Water Radiolysis in Exchanged-Montmorillonites: The H₂ 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 H₂ production under electron irradiation. It is shown that the main factor influencing this H₂ 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 H₂ production measured is very similar to the one obtained in bulk water. For a basal distance smaller than 1.3 nm, the H₂ 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 (H₂ precursor) trapped in the structure. This implies that structural hydroxyl bonds can be broken under irradiation, potentially accounting for the observed H₂ production

Dynamics of Water Confined in Clay Minerals

Ultrafast infrared spectroscopy of the O–D stretching mode of dilute HOD in H₂O 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⁺ and Ca²⁺) 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⁺ cations with two water pseudolayers (2.2–2.3 ps in the case of Ca²⁺ cations) and to 4.7 ps in the case of Li⁺ 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

Electron Transfer at Oxide/Water Interfaces Induced by Ionizing Radiation

The electron transfer from oxide into water is studied in nanoparticle suspensions of various oxides (SiO₂, ZnO, Al₂O₃, Nd₂O₃, Sm₂O₃, and Er₂O₃) by means of pulse and γ radiolysis. The time-resolved and steady-state investigations of the present study demonstrate independently that whatever the band gap and the electron affinity of the oxide, the electron transfer always takes place in these nanometric systems: Irradiation generates hot electrons which have enough energy to cross the semiconductor–liquid interface. Moreover, picosecond measurements evidence that the spectrum of the solvated electron is the same as in water. Lastly, the decay of the solvated electron is similar on the picosecond to nanosecond time scale in water and in these suspensions, but it is clearly different on the nanosecond to microsecond time scale