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 β, defined by the mole fraction vs temperature dependence, (x1 − x2) / 2xc = B(| T − Tc | / Tc)β, 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.