The term salt bridge has two distinct uses in chemistry. The original use described an electrically conductive gel union between two half-cells of a voltaic cell in the field of electrochemistry. The second is the use of an external, slightly polar molecule to create a bridge between sections of a macromolecule that would repel each other without the intervention of a salt bridge. A new field, supramolecular chemistry, in practical development since about 1960, takes advantage of salt bridges to create highly detailed structures.
In a voltaic cell, also called a galvanic cell, an electrochemical reaction takes place in two separate physical locations called half-cells. Half of an oxidation-reduction (redox) reaction occurs in each half-cell. Alessandro Volta demonstrated the basic principle by stacking zinc and silver disks,separated by paper disks saturated in salt water, the bridge, in about 1800. By stacking several of these zinc-bridge-silver disk sets, he was able to detect an electrical shock when he touched both ends simultaneously.
A true battery cell was constructed in 1836 by John Frederick Daniell, who used zinc and copper. A strip of each metal was dipped in a solution of its own metal ion. The two strips were connected by wire and the two solutions by a porous ceramic tubing filled with salt water, the salt bridge.
If a salt bridge is not employed in a battery cell, the reaction occurs directly, and the electron flow cannot be directed through the wire. The salt bridge conducts only the charge on the ion via its salt ions. No ions from the redox reaction travel through the bridge.
Supramolecular chemistry provides an innovative approach to the field of nanotechnology. Nanoscale structures, 1 to 100 nanometers (0.00000004 to 0.0000004 inches), are typically fabricated by whittling down larger structures using electron bombardment or other techniques. Supramolecular chemistry attempts to create structures by mimicking nature's way of self-assembly. Self-assembly occurs when a macromolecule builds itself by adding basic components in a step-wise procedure. It gains new units, which in turn causes the molecule to fold and bend in a way to attract and bond the next component, finally achieving a precise, three-dimensional structure.
Deoxyribonucleic acid (DNA) is self-assembled in the cell by a folding and re-folding process. As each fold is made, new functional groups, side groups of more reactive atoms, are put into a position of attraction or repulsion. As the molecules move to allow the functional groups to be closer or farther apart, a fold is made. Hydrogen bonding, a weak intermolecular, or, in the case of macromolecules, a weak intramolecular attraction between slightly negative hydroxyl groups and slightly positive proton groups directs the folding process.
At times, a fold or bend needs to occur in either a natural or synthetic macromolecule at a place where mild repulsive forces exist. A second small molecule, called a salt bridge, may align itself in the correct spot, where it can bridge the opposing forces. Instead of pushing the fold open, as the unbridged section does, the salt bridge tightens the gap and cinches in the macromolecule. The selection of the salt bridge is very demanding; an exact fit is required physically and in charge distribution. Supramolecular chemists study natural macromolecules to understand and use salt bridges in the construction of useful nanostructures.