Structure, Dynamics, and Adsorption of Charged Guest within the Nanocavity of Polymer-Functionalized Neutral Macrocyclic Host

Tuning Network Connectivity of Silicate and Sodium Borosilicate Glasses by TiO2 for Enhanced Chemical Durability: Molecular Dynamics Simulation Investigations

Extensive molecular dynamics (MD) simulations were performed to disclose the beneficial aspects of TiO₂ doping in SiO₂ and sodium borosilicate (NBS) glasses. Significant amendment in short- and intermediate-range orders of glasses was captured by the radial distribution function, coordination number, bond/angle distribution profiles, structure factor, and probability of linking X-O-X' (where X and/or X' = Si, B, and Ti) structural motifs. Successively, the effect of microscopic structural modification on the macroscopic properties was analyzed in terms of mechanical strength, thermal stability, vibrational characteristics [(vibrational density of states (VDOS)], and chemical durability. The results show that Ti participates in the network chain in the form of TiO₆ and TiO₅ for the Ti-NBS glass whereas in the form of TiO₆, TiO₅, and TiO₄ for the binary TiO₂-SiO₂ glass. The presence of TiO₂ was found to strengthen the glass skeleton. However, the glass-transition temperature was also increased with Ti addition, which indicates increased hurdles during synthesis due to increased cross connections in the glass network with Ti doping. The computed results envisage enhanced chemical durability of Ti-added glasses. In addition, VDOS spectra showed network former-like characteristics of Ti in the glass network with significant contributions up to a vibration frequency of 800 cm^-1. The strong binding of Ti-O-connected Na⁺ in the glass skeleton prevents Na⁺ migration toward the interface or bulk aqueous phase, which contributes to improved chemical stability of Ti-containing glasses. During contact with water, Na⁺ were less likely to leach out from glass to the aqueous solution during Ti doping. In addition, the increased fraction of stable ring structures (5^m-7^m) for Ti-NBS glasses than bare NBS glasses also supports the increased leaching resistivity of Ti-added glasses. Essentially, the elucidation of macroscopic glass properties has been provided in terms of microscopic understanding. The present findings will incite further MD simulations and experiments to disclose more interesting microstructures and dynamics due to the presence of TiO₂ in glasses.

Molecular dynamics simulations of simplified sodium borosilicate glasses: the effect of composition on structure and dynamics

The fusion of valuable material properties has led to the acceptance of sodium borosilicate (NBS) glasses for nuclear waste immobilization. Although popular, the mechanisms associated with these properties are still only partially discovered and need further exploration. Bearing this in mind, the combination of experiments, molecular dynamics (MD) simulations and the Dell, Yuan and Bray model have been used to understand the role of composition variation for structural and physical aspects of vitrified borosilicate glasses. Experiments have been conducted to evaluate the macroscopic glass parameters of density (ρ), glass transition temperature (Tg) and thermal expansion coefficient (TEC). Experimentally observed trends for ρ, Tg and TEC with composition have been found in good agreement with the MD results. MD studies also provide a microscopic understanding of the glass structure and phenomena associated with the change in the glass composition. A detailed view of local structure and medium-range connectivity for the borosilicate glasses has been explored. Owing to a large B4 population, the results showed the abundant presence of BO4-BO4 connections, we hereby omit the generally accepted "B[4] avoidance rule" for glass. The relative propensity for connecting SiO4/BO3/BO4 structural motifs is in line with the predictions made by the Dell, Yuan and Bray model. Furthermore, the effects of composition on the mechanical integrity of NBS glasses, including the elastic nature, plastic distortion, yielding, breaking stress, and brittle fracture, have been explored by MD simulations. In addition, the glass dynamics have been evaluated by diffusion coefficient and the results suggest that Na+ is likely to be more mobile in the case of NBS1 as compared to NBS2 and NBS3 due to significant disruption in the glass network introduced by a larger amount of Na2O network modifier. Also, the diffusivity was reduced with increasing B2O3 due to the altered role of Na+ ions from network modifiers to charge compensators. The combined study of experiments, MD simulations and the Dell, Yuan and Bray model establish the correlation between the microscopic structure and macroscopic properties of NBS glasses with varied composition, which might be of great scientific use for future glasses in various applications including nuclear waste immobilization.

DFT and MD simulation supplemented experiments for isotopic fractionation of zinc compounds using a macrocyclic crown ether appended polymeric resin

Isotope effect is a quantum mechanical phenomenon and thus poses a challenge for the separation of isotopes of an element of interest, especially for heavy elements. Isotopic fractionation of zinc is also quite difficult and challenging but is necessitated due to various applications of its isotopes ranging from nuclear medicine to nuclear power reactors. Here, we developed the dibenzo-18-crown-6 (DB18C6) ether-functionalized poly(methacrylic acid) (PMA) resin by exploiting the ion and isotope recognition ability of the crown ether using DFT/MD simulations followed by experiments for isotopic fractionation of zinc. The PMADB18C6 adsorbent was prepared and suitably characterized. Both computational and experimental findings demonstrate that the adsorption and isotope separation of zinc with PMADB18C6 are due to the molecular recognition effect of the "O" dipole of the crown ether. Furthermore, both MD simulations and experiments suggest Langmuir type adsorption isotherms. The adsorption of Zn2+ ions on the PMA resin is predicted to be endothermic, whereas it is exothermic on the PMADB18C6 resin, as revealed from the experimentally observed enthalpy change. A small scale fixed bed column study was demonstrated to test the scale-up application. The values of the experimental separation factors: 1.0013 for 66/64 and 1.0027 for 68/64 confirm the computationally predicted results of 1.00088 and 1.0010, respectively, thus establishing the combined strength of the theory and experiments for the identification of efficient fractionating agents for a complex quantum isotope effect which eventually helps in planning further experiments in view of medicinal and technological applications of zinc isotopes.