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Statistical
thermodynamics identifies entropy as a measure of the disorder of the thermodynamic
system. This can be expressed in the relationship: S
= k Ln W, where W is the thermodynamic probability
of the state. W is a measure of the number of different ways in which a
particular configuration can be achieved. For a system with n impurity
atoms and (N - n) arranged in a material with N lattice sites: W
= {N!/ (N - n)! (n!)}. Using this expression
for W gives for the entropy of mixing: SM
= k Ln{N!/ (N - n)! (n!)}.
In general, n, N, and (N - n), are large numbers and the factorials may
be expanded using Stirling's approximation. This entropy of mixing is also
known as a configurational entropy.
A
macromolecule can adopt several equivalent geometric configurations that
will give the same distance between its free ends. A thermodynamic probability
is associated with this system also and the entropy contribution related
to the folding of the molecule is also a configurational entropy. |
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