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Feat of clay. When clay is suspended in water and makes up between 1% and 2% of the suspension by weight, it forms a very stable "equilibrium gel" that could be useful in medicine and nanotechnology.

B. Ruzicka et al., Nature Materials, Advance Online Publication (2010)

New State of Matter Seen in Clay

Researchers have observed a new kind of extremely light and stable gel in a suspension of clay at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The so-called equilibrium gel, predicted 4 years ago by physicists, could lead to improved drug-delivery systems and other novel microscopic devices.

A gel is a liquid that is rendered solid by a more or less rigid but disordered network of microscopic particles dispersed throughout its volume. These jellylike materials are extremely common and are used in everything from foods and pharmaceuticals to paints and cosmetics. However, many gels are made by “phase separating” a liquid suspension, which means cooling the liquid down until it splits into two distinct components, the more dense of which is the gel. Unfortunately, this is an unstable process that makes it difficult to control certain properties of the gel, including its density.

In the latest research, carried out over 7 years, physicist Barbara Ruzicka of the University of Rome "La Sapienza" and colleagues have shown how an existing material—the synthetic clay Laponite, which is used as a thickener in many household products—can form a stable gel. The researchers suspended Laponite in water and used the powerful x-ray beams of ESRF to study how the structure of the suspension changes over time and how this evolution depends on the amount of clay present.

At concentrations of up to 1% Laponite by weight, the initial fluid transformed into a gel after a few months, the researchers found. Then about 3 years later, it separated into two phases: one clay-rich and the other clay-poor. However, no such phase separation occurred at concentrations above 1%. Unlike at the lower concentrations, at which the arrangement of the clay particles was continually in flux, at concentrations above 1% the structure eventually stopped changing, indicating that the particles had locked into a stable structure: the equilibrium gel.

According to Ruzicka and co-workers, the clay particles reach an equilibrium because of the way they interact with one another. Typical particles dispersed in a liquid have charges distributed symmetrically across their surfaces and will interact with all of their nearest neighbors when they form a gel. The relatively high density of particles needed to do this will not generally exist in the liquid state, but they can exist if the liquid undergoes phase separation.

Clay particles, in contrast, are disc-shaped and have an asymmetric charge distribution—a net negative charge on their faces and a net positive charge along their edges. So they do not interact with all of their nearest neighbors, allowing them to lock together at lower densities. As such, say the researchers, the material will be able to form a gel without the help of a phase transition. Ruzicka explains that the suspension will change reversibly and continuously from the liquid state into the gel state, a process confirmed by computer simulations developed by the group.

This finding has lots of potential applications, says Ruzicka. One is batteries containing a gel electrolyte, which would produce a relatively high power for a given weight of battery and which could be incorporated into microscopic devices if the gel could be made at a low enough density. Alternatively, equilibrium gels could be used as coatings to deliver drugs into the body. These coatings are needed to protect against the body's immune system and dissolve when the drug reaches its target, so making the coatings lighter would reduce the amount of material that ultimately ends up in the body.

Tom McLeish, a soft condensed matter physicist at Durham University in the United Kingdom, who was not involved with the research, says that the work is important because it provides an experimental demonstration of a new state of matter. And he agrees that the work could also have “applications aplenty.” He argues that the scope for applications could be enhanced enormously by fabricating equilibrium gels artificially—in other words, by making gels that contain particles with specific charge distributions rather than using preexisting materials, as was the case in the current work.