Correlation of aromaticity and molecular weight of oil,asphaltene and preasphaltene

There is a linear relationship between aromaticity (f_a) or H/C and the molecular weight of oil, asphaltene and preasphaltene. The molecular weight of preasphaltene depends most strongly on f_a or H/C and that of oil the least. The molecular weight increases with decrease of f_a or increase of H/C, because the increase of aliphatic structure is favourable to the solubility of each fraction and the larger molecules can be dissolved in the same solvent.