Defined by: \[S^{\unicode[Times]{x3C3}} = S - S^{\unicode[Times]{x3B1} }- S^{\unicode[Times]{x3B2} }=S- V^{\unicode[Times]{x3B1} }\ \frac{S_{\text{m}}^{\unicode[Times]{x3B1} }}{V_{\text{m}}^{\unicode[Times]{x3B1} }}- V^{\unicode[Times]{x3B2} }\ \frac{S_{\text{m}}^{\unicode[Times]{x3B2} }}{V_{\text{m}}^{\unicode[Times]{x3B2} }}\] (S m α V m α) and (S m β V m β) are the entropy densities in the two bulk phases, where S m α and S m β are the mean molar entropies and V m α and V m β are the mean molar volumes of the two phases.