Are Van der Waals interactions holding asteroids together?

A recent post of Derek’s concerned the very weak (high kD) but very important interactions of proteins within our cells.

Most of this interaction is due to Van der Waals forces — Shape shape complementarity (e.g. steric factors) and dipole dipole interactions are also important.

Although important, Van der Waals interactions have always seemed like a lot of hand waving to me.

Well guess what, they are now hypothesized to be what is holding an asteroid together. Why are people interested in asteroids in the first place? [ Science vol. 338 p. 1521 ’12 ] “Asteroids and comets .. reflect the original chemical makeup of the solar system when it formed roughly 4.5 billion years ago.”

[ Nature vol. 512 p. 118 ’14 ] The Rosetta spacecraft reached the comet 67P/Churyumov-Gerasimenko after a 10 year journey becoming the first spacecraft to rendezvous with a comet. It will take a lap around the sun with the comet and will watch as the comet heats up and releases ice in a halo of gas and dust. It is now flying triangles in front of the comet, staying 100 kiloMeters away. In a few weeks it will settle into a 30 kiloMeter orbit around he comet. It will attempt to place a lander (Philae) the size of a washing machine on its surface in November. The comet is 4 kiloMeters long.

[ Nature vol. 512 pp. 139 – 140, 174 – 176 ’14 ] A kiloMeter sized near Earth asteroid called (29075) 1950 DA (how did they get this name?) is covered with sandy regolith (heterogeneous material covering solid rock { on earth } it includes dust, soil, broken rock ). The asteroid rotates every 2+ hours, and it is so small that gravity alone can’t hold the regolith to its surface. An astronaut could scoop up a sample from its surface, but would have to hold on to the asteroid to avoid being flung off by the rotation. So the asteroid must have some degree of cohesive strength. The strength required is 64 pascals to hold the rubble together — about the pressure that a penny exerts on the palm of your hand. A Pascal is 1/101,325 of atmospheric pressure.

They think the strength comes from van der Waals interactions between small (1 – 10 micron) grains — making it fairy dust. It’s rather unsatisfying as no one has seen these particles.

The ultimate understanding of the large multi-protein and RNA machines (ribosome, spliceosome, RNA polymerase etc. etc. ) without which life would be impossible will involve the very weak interactions which hold them together. Along with permanent dipole dipole interactions, charge interactions and steric complementarity, the van der Waals interaction is high on anyone’s list.

Some include dipole dipole interactions as a type of van der Waals interaction. The really fascinating interaction is the London dispersion force. These are attractions seen between transient induced dipoles formed in the electron clouds surrounding each atomic nucleus.

It’s time to attempt the surmount the schizophrenia which comes from trying to see how quantum mechanics gives rise to the macroscopic interactions between molecules which our minds naturally bring to matters molecular (with a fair degree of success).

Steric interactions come to mind first — it’s clear that an electron cloud surrounding molecule 1 should repel another electron cloud surrounding molecule 2. Shape complementarity should allow two molecules to get closer to each other.

What about the London dispersion forces, which are where most of the van der Waals interaction is thought to be. We all know that quantum mechanical molecular orbitals are static distributions of electron probability. They don’t fluctuate (at least the ones I’ve read about). If something is ‘transiently inducing a dipole’ in a molecule, it must be changing the energy level of a molecule, somehow. All dipoles involve separation of charge, and this always requires energy. Where does it come from? The kinetic energy of the interacting molecules? Macroscopically it’s easy to see how a collision between two molecules could change the vibrational and/or rotation energy levels of a molecule. What does a collision between between molecules look like in terms of the wave functions of both. I’ve never seen this. It has to have been worked out for single particle physics in an accelerators, but that’s something I’ve never studied.

One molecule inducing a transient dipole in another, which then induces a complementary dipole in the first molecule, seems like a lot of handwaving to me. It also appears to be getting something for nothing contradicting the second law of thermodynamics.

Any thoughts from the physics mavens out there?

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  • John Wayne  On August 18, 2014 at 1:39 pm

    Chemists tend to invoke Van der Waals forces when we don’t know what is going on, and astrophysicists seem to invoke dark matter when they can’t explain an observation. Leave our hand-waving alone! Is this an example of translational science?

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