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.