Molecular surface potential gradient

To display properties on molecular surfaces, two different approaches are applied. One method assigns color codes to each grid point of the surface. The grid points are connected to lines chicken-wire) or to surfaces (solid sphere) and then the color values are interpolated onto a color gradient [200]. The second method projects colored textures onto the surface [202, 203] and is mostly used to display such properties as electrostatic potentials, polarizability, hydrophobidty, and spin density. 

The potential gradient that exists in the elextrolyte contributes negligibly to the movement of minor ionic species their transport is almost entirely by convection and diffusion. Therefore, the equations that are used for neutral species, such as for dissolved molecular oxygen, are also valid for minor ionic species. Convection refers to the macroscopic movement of a fluid under the influence of a mechanical force (forced convection) or of gravity force (free convection). At solid surfaces the velocity of fluids is zero and as a consequence only diffusion contributes to the flux at the electrode-electrolyte interface. This allows us to write the following expression for the flux of a minor ionic species B ... 

Recently, different approaches to describe the kinetics of oxygen and nitrogen sorption in carbon molecular sieves have been presented. It has been shown that they can lead, as particular cases, to the two above mentioned mechanisms. Srinivasan et al. [18] proposed a surface barrier model of diffusion under a chemical potential gradient, while Trifonov and Golden [19] presented a theoretical approach based on a hopping mechanism, in which the molecules jump from occupied to free adsorption sites. Seaton et al. [20] carried out molecular dynamics simulation of diffusion in individual pores and in pore networks. The experimental selectivity can be reproduced both at the level of individual pores, as well as with pore networks having a wide range of pore sizes. 

Diffusion of molecules which are similar in size to the size of the pores is very restricted because of the effect of potential field of the wall atoms. Diffusion in molecular sieve materials is often of this type. Diffusion in this case is accompanied by relatively large activation energy and can be correlated by assuming that the driving force of diffusion is the chemical potential gradient. Ordinarily, diffusion coefficient is defined in terms of amount adsorbed, q, similar to the case of surface diffusion. 

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