3.5. Evaluation of the molecular state of water by NIRS (Met granulates produced with VA64 and PVP)
The NIR band around 1900–1950 nm shows the molecular state of water, with the water band split into peaks of 1900 nm and 1930 nm, reflecting the bulk water and bound water volumes, respectively. The peak position reflects the moisture mobility of absorbed water [19].
A difference in the tableting properties was observed between VA64 and PVP Met granulates. Therefore, the moisture distribution (bound water and bulk water) of VA64 and PVP Met granulates was evaluated by NIRS. Fig. 4 shows second derivative NIRS spectra, with the peak of the VA64 Met granulates wider compared to the peak of the PVP Met granulates. In particular, the peak position of the VA64 Met granulates around 1930 nm reflecting bound water was wider than the peak position of the PVP Met granulates. Additionally, the peak intensity of the PVP Met granulates around 1930 nm was strong. Therefore, the distributions of bound and bulk water were completely different between the VA64 and PVP Met granulates.