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Lattice gas analogy--we have *N* cells, *N*_{p} particles *A* and
*N*-*N*_{p} particles *B*. We will use *canonical ensemble* (Ising
model for magnetics corresponds to *grand* canonical
ensemble!). Partition function:

(1) |

The number of terms

Average energy: we have(2) |

Stirling formula:

New variables: Free energy:(3) |

System is symmetric--critical point is at . We obtain

or which gives The system is mixed above critical point and demixed below it
Free energy of Flory system below critical point

On spinodal the second derivative is zero:

In critical point binodal & spinodal coincide:

- 1.
- It is not clear how to calculate
*next*approximation(s) - 2.
- Works because of
*compensation of errors*:- We overestimate : correlations
*decrease*average energy - We overestimate : most of states have too small
contribution to
*Q*

- We overestimate : correlations

© 1997

Tue Oct 14 22:58:59 EDT 1997