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Computer simulation adsorption potential

Another special case of weak heterogeneity is found in the systems with stepped surfaces [97,142-145], shown schematically in Fig. 3. Assuming that each terrace has the lattice structure of the exposed crystal plane, the potential field experienced by the adsorbate atom changes periodically across the terrace but exhibits nonuniformities close to the terrace edges [146,147]. Thus, we have here another example of geometrically induced energetical heterogeneity. Adsorption on stepped surfaces has been studied experimentally [95,97,148] as well as with the help of both Monte Carlo [92-94,98,99,149-152] and molecular dynamics [153,154] computer simulation methods. [Pg.268]

The dependences of pH and C-potential on the adsorbed amount of M(H20)2+ at the total metal ion concentrations of 3 x10-3 mol dm-3 are shown in Figures 7 and 8, respectively. The amount adsorbed for each M2+ increases with the pH, and the inflection points are shifted toward the lower pH region in the order of Co2+, Zn2+, Pb2+, Cu2+, which corresponds to the order of the hydrolysis constant of metal ions. To explain the M2+-adsorption/desorption, Hachiya et al. (16,17) modified the treatment of the computer simulation developed by Davis et al. (4). In this model, M2+ binds coordina-tively to amphoteric surface hydroxyl groups. The equilibrium constants are expressed as... [Pg.241]

The main goal of the molecular dynamics computer simulation of ionic solvation and adsorption on a metal surface has been to test the above model and to provide more quantitative information about the different factors that influence the structure of hydrated ions at the interface. Unfortunately, most of the experimental information about these issues has been obtained from indirect measurements such as capacity and current-potential plots, although in recent years in situ experimental techniques have begun to provide an accurate test of the above model. For a recent review of experimental techniques and the theory of ionic adsorption at the water/metal interface, see the excellent paper by Philpott. ... [Pg.145]

Which theory is suitable for a certain application The adsorption theory of Henry is applicable at low pressure. This, however, is natural since it can be viewed as the first term in a series of the adsorption function. A widely used adsorption isotherm equation is the BET equation. It usually fits experimental results for 0.05 < P/P0 < 0.35. For very small pressures the fit is not perfect due to the heterogeneity. For higher pressures the potential theory is more suitable at least for flat, homogeneous adsorbents. It often applies to P/Po values from 0.1 to 0.8. Practically for P/Po > 0.35 adsorption is often dominated by the porosity of the material. A more detailed description of adsorption is obtained by computer simulations [382],... [Pg.195]

This approach was later extended to off-lattice models and a more detailed description of the transfer energy of the different amino acid residues [77]. Magainin, melit-tin, and several other amphipathic peptides were simulated. In these simulations, differences in the interaction of the peptides with the lipid phase were observed. For example, magainin only showed adsorption onto the lipid and no crossing of the lipid occurred, whereas melittin crossed the lipid and formed a stable transmembrane helix. These results are in full agreement with later studies reported by other research groups presented below, involving more elaborate simulation protocols and representations of the peptides and the lipid. These examples show the potential of computer simulations even when some simplifications have to be made to make the system computationally tractable. [Pg.324]

Thus, the surface of this amorphous carbon (which is a model of the surfaces of non-graphitized carbon blacks [23]) differs considerably from the surface of amorphous oxide and the main structural characteristics such as the C-C and 0-0 coordination numbers are also drastically different. Nevertheless, the adsorption properties of heterogeneous surfaces of various nongraphitized carbon blacks with respect to an inert adsorbate such as argon are not that drastically different and actually have many common features. We discuss these properties in the next section. Here we only use this fact to show that subtle structural differences of various models of amorphous oxide surfaces discussed above may be not that important for their adsorption properties in comparison to other factors such as indefiniteness of adsorption potential on oxide surfaces (see below). Because of its generality and in spite of its approximate character, the BS appears to be a convenient model for the computer simulation of adsorption on amorphous, and even more general (see Introduction) heterogeneous oxide surfaces. [Pg.343]

As soon as the expressions and constants in Eq. (4) are fixed one may proceed with calculation of the adsorption energy at a given point of adsorption space. However, to make such a calculation possible one has to know the positions of all the atoms of adsorbent relative to the given point. In other words, one has to know exactly the atomic structure of the adsorbent. This is what is in fact unknown for amorphous oxides. Although one can simulate the atomic structure at the surface of an amorphous oxide as described above, the reliability of the result can at present only be checked by comparison of prediction of adsorption properties with experimental data. But the calculation of adsorption properties (described below) includes, generally speaking, two unknowns the atomic structure of an adsorbent and the adsorption potential. This is the reason why the computer simulation of physical adsorption on amorphous oxides should be preceded by similar simulations on oxides with well defined crystalline structures. [Pg.345]

Physical adsorption on the (001) face of MgO also attracted considerable attention in recent years (see short review and references in Ref. [26]). It provides another opportunity to test methods of adsorption potential calculation which can be used later to simulate adsorption on adsorbents with less reliable atomic structure of surfaces like amorphous oxide. There is a large and rapidly changing electric field near the surface of MgO which should be much stronger than in silicalite due to small cations of Mg " " and larger ionicity of MgO in comparison to Si02. Thus calculations with polar and quadrupole molecules which were carried out on that surface (see Ref. [26] and references therein) necessarily employ methods which may useful for computer simulations on amorphous oxides. [Pg.346]

In recent years, the behavior of fluid molecules in small pores has been well studied by computer simulation [136,137]. Computer simulation can provide us with valuable information about the microscopic behavior of adsorbate molecules confined in small pores in terms of intermolecular and surface forces, thus enabling us to understand the fundamental behaviors of adsorbates in the potential field of the model pores. The current trend in the literature suggests that for physical adsorption in activated carbon, the adsorbate-adsorbate interaction and adsorbate-pore interaction are well represented by the LJ potential theory while the model micropore is a slit-shaped channel of infinite extent. This forms the basis for the appliction of the statistical method in adsorption processes. [Pg.448]


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