the water’s surface to mitigate immediate damage to sensitive shoreline habitats and surface-dwelling wildlife.
The Science of Dispersion: How They Work
Dispersants operate by overcoming the natural tendency of oil and water to remain separate, a phenomenon dictated by interfacial tension. The forces holding the oil molecules together are stronger than the forces attempting to pull them into the water. The active ingredient, known as a surfactant, intervenes to disrupt the slick.
A surfactant molecule is structurally characterized by two distinct ends: one that is attracted to oil (oleophilic) and one that is attracted to water (hydrophilic). When the dispersant is applied to an oil slick, the surfactant molecules migrate to the interface between the oil and the water. They orient themselves at this interface, with the oil-loving end penetrating the oil and the water-loving end remaining in the water.
This alignment lowers the interfacial tension, weakening the forces that hold the oil mass together. With the tension reduced, the mechanical energy from the sea, such as waves and turbulence, can easily fracture the large slick. This action breaks the oil into thousands of tiny, stable droplets, typically measuring less than 100 micrometers in diameter.
These micron-sized droplets are small enough to remain suspended in the water column instead of rising back to the surface and re-coalescing. They become diluted and transported by underwater currents. The immense increase in the oil’s total surface area makes the hydrocarbons more accessible for natural microbial degradation.
Essential Ingredients and Classifications
Modern dispersants are complex chemical formulations composed primarily of two main groups of ingredients: surfactants and solvents. Surfactants act as the active dispersing agent, while solvents ensure the dispersant can effectively penetrate the oil slick and be easily applied. Solvents are often non-aromatic petroleum distillates or glycols, which lower the overall viscosity of the dispersant mixture.
The development of dispersants is categorized into generations based on their chemical makeup, efficacy, and toxicity. Early formulations used in the 1960s were industrial degreasers, proving highly toxic to marine life. Subsequent generations were developed to reduce this toxicity.
The modern classification system defines dispersant types based on their formulation and application method. The second generation, known as Type 1, was hydrocarbon-solvent based, contained a lower concentration of surfactants (around 15 to 25%), and was typically applied undiluted. The most common modern formulations are third-generation, concentrated products classified as Type 2 or Type 3.
These concentrated dispersants contain a higher percentage of surfactants, often ranging from 25% to over 60%, and use less toxic solvents. Type 2 dispersants are water-dilutable concentrates, while Type 3 formulations are designed to be applied neat, or undiluted, often sprayed directly from aircraft. In the United States, all dispersant products are regulated by the Environmental Protection Agency (EPA) under the National Contingency Plan (NCP) Product Schedule.
Environmental Consequences of Using Dispersants
The decision to use dispersants in a spill response involves a trade-off between protecting surface resources and potentially increasing the exposure risk for organisms in the water column. By moving the oil from the surface, dispersants prevent large-scale oiling of coastlines, seabirds, and marine mammals that inhabit the air-water interface. However, the dispersed oil and the dispersant mixture are then introduced directly into the marine environment below.
The dispersed oil droplets are transported deeper into the water column, temporarily increasing the concentration of hydrocarbons available to fish, plankton, and benthic (bottom-dwelling) organisms. Scientific studies indicate that the combination of oil and dispersant can, in some cases, be more toxic to certain marine species than the untreated surface oil alone. Sensitive habitats, such as coral reefs, are particularly vulnerable to the effects of dispersed oil, leading to restrictions on dispersant use in their vicinity.
The primary environmental benefit of chemical dispersion is the acceleration of the oil’s natural breakdown. The increased surface area of the tiny droplets allows naturally occurring, oil-eating microbes to colonize and consume the hydrocarbons faster than they could an intact slick. This microbial degradation is the final fate of the oil, converting the hydrocarbons into harmless carbon dioxide and water over a period of weeks or months.
Bioaccumulation is the potential for toxins to move up the food chain. However, most scientific evidence suggests that the hydrocarbons in dispersed oil are metabolized and excreted by higher vertebrates, including fish. Therefore, significant long-term bioaccumulation or biomagnification of the oil components does not appear to occur in the higher levels of the marine food web.