= Flory interaction parameter
{c}
{title2=$\chi$}
The Flory interaction parameter measures the excess free-energy cost of unlike contacts in <Flory–Huggins solution theory>. For a simple lattice of coordination number $z_c$ and contact energies $\epsilon_{ps}$, $\epsilon_{pp}$ and $\epsilon_{ss}$,
$$
\chi=\frac{z_c}{k_BT}\left[\epsilon_{ps}-\frac{\epsilon_{pp}+\epsilon_{ss}}2\right].
$$
Real effective parameters may also contain entropic contributions and composition dependence. With site volume $v_0$ and composition-independent $\chi$, the effective two-segment <excluded volume> is $v_{\mathrm{eff}}=v_0(1-2\chi)$. Positive, zero and negative $v_{\mathrm{eff}}$ correspond respectively to a <good solvent>, the <theta temperature> condition and a <poor solvent>. In particular, $\chi=1/2$ means cancellation of leading repulsive and attractive contributions; it does not mean that every molecular interaction vanishes.
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