Counter-intuitive: how diluting a solution makes it stronger

A well-known principle in chemistry is that an aqueous gel can be made liquid again by diluting it with water. To produce a gel once more, the solution must instead be concentrated. However, researchers at Eindhoven University of Technology discovered an aqueous solution that turned into a gel precisely when they lowered the concentration further. They published their results in Science.

Molecules such as surfactants can form ordered structures in solution. At higher concentrations, these systems typically become more ordered, with structures transitioning from spherical to elongated forms. The Eindhoven researchers examined a system comprising benzene-1,3,5-tricarboxamide (BTA-EG4) with the surfactant octyltrimethylammonium bromide (OTAB). At high concentrations, BTA-EG4 forms supramolecular polymers in water, giving rise to a hydrogel.

Adding OTAB disrupts the hydrogel, but when the system is subsequently diluted, a gel forms once again, as this dampens the effect of the surfactant. The researchers thus succeeded in creating a gel–sol–gel–sol system as a function of concentration.

The discovery was almost accidental. Two researchers in Bert Meijer’s group stumbled upon the fact in October 2019 that their liquid mixture became a hydrogel when diluted. In the years that followed, they carried out experiments that allowed them to demonstrate the system thoroughly. In a press release from Eindhoven University of Technology, Bert Meijer states that such hydrogels may offer solutions to existing challenges, such as the cultivation of stem cells. The cells can divide safely within the gel, after which it can be diluted to harvest the cells for use.