Dr Henry Craddock, a Consultant in the Oilfield and Chemicals sector, highlights some of the challenges for chemical formulators developing surfactants and surfactant mixtures for use in clean-up and containment methods following oil spills.
 

Use of surfactants
Oil spills and their resulting environmental pollution can cause major ecological damage and the cost of clean-up can be monumental. For example, Exxon states it spent over $2.1 billion on the cleanup after the Exxon Valdez oil spill disaster.¹ Surfactants and surfactant mixtures have formed part of the clean-up and containment methods employed in managing oil spills since the 1980s.
 

Surfactants have varying solubilities in water and various actions with respect to oil and water, as characterized by the hydrophilic–lipophilic balance (HLB) value, which indicates the oil and water solubility of a surfactant. In general, a surfactant with an HLB value of 1-8 promotes the formation of water-in-oil emulsions, and a value of 12-20 promotes oil-in-water emulsions. A surfactant with an HLB value between 8 and 12 may promote either type of emulsion, but more often promotes oil-in-water emulsions. Dispersants are found to have HLB values in this range.
 

Oil dispersants are usually mixtures of surfactants that are formulated to disperse oil slicks into the sea or other body of water. So-called beach cleaners are designed to remove oil from surfaces such as sand and rocks. Emulsion breakers (demulsifiers) and emulsion inhibitors are designed to break water-in-oil emulsions or prevent their formation. 
 

Challenge and controversy
Developing effective oil-spill dispersants is a major challenge to the chemical formulator as crude oil and refined oils are not consistent in their composition, and have varying types and amounts of molecules, with a variety of molecular sizes. What is effective for a polar alkane may not be effective for an aromatic component. The environmental conditions such as the sea state and temperature can also affect the performance of the dispersant.
 

Much controversy has been generated over the use of oil dispersants and this has been re-ignited by their use during the Deepwater Horizon Oil Spill. Here, over seven million litres of dispersant was applied, to very little overall effect.  The major impact was that the dispersant used reduced the amount of floating oil, which reduced the risk for some organisms and environments, but increased the risk for others as the oil was dispersed into the water column.
 

Testing effectiveness and toxicity
Over 100 dispersants have been evaluated over the decades, however, only a handful are approved for use and commercially available.² To obtain approval, effectiveness and toxicity are tested, although the level of toxicity testing in the past has been wholly inadequate.³
 

The comparative effectiveness of dispersants is relatively easy and straightforward to measure,⁴ however testing effectiveness in real conditions is problematic as it is impossible to represent all the environmental conditions and oil compositional factors. Over 50 different tests have been developed to attempt to assess more realistically a dispersant’s performance.^5,6
 

Toxicity testing has become more sophisticated, and now the dispersants in use are less toxic than the oils being dispersed. However, the dispersion of the oil and chemical mixture can have serious toxic effects on the biota if applied in shallow waters, particularly the fauna on the seabed. Toxicity testing for the use of oil spill dispersants in regulated sea areas has become more defined⁷ and these have become standards for the international use of oilfield dispersants.
 

In practice
A typical oilfield dispersant formulation consists of a pair of non-ionic surfactants usually formulated with an additional anionic surfactant, in proportions to yield an HLB value of around 10. Studies have been conducted on this type of mixture to optimize the proportions and determine the best three ingredients.
 

To be effective, the chemical dispersant must be applied as soon as possible after the oil spill has occurred. This is because if the oil ages, is weathered and/or thins out, it will disperse poorly as the chemical dispersant is applied. There are a large number of other physical and environmental factors that affect the dispersant performance as well as how the dispersant is physically applied, including droplet size.
 

Fundamentally no one wants to have any oil spills, however, unless we stop extracting, using, storing and transporting oil the risk of a large oil spill is always present. The decision to use dispersants or not will always be problematic and judgmental. Hopefully as we progress the understanding of the physical process and progress green chemistry and biosurfactants, in tandem with similar advances in dispersion effectiveness, the use of dispersants will be recognised as a useful and environmentally responsible choice.