A
crystal has a defined structure that cannot be altered much once it is
formed-or such was the belief until now. Spanish chemists have recently
demonstrated this assertion to be false; they have produced a layered material
whose
porosity can be changed from one moment to the next. This could signal the
birth of a new type of material with tailored
pores that respond to external
signals. Microporous materials can take up "guest"
molecules, and are highly
coveted for selective
catalysts,
ion exchangers, or
storage for
pharmaceuticals
or other chemical agents.
The team, led by Ernesto Brunet, selected gamma-
zirconium phosphate, which is
known to have a layered structure, as their starting material. The surfaces of
the individual layers have
phosphate groups [PO4] sticking out of them, and
these can easily be replaced without changing the structure of the layers. The
researchers replaced some of the
phosphates with short
polyethylene glycol
chains that had a
phosphonic acid group [PO(OH)2] at each end. These
"di
phosphonates" forced out one phosphate on each of two adjacent layers,
bridging the layers together. The chains support the space between the layers
like
columns in an arcade. The researchers then
Exchange the remaining phosphate
groups for hypophosphite groups [H2PO2]. These are embedded in the surface of
the layer so that their apolar PH2 side protrudes into the space between the
layers. In contrast to the product of the precursor step, the apolar
interact
ions between the layers now cause the columns to lie flat (parallel to
the layers), which significantly decreases the distance between the layers.
If this material is then treated with basic
methylamine, it abruptly soaks it
up-within a narrow pH range-and the
separation between layers increases by
almost 70%. How? Polar molecules like methylamine initially cannot enter into
the apolar interlayer spaces. However, the anchors for the columns, the
phosphonate groups, are polar. Once the pH value reaches about 4.5, the
attraction between the basic methylamine and the acidic phosphonates is so
strong that individual methylamine ions force their way in at the edges of the
crystals. These ions are relatively large compared to the distance between the
layers. Like a wedge, they drive the rigid layers apart and cause the columns to
stand upright. A few wedges are enough to completely neutralize the apolar
attractive forces between the layers and bring all of the columns into an
upright position. The distance between the layers-and with it the
porosity-increases abruptly. "Such a high sensitivity of microcrystalline
porosity towards the incorporation of
small molecules is thus far unique," says
Brunet.