(Solid + liquid) phase equilibria and enthalpy changes in (water + 2,2-dimethyl-1,3-propanediol)

Abstract

The (solid + liquid) phase diagram has been determined for (water + 2,2-dimethyl-1,3-propanediol). A solid hydrate containing 6H 2O for each C(CH 3) 2(CH 2OH) 2 forms, and melts congruently at 272.44 K. A solid phase transition occurs in C(CH 3) 2(CH 2OH) 2 at 310.15 K. D.s.c. was used to determine the enthalpies of transition (12.5 kJ · mol −1) and of fusion (4.59 kJ · mol −1) of the pure C(CH 3) 2(CH 2OH) 2, and the enthalpy of fusion of the hexahydrate (44.7 kJ · mol −1). These values suggest that C(CH 3) 2(CH 2OH) 2 forms a plastic crystal above the transition temperature. Titration calorimetry was used to determine ΔH at the melting temperature of the hexahydrate (272.44 K) for the reaction of {C(CH 3) 2(CH 2OH) 2(s) + 6H 2O(1)} to form the liquid solution with the hydrate composition. The value obtained was 7.697 kJ · mol −1. The ΔH at 272.44 K for the formation of the solid hydrate from H 2O(s) and C(CH 3) 2(CH 2OH) 2(s) is −(1.7 ± 0.7) kJ·mol −1, suggesting very little energy stabilization in the formation of the hydrate.