Most concepts aimed at achieving directional motion at the molecular level use molecular switches and machines incorporated in or on a matrix such as surfaces, particles, liquid crystals, gels, or polymers, which provide the order required for concerted action of the molecular machines. Sophisticated synthetic effort is often also required to prepare the molecular machines. Michael Gradzielski and colleagues from the TU Berlin and FU Berlin, Germany report in Advanced Materials that directional motion and transport can be achieved using a salt concentration gradient and an appropriately designed self‐assembling system, without substantial synthetic effort or a matrix.
The researchers show that in water solutions of the structurally very simple CAMCl2 monomer (shown to the left) flexible, partially amorphous ribbons form above a critical concentration in the absence of salt, NaCl, but when NaCl is added it strongly induces the formation of mechanically rigid and straight rods, which then contain fully crystallized CAMCl2.
With salt gradients defining the direction, the mechanical work generated from the crystallization process under these non-equilibrium conditions during the supramolecular polymerization can be used to push 100 μm‐sized particles forward over distances of hundreds of micrometers or even millimeters (shown below). It is a continuous process, which follows the moving salt gradient and—once the salt gradient is established—proceeds autonomously until the salt gradient has leveled out.