X-ray Diffraction of Water in Polyvinylpyrrolidone.
Molecular pharmaceutics(2023)
Abstract
PVP is a hydrophilic polymer commonly used as an excipient in pharmaceutical formulations. Here we have performed time-resolved high-energy X-ray scattering experiments on pellets of PVP at different humidity conditions for 1-2 days. A two-phase exponential decay in water sorption is found with a peak in the differential pair distribution function at 2.85 Å, which is attributed to the average (hydrogen bonded) carbonyl oxygen-water oxygen distance. Additional scattering measurements on powders with fixed compositions ranging from 2 to 12.3 wt % HO were modeled with Empirical Potential Structure Refinement (EPSR). The models reveal approximately linear relations between the carbonyl oxygen-water oxygen coordination number () and the water oxygen-water oxygen coordination number () versus water content in PVP. A stronger preference for water-water hydrogen bonding over carbonyl-water bonding is found. At all the concentrations studied the majority of water molecules were found to be randomly isolated, but a wide distribution of coordination environments of water molecules is found within the PVP polymer strands at the highest concentrations. Overall, the EPSR models indicate a continuous evolution in structure versus water content with =1 occurring at ∼12 wt % HO, i.e., the composition where, on average, each water molecule is surrounded by one other water molecule.
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Key words
polyvinylpyrrolidone,Kollidon,X-ray diffraction,amorphous structure,pair distribution function,EPSR modeling
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