Researchers in Italy and France are the first to have observed an arrested “empty” liquid in the lab. The gel is a new state of soft matter and consists of sparse networks of bonded nanoparticles. It could be used to make lightweight, extremely stable materials that do not age for applications in electrolyte batteries, nanocomposites and even biomedicine.
Gels are common materials but the way they are obtained depends on the colloidal- or nanoparticles that make them up. Until now, gels could only be produced using “out-of-equilibrium” processes – for example, through a technique called interrupted phase separation in which the system first irreversibly separates into two parts with the denser part alone then “freezing”, or arresting. The problem here though is that the gel is unstable and obtained in an uncontrolled way.
Now, Barbara Ruzicka and colleagues of the Consiglio Nazionale delle Ricerche together with co-workers at the ESRF in Grenoble, France, have discovered a new, controllable way to make gels. The researchers studied a colloidal clay called laponite, which is widely used to thicken many products such as cleansers, surface coatings, ceramic glazes and cosmetic products. They found that it forms a low-density (or empty) liquid that does not separate into two phases but which becomes a stable gel over time. This liquid can reversibly and continuously transform into an arrested structure, then known as an “equilibrium gel”. The researchers had predicted such behaviour in computer simulations but it had never actually been observed in an experiment until now.
T-bonds
The liquid forms thanks to interactions between the constituent particles, in this case clay platelets, that drive gel aggregation through only a few connections, or T-bonds, per particle. These bonds are so-called because the rim of one nanoparticle touches the face of another.
“Our equilibrium gel is a state where a few colloidal particles are ‘frozen’ in a T-connected network that has an energy very close to the ground state of the system,” team member Emanuela Zaccarelli told nanotechweb.org. “This is possible because particles can only gain in energy by forming a limited number of bonds, so there is no need for the system to become (too) dense for it to form a liquid or gel structure.”
The density of the liquid is controlled by the number of bonds per particle. This means that the arrested empty liquid is an extremely simple network in which most particles are part of long chains (two bonds). Sometimes, a branching particle (three bonds) makes the network more rigid.
Seven year study
The researchers actually followed their experiment over a long time – seven years – to observe the very low density phase separation process and to make sure that the gel was indeed highly stable. However, they assure us that the gel becomes stable much quicker – in about a year. Ruzicka says that changing parameters such as temperature or salt content in the laponite would speed up things even more. Indeed, the team is currently investigating ways to prepare equilibrium gels in shorter periods of time.
The gels could be used in any application where stable materials that do not degrade are needed – for example, in electrolyte gel batteries and nanocomposites. Their light weight could also be important in applications involving transport, like drug delivery and tissue repair. Medicine and pharmaceutics could particularly benefit since the clay material studied is biocompatible.
The team is now looking at other suitable soft-matter suspensions other than clays for making equilibrium gels.
The present results were reported in Nature Materials.