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Related Concept Videos

X-ray Diffraction of Biological Samples01:10

X-ray Diffraction of Biological Samples

X-ray diffraction or XRD is an analytical tool that utilizes X-rays to study ordered structures such as crystalline organic and inorganic samples, polycrystalline materials, proteins, carbohydrates, and drugs.
According to Bragg's law, when X-rays strike the sample positioned on a stage, the rays are  scattered by the electron clouds around the sample atoms. The  X-ray diffraction or scattering is caused by constructive interference of the X-ray waves that reflect off the internal crystal...
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Polymer Classification: Crystallinity

Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.
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X-ray Crystallography02:18

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The size of the unit cell and the arrangement of atoms in a crystal may be determined from measurements of the diffraction of X-rays by the crystal, termed X-ray crystallography.
Diffraction
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Polymerization produces macromolecules with a range of chain lengths due to the random nature of molecular growth processes. As chains form and terminate at different stages, a single polymer sample contains molecules of varying sizes rather than a uniform structure. This variability is described using average molar masses and distribution-related parameters, which together provide a comprehensive understanding of polymer characteristics.The distribution of molar masses plays a critical role in...
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Polymorphism refers to the existence of a drug substance in multiple crystalline forms, known as polymorphs. Recently, this term has been expanded to include solvates (forms containing a solvent), amorphous forms (non-crystalline forms), and desolvated solvates (forms from which the solvent has been removed).
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Characterization of amorphous API:Polymer mixtures using X-ray powder diffraction.

Ann Newman1, David Engers, Simon Bates

  • 1SSCI, An Aptuit Company, West Lafayette, Indiana, USA. ann.newman@aptuit.com

Journal of Pharmaceutical Sciences
|March 21, 2008
PubMed
Summary

A new X-ray powder diffraction (XRPD) method using pair distribution functions (PDF) offers a more accurate assessment of amorphous API-polymer miscibility than traditional differential scanning calorimetry (DSC). This advanced technique reveals phase separation in systems where DSC may fail, providing crucial insights into material stability.

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Area of Science:

  • Materials Science
  • Pharmaceutical Science
  • Physical Chemistry

Background:

  • Determining amorphous API-polymer miscibility traditionally relies on differential scanning calorimetry (DSC) to identify glass transition temperatures (Tg).
  • Limitations exist with DSC, particularly for systems with nanoscale phase separation, potentially leading to misinterpretations of miscibility.
  • Accurate miscibility assessment is critical for predicting physical stability and performance of amorphous solid dispersions.

Purpose of the Study:

  • To develop and validate a novel X-ray powder diffraction (XRPD) method, coupled with pair distribution function (PDF) analysis, for assessing amorphous API-polymer miscibility.
  • To compare the efficacy of the XRPD-PDF method against traditional DSC measurements.
  • To investigate the miscibility and phase behavior of specific API-polymer systems.

Main Methods:

  • Utilized X-ray powder diffraction (XRPD) to analyze amorphous mixtures of dextran-poly(vinylpyrrolidone) (PVP), trehalose-dextran, and indomethacin-PVP.
  • Computed pair distribution functions (PDF) from XRPD data to characterize the local atomic arrangements.
  • Correlated PDF profiles with differential scanning calorimetry (DSC) measurements of glass transition temperatures (Tg).

Main Results:

  • The XRPD-PDF method successfully identified phase-separated systems, such as dextran-PVP and trehalose-dextran, by matching component PDFs to the mixture PDF.
  • Indomethacin-PVP was confirmed as miscible, showing a unique PDF profile distinct from its components, aligning with DSC results.
  • The XRPD-PDF method revealed phase separation in trehalose-dextran, a system where DSC indicated miscibility due to nanoscale domains (<30 nm).

Conclusions:

  • The XRPD-PDF method provides a more comprehensive assessment of amorphous API-polymer miscibility than DSC, especially for systems with nanoscale phase separation.
  • Trehalose-dextran forms a phase-separated solid nanosuspension, not a true miscible system, highlighting the limitations of DSC for such materials.
  • Understanding nanoscale phase separation is crucial for predicting the physical stability and crystallization tendency of amorphous solid dispersions.