PHASE-TRANSITIONS AND THERMODYNAMIC PROPERTIES OF ANHYDROUS CAFFEINE

Caffeine has been found to display a low-temperature β- and a high-temperature α-modification. By quantitative DTA the following data were determined: transformation temperature 141±2°; enthalpy of transition 4.03±0.1 kJ·mole-1; enthalpy of fusion 21.6±0.5 kJ·mole-1; molar heat capacity {Mathematical expression} in good accord with drop-calorimetric data. For the constants of the equation log (p/Pa)=-A/T+B, static vapour pressure measurements on liquid and solid α-caffeine, and effusion measurements on solid β-caffeine yielded: {Mathematical expression}. The evaporation coefficient of β-caffeine is 0.17±0.03. © 1979 Wiley Heyden Ltd., Chichester and Akadémiai Kiadó, Budapest.