Pore diameter interactions

We have studied, by MD, pure water [22] and electrolyte solutions [23] in cylindrical model pores with pore diameters ranging from 0.8 to more than 4nm. In the nonpolar model pores the surface is a smooth cylinder, which interacts only weakly with water molecules and ions by a Lennard-Jones potential the polar pore surface contains additional point charges, which model the polar groups in functionalized polymer membranes. 

In bulk diffusion, the predominant interaction of molecules is with other molecules in the fluid phase. This is the ordinary kind of diffusion, and the corresponding diffusivity is denoted as a- At low gas densities in small-diameter pores, the mean free path of molecules may become comparable to the pore diameter. Then, the predominant interaction is with the walls of the pore, and diffusion within a pore is governed by the Knudsen diffusivity, K-This diffusivity is predicted by the kinetic theory of gases to be... 

Surprisingly, an enhanced conductivity is shown for a certain range of confinement, as seen in Figure 19. This can be explained by the decrease in coulombic attractive interaction among the ions in a one-dimensional configuration. But when the pore diameter is... 

The decrease in the heat of adsorption as the pore size is increased beyond this size is not surprising dispersive interactions with the zeolite pore decrease. The behavior at lower pore dimensions is explained by considering the location of the sorbed molecules. In the cases of zeolites rho and A, the alkanes were found to adopt highly coiled conformations in the centers of the a-cages that form these structures. The alkanes thus are located in pores with a larger diameter than that usually used to characterize the zeolite (namely, the diameter of the windows that connect the cages). If the heat of adsorption as a function of pore diameter is replotted to reflect the locations of the sorbed molecules, a more straightforward inverse relationship is obtained. 

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