(194a) A New Approach Based On Accessibility in the Characterization of Porous Media

In this talk we will present a new and consistent approach in the characterization of a porous solid for its void volume, surface area and pore size. Since solids that are useful for gas phase adsorption have pores in molecular dimensions and more often than not these pores do not adopt simple geometry, such as slit, cylinder or sphere, it is necessary for us to establish a new strategy on how to deal with them. Here we argue that the key concept is the accessibility. The accessible void volume is defined as the space accessible to the centre of mass (COM) of a given molecular probe, and the accessible geometrical surface area as one that is traced by the COM of that probe subject to zero solid-fluid potential energy. Unlike the volume and surface area, the accessible pore size is a local variable, and it is a property of any point in the accessible volume and it has an associated local accessible volume. The relationship between the accessible pore size and its associated accessible pore volume is called the accessible pore size distribution (APSD) to distinguish it from the traditional pore size distributions that are commonly used in the literature. The consistency in these structural variables will be discussed, and we demonstrate this new concept of characterization with a number of examples ? bundle of carbon nanotubes, disordered solids, metal organic frameworks, etc.