No abstract

Correct modeling of framework flexibility plays a major role in obtaining accurate results when performing atomistic simulations of guest molecule diffusion within ZIF crystal structures. Here we present a full set of force field parameters, based on the AMBER database and on previously computed partial charges, well reproducing the ZIF-8 structural properties over a wide range of temperatures and pressures. To test our model, the self-diffusivity for sorbed carbon-dioxide is computed and is found to be in good agreement with experimental measurements. Our results are also compared to the ones obtained with other charge models and are found to be more accurate. Finally, an estimate of the influence on self-diffusion of various simulation details is given.

Classical molecular dynamics simulations of water adsorbed in silicalite, a hydrophobic all-silica zeolite, were performed at different temperatures in the range 100−580 K, to explore possible phase transitions and to compare the behavior of adsorbed water with that of bulk water or water confined in nanopores of different geometry. We used a potential model including full flexibility both of water molecules and of the silicate framework. The results show an unexpected complexity. At very low temperatures (below 225 K), water appears to be mostly in form of amorphous solidlike clusters among which a slow molecule interchange occurs, giving rise to a single-file like diffusion on the time scale of our simulations. At intermediate temperatures, in the approximate range of 225−350 K, the behavior of water is almost liquidlike, whereas at higher temperatures, there are evidences of a vaporlike features, in agreement with the suggestions of previous theoretical and experimental works. This behavior is discussed by considering, among others, the average distribution of water in the channels, the size and lifetime of the hydrogen bonded clusters, and the water−water interaction energy. The results are compared with the available experimental data, previous simulations, and statistical mechanical studies.

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.

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