Melting mechanisms of ionic salts. Tetra-n-amyl ammonium thiocyanate

Abstract

A study is reported of the melting of (n-amyl)₄N⁺SCN^–(m.p. 49.5°C) using dilatometry, phase-transition calorimetry, and microscopy. The changes in the electrical conductance, and in u.v.-absorption for the fusion process are also reported. A solid-state transition occurs at 42°C, with ΔH_tr and ΔS_tr being 5400 cal/mol and 17 cal/deg. mol respectively. The enthalpy and entropy of fusion are 4600 cal/mol and 14 cal/deg. mol respectively. The molar volume change, ΔV_m/V_25°C is 4.8 % and occurs almost entirely at the solid state transition. Rapid cooling results in incompletely crystallized samples, with corresponding variations in the temperatures and enthalpies for the solid state transition and for fusion. The melting mechanism is examined relative to the fusion of paraffinic hydrocarbons and ionic crystals. The solid-state transition (42°C) may be attributed to a kink-block type transition within the amyl groups of the quaternary ammonium cation, and that the process at the melting point (49.5°C) is essentially the “normal” fusion of ionic salts.