2 research outputs found
Water Radiolysis in Exchanged-Montmorillonites: The H<sub>2</sub> Production Mechanisms
The
radiolysis of water confined in montmorillonites is studied
as a function of the composition of the montmorillonite, the nature
of the exchangeable cation, and the relative humidity by following
the H<sub>2</sub> production under electron irradiation. It is shown
that the main factor influencing this H<sub>2</sub> production is
the water amount in the interlayer space. The effect of the exchangeable
cation is linked to its hydration enthalpy. When the water amount
is high enough to get a basal distance higher than 1.3 nm, then a
total energy transfer from the montmorillonite sheets to the interlayer
space occurs, and the H<sub>2</sub> production measured is very similar
to the one obtained in bulk water. For a basal distance smaller than
1.3 nm, the H<sub>2</sub> production increases with the relative humidity
and thus with the water amount. Lastly, electron paramagnetic resonance
measurements evidence the formation of a new defect induced by ionizing
radiation. It consists of a hydrogen radical (H<sub>2</sub> precursor)
trapped in the structure. This implies that structural hydroxyl bonds
can be broken under irradiation, potentially accounting for the observed
H<sub>2</sub> production
Dynamics of Water Confined in Clay Minerals
Ultrafast infrared spectroscopy of the O–D stretching
mode
of dilute HOD in H<sub>2</sub>O probes the local environment and the
hydrogen bond network of confined water. The dynamics of water molecules
confined in the interlayer space of montmorillonites (Mt) and in interaction
with two types of cations (Li<sup>+</sup> and Ca<sup>2+</sup>) but
also with the negatively charged siloxane surface are studied. The
results evidence that the OD vibrational dynamics is significantly
slowed down in confined media: it goes from 1.7 ps in neat water to
2.6 ps in the case of Li<sup>+</sup> cations with two water pseudolayers
(2.2–2.3 ps in the case of Ca<sup>2+</sup> cations) and to
4.7 ps in the case of Li<sup>+</sup> cations with one water pseudolayer.
No significant difference between the two cations is noticed. In this
2D confined geometry (the interlayer space being about 0.6 nm for
two water pseudolayers), the relaxation time constants obtained are
comparable to the ones measured in analogous concentrated salt solutions.
Nevertheless, and in strong opposition to the observations performed
in the liquid phase, anisotropy experiments evidence the absence of
rotational motions on a 5 ps time scale, proving that the hydrogen
bond network in the interlayer space of the clay mineral is locked
at this time scale