Study of molecular sieve carbons. Part 2.—Estimation of cross-sectional diameters of non-spherical molecules

Abstract

The estimation of molecular dimensions by various experimental methods is discussed and their relevance to non-spherical molecules is evaluated. Liquid molar densities provide an average dimension which is adequate for spherical molecules but completely insensitive to molecular shape. The combination of bond lengths and van der Waals radii enables one to estimate satisfactorily the length of linear molecules but not their width. The kinetic diameters calculated from different physical properties of gases diverge significantly and are insensitive to molecular shape. Adsorption in molecular sieve(MS) solids exhibit high sensitivity to the width or smallest dimension of the molecule. Molecular sieve carbons (MSC) seem promising in this respect since their average pore diameter can be tailored to the exact critical dimension of any molecule in the range 3–5.5 Å studied so far. The combination of adsorption stereospecificity data and liquid molar volumes provides reasonable numerical estimates of the width of non-spherical molecules. Polar molecules may have different dimensions depending on whether the carbon surface is polar (oxidised) or non-polar. Hydrogen acquires a surprisingly large width which is in accordance with its high liquid molar volume. Adsorbent–adsorbate interactions play a crucial role in determining molecular dimensions and serve to elucidate the unusual behaviour of both hydrogen and polar molecules. An overview of the concept molecular dimension is given in terms of the effect of intermolecular forces.