A model potential in two forms (harmonic and anharmonic) is proposed to be used in molecular dynamics simulations of silicate frameworks. This model is applied to the calculation of structural and vibrational properties of the anhydrous phase of Linde 2Mite 4 A. Our system is formed by a cubic box corresponding to one crystallographic cell containing 662 atoms without constraints. The results are compared with experimental data. The proposed models satisfactorily reproduce the main features of the aluminosilicate framework structure and dynamics.

Surface hydroxy groups are known to play a significant role in the photocatalytic behavior of TiO2. The molecular or dissociative character of H2O adsorption on titania is strongly affected by the local surface structure, but dissociation, producing −OH, should prevail on surface sites with a high degree of coordinative unsaturation. Here we report an IR investigation of hydroxy groups on low coordination sites of TiO2 P25 (Degussa), a commercial titania powder considered a staple in the field of heterogeneous photocatalysis. The presence of coordinatively defective surface terminations well agrees with the roughness of the border of TiO2 particles observed by high resolution transmission electron microscopy. Furthermore, IR spectra of adsorbed CO were used to probe the Lewis acidity of surface Ti4+ ions exposed after the removal of water molecules and hydroxy groups. Evidence for two additional types of surface cationic sites with respect those reported in the literature was found.

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30)K = exp Accepting that yc = ya i.e., Ay = 0 and n, = ns, we deduce that or Mazer et report an aggregation number of 10 for the formation of a primary micelle, Le., n, = 10. Accepting our Awo values and AGO = -RT In K, it is deduced that AGO varies in the range -1.3 to -6.7 kcal/mol. Although several simplifications are involved, such values are very close to those obtained by Mazer et al. (from -3.8 to -6.7 depending on experimental conditions) for the above-mentioned bile salts. Giglio et al." have demonstrated the advantages of the helical model compared with the Small model for deoxycholate derivatives. The previous aggreement could mean that such a model is also valid for other bile salts.The effect of the electrolyte anion on the value of Ay found in this paper has not been considered in KS model and further developments are required.Acknowledgment. We thank the Xunta de Galicia for financial support (project XUGA29906A90). cmc critical micelle concentration QLS quasi-elastic light scattering NaDC sodium deoxycholate NaC sodium cholate NaDHC sodium dehydrocholate NaTDC sodium taurodeoxycholate TC taurocholic acid TDC taurodeoxycholic acid TCDC taurochenodeoxycholic acid TUDC tauroursodeoxycholic acid Salsalicylate anion M mol d n i 3 mM mol m-' Nasal, 54-21-7.The effect of the temperature on the diffusion of methane in silicalite was studied by molecular dynamics both using a model where the vibrations of the zeolite framework are taken into account and keeping the framework Fled. Methane molecules were represented by Lennard-Jones particles. The diffusion coefficients were evaluated at four different temperatures in the range 150450 K and resulted in good agreement with experiment. The effect of the vibrating framework on the diffusive process is discussed and a detailed analysis of the behavior of methane molecules in silicalite is reported. IntroductionThe elucidation of the behavior of fluids within narrow pores

Car-Parrinello structural optimizations of realistic models of the Maya Blue (MB) hybrid material are combined with TDDFT calculations of the electronic excitation spectra to identify the nature of the fundamental guest-host interactions leading to the unusual stability of this pigment. The comparison with the features of the experimental visible spectrum reveals that the main mode of interaction between the host solid (the palygorskite clay) and the guest molecule (the organic indigo dye) involves the coordination of the carbonyl group of the dye by Al3+ ions exposed at the edge of the palygorskite tunnels. Analogous Mg2+-dye interactions which do not strongly affect the MB visible spectrum can also be present. Thermal treatment used in the preparation of the pigment appears therefore essential to release some of the structural water molecules tightly bound to the Al3+ ions in the internal clay surface, thus leaving them available to coordinate the organic molecule. Moderate heating also favors the oxidation of indigo to dehydroindigo (DHI): the spectral features of the latter complex with Al3+ are in remarkable agreement with the experimental spectrum, thus confirming the substantial role of DHI in the properties of Maya Blue

The development of functional materials based on the supramolecular organization of photoactive species in nanosized porous matrices requires a deep knowledge of host−guest interactions and of their influence on material properties and stability. Extensive first-principles investigations on the fluorescent dye fluorenone inside zeolite L, both at dry conditions and in the presence of water, have unraveled the molecular origin of the peculiar stability of this composite in humid environments, a fundamental prerequisite for practical applications. Results of first-principles molecular dynamics simulations, structural optimizations, and TDDFT calculations, validated by comparison with experimental data, provide a comprehensive picture of the structure, energetics, electronic excitation properties, and room-temperature behavior of the fluorenone/zeolite L composite and predict a substantial optical anisotropy for this material also maintained upon contact with water. The interaction of the fluorenone carbonyl group with the zeolite extraframework potassium cations is responsible for the dye stabilization in zeolite L nanochannels and features itself as a general leitmotiv regarding important properties of carbonyl functionalized photoactive species in hydrophilic matrices

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Integrated studies of CO on truncated bipyramidal TiO 2 anatase nanoparticles mainly exposing smooth (101) surfaces provide the missing link between TiO 2 single crystals and commercial TiO 2 nanopowders with complex morphology. The synergy among high resolution transmission electron microscopy, IR spectroscopy and modeling correlates adsorbed CO stretching frequency to anatase surface type, and reveals how disorder of the adsorbed CO layer affects CO/TiO 2 IR bands.