The name is absent

potential due to association is given by

if r < r_c∙, Θ_ai < θ_c-, θ_β2 < θ_c

otherwise

Eqn. 1.2 essentially means that if the molecules 1 and 2 are close enough and their as-
sociating sites are oriented towards each other, they associate with the strength of the
association potential given by ε_as. These positional and orientational constraints are
prescribed by the parameters, r_c and θ_c. respectively. Wertheim introduced separate
singlet densities for each possible associating state of a molecule. For example, for
molecules with one associating site, the individual singlet densities are the densities
of associated molecules and the non-associated molecules. For molecules with mul-
tiple associating sites, the associating state of a molecule increases depending upon
the number of sites that are associated. Using graph theory in statistical mechanics,
Wertheim derived the thermodynamic variables as functionals of the singlet densi-
ties. Additional constraints on the associating molecules were introduced to reduce
the number of graphs, as shown in fig. 1.4.

• If two molecules are associated at their sites A and B respectively, then other
molecules cannot associate at either A or B.

• A site on a molecule cannot associate with two sites on another molecule, si-
multaneously.

• Two sites on a molecule cannot associate with two sites on another molecule,

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