(a) Consider the substances shown in parts (a), (c), and (d) in the second illustration in Box 8F.1.

(b) In which of these three parts of the figure will the interactions be the strongest?

(c) In which will they be the weakest?

Transcribed Image Text:

BOX 8F.1 FRONTIERS OF CHEMISTRY: SELF-ASSEMBLING MATERIALS A major concern of scientists working with nanomaterials is how to coax molecules into organizing themselves into the desired nanoscale arrangements. Many biological processes operate at the nanoscale level. Proteins, for example, fold up into the shape that optimizes their function (Topic 11E), cell mem- branes form spontaneously when certain molecules called phos- pholipids encounter water (Topic 5D), and the DNA molecules of our genes replicate themselves every time a cell divides. Although the expression has a variety of definitions, the com- mon meaning of self-assembly of molecules is the spontaneous formation of organized structures from individual parts. Self-assembly can be static or dynamic. In static self-assembly, the structure formed is stable and the process does not readily reverse. Two examples are the folding of polypeptide chains into a protein molecule and the formation of the double helix of DNA. Dynamic self-assembly involves interactions that dissipate energy and can easily be reversed. Two examples are oscillating chemical reactions and convection patterns. On the macroscopic scale, the coordinated motion of a school of fish or a flock of migrating birds is regarded as an example of dynamic self-assembly. However, the most promising area of self-assembly is in the inter- mediate realm between molecular and macroscopic, where it may become possible to design nanoscale sensors and machines to carry out specific tasks. Designing materials that can assemble themselves is a strategy that is beginning to result in materials that can respond to stimuli and act in seemingly intelligent ways. -NH₂* (b) Water (c) (d) Nonpolar medium These four types of forces are responsible for the adaptive behavior of smart gels. The different forces come into play when the network of polymer chains composing a gel is disturbed. (a) Charged ionic regions can attract or repel each other. (b) Nonpolar hydrophobic regions exclude water. (c) Hydrogen bonds may form from one chain to another. (d) Dipole-dipole interactions can attract or repel chains. polymers, silicone oil (a polymer with a-(-O-Si-O-Si—),— backbone and hydrocarbon side chains), and salts. When exposed to an electric field, the molecules of the soft gel rearrange themselves