ACCESSIBILITY OF GEOMETRICALLY-ROUGH (FRACTAL) SURFACES OF NATURAL SORBENTS TO PROBE MOLECULES

Vapor sorption of a series of probe molecules was studied to investiga te the accessibility of surfaces for adsorption. The hypothesis was th e amount of probe molecules required to cover a surface with monolayer (i.e., monolayer value, N-m, mmoles/g sorbent is affected by the mole cular level roughness of the surface. In particular, the applicability of the concept of fractal dimension was considered as the conceptual guideline. The experimental results indicated that the surface structu re of simple sorbents, such as kaolinite and silica-gel, can be interp reted as fractal-like. However, several factors severely limited the a ccurate estimation of the fractal dimension from sorption data; these include 1) unknown surface configuration/packing of adsorbed molecule, 2) experimental difficulty (low vapor sorption) associated with large probe molecules, 3) unrealistic assumptions inherent in vapor sorptio n theory (B.E.T. theory), and 4) slow sorption/desorption kinetics and hysteresis. The result for Borden sand showed that sorption of variou s probe molecules by this medium could not be explained from fractal t heory.