= Solution
Write $z_c$ for the lattice coordination number and $\epsilon_{AA},\epsilon_{BB},\epsilon_{AB}$ for contact energies. Counting the excess of unlike contacts relative to the unmixed reference gives the <Flory interaction parameter>
$$
\boxed{\chi=\frac{z_c}{k_BT}\left[\epsilon_{AB}-\frac{\epsilon_{AA}+\epsilon_{BB}}2\right].}
$$
Unlike contacts that are less favorable than the mean like contact give $\chi>0$. For temperature-independent contact energies, \b[$\chi$ is proportional to $1/T$], so heating weakens this energetic preference relative to the <entropy of mixing>. Effective parameters in real solutions can also include entropic terms, often represented schematically by $A/T+B$; the pure contact-energy model does not explain every possible temperature dependence.
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