Medical, edible cat litter: Sepiolite succumbs to XRD

Mineral solution

Sepiolite has been known since Roman times when it was used to filter and purify wine, today it's commonly found in cat litter trays. It absorbs huge amounts of liquid as it is so porous now an X-ray diffraction study could help explain why and perhaps lead to more technological applications, such as the development of food binders and drug-delivery agents.

"The future of sepiolites in the household is outside the litterbox," says Manuel Sanchez del Rio of the European Synchrotron Radiation Facility (ESRF). "Already today, they absorb liquid spillages and odours and stabilise aqueous products like paints, resins and inks. In synthetic form, they could bind food products and stabilise drugs, extending their shelf life and making sepiolite an edible product." Before that is possible a clearer understanding of this rather enigmatic mineral is needed.

Sepiolite is a porous and low-density, magnesium silicate mineral with a typical formula of Mg₄Si₆O15 (OH) ₂?6H2O. It exists as fibrous, particulate and solid forms. The name derives from its superficial resemblance to the porous "bone" of the cuttlefish, Sepia latimanus known in Greek as the "sepion". While the mineral has been known since at least the Roman era, what has remained elusive is an insight at the atomic scale into just how this porous substance is able to absorb such enormous quantities of liquid in such tiny crystals.

Sepiolite succumbs

A paper from researchers in France and Spain to be published in the October 2011 issue of the American Mineralogist will reveal for the first time a single-crystal X-ray diffraction image of sepiolite. The study might allow materials scientists to develop a synthetic route to sepiolite and its analogues with properties improved or tailored for specific applications from food additives to drug-delivery agents.

The team includes Manuel Sanchez del Rio, Emilia Garcia-Romero, Mercedes Suarez, Ivan da Silva, Luis Fuentes Montero, and Gema Martinez-Criado from the Universities of Madrid and Salamanca in Spain, of the Institut Laue-Langevin (ILL), the European Synchrotron Radiation Facility (ESRF), and the Spanish CRG Beamline at the ESRF (SpLine), all in Grenoble, France.

No other mineral is known to absorb as much water or other liquid in the same volume of material as sepiolite. Indeed, the surface area of sepiolite minerals ranges from 75 to 400 square metres per gram of mineral. In the age-old custom of comparative descriptions, just 20 grams of sepiolite has an internal surface equivalent to that of a football field. This fact alone might be sufficient to explain how sepiolite can absorb 2.5 times its mass of water. The trite explanation for its absorptive properties is that the mineral is highly porous at the nanoscale containing tunnels and caverns within its crystal form and the fact that its elongated, needle-like crystals pack only very loosely into a porous material of low density. The tunnels in the crystal structure combined with the empty space between the needles form a capillary network through which liquids can easily flow deep inside the bulk material where water molecules might then attach to the internal surfaces of the crystals.

Until now, sepiolite has presented a crystallographic problem. It forms only tiny crystals. However, by collecting different samples of sepiolite fibres from twenty deposits across the globe, the researchers have found adequate crystals that could be investigated with electron microscopy and X-ray powder diffraction using synchrotron radiation.

The littlelest crystal

"To study these very small crystals, the ESRF uses an X-ray beam with just 2 by 5 micrometres cross section, "explains Sanchez del Rio. In the end, we collected X-ray diffraction data for two fibres." He admits that the data were not easy to interpret and so required extensive computer simulations to confirm and refine the information gathered by electron diffraction experiments done in parallel at the University Complutense of Madrid.

By studying sepiolites from a wide variety of sources, the team was able to correlate the physical and chemical properties of a given type with its revealed atomic structure. "Today, no synthetic clay surpasses natural sepiolite," says University of Salamanca's Suárez. "This is about to change as our understanding of their atomic structure will guide the synthesis of sepiolites from other, more abundant clay minerals and the design of completely new materials for use in catalysis and batteries."

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