Particles, Spores & Van Der Waals Forces
by Ed Lake
NOTE: The questions posed on this web page are largely answered by a new web page which can be accessed by clicking HERE

I've been trying for years to describe - in layman's terms - exactly why dry anthrax spores are not drawn together by van der Waals forces in exactly the same way as various types of particles of the same size.  Manufactured particles of lactose are the prime example. 

If anyone has any suggestions, comments, ideas, thoughts or scientific articles about this subject, please send them to me at  I can also set up a link-serve forum if people wish to discuss this particular subject with others.  No names of people helping me with this problem will be used on this site without permission.

A description of the problem:

Suppose we have two objects of the same size, a particle of lactose and an anthrax spore.  Each is 1 micron in diameter:

It is known by chemists that when you have multiples of the lactose particle, they will tend to bind tightly together due to van der Waals forces:

It is known by microbiologists that when you have multiple dry spores, the effects of van der Waals forces between the spores are negligible.  Any small force can separate them.
Factors to consider and discuss:

Van der Waals forces are intermolecular forces, i.e., forces acting between molecules.  How these forces also affect particles and spores is the question here.  (An article titled "Theory of van der Waals Forces as Applied to Particlate Materials" may hold answers, but the article is difficult to translate into layman's language.)

If lactose particles are made from molecules bound together by Dipole-Dipole attraction, do the particles also have some Dipole-Dipole attraction binding them together?  How does it work?

If other particles are made from molecules bound together by temporary fluctuating dipoles, can that force somehow also bind particles together?  How?

If dry spores are made from many types of molecules, what kind of van der Waals force would bind them together and how would that force work?  It is known that spores can cling to surfaces for a variety of reasons, but do van der Waals forces cause dry spores to bind to each other in any significant way? 

Anthrax spores are known to have a hydrophobic outer layer known as the exosporium.  According to Wikipedia, hydrophobic molecules tend to be non-polar.   According to one source

The hydrophobic nature of spores may allow for their rapid concentration and partial
purification from contaminating materials.
According to another source:
The exosporium confers, then, particular adherence and hydrophobic (water-hating) properties. A predominant trait of bacterial spores - surface hydrophobicity - plays a role, therefore, in the process of spore drying, and has been featured in various Bacillus species, with special reference to influence attributed to the exosporium.
How significant is the fact that dry spores exist in air while many problems with the effects of van der Waals forces on manufactured particles occur in a vacuum?

If you put a pile of hydrophobic spores in a chamber next to a pile of hydrophilic particles of silica or lactose, what effect will the air have on the two different piles?  Would moisture in the spores evaporate into the air?  Would moisture in the air be absorbed by the silica?  

Some people argue that the only factor to be considered here is size, and that all particles of the same size are attracted to each other by van der Waals forces in an identical way.  Some of those same people argue that, "Since anthrax spores and lactose particles are composed of very similar molecules made from the same atoms, their Hamaker constants are almost identical."  And since their Hamaker constants are "almost identical," they must be bound together by van der Waals forces in an almost identical way.  But, since van der Waals forces are forces between molecules, and molecules have many different characteristics, logic says the attraction between spores or even between particles of different substances cannot be identical.   And logic says that Hamaker's constant is of little or no value for determining van der Waals forces affecting a particle the size of a spore.  But logic is not enough.  A solid scientific explanation is needed.

Size definitely is a factor which causes many tiny particles to bind together, but spores do not fall into the same size category as nanoparticles, which are typically much smaller than 1-micron.  Furthermore, many nanoparticles consist of pure elements such as gold or aluminum.  There are no molecules involved, only atoms.  One metal atom is held to another metal atom by metallic bonding, not by van der Waals forces.  And, when dealing with nanoparticles, the forces which bind one tiny nanoparticle of gold to another nanoparticle of gold are the same forces of metallic bonding, not van der Waals forces.

Metallic bonding and van der Waals forces are very different from one another.  Metallic bonding involves the sharing of electrons, whether that sharing is binding atoms together or binding particles together.  The smaller the nanoparticle, the easier it is for one atom in a particle to bind with and share electrons with atoms in another particle.  Van der Waals forces have to do with the shape and polarity of molecules and are electrodynamic in nature.  There is no sharing of electrons.  The positive end of one molecule is attracted to the negative end of another molecule and they are bound together like tiny magnets.  The only atoms which can be bound together by van der Waals forces are the atoms of noble gases (Helium, Neon, Argon, Krypton, Xenon and Radon, which cannot share electrons.)

Except for noble gases, van der Waals forces bind two or more molecules together  That is also very different from covalent bonding and ionic bonding which are the forces which hold two or more different atoms together to form a molecule.

Nanopowders of gold, diamond, aluminum, silica and other materials can be manufactured with diameters much smaller than 1-micron.  Some have diameters less than 1/100th the diameter of a spore.  An anthrax spore is a living entity created by Nature with a diameter that is approximately 1-micron, and it cannot be made significantly smaller.  (Viruses are also made by Nature.  Viruses are significantly smaller than anthrax spores.  Do viruses bind together with significantly greater force than anthrax spores?  What about lactose particles the size of a virus?)

Since the individual molecules which form a given substance are also of a fixed size, when a particle of that given substance is manufactured smaller and smaller, the percentage of the molecules which are part of the surface of the particle will increase.   Does the percentage of molecules forming the surface of a particle affect how it will be attracted (or repulsed) by van der Waals forces to other particles of the same type? 

As the percentage of molecules at the surface increases, wouldn't it become easier for molecules in adjacent particles to align and bind together in the same way and with the same force as molecules within each particle?   The size and shape of the molecules will also determine how easily molecules in adjacent particles can bind together.  Size, shape and polarity must affect how particles bind together, and that would again indicate that two particles of the same size but of a different substance would not bind together with the same force.

The smaller the particle, the lighter it becomes.  As a particle is reduced in size, at some point the force of gravity (the mass of the particle) will become less than the van der Waals force binding one particle to another.  At that point, gravity would not be able to separate the particles (they'll bind like a bunch of grapes as in the illustration above).  But, the differences in size, shape and polarity of molecules will determine how small particles of different substances must be before gravity becomes the lesser force. 

Lactose is in a liquid form before it is turned into a spherical particle by drying.  The phenomenon of surface tension would create a very different alignment of molecules than would be found in a living object such as an anthrax spore.  Surface tension is described in Wikipedia this way:

Surface tension is caused by the attraction between the molecules of the liquid by various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore all of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. This inward pull tends to diminish the surface area, and in this respect a liquid surface resembles a stretched elastic membrane. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.
Lactose is a combination of glucose (sugar), which is a polar molecule, and galactose,  which is another polar molecule.  Wouldn't a spherical particle made totally of polar molecules have a significantly different effect on neighboring particles than a particle or spore made from many different kinds of molecules? 

An anthrax spore is a living object made from many kinds of organic molecules including lipids, amino acids and carbohydrates.  Amino acids have polar and nonpolar aspectsProteins are vast molecules built from enormous chains of amino acids.  Protein folding is also involved in the formation of any living entity.

A polar molecule will repel or attract another molecule depending upon the alignment of the two molecules:

How are molecules aligned in a lactose particle?  How are molecules aligned in a spore?  Would the differences have an effect on particle to particle interaction?

A dry anthrax spore consists of different layers.  The outermost layer is called the exosporium which is lattice-like in structure.  There are typically gaps of open space between the exosporium and the coat of the actual spore.  So, while a lactose particle is solid, a dry spore is not solid.

A lactose particle is a perfect sphere.  An anthrax spore is a rough surfaced ovoid.  Rough surfaces reduce the effects of van der Waals forces.  That rough surface also has the remnants of tiny, hair-like tendrils.

There are probably many other factors which seem to show logically that a spore cannot be affected by van der Waals forces in the same way as a lactose particle.  But logic is not enough to resolve this question.

Why is there a question?

The November 28, 2003, issue of Science Magazine contained an article by Gary Matsumoto titled "Anthrax Powder - State of the Art?".  The article included this information: 

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 article shows how tiny particles must be kept separated by a supersophisticated coating in order to keep them from binding together, and concludes that
only a state-run facility or a corporation has the resources to make an anthrax powder as good as the one mailed to the Senate.
The August 2006 issue of Applied and Environmental Microbiology contained an article by Douglas J. Beecher of the Hazardous Materials Reponse Unit of the FBI laboratories in Quantico, Virginia, titled "Forensic Application of Microbiological Culture Analysis to Identify Mail Intentionally Contaminated with Bacillus Anthracis Spores."   The article contained this information:
Individuals familiar with the compositions of the powders in the [anthrax] letters have indicated that they were comprised simply of spores purified to different extents.   However, a widely circulated misconception is that the spores were produced using additives and sophisticated engineering supposedly akin to military weapon production.  This idea is usually the basis for implying that the powders were inordinately dangerous compared to spores alone.  The persistent credence given to this impression fosters erroneous preconceptions, which may misguide research and preparedness efforts and generally detract from the magnitude of hazards posed by simple spore preparations.
The "widely circulated misconception" is that the anthrax used in the attacks of 2001 must have been coated with silica because spores will stick together due to van der Waals forces in the same way as medicinal particles of the same size if they are not coated.

However, uncoated dry anthrax spores have been killing people worldwide for countless centuries. 

What am I going to do with the answer?

I don't know.  It depends upon the response.  If I'm provided with any help at all, at minimum I'll add an "RESPONSES" section below.   If I get a very good response, I might replace this page with a new web page where the answer is laid out in detail. 

The main objective here is to provide a scientific explanation - using layman's language - for why dry spores do not have to be coated with silica or some similar substance to keep them from binding together in large clumps due to van der Waals forces.  (It is understood that dry spores are often mixed with some form of silica to keep the dry spores from absorbing moisture, which would cause them to clump because the water molecules will cling together due to van der Waals forces.)

First version posted October 21, 2007.
Revised Oct. 28, 2007, to include new questions about forces holding particles to other particles.
Revised Nov. 4, 2007, to include comments about Hamaker's constant.
Revised Dec. 18-19, 2007, to distinguish metallic bonding from van der Waals forces.
Revisied May 28, 2008, to show that the questions are largely answered on a new web page.
Revised July 23, 2008, to include discussion about the hydrophobic exosporium on anthrax spores.