Sepiolite is a lightweight porous mineral used in cat litter and
other applications. The extraordinary properties of this clay make
it a highly sought after mineral, despite its scarcity in the
Earth's crust: only a few mines worldwide extract it, several of
them clustered near Madrid in Spain, the world's biggest exporter
of this material.
Sepiolite has been known since Roman times when it was used to
filter and purify wine, but our understanding at the atomic scale
of how these tiny crystals absorb enormous amounts of liquid has
remained elusive until now. A team of scientists from Spain and
France has obtained for the first time single-crystal X-ray
diffraction images of sepiolite, opening the path to industrial
synthesis and further improvement of its properties. The results
will be published in the October 2011 issue of the
journalAmerican
Mineralogist.
The team included scientists 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 more water or other liquids as
efficiently as sepiolites. The reasons are its structural
nanoporosity due to tunnels in the crystals, and the fact that the
elongated, needle-shaped sepiolite crystals pack very loosely into
a lightweight porous material. The surface area ranges between 75
and 400 m2/g, meaning that 20g of mineral have an internal surface
equivalent to that of a football court. This is why sepiolite can
absorb 2.5 times its weight in water. The tunnels in the crystal
structure along with the empty space between the needles form a
capillary network through which liquids can easily flow deep inside
the bulk where the molecules attach to the surface of the
crystals.
The tiny size of these crystals—they measure a few micrometres in
length and as little as some dozen atoms across—has been the main
obstacle to their being studied with single-crystal diffraction
techniques. For this experiment, the scientists collected samples
of sepiolite fibres from twenty different deposits around the
world. These fibres, each made of many crystals, were first imaged
with electron-microscopy and then studied using X-ray powder
diffraction.
However, the most accurate technique to resolve the
three-dimensional structure of a crystal is single-crystal
diffraction with either X-rays or electrons as probe. "To study
very small crystals, the ESRF uses an X-ray beam with just 2 by 5
micrometres cross section. In the end, we collected X-ray
diffraction data for two fibres", says Manuel Sanchez del Rio from
the ESRF, "but the data were not easy to interpret, and needed
extensive computer simulations to confirm and refine the
information gathered by electron diffraction experiments done in
parallel at the University Complutense of Madrid".
The wide variety of sepiolites studied is now enabling the team to
correlate between the physical and chemical properties of a given
type with its atomic structure. "Today, no synthetic clay surpasses
natural sepiolite. 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", says Mercedes Suárez
from the University of Salamanca.
"The future of sepiolites in the household is outside the
litterbox. 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",
concludes Manuel Sanchez del Rio.
###
European Synchrotron Radiation Facility
Thanks to European Synchrotron Radiation Facility for this
article.
