  Water is the most common substance on Earth.The discovery of heavy water and
its further study have shown that the change of hydrogen for deuterium leads to
the significant differences in their properties.The triple point temperature of
heavy water is higher,at the same time the critical temperature is
lower.Experimental values of the second virial coefficient of the EOS for the
vapor of normal and heavy water differ at all temperatures.This fact can
influence the values of the dimerization constant for the heavy water vapor.The
equilibrium properties of the dimerization process are described with the
methods of chemical thermodynamics.The chemical potentials for monomers (m) and
dimers (d)are the functions of their concentrations.The interactions of
monomer-dimer and dimer-dimer types are taken into account within the solution
of equation for chemical potentials.The obtained expression for the
dimerization constant contains the contributions of these types.The averaged
potentials are modeled by the Sutherland potential.Theoretical values of the
dimerization constant for the heavy water vapor at different temperatures are
compared to those for normal water.We see the exceeding of the values for the
heavy water at all temperatures.This fact is in good agreement with all
experimental data that is available.The excess is related to the differences in
the character of the heat excitations of the dimers of normal and heavy
water,their rotational constants and energy of their vibrational
excitations.Significant role is also played by the monomer-dimer and
dimer-dimer interactions.
