“Structural” water makes for strong new glue

Scientists have used specially arranged water molecules to create a powerful glue that relinquishes its adhesive properties when heated and can potentially be reused several times without losing strength.

A Chinese-German team led by Shengyi Dong from China’s Hunan University has created the new glue by utilising the ability of water, under certain circumstances, to organise itself and behave in ways that it normally doesn’t.

The concept of “structural water” has a long history. It is well known that water molecules near a suitable surface are compelled to ordered themselves into hexagonally shaped, single-layer sheets. In biological systems, molecules can bind to the surface or interior of proteins, which helps make them stable. This has implications for drug design.

(Structural water is quite different from “structured” water, by the way. The latter term is used on plethora of New Age products, making evidence-free claims for health benefits.)

Dong and colleagues exploited another phenomenon that emerges when water molecules bind with others. Linked to the right type, the result is a polymer that has “unusual dynamics, flexibility, elasticity, and adhesive properties”. The research is reported in Science Advances.

Water is able to bind the organic molecules together through an effect called hydrogen bonding. H-bonding occurs because the hydrogen atoms in water have a slight positive charge, while oxygen and nitrogen atoms in the organic molecules are slightly negative.

The positive and negative charges have an electrostatic attraction which holds the molecules together, with the water ones able to act as a bridge between the organic ones, forming long alternating chains.

The key to making hydrogen bonds strong enough to form a sticky polymer was to use a type of compound called crown ethers. These are ring-shaped molecules that contain numerous oxygen atoms, with two carbon atoms between each. The oxygen atoms are able to form many hydrogen bonds with water. Individually they are weak, but become strong when added together.

The team found that their best version of the crown ether molecule, which they called TC7, was a non-sticky powder when dry, but formed a strong adhesive when a small amount of water was added.

The glue could hold glass surfaces together with an adhesion strength of roughly 10 times that of common polyvinyl acetate (PVA) glue.

Unlike PVA, TC7 rapidly loses its strength at higher temperatures. This could be a problem for many applications, but could also be an advantage.

Tests showed that glass surfaces bound with TC7 could be separated by heating, but stuck back together with the same strength when cooled. The glue can be reused many times, constituting, the researchers conclude, “a successful resource-saving technology”.