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 . Its spinodal satisfies . At the symmetric thermodynamic critical point, the third composition derivative also vanishes, giving and . If , the critical temperature is . This is the equal-size limit of Flory–Huggins solution theory.
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A regular solution is a concept in thermodynamics and materials science that describes a mixture of two or more components that behave ideally at certain conditions. In a regular solution, the interactions between like molecules (molecules of the same species) and unlike molecules (molecules of different species) are taken into account. The model helps in understanding the thermodynamic properties of liquid mixtures, particularly in solutions where deviations from ideal behavior may occur due to differences in size, shape, and energies of the interacting species.