Heat capacity and glass transition in P2O5-H2O solutions: support for Mishima’s conjecture on solvent water at low temperature

CORTI, HORACIO R.; NORES PONDAL, FEDERICO J.; ANGELL, C. AUSTEN

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

ROYAL SOC CHEMISTRY

Año: 2011 vol. 13 p. 19741 - 19748

1463-9076

The P 2 O 5 –water system has the widest range of continuously glass-forming compositions knownfor any glassformer + water binary system. Despite the great range of structures explored by theglasses and liquids in this system, the glass transition temperature (T g ) itself varies in a simplemonotonic fashion. However the values of  T g reported in the literature show wide disagreement,linked to the different methods of measurement employed. In this work we use differentialscanning calorimetry (DSC) to obtain both T g itself and the jump in heat capacity that occurs asthe metastable equilibrium of the supercooled liquid relieves the non-ergodic glassy state. Ourstudy covers the molar ratio range of  H 2 O/P 2 O 5 from 1.5 to 14 (corresponding to the massfraction of  P 2 O 5 between 0.36 and 0.84), which includes the compositions corresponding topyrophosphoric acid (H 4 P 2 O 7 ) and orthophosphoric acid (H 3 PO 4 ). The theoretical model ofCouchman and Karasz predicts very well the glass transition temperatures of the P 2 O 5 –H 2 Osystem over the whole composition range if the relatively large heat capacity change associatedwith water in aqueous solutions at the glass transition temperature is adopted, instead of thevanishingly small value observed for vapor deposited or hyperquenched pure water. Therefore,solvent water in this ambient pressure P 2 O 5 –H 2 O system behaves like a different liquid, moreclosely resembling a high-density liquid (HDL) polyamorph, as suggested by Mishima forelectrolytes at high pressures.