Reactions: Mechanochemistry in geochemical systems

Letters to the editor

Mechanochemistry in geochemical systems

The C&EN article by Fernando Gomollón Bel titled “Mechanochemists Want to Shake Up Industrial Chemistry” shines a welcome light on the little-known processes whereby mechanical forces can break or create chemical bonds. The article focused on the environmentally friendly characteristics of this phenomenon. We wish to point out that such reactions can occur naturally in geochemical systems, most obviously those involving aqueous processes in earth and ocean science.

The scale of such processes is large, including the glacial grinding of rocks to form the chemistry of meltwaters, and the swimming of tiny microbes. For water, the essential characteristic is that it exists as a hydrogen-bonded structure. This is typically glossed over in the earth sciences.

In recent work, we have shown that the hydrogen-bonded structure of water can be quantitatively determined with a precision of 1–2% over a wide range of temperature and pressure. It is the existence of the simple H₂O species that is the primary driver of chemical reactions, whether in attacking rocks or controlling viscous flow. Flow processes are essential and ubiquitous in earth and ocean systems.

Svante Arrhenius pointed out in 1917 that for viscous flow to occur, an activation energy barrier must be overcome. The specific problem for water was addressed by Raymond Ewell and Henry Eyring in 1937, when it was shown that it is the presence of the single H₂O molecule that controls flow. This finding was neglected for many years, and thus when efforts were made by Edward Purcell in 1977 to analyze the efficiency of microbial swimming by considering purely dynamical forces, the unusual conclusion that microbes swam with only 1% efficiency was reached. This makes little sense, for microbes have had over 1 billion years of evolutionary development to achieve efficiency in all aspects.

Our analyses show that the activation energy of viscous flow in water and seawater is consistent with the mechanical breaking apart of hydrogen bonds in the dominant water pentamer (H₂O)₅. Simple extension shows that this is also achieved by the work done by microbial flagella and that the formation of a boundary layer of single H₂O species greatly reduces drag. While an exact efficiency of microbial swimming cannot yet be calculated, it is most assuredly far higher than 1%. Swimming microbes are excellent mechanochemists.

We thank C&EN for bringing this topic to the fore.

Peter G. Brewer and Edward T. Peltzer
Moss Landing, California