COMPUTER MODELING OF PARTIAL SPECIFIC VOLUMES OF HUMIC SUBSTANCES

The determination of the structure of humic substances from peat soils and natural waters is a major problem in soil science. Humic substanc es are chemically heterogeneous and consist of molecules covering a wi de distribution of molecular size. A quantity related to both molecula r size and structure in the hydrated state is the volume of the humic molecules per unit mass or partial specific volume (<(nu)over bar>(2)) . The partial specific volumes of humic substances have been estimated from atom and chemical group volume contributions for 17 proposed str uctures of fulvic and humic acids taken from the literature. The resul ts show that the partial specific volume reflects the composition of h umic substances, decreasing with increase in the carbon: hydrogen (C:H ) molar ratio. The more oxidized the humic material the smaller is its partial specific volume. These trends agree with our experimental mea surements of partial specific volumes for a range of humic materials ( aquatic fulvics, aquatic and peat humics). The predicted values of par tial specific volumes for the proposed structures, however, are overes timated by approximately 15% compared to the experimental values. Ther e is little difference between values of <(nu)over bar>(2) calculated from the detailed humic structures and those calculated from their emp irical formulae; calculations of partial specific volumes using a rand om modelling approach leads to overestimation by approximately 37%. It seems that the estimates based on additivity of atom and chemical gro up volumes do not take into account an overall contraction of the stru ctures because of hydrophobic interactions. In general terms the value s of the partial specific volumes of humic substances suggest they are more compact molecules than globular proteins, and intrinsic viscomet ry studies suggest that aqueous salt solutions approach theta-solvent conditions for humic substances.