The densities of vitrains of twelve Japanese coals and five foreign coals were measured in methanol, n-hexane and mercury at 25ยฐC using a vacuum apparatus. All the densities showed higher values in lower and higher rank coals and minima at about 87% carbon content. The pore volumes deduced from the differences between the specific volumes measured in the liquids and in helium, and from carbon dioxide adsorption, are discussed and compared.
It seems that the micropore diameter or entrance diameter is small in the lower and higher rank coals, and shows a minimum at about 87% carbon content.
The ease of accessibility of molecules to the micropores of coals decreased in the order methanol, carbon dioxide and helium. The excess of density in methanol over that in helium is too large to be interpreted only by contraction of methanol in the pores of coal. It is suggested that helium may be restricted in penetrating very narrow pore entrances by the activated diffusion effect, while methanol can penetrate into these pores by virtue of an imbibition effect.
The volume of micropores accessible to methanol but inaccessible to carbon dioxide is directly proportional to the content of hydroxyl groups.