The term “salt bridge” has two meanings in chemistry: a conductive gel bond in voltaic cells and a molecule used to bridge sections of a macromolecule. Supramolecular chemistry uses salt bridges to create nanostructures. Without a salt bridge in a voltaic cell, electrons cannot flow through the wire. In self-assembly, a salt bridge can bridge opposing forces and squeeze a macromolecule into a fold. Supramolecular chemists study natural macromolecules to use salt bridges in constructing nanostructures.
The term salt bridge has two distinct uses in chemistry. The original usage described an electrically conductive gel bond between two half cells of a voltaic cell in the field of electrochemistry. The second is the use of a slightly polar external molecule to bridge sections of a macromolecule that repel each other without the intervention of a salt bridge. A new field, supramolecular chemistry, in practical development since about 1960, exploits 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 discs, separated by paper discs saturated in salt water, the bridge, circa 1800. By stacking several of these zinc-bridge-silver disc sets, he was able to detect an electric shock when he touched both ends simultaneously.
A real battery cell was built in 1836 by John Frederick Daniell, using zinc and copper. A strip of each metal was dipped in a solution of its metal ion. The two strips were connected by wire and the two solutions by a porous ceramic tube filled with salt water, the salt bridge.
If a salt bridge is not employed in a battery cell, the reaction occurs directly and the flow of electrons cannot be directed through the wire. The salt bridge only conducts 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 reducing 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 is built by adding building blocks in a step-by-step procedure. It acquires new units, which in turn cause the molecule to bend and bend in a way that attracts and binds the next component, ultimately resulting in a precise three-dimensional structure.
Deoxyribonucleic acid (DNA) is self-assembled in the cell by a process of folding and folding. As each fold is made, new functional groups, side groups of more reactive atoms, are placed in an attractive or repelling position. As the molecules move to allow functional groups to be closer or further apart, a fold is created. 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.
Sometimes, a bend or fold must occur in a natural or synthetic macromolecule in a place where slight repulsive forces exist. A second small molecule, called a salt bridge, can line up 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 squeezes the gap and squeezes the macromolecule. The selection of the salt bridge is very demanding; an exact measure is required physically and in responsible distribution. Supramolecular chemists study natural macromolecules to understand and use salt bridges in the construction of useful nanostructures.
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