Mesopore Structure of Microcrystalline Cellulose Tablets Characterized by Nitrogen Adsorption and SEM: The Influence on Water-Induced Ionic Conduction

Abstract

Tablets of microcrystalline cellulose were formed at different compaction pressures and physical properties, such as pore size distribution, surface area, and pore surface fractality, were extracted from N2 adsorption isotherms. These properties were compared to previously published data on the water-induced ionic conductivity of the tablets. The conduction process was shown to follow a percolation model with a percolation exponent of 2 and a porosity percolation threshold of approximately 0.1. The critical pore diameter for facilitated charge transport was shown to be in the 5-20 nm range. When the network of pores with a diameter in this interval is reduced to the point where it no longer forms a continuous passageway throughout the compact, the conduction process is dominated by charge transport on the surfaces of individual microfibrils mainly situated in the bulk of fibril aggregates. A fractal analysis of nitrogen adsorption isotherms showed that the dominant interface forces during adsorption is attributed to surface tensions between the gas and the adsorbed liquid phase. The extracted fractal dimension of the analyzed pore surfaces remained unaffected by the densification process at low compaction pressures (< approximately 200 MPa). At increased densification, however, pore-surface structures smaller than approximately 100 nm become smoother as the fractal dimension decreases from approximately 2.5 at high porosities to approximately 2.3 for the densest tablets under study.