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Droplet Size, Which BP Cannot Control, Is Critical to Biological Effects of Dispersed Oil


As the controversy over BP’s refusal to switch to a less toxic dispersant rages on, it seems reasonable to take a look at the underlying chemistry and biochemistry of dispersants and oil which has been dispersed into water.

Virtually all of the biochemistry of life takes place in water-based, or aqueous, solutions. In order to control the conditions under which these chemical reactions take place, individual cells must separate their own internal aqueous environment from any water surrounding them.

Individual water molecules are polar, meaning that localized areas of positive and negative charge exist because the oxygen atom in each water molecule holds onto electrons more strongly than the two hydrogen atoms attached to it. As a result, there is a partial negative charge on the oxygen atom and partial positive charges on each of the two hydrogen atoms in a water molecule. These charges dictate how water molecules interact with one another, and in this case, the old adage that “opposites attract” is the rule.

The opposite of a polar compound, like water, where charges develop, is a nonpolar compound, where charges do not develop. The most common nonpolar compounds have long, “straight” chains of carbon atoms attached to one another, where the repeating unit in this chain is one carbon atom with two hydrogen atoms attached to it, commonly referred to as hydrocarbons. Oils are a class of hydrocarbons. In biochemistry, fats, or lipids, also have hydrocarbon characteristics.

The reason oil and water don’t mix is that in seeking out opposite charges, the polar water molecules aggregate with one another while excluding the nonpolar oil molecules which can’t join the mix because they don’t carry a partial charge.

Cells take advantage of this separation of oil and water to protect their internal environment from the outside world. Through the use of special molecules classed as phospholipids, cells surround themselves with an oil-like coating that prevents free diffusion of material dissolved in the internal aqueous environment to the outside aqueous environment. Phospholipids are large molecules with both polar and nonpolar regions. The polar ends of phospholipids orient themselves into aqueous environments while the nonpolar ends orient themselves away from water. Cell membranes have two layers of phospholipids, with the net result being that the cell contents are surrounded by a layer of lipid with one polar side of the bilayer being the inside of the cell and the other polar side of the bilayer being the outside of the cell.

The following illustration from Wikimedia Commons shows a schematic of a phospholipid bilayer and fortuitously leads us into the related chemistry of dispersants. For now, look only at part 1 of the illustration. The red balls represent the charged portions of the phospholipid molecules, and the long chains emanating from them are the lipid component with the repeating units of carbons with two hyrdogens:

bilayer and micelle
(Image: Steven Gilbert on Wikimedia Commons)

Although the illustration shows only a cross-section of a small stretch of phospholipid bilayer, the viability of cells is highly dependent on an intact bilayer surrounding the entire cell.

With that long introduction to the immiscibility of oil and water and the structure of cell membranes, we are ready to consider the dispersion of oil in water through the use of dispersants. We can rely here on a book from the National Academy of Sciences Press, “Oil Spill Dispersants: Efficacy and Effects“.

Fortunately, NAS provides a widget for embedding the book. The discussion below addresses pages 52 to 56 of the book.

Looking at Figure 3-1 on page 53, what should stand out most is that the schematic drawing of a surfactant molecule, which is the active ingredient in a dispersant mixture, is the same as the schematic of a phospholipid. The surfactant molecules, like phospholipids, have both polar (hydrophilic or water-attracted) and nonpolar (lipophilic, or lipid-attracted) regions.

When mixed into water with no oil present, the surfactant molecules from a dispersant mix would naturally form micelles as shown in part 2 of the Steven Gilbert illustration above. As shown in Figure 3-1 in the book, dispersant in the presence of a surface slick of oil would be expected to “break off” droplets of oil with surfactant coating.

In the deepwater situation, as most of you have seen on the web cam of the spill, a stream of dispersant is directed at the oil leak, relying on the turbulence of the spewing oil to mix the oil and dispersant.

Mixtures with dispersed droplets of surfactant-coated oil in water are also called emulsions. Biologists growing, or culturing, organisms in solution rely on emulsions to provide lipids if the organisms require lipids for growth. From my own direct experience with emulsifying lipids into growth medium, I can state that creating emulsions is a very “tricky” process highly dependent on both the chemical and physical conditions under which they are being made. The surfactant chosen, its concentration, temperature, presence of other compounds in the water phase and the method used for mixing all have profound effects on the type of emulsion formed, its stability and the size of the droplets. Especially for the dispersant release at the Gulf floor, it is not a stretch to say that BP cannot control all of the factors which affect droplet formation, and therefore, BP cannot control droplet size as the streams of oil and dispersant mix.

Going back to the issue of emulsions used to deliver lipids to organisms for growth, droplet size in these emulsions is perhaps the most critical parameter in how available the lipid will be for the organism to take it up as a nutrient. In the Gulf, this same argument holds, but it has both good and bad sides. Some of the naturally occurring bacteria in the Gulf will be able to metabolize the oil, providing bioremediation of the spill. However, the oil itself is quite toxic to virtually all other organisms, and making it available to them for uptake means that the dispersed oil will be much more toxic than oil that remains in large slicks. Hence this statement on page 49 of the embedded book:

Ironically, as the effectiveness of dispersant increases, so does the potential threat to organisms exposed to the dispersed plume, due to the increased concentration of dissolved compounds and dispersed droplets in the water column. In open deep water, it may be reasonable to assume rapid dilution of the plume would take place. It is a generally held view, however, that such dilution should not be expected in shallower waters; hence a general avoidance of the use of dispersants in shallower waters exists.

I would add to that statement that the longer a high rate of oil release continues, the less reliable is the assumption that the dispersed plume in deep water will dilute rapidly. Considerations along these lines most likely were behind the call by the EPA Monday afternoon to reduce the amount of dispersant being used.

Finally, there is one more area where biologists use surfactants. When biochemists wish to isolate compounds from cells, they must first break the cells open. Often, surfactants are employed to accomplish cell lysis. See, for example, this listing of such surfactants by a scientific supplier. One of the more commonly used agents on that list is a surfactant called Tween 80. Going back to the dispersant book, we see on page 55 that Corexit 9500 has about 48% “ethoxylated sorbitan mono- and trioleates and sorbitan monooleate”. Referring to the MSDS for Tween 80, we see this list of synonyms: “Polyoxyethylene 20 sorbitan monooleate; Polyethylene oxide sorbitan mono-oleate; Polyoxyethylene sorbitan monooleate; Polyoxyethylene sorbitan oleate;”, meaning that Corexit 9500 contains a large amount of material very similar to the Tween 80 product that is used to lyse cells. As an aside, Corexit 9500 also is stated on page 55 to contain 35% sodium dioctyl sulfosuccinate, which we see on Wikipedia is used in pharmacology as a stool softener under the name Docusate.

This ability of the surfactants in Corexit 9500 to lyse cells may well be the primary route to the high toxicity seen for this dispersant when it is compared to other dispersants, although there probably are toxic effects from the hydrocarbon solvents employed, as well.

In summary, then, to understand the biological impacts of dispersed oil from this spill, it would be necessary to understand the spectrum of droplet sizes being generated and the relative sensitivities of the organisms present to these dispersed oil droplets of the stated sizes. Further, it would be necessary to determe the expoure of the organisms present to any free dispersant that does not get bound up in emulsions and disperses as micelles. There is much more material in the embedded book which addresses the current state of scientific understanding for many of these issues surrounding dispersant use. In future diaries, I hope to return to those portions of that material that seem relevant to the ongoing situation.

CommunityMy FDLSeminal

Droplet Size, Which BP Cannot Control, Is Critical to Biological Effects of Dispersed Oil

As the controversy over BP’s refusal to switch to a less toxic dispersant rages on, it seems reasonable to take a look at the underlying chemistry and biochemistry of dispersants and oil which has been dispersed into water.

Virtually all of the biochemistry of life takes place in water-based, or aqueous, solutions. In order to control the conditions under which these chemical reactions take place, individual cells must separate their own internal aqueous environment from any water surrounding them.

Individual water molecules are polar, meaning that localized areas of positive and negative charge exist because the oxygen atom in each water molecule holds onto electrons more strongly than the two hydrogen atoms attached to it. As a result, there is a partial negative charge on the oxygen atom and partial positive charges on each of the two hydrogen atoms in a water molecule. These charges dictate how water molecules interact with one another, and in this case, the old adage that "opposites attract" is the rule.

The opposite of a polar compound, like water, where charges develop, is a nonpolar compound, where charges do not develop. The most common nonpolar compounds have long, "straight" chains of carbon atoms attached to one another, where the repeating unit in this chain is one carbon atom with two hydrogen atoms attached to it, commonly referred to as hydrocarbons. Oils are a class of hydrocarbons. In biochemistry, fats, or lipids, also have hydrocarbon characteristics.

The reason oil and water don’t mix is that in seeking out opposite charges, the polar water molecules aggregate with one another while excluding the nonpolar oil molecules which can’t join the mix because they don’t carry a partial charge.

Cells take advantage of this separation of oil and water to protect their internal environment from the outside world. Through the use of special molecules classed as phospholipids, cells surround themselves with an oil-like coating that prevents free diffusion of material dissolved in the internal aqueous environment to the outside aqueous environment. Phospholipids are large molecules with both polar and nonpolar regions. The polar ends of phospholipids orient themselves into aqueous environments while the nonpolar ends orient themselves away from water. Cell membranes have two layers of phospholipids, with the net result being that the cell contents are surrounded by a layer of lipid with one polar side of the bilayer being the inside of the cell and the other polar side of the bilayer being the outside of the cell.

The following illustration from Wikimedia Commons shows a schematic of a phospholipid bilayer and fortuitously leads us into the related chemistry of dispersants. For now, look only at part 1 of the illustration. The red balls represent the charged portions of the phospholipid molecules, and the long chains emanating from them are the lipid component with the repeating units of carbons with two hyrdogens:

bilayer and micelle
(Image: Steven Gilbert on Wikimedia Commons)

Although the illustration shows only a cross-section of a small stretch of phospholipid bilayer, the viability of cells is highly dependent on an intact bilayer surrounding the entire cell.

With that long introduction to the immiscibility of oil and water and the structure of cell membranes, we are ready to consider the dispersion of oil in water through the use of dispersants. We can rely here on a book from the National Academy of Sciences Press, "Oil Spill Dispersants: Efficacy and Effects".

Fortunately, NAS provides a widget for embedding the book. The discussion below addresses pages 52 to 56 of the book.

Looking at Figure 3-1 on page 53, what should stand out most is that the schematic drawing of a surfactant molecule, which is the active ingredient in a dispersant mixture, is the same as the schematic of a phospholipid. The surfactant molecules, like phospholipids, have both polar (hydrophilic or water-attracted) and nonpolar (lipophilic, or lipid-attracted) regions.

When mixed into water with no oil present, the surfactant molecules from a dispersant mix would naturally form micelles as shown in part 2 of the Steven Gilbert illustration above. As shown in Figure 3-1 in the book, dispersant in the presence of a surface slick of oil would be expected to "break off" droplets of oil with surfactant coating.

In the deepwater situation, as most of you have seen on the web cam of the spill, a stream of dispersant is directed at the oil leak, relying on the turbulence of the spewing oil to mix the oil and dispersant.

Mixtures with dispersed droplets of surfactant-coated oil in water are also called emulsions. Biologists growing, or culturing, organisms in solution rely on emulsions to provide lipids if the organisms require lipids for growth. From my own direct experience with emulsifying lipids into growth medium, I can state that creating emulsions is a very "tricky" process highly dependent on both the chemical and physical conditions under which they are being made. The surfactant chosen, its concentration, temperature, presence of other compounds in the water phase and the method used for mixing all have profound effects on the type of emulsion formed, its stability and the size of the droplets. Especially for the dispersant release at the Gulf floor, it is not a stretch to say that BP cannot control all of the factors which affect droplet formation, and therefore, BP cannot control droplet size as the streams of oil and dispersant mix.

Going back to the issue of emulsions used to deliver lipids to organisms for growth, droplet size in these emulsions is perhaps the most critical parameter in how available the lipid will be for the organism to take it up as a nutrient. In the Gulf, this same argument holds, but it has both good and bad sides. Some of the naturally occurring bacteria in the Gulf will be able to metabolize the oil, providing bioremediation of the spill. However, the oil itself is quite toxic to virtually all other organisms, and making it available to them for uptake means that the dispersed oil will be much more toxic than oil that remains in large slicks. Hence this statement on page 49 of the embedded book:

Ironically, as the effectiveness of dispersant increases, so does the potential threat to organisms exposed to the dispersed plume, due to the increased concentration of dissolved compounds and dispersed droplets in the water column. In open deep water, it may be reasonable to assume rapid dilution of the plume would take place. It is a generally held view, however, that such dilution should not be expected in shallower waters; hence a general avoidance of the use of dispersants in shallower waters exists.

I would add to that statement that the longer a high rate of oil release continues, the less reliable is the assumption that the dispersed plume in deep water will dilute rapidly. Considerations along these lines most likely were behind the call by the EPA Monday afternoon to reduce the amount of dispersant being used.

Finally, there is one more area where biologists use surfactants. When biochemists wish to isolate compounds from cells, they must first break the cells open. Often, surfactants are employed to accomplish cell lysis. See, for example, this listing of such surfactants by a scientific supplier. One of the more commonly used agents on that list is a surfactant called Tween 80. Going back to the dispersant book, we see on page 55 that Corexit 9500 has about 48% "ethoxylated sorbitan mono- and trioleates and sorbitan monooleate". Referring to the MSDS for Tween 80, we see this list of synonyms: "Polyoxyethylene 20 sorbitan monooleate; Polyethylene oxide sorbitan mono-oleate; Polyoxyethylene sorbitan monooleate; Polyoxyethylene sorbitan oleate;", meaning that Corexit 9500 contains a large amount of material very similar to the Tween 80 product that is used to lyse cells. As an aside, Corexit 9500 also is stated on page 55 to contain 35% sodium dioctyl sulfosuccinate, which we see on Wikipedia is used in pharmacology as a stool softener under the name Docusate.

This ability of the surfactants in Corexit 9500 to lyse cells may well be the primary route to the high toxicity seen for this dispersant when it is compared to other dispersants, although there probably are toxic effects from the hydrocarbon solvents employed, as well.

In summary, then, to understand the biological impacts of dispersed oil from this spill, it would be necessary to understand the spectrum of droplet sizes being generated and the relative sensitivities of the organisms present to these dispersed oil droplets of the stated sizes. Further, it would be necessary to determe the expoure of the organisms present to any free dispersant that does not get bound up in emulsions and disperses as micelles. There is much more material in the embedded book which addresses the current state of scientific understanding for many of these issues surrounding dispersant use. In future diaries, I hope to return to those portions of that material that seem relevant to the ongoing situation.

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Jim White

Jim White

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