Phase Separation and the Critical Index for Liquid–Liquid Coexistence in the Sodium–Ammonia System

A precise determination of the liquid–liquid coexistence curve has been made for solutions of sodium in liquid ammonia. The critical index $\beta$ , defined by the mole fraction vs temperature dependence, ${\mathrm{(x}}_1{\mathrm{\hspace{0.17em}-\hspace{0.17em}x}}_2{\text{)\hspace{0.17em}/\hspace{0.17em}2x}}_c{\mathrm{\hspace{0.17em}=\hspace{0.17em}B(|\hspace{0.17em}T\hspace{0.17em}-\hspace{0.17em}T}}_c{\mathrm{\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}T}}_c{)}^{\beta }$ , undergoes a rather abrupt change from 0.502 ± 0.010 to 0.34 ± 0.02 when $T$ approaches to within 1.8°C of the consolute temperature. The validity range of Landau molecular field theory is used to deduce an interaction cluster extending over at least 140 ammoniated sodium centers.