Water diffusion in nanoporous glass: an NMR study at different hydration levels

By pulsed field gradient nuclear magnetic resonance measurements, we investigated the translational diffusion of water confined in the 200 A diameter pores of a sol-gel silica glass. The experiments, performed as a function of the hydration level, showed an enhancement of the self-diffusion coefficient when the water content corresponds to one or fewer monolayers. An explanation for this occurrence has been given in terms of a two-phase process involving a fast molecular exchange between the liquid and the vapor phase. Moreover, in partially filled pores, the surface water diffusion coefficient was measured, and was 4 times lower than the diffusion of liquid confined water in saturated spaces.     

A new self-consistent empirical interatomic potential model for oxides, silicates, and silica-based glasses

A new empirical pairwise potential model for ionic and semi-ionic oxides has been developed. Its transferability and reliability have been demonstrated by testing the potentials toward the prediction of structural and mechanical properties of a wide range of silicates of technological and geological importance. The partial ionic charge model with a Morse function is used, and it allows the modeling of the quenching of melts, silicate glasses, and inorganic crystals at high-pressure and high-temperature conditions. The results obtained by molecular dynamics and free energy calculations are discussed in relation to the prediction of structural and mechanical properties of a series of soda lime silicate glasses.     

Medium-range order in phospho-silicate bioactive glasses: Insights from MAS-NMR spectra,chemical durability experiments and molecular dynamics simulations

The medium-range order of phospho-silicate bioactive glasses (with compositions (2 ? p)SiO₂ · 1Na₂O · 1.1CaO · pP₂O₅, in which p = 0.10, 0.20, 0.26) has been studied by means of a combined-experimental (MAS-NMR, chemical durability measurements) and computational (classical molecular dynamics (MD)) approach. The structural model obtained by MD is showed to be helpful in the interpretation of the NMR spectra. A small amount of Si–O–P link units has been detected in glasses with low P₂O₅-content, but at high P₂O₅ concentration the percentage of Si–O–P bridges becomes important. However, Qⁿ distributions show that the HP5 (p = 0.20) glass structure is less polymerized with respect to the H (p = 0.10) and HP6.5 (p = 0.26) glasses. These results provide useful explanation of the behavior of these glasses in water and highlight the influence of the medium-range order on a very important property of potential bioactive glasses such as the chemical durability.     

Endor and eseem study of the radical obtained by γ irradiation of a single crystal of ammonium tartrate

The radical obtained by γ irradiation of a single crystal of ammonium tartrate has been identified by paramagnetic resonance spectroscopies. The proton hyperfine coupling tensors have been obtained, from the angular dependence of the ENDOR transition frequencies. ESEEM spectra of the radical have been also obtained. The intensities of the ESEEM lines are related to the depth of the modulation, which has a dramatic angular dependence.     

Molecular Ordering in Liquid Crystals from Incoherent Quasi-Elastic Neutron Scattering

The Elastic Incoherent Structure Factor (EISF) for neutron scattering in liquid crystals has been interpreted theoretically by evaluating the correlation functions for the molecular reorientation according to the diffusion model for highly ordered fluids. It is shown that the available experimental data are compatible with the orientational distribution function predicted by the Maier-Saupe model for uniaxial mesophases provided that the long time behaviour of the correlation functions within the time scale of the experiment be properly considered.     

Phenomenological representation of triplet-exciton kinetics

A general density-matrix formulation is derived in a way suitable for the analysis of optical and magnetic effects due to the triplet-exciton kinetics in molecular crystals, in the presence of a microwave field inducing transitions among the spin levels. The expansion of the density matrix as a power series of the microwave field intensity provides a hierarchy of systems of equations, the knowledge of higher-order corrections requiring the solutions of all lower-order terms. The solutions of the zeroth-, first- and second-order equations are relevant for the calculation of polarization and delayed fluorescence effects. ESR lineshapes and DF ODMR spectral profiles, respectively.