= Regular solution
{title2=$f/(k_BT)=\phi\log\phi+(1-\phi)\log(1-\phi)+\chi\phi(1-\phi)$}
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= Regular solution model
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A regular solution keeps the ideal combinatorial <entropy of mixing> while allowing a nonzero energetic preference for unlike contacts. For a symmetric binary <mixture> with equal molecular sizes, the displayed free energy per site combines mixing entropy with a contact penalty $\chi\phi(1-\phi)$. Its <spinodal> satisfies $1/\phi+1/(1-\phi)-2\chi=0$. At the symmetric <thermodynamic critical point>, the third composition derivative also vanishes, giving $\phi_c=1/2$ and $\chi_c=2$. If $\chi=A/T$, the critical temperature is $A/2$. This is the equal-size limit of <Flory–Huggins solution theory>.
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