Vapor Pressure and Solid Phases of Methanol below Its Triple Point Temperature

We present an experimental work devoted to study of the thermodynamical properties of solid methanol. We combine Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS) to measure, for the first time, the vapor pressure of various methanol solid phases and determine their Clausius−Clapeyron equations. We perform our experiments between T = 130 K and the triple point temperature Tt = 175.61 K. When methanol is condensed from its vapor below Tt, we observe three different solid phases depending on temperature. A condensation at T = 130 K forms a metastable phase with an enthalpy of sublimation ΔHmetastable-vapor = 42.9 ± 0.5 kJ·mol-1. Upon heating, this phase transforms itself at T ≈ 145 K to the α-phase that has an enthalpy of sublimation ΔHα-vapor = 46.9 ± 0.2 kJ·mol-1. Cooling the α-phase does not lead back to the metastable phase, whereas heating this α-phase leads to the β-phase occurrence at Tα-β = 157.36 K. This latter one is stable until Tt and has an enthalpy of sublimation ΔHβ-va...