CNR researchers (CNR-ISC and CNR-IPCF) in collaboration with University La Sapienza and ESRF (Grenoble) have observed a new kind of extremely light and stable gel in a suspension of colloidal clay. The so-called equilibrium gel, predicted 4 years ago by theoretical calculations by members of the same research team for a simplified model, 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, Barbara Ruzicka (CNR-IPCF), Emanuela Zaccarelli (CNR-ISC) 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 studied 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 an extremely stable structure: the equilibrium gel.
This is made possible by the type of anisotropic interactions governing the clay disks behaviour. Indeed, typically colloidal particles interact in an isotropic way 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, but tend to form so-called T-bonds, in which faces and rims are connected in a sort of chain. Hence, the number of nearest neighbours with which they interact is limited, allowing them to form a gel at low density without the need for an intervening phase transition. This is a result of the limited valence of the particles in agreement with theoretical and numerical predictions for patchy colloidal spheres.
B. Ruzicka, E. Zaccarelli, L. Zulian, R. Angelini, M. Sztucki, A. Moussaid, T. Narayanan and F. Sciortino Observation of empty Liquids and equilibrium gels in a colloidal clay Nature Materials 10, 56-60 (2011)
and related News and Views article Colloidal gels: clays go patchy Nature Materials 10, 5-6 (2011)
 E. Bianchi, J. Largo, P. Tartaglia, E. Zaccarelli, and F. Sciortino Phase diagram of patchy colloids: towards empty liquids Phys. Rev. Lett. 97, 168301 (2006)
 P. J. Lu, E. Zaccarelli, F. Ciulla, A. B. Schofield, F. Sciortino and D. Weitz. Gelation of Particles with Short -Ranged Attraction. Nature 453, 499 503 (2008)