Category: Formulation and Quality
Purpose: Polyalcohols are important in food and drug industries and model glass-formers. The ability to form extensive hydrogen bonds controls polyalcohols’ properties and applications. Despite previous work, much is unknown (even controversial) about the local structures of polyalcohols. A member of this family of molecules, D-mannitol forms a second amorphous phase with a lower density and stronger hydrogen bonds (HBs). The structure of Phase X remains unknown. In this work, the structures of four polyalcohols (glycerol, sorbitol, maltitol and mannitol) are studied by synchrotron X-ray scattering and the Pair Distribution Function (PDF) method over a wide temperature range that covers both the liquid and the glass state.
Methods: The liquid polyalcohols were filled in 2 mm O.D. quartz capillary tubes with wall thickness of 0.01 mm. X-ray total scattering was measured using a two-dimensional detector in transmission at beamline 6-ID-D at the Advanced Photon Source, Argonne National Laboratory. Temperatures of measurement were 400 K to 100 K at a step of 25 K. The scattering data were corrected to obtain the structure factors S(q) and PDFs.
Results: Density change explains most of the structural difference in the intermolecular spacing. Significant variation was observed between the polyalcohols in the distance over which intermolecular positions are corrected. Glycerol has the largest correlation length (~ 7 molecular layers). The systematic changes near 2.8 Åindicate more extensive intermolecular hydrogen bonds with cooling. The large hydrogen-bonded “clusters” of glycerol are expected to provide greater resistance to structural changes with temperature and may explain its low fragility. Both amorphous phases of mannitol show 1.8 hydrogen bonds per OH (2 in crystals). IR shows that LDA has stronger hydrogen bonds. This may be a result of looser packing.
Conclusion: Density change accounts for most of the structural changes in intermolecular distance. Polyalcohols form more intermolecular hydrogen bonds with cooling. Glycerol has the largest correlation length among the systems studied, possibly related to its low fragility. Mannitol’s LDA and HDA have similar numbers of HBs but LDA has stronger HBs, as a result of looser packing.
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Chengbin Huang– University of Wisconsin-Madison, Jersey City, New Jersey
Men Zhu– Madison, Wisconsin
Chris Benmore– Argonne, Illinois
Mark Ediger– Madison, Wisconsin
John Perepezko– Madison, Wisconsin
Lian Yu– Madison, Wisconsin