title: Possible existence of two amorphous phases of D-mannitol related by a first-order transition.
abstract: We report that the common polyalcohol D-mannitol may have two amorphous phases related by a first-order transition. Slightly above its glass transition temperature Tg (284 K), the supercooled liquid (SCL) of D-mannitol transforms to a low-energy, apparently amorphous phase with stronger hydrogen bonds. The enthalpy of this so-called Phase X is approximately halfway between those of the known amorphous and crystalline phases, a position low for glass aging and high for crystal polymorphs. Similar to the SCL, Phase X is transparent with broad X-ray diffraction and Raman scattering; upon temperature cycling, it exhibits a glass-transition-like change of heat capacity. On fast heating, Phase X transforms back to the SCL near Tg + 50 K, enabling a determination of their equilibrium temperature. The presence of D-sorbitol as a plasticizer enables observation of a first-order transition from the SCL to Phase X entirely in the liquid state (liquid-liquid transition). The transition from D-mannitol's SCL to Phase X has intriguing similarities with the formation of the glacial phase of triphenyl phosphite (TPP) and the conversion from high-density to low-density amorphous ice, both studied intensely in the context of polyamorphism. All three processes occur near Tg with substantial enthalpy decrease toward the crystalline phases; the processes in water and D-mannitol both strengthen the hydrogen bonds. In contrast to TPP, D-mannitol's Phase X forms more rapidly and can transform back to the SCL. These features make D-mannitol a valuable new model for understanding polyamorphism.
authors.affiliation: Department of Chemistry and School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, USA.
authors.forename: Men
authors.initials: M
authors.lastname: Zhu
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Jun-Qiang
authors.initials: JQ
authors.lastname: Wang
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: John H
authors.initials: JH
authors.lastname: Perepezko
authors.affiliation: Department of Chemistry and School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, USA.
authors.forename: Lian
authors.initials: L
authors.lastname: Yu
mesh_terms: D005898:Glass; D006358:Hot Temperature; D007700:Kinetics; D008353:Mannitol / Q000737:chemistry*; D044367:Phase Transition
chemical_list: D008353:Mannitol