Phase transitions and thermodynamic properties of anhydrous caffeine

Caffeine has been found to display a low-temperatureβ- and a high-temperatureα-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 $$begin{array}{*{20}c} {{vartheta mathord{left/ {vphantom {vartheta {^circ C}}} right. kern-nulldelimiterspace} {^circ C}}} & {100(beta )} & {100(alpha )} & {150(alpha )} & {100(alpha )} {{{C^circ _mathfrak{p} } mathord{left/ {vphantom {{C^circ _mathfrak{p} } {J cdot K^{ - 1} cdot mole^{ - 1} }}} right. kern-nulldelimiterspace} {J cdot K^{ - 1} cdot mole^{ - 1} }}} & {271 pm 9} & {287 pm 10} & {309 pm 11} & {338 pm 10} end{array} $$ 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 solidα-caffeine, and effusion measurements on solidβ-caffeine yielded: $$begin{array}{*{20}c} {A = 3918 pm 37; 5223 pm 28; 5781 pm 35K^{ - 1} } {B = 11.143 pm 0.072; 13.697 pm 0.057; 15.031 pm 0.113} end{array} $$ . The evaporation coefficient ofβ-caffeine is 0.17±0.03.