Adsorption of water in Na-LTA zeolites: an ab initio molecular dynamics investigation.

The very wide range of applications of LTA zeolites, including the storage of tritiated water, implies that a detailed and accurate atomic-scale description of the adsorption processes taking place in their structure is crucial. To unravel with an unprecedented accuracy the mechanisms behind the water filling in NaA, we have conducted a systematic ab initio molecular dynamics investigation. Two LTA structural models, the conventional Z4A and the reduced one ZK4, have been used for static and dynamic ab initio calculations, respectively. After assessing this reduced model with comparative static DFT calculations, we start the filling of the α and ß cages by water, molecule by molecule. This allowed us to thoroughly study the interaction of water molecules with the zeolite structure and between water molecules, progressively forming H-bond chains and ring patterns as the cage is being filled. The adsorption energies could then be calculated with an unprecedented accuracy, which showed that the interaction of the molecules with the zeolite weakens as their number increases. By these methods, we have been able to highlight the primary role of Na+ cations in the interaction of water with zeolite, and inversely, the role of water in the displacement of cations when it is sufficiently solvated, allowing the passage between the α and ß cages. This phenomenon is possible thanks to the inhomogeneous distribution of water molecules on the cationic sites, as shown by our AIMD simulations, which allows the formation of water clusters. These results are important because they help in understanding how the coverage of cationic sites by water will affect the adsorption of other molecules inside the Na-LTA zeolite.

One-pot electrosynthesis of polyglycine-like thin film on platinum electrodes as transducer for solid state pH measurements.

The anodic oxidation of concentrated glycine based aqueous electrolyte on smooth platinum electrode leads to a strongly grafted polyglycine-like coating on the surface in an irreversible way. Due to the proton affinity towards amino groups of polyglycine (PG), the electrodeposited thin film was used as receptor for solid potentiometric pH sensor. In order to reach local pH measurement, we developed miniaturized microelectrodes on glass substrate thanks to photolithography process. We used silver chloride on silver as the reference electrode. The couple (silver chloride, PG based platinum electrode) of microelectrodes gives linear potentiometric response vs. pH in the range [2-12], reversibly and with a sensitivity of 52.4 mV/pH (for 1mm electrode size). PG based pH electrode is compared to other organic polymer based pH receptor such as linear polyethylenimine (L-PEI), polyaniline (PANI) and glass membrane.