Mechanochemistry, Mechanosynthesis,
and Molecular Machinery

Industrial-scale
mechanosynthetic machinery
(probably not optimized for this)

Ball mill, Zhengzhou Hengxing Heavy Equipment Co.,Ltd

Volume 1, Number 1 of Nature Chemistry is now out, and the next issue will include an article titled “Activating catalysts with mechanical force”. This article reports a nice experimental result and helps to illustrate the broad range of physical processes included under the umbrella terms of “mechanochemistry” and “mechanosynthesis”.

The authors demonstrate two examples of what should be a widely applicable technique: preparing metal coordination compounds with ligands attached to polymer chains, then yanking the chains to pull a ligand loose, causing what they term “mechanochemical catalyst activation”. Intense ultrasound provides the necessary mechanical force without machinery. This is a nice example of using force to do mechanical work on a molecule, thereby raising its energy and changing its structure and activity.

This example falls somewhere in the middle of a spectrum of very different processes.

Mechanochemistry with unmechanized force

A more familiar form of mechanochemistry works by pounding materials in a ball mill. The substances to be reacted (relatively soft solids) are placed in a chamber together with hard balls (e.g., steel), which are then vibrated or tumbled. Flakes of solid that are unfortunate enough to be piled on a spot where two balls collide are smashed together enormous pressure, binding, flattening, and fragmenting them. The resulting laminated flakes undergo this again and again. To understand the general nature of the process, imagine stacking, flattening, breaking apart, and re-stacking what are initially two pieces of different material: Each cycle doubles the number of layers, and a few dozen cycles will thin the layers to less than an atomic diameter — which means that there are, in fact, no longer any layers, because the materials have been mixed at the molecular level. Ball-mill processing has long been used to make alloy powders of incompatible metals, and to mix compounds without a solvent. Mixing, impact heating, and mechanical force itself can activate or drive reactions. This sort of mechanochemical process can be used to perform mechanosynthesis (without machinery).

Biosphere-scale
mechanosynthetic machinery
(fine-tuned)

ATP synthase
Review article [pdf 1.7 MB]

Mechanochemistry with forceful machines

Querying Google Scholar about “mechanochemistry” returns results dominated by papers discussing mechanical forces in biological molecular machines. An outstanding example is ATP synthase, which consists of two coupled motors, one driven by the breakdown of ATP, the other driven by the flow of protons across a membrane. The motors are coupled in a way that always forces one of them to run in reverse, and as the name suggests, the normal mode of operation uses the proton-driven machine to drive the ATP-driven machine in reverse. This results in the synthesis of ATP, and thereby enables you to move, think, and so on, from moment to moment. (For more on this remarkable device, see Reverse engineering a protein: the mechanochemistry of ATP synthase [pdf, 1.7 MB].)

Mechanochemistry with (sometimes) force-free machines

The above forms of mechanochemistry and mechanosynthesis all employ force, while only the molecular biological instances employ both force and machines. The forms of mechanosynthesis that I discussed in the preceeding post are closer to the biological examples, in that they employ machines, but the crucial aspect in this instance is the use of machines and mechanical constraints to guide reactive encounters on a molecular scale. As we’ve seen this does not necessarily require the use of mechanical force (or motive power of any sort), although guidance and applied force can be a potent combination.

Thus, the terms “mechanochemistry” and “mechanosynthesis” embrace processes that involve enormous force, but no machines; others that involve machines, but no force; and yet others that involve both. The entire spectrum offers lessons for the future of nanomechanical mechanochemistry. The first, perhaps, is the unity of chemistry and mechanics, and the danger of trying to divide that unity with sharp definitions where there are no sharp lines to be drawn.

When I offer a more specialized definition of “mechanosynthesis”, it should be read as explaining what the word means in the context of molecular machine systems used for molecular fabrication.

Related posts:

1. Modular Molecular Composite Nanosystems
      Biomolecular engineering for atomically precise nanosystems
2. Toward Advanced Nanotechnology: Nanomaterials (1)
      Why I’ve never advocated starting with diamond
3. Toward Advanced Nanotechnology: Nanomaterials (2)
      Stiffness matters (and protein isn’t remotely like meat)
4. Self-Assembly for Nanotechnology
      The virtues of self-assembly and the benefits of external guidance
5. From Self-Assembly to Mechanosynthesis
      Mechanosynthesis begins with soft machines
6. Toward Advanced Nanotechnology: Nanomaterials (3)
      Mechanical engineering meets thermal fluctuations
7. Toward Advanced Nanotechnology: Nanomaterials (4)
      Nanostructures, Nanomaterials, and Lattice-Scaled Stiffness
8. Toward Advanced Nanotechnology: Nanomaterials (5)
      Nanomachines, Nanomaterials, and K_lm

  

Tagged as: chemistry, mechanochemistry, mechanosynthesis, nanomachines, Nanotechnology