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HOME > PAST ISSUE > March-April 2014 > Article Detail

PERSPECTIVE

Little Interactions Mean a Lot

Noncovalent bonds are weaklings compared to familiar chemical reactions, but they add up to strongly influence the shape and behavior of molecules.

Roald Hoffmann

A Really Recyclable Molecular Filter

2014-03PerspectiveFp97.jpgClick to Enlarge ImageNoncovalent bonds abound in nature, but there are clever new ways to exploit them artificially as well. Separating the small from the large is important in science, for instance, in gel electrophoresis and centrifugation. The development of nanofilters has opened up new ways to quickly separate small particles from even smaller ones, but most do not exploit noncovalent bonds because materials that rely on weak interactions often are not stable.

In 2011, Boris Rybtchinski and his collaborators at the Weizmann Institute of Science made a nanofilter held together by noncovalent bonds. The nanofilter has the additional intriguing property of being recyclable.

To make this nanofilter, Rybtchinski and colleagues took a beaker with a solution of a molecule in it, call it PP2b (you don’t want to know its systematic name), and poured it on a membrane, essentially an inert support. PP2b transformed into a colored gel on the support. That gel could separate the two kinds of nanoparticles. Small ones could go through readily, whereas large ones stayed on top. Then, when they poured a mixture of water and ethanol on the filter (roughly the strength of a good vodka), the gel dissolved and passed through the membrane, along with the large nanoparticles. PP2b and the large particles were easily separated, and then the PP2b was ready to be used again.

A balance of hydrophobic and hydrophilic bonding—that is, two types of weak, noncovalent bonds, working together—explains the magic of supramolecular polymers such as PP2b. Hydrophobic and hydrophilic are terms describing the proclivity of molecules or pieces of molecules to prefer, on a microscopic level, an aqueous environment or its opposite, represented emblematically by a long hydrocarbon chain.

If a piece of a molecule is polar, a local separation of positive and negative charge, that polarity will be attracted to a watery environment; a nonpolar segment would prefer the hydrocarbon surroundings. Soaps and detergents work through providing regions of both hydrophobic and hydrophilic bonding; the balance is also important in the preparation of many good things in this world, such as mayonnaise.

The platelet-like hydrocarbon rings in the middle of PP2b (see above) are nonpolar; the CH2CH2O, repeated on average 19 times, is the polar part, which likes water. In a number of solvents, including the common THF (tetrahydrofuran), PP2b is a biggish molecule with two tails. But in the presence of substantial amounts of water, the hydrophilic and hydrophobic interactions turn on, because the hydrophobic parts (the central organic platelets) want to be near each other.

The organic platelets, with pendant ethylene glycol tails, aggregate. On a molecular level one sees the formation of segments, then fibers, which can be viewed with a scanning electron microscope.

The performance of PP2b in solution is pretty neat—usually, one adds water and a solid dissolves; here, Rybtchinski and his colleagues added water, and a gel, a solid-like material with a lot of water still in it, formed. That’s the filter.

Notice that hydrogen bonds and dispersion forces are involved; small interactions that, as I said, are 10 to 30 times smaller than the energies of carbon–carbon, carbon–oxygen, or carbon–hydrogen bonds. Yet these small interactions add up, with a vengeance, to create three-dimensional aggregates, the network of fibers. This network is robust—stable to approximately 70 degrees Celsius—until you pour alcohol on it.

From Molecular to Macroscopic Scales

After all that, I still like my energies big. And, for that matter, I also like to follow individual molecules through their collisions with other molecules as bonds are broken and remade. My desire for the big and the particular—and my resistance to calculating averages over billions of molecular trajectories—comes with a price, however. It prevents me from moving from the angstrom scale of molecules to the macroscopic world where we experience the practical results of chemistry. It keeps me away from ferromagnetism and viscosity, and from Esther Williams’s interaction with water.

It’s an irrational desire, I know. The accumulation of tiny differentials rules nature. How could it be otherwise in a molecular world where it takes 6 x 1023 water molecules to make a single slurp of refreshing liquid? Change happens more readily through shuffling small pieces than by completely rearranging big ones, as evolution has discovered.

On the human level, as in chemistry, a big number of small actions can bring about large-scale change: Within our imperfect approaches to democracy, the small contributions of many can accomplish real change—be it in the abolition of slavery, the empowerment of women, or the limitation of automobile emissions.

I waver. It is so clear that small interactions of molecules can hardly be ignored, even if a weak theoretician with imperfect tools decides he cannot work on them. I like stories—science, and not only science, lives in them. The stories of modern chemistry that I have retold here show ever so clearly that aggregation, the action of many tiny gobs of energy in concert, can lead to essential change and a tangible, macroscopic consequence.

Bibliography

  • Ball, P. 1999. H2O: A Biography of Water. London: Weidenfeld & Nicolson.
  • Boys, C. V. 1958 (1911). Soap Bubbles: Their Colors and Forces which Mold Them. New York: Dover.
  • Chandler, D. 2005. Interfaces and the driving force of hydrophobic assembly. Nature 437:640–647.
  • De Greef, T. F. A., M. M. J. Smulders, A. P. H. J. Schenning, R. P. Sijbesma, and E. W. Meijer. 2009. Supramolecular polymerization. Chemical Reviews 109:5687–5754.
  • Goodman, J. M. 1997. What is the longest unbranched alkane with a linear global minimum conformation? Journal of Chemical Information and Computer Sciences 37:876–878.
  • Krieg, E., H. Weissman, E. Shirman, E. Shimoni, and B. Rybtchinski. 2011. A recyclable supramolecular membrane for size-selective separation of nanoparticles. Nature Nanotechnology 6:141–146.
  • Krieg, E., E. Shirman, H. Weissman, E. Shimoni, S.G. Wolf, I. Pinkas, and B. Rybtchinski. 2009. Supramolecular gel based on a perylene diimide dye: Multiple stimuli responsiveness, robustness, and photofunction. Journal of the American Chemical Society 131:14365–14373.
  • Lüttschwager, N. O. B., T. N. Wassermann, R. A. Mata, and M. A. Suhm. 2013. The last globally stable extended alkane. Angewandte Chemie-International Edition 52:463–466.








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