Speaker
Prof.
Fabio Bruni
(Università di Roma Tre)
Description
Laponite is a synthetic disc-shaped crystalline clay belonging
to the family of swelling smectites. To date it is the most
widely studied synthetic clay on account of its special
chemical and physical properties, making it promising both as
''smart'' material suitable for several industrial and
biomedical applications and as model system for fundamental
studies on phase transitions and water molecules-charged
surfaces interaction [1].
We have performed dielectric relaxation experiments on
laponite powder with about 4 water layers of hydration
around each laponite disc (Fig. 1A), over a broad interval of
frequencies, ranging from 10-3 to 107 Hz, and over the
temperature range between 150 and 300 K. The aim of this
work is to study the orientational dynamics of water
molecules close to the laponite surface. In particular, we
examined the temperature dependence of the relaxation
process assigned to the collective dynamics of the hydrogen
bond (HB) network.
Preliminary results indicate the presence of two dynamical
crossovers of this collective relaxation time (Fig. 1B): the first
one (at about 260 K) identifies two different Vogel-Fulcher-
Tamman (VFT) regimes, while at about 170 K a second
crossover identifies a dynamical transition from a VFT to a
Arrhenius temperature dependence of the relaxation time.
This situation is quite similar to what previously observed for
water molecules in the first hydration shell of a globular
protein [2]. In this latter case, a coarse-grained model of
water molecules helped us to attribute the observed
crossovers to the thermodynamics of the HB network, with
two specific heat maxima. The high temperature maximum is
caused by fluctuations in the HB formation, while the low
temperature maximum is due to the cooperative reordering
of the HB network.
[1] Ruzicka B. et al., Soft Matter 7, 1268-1286 (2011).
[2] Mazza et al., PNAS, 108, 19873 (2011).
