Molecular dynamics studies on zeolites. 6. Temperature dependence of diffusion of methane in silicalite

Abstract

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

United Atom Force Field for Alkanes in Nanoporous Materials

“…There seems to be some indication that framework vibrations can alter the diffusivities of tightly fitting molecules, 56,57 even though this appears not to be the case for the diffusion of small alkanes through cation-free sieves. 58,59 We stress that to compare a flexible framework with a rigid framework the flexibility should be modeled in such a way that the two structures are on average identical. This implies that the reference bond lengths should be taken from the rigid structure.…”

Diffusion and vibrational relaxation of a diatomic molecule in the pore network of a pure silica zeolite: A molecular dynamics study

“…, which, in previous simulations, 4 provided an adequate description of methane-silicalite interaction, and are consequently fairly suitable for describing ͑CH 3 ͒-O interactions.…”

“…Simulations with a vibrating lattice showed that the framework can act as an effective ''heat bath'' for the sorbates; at any time their velocities are thermalized to a Boltzmann distribution through a continuous energy exchange with the lattice atoms. 4 This fact can have some effects on the diffusion and other properties of the guest species; until now MD studies on this topic have mainly concerned with adsorbed Lennard-Jones ͑LJ͒ spheres, representing methane or xenon. [4][5][6][7][8][9] The effects of framework vibrations should be greater on nonspherical sorbates larger than methane.…”

“…4 This fact can have some effects on the diffusion and other properties of the guest species; until now MD studies on this topic have mainly concerned with adsorbed Lennard-Jones ͑LJ͒ spheres, representing methane or xenon. [4][5][6][7][8][9] The effects of framework vibrations should be greater on nonspherical sorbates larger than methane. These species move in a potential with higher energy barriers to diffusion compared to methane, and the framework vibrations, which can lower these barriers, could have more important effects.…”

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…90 The crossing of the lines gives the parameters for which both the Henry coefficient and heat of adsorption agree with the experimental results. For comparison, the stars are the parameters of the model proposed by Bezus et al, 74 Goodbody et al, 91 Hufton et al, 92 and Demontis et al 93,74,[91][92][93] The triangle gives the model of Dubbeldam et al 94 Pascual and co-workers 95,96 used an approach that relies less on the availability of accurate experimental data. This method relies on the assumption that alkane-zeolite interactions can be estimated from the Lorentz-Berthelot mixing rules for the anisotropic united-atom model:…”