X-ray Diffraction of Water in Polyvinylpyrrolidone

PVP is a hydrophilic polymer commonly used as an excipientin pharmaceuticalformulations. Here we have performed time-resolved high-energy X-rayscattering experiments on pellets of PVP at different humidity conditionsfor 1-2 days. A two-phase exponential decay in water sorptionis found with a peak in the differential pair distribution functionat 2.85 angstrom, which is attributed to the average (hydrogen bonded)carbonyl oxygen-water oxygen distance. Additional scatteringmeasurements on powders with fixed compositions ranging from 2 to12.3 wt % H<INF>2</INF>O were modeled with Empirical Potential StructureRefinement (EPSR). The models reveal approximately linear relationsbetween the carbonyl oxygen-water oxygen coordination number(n <INF>O<INF>C</INF>-O<INF>W</INF> </INF>)and the water oxygen-water oxygen coordination number (n <INF>O<INF>W</INF>-O<INF>W</INF> </INF>) versus watercontent in PVP. A stronger preference for water-water hydrogenbonding over carbonyl-water bonding is found. At all the concentrationsstudied the majority of water molecules were found to be randomlyisolated, but a wide distribution of coordination environments ofwater molecules is found within the PVP polymer strands at the highestconcentrations. Overall, the EPSR models indicate a continuous evolutionin structure versus water content with n <INF>O<INF>W</INF>-O<INF>W</INF> </INF>=1 occurring at similar to 12 wt %H<INF>2</INF>O, i.e., the composition where, on average, each watermolecule is surrounded by one other water molecule.