Abstract
There is a controversy about the extent to which the primary and secondary dielectric relaxations influence the crystallization of amorphous organic compounds below the glass transition temperature. Recent studies also point to the importance of fast molecular dynamics on picosecond-to-nanosecond time scales with respect to the glass stability. In the present study we provide terahertz spectroscopy evidence on the crystallization of amorphous naproxen well below its glass transition temperature and confirm the direct role of Johari-Goldstein (JG) secondary relaxation as a facilitator of the crystallization. We determine the onset temperature Tβ above which the JG relaxation contributes to the fast molecular dynamics and analytically quantify the level of this contribution. We then show there is a strong correlation between the increase in the fast molecular dynamics and onset of crystallization in several chosen amorphous drugs. We believe that this technique has immediate applications to quantify the stability of amorphous drug materials.
Highlights
Amorphous solids are high energy materials compared to their crystalline counterparts and characterized by the absence of a long-range three-dimensional molecular order.[1]
We show there is a strong correlation between the increase in the fast molecular dynamics and onset of crystallization in several chosen amorphous drugs
This study showed that the terahertz molecular dynamics is strongly related to the molecular mobility governing the stability of amorphous drugs
Summary
Amorphous solids are high energy materials compared to their crystalline counterparts and characterized by the absence of a long-range three-dimensional molecular order.[1]. Because of the high internal energy, amorphous solids generally have a higher kinetic solubility and dissolution rate.[1,4] This can be exploited for applications such as in the pharmaceutical field to increase bioavailability of poorly soluble drugs when administered orally in the form of tablets or capsules. From a thermodynamic point of view, these systems are unstable and experience an intrinsic drive to convert to a lower energy and stable crystalline form over time. In order to be able to exploit the properties of the amorphous form practically one needs to ensure physical stability at least for the duration of the shelf life of e.g. pharmaceutical formulations
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