X-ray Diffraction of Water in Polyvinylpyrrolidone
Chris J. Benmore, Sophie R. Benmore, Stephen K. Wilke, V. J. Menon, S.R. Byrn, J. K. Richard Weber
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 % H 2 O were modeled with Empirical Potential Structure Refinement (EPSR). The models reveal approximately linear relations between the carbonyl oxygen–water oxygen coordination number ( n O C –O W ) and the water oxygen–water oxygen coordination number ( n O W –O W ) 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 n O W –O W =1 occurring at ∼12 wt % H 2 O, i.e., the composition where, on average, each water molecule is surrounded by one other water molecule.