Oil dispersant

Oil dispersant mechanism of action

Oil dispersant are a mixture of surfactants and solvents that break up an oil spill into droplets. By breaking it up, microbes and the environment can more easily biodegrade the oil. A mixture of oil and water is normally unstable, but can be stabilized with the addition of surfactants. Surfactants improve interaction at the oil-water junction, decreasing surface energy. Dispersants have had negative environmental effects due to their toxicity; however, reformulated dispersants have been accepted by the United States Environmental Protection Agency (EPA).

History

In 1967, the supertanker Torrey Canyon leaked oil onto the English coastline. Alkylphenol surfactants were primarily used to break up the oil, but proved very toxic in the marine environment; all types of marine life were killed. This led to a reformulation of dispersants to be more environmentally sensitive. After the Torrey Canyon spill, new boat-spraying systems were developed. Later reformulations allowed more dispersant to be contained (at a higher concentration) to be aerosolized.

Theory

Requirements

There are five requirements for surfactants to successfully disperse oil:

• Dispersant must be on the oil's surface in the proper concentration
• Dispersant must penetrate (mix with) the oil
• Surfactant molecules must orient at the oil-water interface (hydrophobic in oil and hydrophilic in water)
• Oil-water interfacial surface tension must be lowered (so the oil can be broken up).
• Energy must be applied to the mix (for example, by waves)

The hydrophilic-lipophilic balance (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 20 is the most hydrophilic for a non-ionic surfactant.

Oil reacting with surfactant in water

Dispersion models

Developing well-constructed models (accounting for variables such as oil type, salinity and surfactant) are necessary to select the appropriate dispersant in a given situation. Two models exist which integrate the use of dispersants: Mackay's model and Johansen's model. There are several parameters which must be considered when creating a dispersion model, including oil-slick thickness, advection, resurfacing and wave action. A general problem in modeling dispersants is that they change several of these parameters; surfactants lower the thickness of the film, increase the amount of diffusion into the water column and increase the amount of breakup caused by wave action. This causes the oil slick's behavior to be more dominated by vertical diffusion than horizontal advection.

One equation for the modeling of oil spills is:

$({\frac {\partial h}{\partial t}})\bigtriangledown (h({\vec {U}}+({\frac {\vec {t}}{f}})-\bigtriangledown (E\bigtriangledown h)=R$

where

• h is the oil-slick thickness
• ${\vec {U}}$ is the velocity of ocean currents in the mixing layer of the water column (where oil and water mix together)
• ${\vec {t}}$ is the wind-driven shear stress
• f is the oil-water friction coefficient
• E is the relative difference in densities between the oil and water
• R is the rate of spill propagation

Mackay's model predicts an increasing dispersion rate, as the slick becomes thinner in one dimension. The model predicts that thin slicks will disperse faster than thick slicks for several reasons. Thin slicks are less effective at dampening waves and other sources of turbidity. Additionally, droplets formed upon dispersion are expected to be smaller in a thin slick and thus easier to disperse in water. The model also includes:

• An expression for the diameter of the oil drop
• Temperature dependence of oil movement
• An expression for the resurfacing of oil
• Calibrations based on data from experimental spills

The model is lacking in several 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.

Thermodynamics

Non-ionic surfactants

To determine the Gibbs free energy of micellization, the change in chemical potential for the surfactant going from a single solvated molecule to a micelle is measured. There are many approaches to this, one of which is the phase-separation model. This model takes advantage of the fact that micellization resembles two phases separated by a monolayer. However, the model does not take into account changes in energy associated with the interactions of charges and is suitable only for describing non-ionic surfactants. In the phase-separation model, there are two distinct phases: alpha(α) and beta(β). The surface tension between the two phases is described by the Laplace Equation, which relates the change in pressure across two phases to the curvature and surface tension.

$\Delta P=\gamma (C_{1}+C_{2})$

where:

$\Delta P$ is the change in pressure across the interface

$\gamma$ is the surface tension

$C_{1}$and $C_{2}$ are the curvatures of the selected interface

The chemical potential, μ, at low concentrations can be described by the equation:

$\mu _{sur}(micelle)=\mu _{sur}^{0}+RTln[S]$

When [S] (the concentration of surfactant) reaches the critical micelle concentration, the chemical potential of the surfactant in the micelle is equal to the chemical potential of the surfactant when it is solvated. Thus, the Gibbs free energy of micelle formation is described by the equation:

$\Delta G_{m}^{mic}=\mu _{micelle}-\mu _{sur}^{0}=RTlnCMC$

The main factor driving the formation of micelles, the movement of aliphatic compounds out of water and into an environment where they can interact with other non-polar groups, is driven by entropy. Although there is ordering occurring by means of a phase separation, the entropy gained by the water molecules interacting with other water molecules is far greater in magnitude.

Ionic surfactants

U.S. Air Force C-130 plane releases dispersants over the [[Deepwater Horizon oil spill]].

Describing the formation of micelles mathematically for ionic surfactants is far more difficult because of the repulsion that occurs between the head groups as the micelle is formed. The surfactant molecules must also be dehydrated prior to micelle formation (which decreases the shielding of each head group, thus increasing the repulsion between two molecules). For this reason, the critical micelle concentration (CMC) of ionic surfactants tends to be higher than that of their non-ionic counterparts. When addressing ionic surfactants, one must consider the electric double layer that forms at the surface of the micelle. This double layer has the effect of stabilizing the micelle by shielding the like charges from each head group. Adding salt to ionic surfactants has the effect of drastically reducing the CMC. The salt increases the concentration of ions available to screen the charge of the ionic head groups, and will thus make it easier for particle aggregation to occur. Another method to reduce the CMC of an ionic micelle is to increase the length of the alkyl chain, increasing the amount of dispersion interactions and thus making micelle formation more energetically favorable.

Ionic micelles are very difficult to describe mathematically due to the repulsion occurring between all head groups, the number of variables and the fact that the electric potential felt by each head group is dependent on the other groups.

Surfactant types

There are four main types of surfactants, 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. In general, compounds with an HLB between one and four will not mix with water. Compounds with an HLB value above 13 will form a clear solution in water. Oil dispersants usually have HLB values from 8–18.

HLB values for various surfactants

Surfactant	Structure	Avg mol wt	HLB
Arkopal N-300	C9H19C6H4O(CH2CH2O)30H	1,550	17.0
Brij 30	polyoxyethylenated straight chain alcohol	362	9.7
Brij 35	C12H25O(CH2CH2O)23H	1,200	17.0
Brij 56	C16H33O(CH2CH2O)10H	682	12.9
Brij 58	C16H33O(CH2CH2O)20H	1122	15.7
EGE Coco	ethyl glucoside	415	10.6
EGE no. 10	ethyl glucoside	362	12.5
Genapol X-150	C13H27O(CH2CH2O)15H	860	15.0
Tergitol NP-10	nonylphenolethoxylate	682	13.6
Marlipal 013/90	C13H27O(CH2CH2O)9H	596	13.3
Pluronic PE6400	HO(CH2CH2O)x(C2H4CH2O)30(CH2CH2O)28-xH	3000	N.A.
Sapogenat T-300	(C4H9)3C6H2O(CH2CH2O)30H	1600	17.0
T-Maz 60K	ethoxylated sorbitan monostearate	1310	14.9
T-Maz 20	ethoxylated sorbitan monolaurate	1226	16.7
Triton X-45	C8H17C6H4O(CH2CH2O)5H	427	10.4
Triton X-100	C8H17C6H4(OC2H4)10OH	625	13.6
Triton X-102	C8H17C6H4O(CH2CH2O)12H	756	14.6
Triton X-114	C8H17C6H4O(CH2CH2O)7.5H	537	12.4
Triton X-165	C8H17C6H4O(CH2CH2O)16H	911	15.8
Tween 80	C18H37-C6H9O5-(OC2H4)20OH	1309	13.4

Comparative industrial formulations

Two formulations of different dispersing agents for oil spills, DISPERSIT and Omni-Clean, are shown below. A key difference between the two is that Omni-Clean uses ionic surfactants and DISPERSIT uses entirely non-ionic surfactants. Omni-Clean was formulated for little or no toxicity toward the environment. DISPERSIT, however, was designed as a competitor with COREXIT. DISPERSIT contains non-ionic surfactants, which permit both primarily oil-soluble and primarily water-soluble surfactants. The partitioning of surfactants between the phases allows for effective dispersion.

Omni-Clean OSD				DISPERSIT
Category	Ingredient		Function	Category	Ingredient		Function
Surfactant		Sodium lauryl sulfate	Charged ionic surfactant and thickener	Emulsifying agent		Oleic acid sorbitan monoester	Emulsifying agent
Surfactant		Cocamidopropyl betaine	Emulsifying agent	Surfactant		Coconut oil monoethanolamide	Dissolves oil and water into each other
Surfactant		Ethoxylated nonylphenol	Petroleum emulsifier & wetting agent	Surfactant		Poly(ethylene glycol) monooleate	Oil-soluble surfactant
Dispersant	File:Cocamide DEA.png	Lauric acid diethanolamide	Non-ionic viscosity booster & emulsifier	Surfactant		Polyethoxylated tallow amine	Oil-soluble surfactant
Detergent		Diethanolamine	Water-soluble detergent for cutting oil	Surfactant		Polyethoxylated linear secondary alcohol	Oil-soluble surfactant
Emulsifier		Propylene glycol	Solvent for oils, wetting agent, emulsifier	Solvent		Dipropylene glycol methyl ether	Enhances solubility of surfactants in water and oil.
Solvent	H2O	Water	Reduces viscosity	Solvent	H2O	Water	Reduces viscosity

Degradation and toxicity

Both the degradation and the toxicity of dispersants depend on the chemicals chosen within the formulation. Compounds which interact too harshly with oil dispersants should be tested to ensure that they meet three criteria:

• They should be biodegradable.
• In the presence of oil, they must not be preferentially utilized as a carbon source.
• They must be nontoxic to indigenous bacteria.

Applications

Oil treatment

The effectiveness of the dispersant depends on the weathering of the oil, sea energy (waves), salinity of the water, temperature and the type of oil. Dispersion is unlikely to occur if the oil spreads into a thin layer, because the dispersant requires a particular thickness to work; otherwise, the dispersant will interact with both the water and the oil. More dispersant may be required if the sea energy is low. The salinity of the water is more important for ionic-surfactant dispersants, since they will preferentially interact with the water more than the oil. The viscosity of the oil is another important factor; viscosity can retard dispersant migration to the oil-water interface and also increase the energy required to shear a drop from the slick. Viscosities below 2,000 centipoise are optimal for dispersants. If the viscosity is above 10,000 centipoise, no dispersion is possible.

Methods of use

Dispersants are delivered in concentrated, dilute solutions and are aerosolized by aerial spraying (typically by an aircraft or boat). Sufficient dispersant with droplets in the proper size are necessary; this can be achieved with an appropriate pumping rate. Droplets larger than 1,000 µm are preferred, to ensure they are not blown away by the wind. The ratio of dispersant to oil is typically 1:20.

Oil spills and dispersants used

Exxon Valdez

Main article: Exxon Valdez oil spill

Alaska had fewer than 4,000 gallons of dispersants available at the time of the Exxon Valdez oil spill, and no aircraft with which to dispense them. The dispersants introduced were relatively ineffective due to insufficient wave action to mix the oil and water, and their use was shortly abandoned.

Deepwater Horizon

During the Deepwater Horizon oil spill, an estimated 1.84 million gallons of oil dispersants were used in an attempt to reduce the amount of surface oil and mitigate the damage to coastal habitat. Nearly half (771,000 gallons) of the dispersants were applied directly at the wellhead. The primary dispersant used was Corexit, which was controversial due to its toxicity.