Observing a Single Solvent–Solute Association

By Jason Socrates Bardi

Dissolved molecules, like misery, love company.

Typically any "solute" molecule that is dissolved in liquid phase—the "solvent"—will be surrounded by a cage of hundreds or even thousands of such solvent molecules. These associations are fickle. The cages constantly form and dissipate through diffusion, and have lifetimes of less than a nanosecond. This makes studying the interactions between one solvent and one solute molecule difficult.

Or perhaps exceedingly difficult.

In the 1960s, scientists studied these sorts of detailed interactions by blasting streams of solute and solvent molecules in the gas phase into each other under extremely high vacuums. But nobody has ever been able to look at the detailed interactions between one solvent and one solute molecule in the liquid phase.

Now a technique invented at the Skaggs Institute for Chemical Biology at The Scripps Research Institute (TSRI) allows scientists to do just that.

In a study funded by The Skaggs Institute for Research and the National Institute of General Medical Sciences, TSRI Research Associates Alessandro Scarso and Alex Shivanyuk and TSRI Professor Julius Rebek, who is the director of the Skaggs Institute for Chemical Biology, studied the fundamental forces of biological chemistry between the solvent and the solute in the liquid phase.

The new method, which they call coencapsulation, temporarily places one molecule each of solute and solvent into a self-assembled capsule. The capsule is something like a drug capsule on the nanometer scale—with two half nano-capsules coming together to form a nanocapsule around a solute and a solvent. This coencapsulation is reversible and typically lasts for about one second.

Using nuclear magnetic resonance spectroscopy, Scarso, Shivanyuk, and Rebek studied different solutes with a panel of 15 common organic solvents—like chloroform, benzene, acetone, propanol—at room temperature in the liquid phase, and they were able to observe the weak attractive and repulsive intermolecular forces, such as hydrogen bonding, van der Waals interactions, and dipole–dipole coupling, between a single solvent and a single solute molecule. They were even able to determine the orientation of the solvent with respect to the solute that results from these attractive and repulsive forces.

This is the first time the interactions of an individual solvent molecule with an individual solute molecule have been observed under such favorable conditions.

To read the article, "Individual Solvent/Solute Interactions through Social Isomerism" by Alessandro Scarso, Alexander Shivanyuk, and Julius Rebek, Jr., please see the November 19, 2003 issue of the Journal of the American Chemical Society or go to: http://pubs.acs.org/cgi-bin/asap.cgi/jacsat/asap/abs/ja037808e.html.



One solute molecule is normally surrounded by a field of solvent (top). But the technique described by TSRI researchers allows them to sequester one solvent and one solute molecule and study the interactions between them. Picture courtesy of Alessandro Scarso.