ACS Nanohttp://doi.org/hzq (2012)

The ionic bond is one of the most fundamental interactions in chemistry and is of primary importance in nature for building macromolecular structures and controlling cell metabolism. However, the strength of this interaction remains essentially unknown, unless it is probed in a vacuum. Evan Spruijt and colleagues at Wageningen University have now shown that force spectroscopy can be used to directly measure the strength of single ionic bonds in aqueous solution, where hydration and dielectric effects — absent in a vacuum — come into play.

Spruijt and co-workers deposited a negatively charged polyelectrolyte on a surface and attached a positively charged polyelectrolyte to the tip of an atomic force microscope. They then put the two in close proximity so that ionic bonds form. As the tip of the microscope is pulled away from the surface, a force versus distance curve is obtained. The curve has either a step-like shape, in which each step corresponds to the rupture of one ionic bond, or a single peak, in which the energy is released simultaneously by multiple interacting charges.

The measured strength of single ionic bonds ranges between one and six times the thermal energy at room temperature, values that correspond to lifetimes of milliseconds to years, respectively. Given such a large span of energy and tunability, which is chiefly controlled by salt concentration, ionic bonds could be useful for building large and reversible molecular structures.