Solvent-induced changes in the visible transmission spectrum of illinois no. 6 coal. Enhancement of light transmission through a microporous solid by solvent-induced index matching of the pores

Abstract

Coal is a high-surface-area microporous solid with an optical density considerably greater than that expected from an organic matrix containing only simple aliphatic and aromatic hydrocarbons. Mechanisms proposed to explain the anomalously high optical density of the dry carbonaceous material include the presence of large polycyclic aromatic species, charge-transfer complexes and light scattering from inhomogeneities. Initial experiments testing these mechanisms have been performed by examining the changes induced in the optical transmission of Illinois no. 6 coal when the dry coal is saturated with solvents of different swelling and optical properties. Over the spectral region from 4000 to 7000 Å, transmission through thin sections of coal increases substantially upon addition of solvent. Initial analysis of the data shows that the transmission increase and wavelength dependence of the resulting transmission spectra correlate with a model in which transmission losses are dominated by light scattering from the pores. The principal effect of the solvent is to index match partially the pores to the organic matrix, thereby reducing the scattering cross-section and increasing the transmission through the coal.