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Table of Contents
|Van der Waals Forces
Parts of an Atom
Protons, Neutrons & Electrons
|The Triboelectric Series
Particles Charged with Static Electricity
Spores and Fumed Silica
The Heart of the Debate
What Happens in a Ball Mill?
|Chemists and engineers often work with particles in the 1-5 micron
range, which is also the range where inhaled Bacillus anthracis
spores can result in inhalation anthrax. In most cases, however,
the chemists and engineers are working with tiny particles made from a
single substance. In the pharmaceutical industry, for example, they
may work with tiny particles of lactose or corn starch. These particles
tend to bind together due to van der Waals forces. It appears that
the assumption is made that van der Waals forces will cause spores to bind
together in a similar manner. That assumption is untrue.
According to Wikipedia,
In physical chemistry, the name van der Waals force refers to the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. The term includes:and
All intermolecular/van der Waals forces are anisotropic (except those between two noble gas atoms), which means that they depend on the relative orientation of the molecules.Dipole-dipole forces pertain to polar molecules. Polar molecules are like little magnets. When two magnets are oriented with a positive end of one near the negative end of the other, they will attract each other and bind tightly together. If they are aligned with ends of the same polarity near each other (positive to positive or negative to negative) they will repulse each other.
Dipole-induced dipole forces are similar, except that they are less like permanent magnets and more like electro-magnets. The molecules can switch polarity in order to connect with nearby molecules.
Also, molecules do not just bind together end to end in long strings, they also bind together side to side:
The key point for the purposes of this paper is that Van der Waals forces are forces which act between molecules (and between atoms of noble gases - helium, neon, argon, krypton, xenon, radon, which do not relate to this discussion). While it may sometimes appear that van der Waals forces also bind particle to particle, particularly if the particle consists of a single chemical substance comprised of polar molecules, but, in reality, that is a misunderstanding or distortion of what happens.
When a particle of lactose, for example, touches another particle of lactose, there is no inherent "particle force" to bind them together (other than gravity). However, the touching of the particles means the molecules of lactose within one particle are touching molecules of lactose in another particle. That can have a binding effect. The binding force is the molecule to molecule force, not something that pertains to the lactose particle itself or to particles in general.
What happens between two particles of lactose is very similar to
what happens between two droplets of water of the same size. It just
happens a lot slower because lactose does not have the high viscosity (flowability)
of water. Here is a series of illustrations which show how such particles
bind together because of the way the molecules within them bind
|This binding effect due to van der Waals forces creates problems
in the pharmaceutical industry where companies have developed unique
processes to coat spheres of chemical substances with fumed silica to prevent
the spheres from touching, binding and merging. Examples: corn
starch and lactose
as seen in this SEM image:
In the field of microbiology, droplets of nutrients are coated with silica in a patent filed by Charles Bailey and Ken Alibek to create tiny cultures where needed bacteria can grow and reproduce in a process that greatly reduces the risk of contamination of an entire production run by other unwanted bacteria.
But Bacillus spores are nothing like those particles of chemicals
Dry objects such as a Bacillus spores thoroughly and individually dried by spray drying or similar methods will not bind together due to van der Waals forces in any significant way for at least four reasons:
#1. A spore's dryness and semi-rigidity reduces its ability to distort and merge with the other object.
Whenever I discuss how dry spores are different from lactose particles, scientists will inevitably remind me that truly dry spores exist only in a vacuum. As soon as dry spores are exposed to the air, they are also exposed to moisture in the air.
Just as moisture in the air will cause your instant coffee crystals to clump if you do not keep the jar tightly sealed, moisture will also bind together spores if the spores are left in the open air for a long period of time where they can absorb moisture from the air. On the other hand, the outer coating of an anthrax spore (the exosporium) is relatively hydrophobic, which means it will not absorb moisture as easily as many other substances or objects. Nevertheless, if and when spores absorb moisture from the air, the molecules of water on the outer surface of one spore will attract the molecules of water on the outer surface of another spore via capillary action, and the two spores will stick together due to surface tension force.
To avoid the binding effect moisture will have on spores, water-absorbant silica is often added to the spore powder to absorb the moisture before it can affect the spores. The silica will not only capture moisture from the air more easily, it can also extract moisture from the spores.
In the attack anthrax situation, the dry spores were put into envelopes which provided some protection against absorbing moisture. (The New York Post letter was left in a humid area for weeks before it was found. The powder inside had turned into something described by General Parker as " looking like Purina Dog Chow, clumpy like a pellet.")
In summary, gravity is too weak to bind together spores in any significant way, and the binding effect of van der Waals forces between molecules in adjacent spores are also too weak to have any significant affect -- as long as the spores are totally dry.
The primary force that will bind spores together under common
circumstances is a very different kind of force.
It appears that many of the misunderstandings about spores result from confusing van der Waals forces with static electricity, a.k.a. electrostatic interactions, which are not one of van der Waals forces. Static electricity can be a strong particle to particle binding force, as long as the particles are not of the same substance. Static electricity will bind spores to other objects if measures aren't taken to prevent it. This is particularly true when it comes drying spores in hot air, and even more so when spores are freeze dried into pellets of trillions of spores and the pellets are then milled to break the pellets down into individual spores. Charged spores can stick to every surface they touch.
Everyone knows about "static cling" which results from drying clothes in a dryer without the use of some kind of static reducer. And they know it is an effect of static electricity. But what is static electricity? What is an electrical charge? In order to answer that, we need to understand the construction of an atom.
An atom consists of a "nucleus" and electrons orbiting around it. The nucleus at the center is made up of two different types of tiny particles: protons and neutrons. The 115 kinds of atoms are different from each other because they have different numbers of protons, neutrons and electrons.
Protons, neutrons and electrons are very different from each other. They have their own properties, or characteristics. One of these properties is called an electrical charge. Protons have a "positive" (+) charge. Electrons have a "negative" (-) charge. Neutrons have no charge, they are neutral. The charge of one proton is equal in strength to the charge of one electron. When the number of protons in an atom equals the number of electrons, the atom itself has no overall charge, it is neutral.
The protons and neutrons in the nucleus of an atom are held together very tightly and do not normally change. Breaking a nucleus into parts is called nuclear fission. Adding something to a nucleus is called nuclear fusion. Moving electrons from one atom to another, however, is done relatively easily. That's because some of the outer electrons orbiting a nucleus are held very loosely in their orbits. When electrons move from one atom to another, the effect is not as dramatic as nuclear fission or nuclear fusion. An atom that loses electrons simply has more protons (positive charges) than electrons (negative charges). It is therefore positively charged. An atom that gains electrons simply has more negative than positive particles. It has a negative charge.
A negative or positive charged atom is called an "ion."
Anyone can create ions. It's easiest to do in dry weather when electrical charges in materials are not easily adjusted by the atoms which make up moisture in the air.
One common way to create ions in dry weather is with friction, by rubbing items made from two different materials together. This causes electrons to move from one object to another. The more rubbing, the more electrons move, and the larger the static charge that builds up. ( Scientists believe that it is not the rubbing or friction that causes electrons to move. It is simply the contact between two different materials. Rubbing just increases the contact area between them.) Static charges can also be induced by pulling materials apart. Pulling a strip of Scotch tape off of an object will put a static charge into the tape.
How many electrons get transferred when two materials are rubbed together (or pulled apart) depends upon the materials. Different materials cause different numbers of electrons to be transferred in different directions. The more electrons that get moved, the greater the static electrical charges that result.
triboelectric series is a list that ranks various materials according
to their tendency to gain or lose electrons. It usually lists materials
in order of decreasing tendency to charge positively (lose electrons),
and increasing tendency to charge negatively (gain electrons). Somewhere
in the middle of the list are materials that do not show strong tendency
to behave either way. It's important to understand that the tendency
of a material to become positive or negative after triboelectric charging
has nothing to do with the level of conductivity
(or ability to discharge) of the material.
A charged object will also attract something that is neutral. Think about how you can make a balloon stick to the wall. If you charge a balloon by rubbing it on your hair, it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very slightly to one side, away from the balloon. In either case, there are more positive charges closer to the negative balloon. Opposites attract. The balloon sticks. (At least until the electrons on the balloon slowly leak off.) It works the same way for neutral and positively charged objects.
And everyone who does any milling knows that static electricity is
generated and can have catastrophic effects if precautions aren't taken.
(Milling grains will put tiny particles into the air which can cause an
explosion if a static spark ignites the particles.)
It's also important to understand that static electricity works over significantly greater distances than van der Waals forces. As with gravity, an object charged with static electricity will attract other objects without actually touching them. An object with an electrical charge has an electrical field around it.
An electric field is a region where electrical force acts on a charge. The direction of a field is always outward from a positive charge and inward towards a negative charge.
The illustration above left shows the electric field of a positive charge. The one above right shows that of a negative charge. The arrows indicate field lines which go out from the positively charged object and into the negatively charged object. Field lines are a way of illustrating the direction of the electrical field.
Fumed silica is frequently used to help keep spores dry. The fumed silica particles will more readily absorb moisture due to their structure and their chemical composition.
A typical fumed silica particle is feather-like or cotton-like in appearance. It consists of tiny strands of nano-spheres of silica fused end to end. Under extreme magnification, a few of the strands would look like this:
When affixed to a spore, the cotton-like particles look like the image on the right below:
In September of 2001, someone sent five (or more) letters filled with a crude form of dry anthrax powder to media organizations in New York City and in Florida. When those letters failed to accomplish what the sender wanted, the same person then sent two more letters -- this time filled with pure spores -- to two U.S. Senators, Tom Daschle of South Dakota and Patrick Leahy of Vermont. As a result of these two attacks, 22 people were infected with anthrax, and five of them died. None were the intended recipients of the deadly letters.
It took awhile for the two different anthrax powders to get to their destinations and to have their affect, and it took awhile for the anthrax cases to be correctly diagnosed, since anthrax infections are very rare, particularly in New York City and Florida.
When the first case was properly diagnosed (Bob Stevens in Florida) and the first letters were found (first in Washington D. C., then in New York City), debates began almost immediately over whether or not the powders in the letters (particularly the Senate letters) were "weaponized." Those who assumed the letters were sent by the 9/11 terrorists or by an agent of some foreign government looked at the facts and saw what they wanted to see. Conspiracy theorists with a different political agenda argued that the spores must have been "weaponized" in a sophisticated and illegal U.S. Government bioweapons lab, and they saw what they wanted to see when they looked at the facts.
Matters weren't helped by the fact that the scientists examining the contents of the letters had little or no experience with anthrax powders, nor by the fact that the media was ready to report on any rumor or belief as if they were solid facts. Some initial reports were totally wrong. Others were misinterpretations of misunderstandings.
In spite of the countless victims of inhalation anthrax who had died from aerosolized anthrax spores over the millennia, and in spite of the fact that anthrax was the first disease to be proven to come from bacteria, and in spite of the fact that anthrax spores were first observed in a lab by Robert Koch in the 1870's, there were many scientists and others who insisted that the spores in the senate letters must have been "weaponized" for them to do what they did -- and the weaponization must have been accomplished in a super-sophisticated government lab in violation of international law.
The basis for their argument was a false belief that spores would stick together due to van der Waals forces if the spores were not "weaponized" with a coating of silica or a similar substance such as bentonite. When Science magazine printed a description of exactly how this imaginary process was accomplished, countless scientists accepted it as a fact, even though the article was just a conspiracy theory written by a reporter, not a scientist. The article, "Anthrax Powder - State of the Art?" is probably still being cited, quoted and believed today. (The equally bad Washington Post article titled "FBI's Theory On Anthrax Is Doubted" may have been believed by even more people.)
In the Science Magazine article, the author identified the two factions involved in the dispute this way:
One group, comprised mostly of microbiologists and molecular biologists, argues that this material could have been a do-ityourself job, made by someone knowledgeable but with run-of-the-mill lab equipment on a modest budget. This contingent includes one well-known bioweaponeer, Ken Alibek, who defected from Russia to the United States in 1992.The "scientific problem" was described this way:
Anthrax spores cling to one another if they get too close; sticky chains of proteins and sugar molecules on their surfaces latch onto each other, drawn by van der Waals forces that operate at a distance of a few tens of angstroms. Untreated spores clump into larger particles that are too heavy to stay airborne or reach the narrowest passages in the lung.The beliefs began with this:
The Senate anthrax spores carried like electrical charges, and some experts believe that they were added deliberately to aid dispersal.And the beliefs were described in greater detail this way:
More revealing than the electrostatic charge, some experts say, was a technique used to anchor silica nanoparticles to the surface of spores. About a year and a half ago, a laboratory analyzing the Senate anthrax spores for the FBI reported the discovery of what appeared to be a chemical additive that improved the bond between the silica and the spores. U.S. intelligence officers informed foreign biodefense officials that this additive was “polymerized glass.” The officials who received this briefing—biowarfare specialists who work for the governments of two NATO countries—said they had never heard of polymerized glass before.The article actually has drawings showing how silica particles were glued to spores with polymerized glass to act as "spacers" to help defeat van der Waals forces. According to this argument, it was all based upon the way they weaponized spores during the Cold War. And those who believed this way argued their beliefs for years.
Then some facts about how they actually weaponized anthrax spores during the Cold War were published in a scientific article. The weaponized anthrax spores did indeed have a "coating" of fumed silica. But everything else was different from what was believed, including how and why the spores were coated with silica.
The scientific article was published in the 1 March, 2008, issue of Aerosol Science and Technology, and it was titled "Development of an Aerosol System for Uniformly Depositing Bacillus Anthracis Spore Particles on Surfaces." The authors were scientists from Dugway Proving Grounds in Utah, where the "weaponized" simulants used in the sampling tests had been made, and scientists from the CDC's National Institute for Occupational Safety and Health in Cincinnati who were interested in sampling methods. The authors believed what they had read in the media about the anthrax spores used in the 2001 attacks being coated with silica, and they based their study upon that belief.
The article contained two very enlightening images of weaponized simulant spores. Here they are:
|The text of the article also described a mystery they hadn't been
able to (and/or hadn't found any need to) solve:
Figure 7a shows a particle potentially containing a single BG spore; since no uncoated single spores were observed, this suggests that virtually all single spores remained coated with silica. The coating apparently solidified from exposure to water in the air over the years of sample storage and use. However, multiple spores or clumps were found frequently and these were often largely uncoated as indicated in Figure 7b. The reason for the difference in coating adherence to different sized particles is unclear.In an exchange of emails with one of the authors, I was told that the authors also could not agree on what caused the silica particles to bind to the single spores. It was, evidently, another mystery which didn't really require an answer in order for them to do their study of sampling methods.
The solution to these two "mysteries," however, could be the solution to the debate over whether or not the spores in the anthrax attacks of 2001 must have been "weaponized" in order to do what they did.
No one actually saw any silica coatings or additives when the attack spores were examined under either a Transmission Electron Microscope (TEM) or a Scanning Electron Microscope (SEM). However, that hasn't resolved the debate because the conspiracy theorists have been arguing physics. They claim, in effect, if the physics of tiny particles require that the spores be coated with silica in order to do what they did, then the eye-witness testimony (the empirical evidence) must be wrong, i.e., the eye-witnesses must either be incompetent or part of some vast criminal conspiracy to mislead the American people about some secret and illegal bioweapons program.
That's the heart of the debate: Physics.
Photographs of the attack spores have never been released. All we've seen is an image of a "reference sample" similar in character to what was in the Daschle envelope. It was published in Richard Preston's book "The Demon In The Freezer." Here it is:
The particles of silica clearly visible in the Dugway images are nowhere to be seen in this "reference sample."
But what about the questions posed by the Aerosol Science article?
1. Why were single spores coated while spore clumps were not?One answer becomes immediately clear: The force in question #2 cannot be van der Waals forces because van der Waals forces would affect multiple spores the same way they'd affect single spores. In other words, van der Waals forces in a single spore would be no different than van der Waals forces in a spore stuck to another spore. Every spore should attract silica particles equally.
If the silica particles are not bound to the spores by van der Waals forces, what force does bind the silica to the spores?
The answer to that question can be found in how the spores were "weaponized" as described in the Aerosol Science article:
Spores were collected by simple centrifugation to remove spent media. The pelleted material was dried by a proprietary azeotropic method. Ten percent (by weight) of an amorphous silica-based flow enhancer was added to the dried spores. The dried material was milled using an exclusionary ball mill. In this process the material passed through a series of stages separated by increasingly finer mesh screens. In each stage 0.01 m diameter steel balls forced the product through the screen separators. A pneumatic vibrator actuated the entire mill.Milling causes friction, as does the pneumatic vibrator which actuated the entire mill.
Two substances were put into the mill: fumed silica and spores. In the Triboelectric Series chart above, spores would be among the "most positive" and fumed silica particles would be among the "most negative." When friction is applied in a ball mill, that is literally a recipe for binding two substances together with static electricity.
But something else also happens to the fumed silica inside the ball mill: The fragile strands of nano-spheres are crushed and broken into tiny pieces. The resulting tiny pieces are considerably smaller than a spore. The friction of the milling process will result in spores with positive charges and tiny silica particles with negative charges:
And this, of course, will result in the negative charged silica particles clinging to the positive charged spore:
And because the charged spore is much larger than the tiny silica particles, its electrical field should also be much larger. The tiny particles of crushed silica will not only cling to the spore, they will also pile onto the spore as every silica particle which comes within the spores positive electrical field is drawn in. If there are more than enough particles to completely cover the spore, the end result looks like this:
That still leaves one unanswered question:
1. Why were single spores coated while spore clumps were not?The answer seems obvious: The uncoated clumps were small enough to fall through the mesh screens before there was time for a static charge to develop.
There may be other reasons as well, answers having to do with the shapes of the spore clumps and how those shapes affect an electrical field, or how a static charge is dispersed in a clump of spores. However, having multiple explanations is better than having no explanation for why van der Waals forces didn't bind silica to the spore clumps the same way it did to individual spores. An argument might be made for a why a single clump or two were not affected, but not for a pattern of coated individual spores and largely uncoated clumps.
Furthermore, the coated Dugway spore shows the "piling on" effect that is associated with static electricity. The spore had a positive electrical field strong enough to not only attract silica particles which could directly touch the spore's surface, but it also held in place silica particles which couldn't actually touch the spore's surface due to other silica particles being in the way. That's an effect that cannot be explained with by der Waals forces. If van der Waals forces caused silica particles to "pile on" a spore, the silica particles would also pile on each other and everything would stick to everything.
Lastly, in the Dugway process, an additional 10% by weight of fumed silica was added to the powder after it had exited the ball mill. Van der Waals forces would be identical on both sides of the mesh filters. If van der Waals forced could bind silica to the spores in the mill, those forces could also bind the silica to the spores after the milling process was completed. It didn't happen. On the other hand, if there was not enough static charge on the spore to bind silica to the spore before it gets through the mesh filter, there wouldn't be enough on the exit side of the mesh filter, either.
The argument that silica particles are glued to spores with "polymerized glass" in order to defeat van der Waals forces is scientifically invalid because (1) there is no reason for such a process, (2) there is no known technology to do it, and (3) the Dugway process did not apply any glue.
The argument that the silica particles in the Dugway photographs cling to the spores due to van der Waals forces is invalid because the argument provides no explanation for why the silica does not stick to the clumps. Milling doesn't generate any van der Waals forces. And additional silica is added after milling, where van der Waals forces in each particle would still be in full effect (whatever that effect is), yet that additional silica didn't stick either. If van der Waals forces were the binding forces, the silica should totally coat the spores in the clumps just as happens with the individual spores.
Conclusion: The Dugway spores were coated with silica due to the effects of static electricity generated during milling, not due to van der Waals forces. And where van der Waals forces were the only binding forces present - in the clumps - the binding effect between the spores and the particles of silica was insignificant.
If anyone sees any errors in this logic and in this explanation, please contact me at firstname.lastname@example.org to explain exactly where and how I'm wrong.
June 7, 2008: Added more information about moisture. Added more information about how spore surfaces affect van der Waals forces. Added the Table of Contents.
June 12, 2008: Added information to the "Moisture" section about the exosporium being hydrophobic and how silica can extract moisture from a spore.
October 1, 2008: Changed comments about moisture to show that surface tension force would bind wet spores together, not van der Waals forces.
© 2008 by Ed Lake
All Rights Reserved