Differential thermal analysis under high pressure IV: Low‐temperature DTA of solid‐solid and solid‐liquid transitions of several hydrocarbons up to 3 kbar

A DTA apparatus for the temperature range −200°C to +150°C and up to 3000 bar is described. The apparatus is applied to the calorimetric investigation of the phase behaviour of pentane, hexane, heptane, octane, nonane, decane, isopentane, 2,2‐dimethylbutane, 2,3‐dimethylbutane, cyclopentane, cyclohexane, and methylcyclohexane. A new solid phase has been found in cyclohexane at elevated pressures. A strange phase behaviour is observed in 2,3‐dimethylbutane that exhibits endothermic and exothermic transitions. The well‐known odd‐even effect of the melting temperatures of the n‐alkanes is correlated with the slopes of the melting curves: For sufficiently high pressures the melting curve of an even n‐alkane intersects the melting curve of the following odd n‐alkane. Volume changes accompanying the phase transitions have been calculated using the Clausius‐Clapeyron equation.