The purpose of this study was to investigate the energetic state of water in silicified microcrystalline cellulose (SMCC) and α-lactose monohydrate wet masses using near-infrared (NIR) spectroscopy. The applicability of NIR spectroscopy to studying pharmaceutical wet masses at a wide moisture range was evaluated in comparison with mixer torque rheometry (MTR). With increasing moisture content changes in the physical properties of the samples resulted in an apparent increase in log (1/R) throughout the whole spectrum. The upward displacement of baseline and the relative height of water bands were greatest with materials that had a poor liquid-retention capacity. In the case of SMCC and 1:1 mixture of SMCC and α-lactose monohydrate, the height of the baseline-corrected water bands increased linearly at low moisture contents, thereafter achieving a plateau stage. According to the MTR results, the plateau stage of the band heights indicated a capillary state of liquid saturation. The second derivative spectrum was capable of distinguishing monohydrate, absorbed, and adsorbed water, which overlapped in the absorbance spectrum. When water was absorbed to the internal structure of the material (SMCC), the water bands were first seen at higher wavelengths, then followed by a shift to lower wavelengths. When water was only adsorbed onto the surface of the particles (glass ballotini), the water bands were seen directly in the region of bulk water.
Luukkonen, P., Rantanen, J., Mäkelä, K., Räsänen, E., Tenhunen, J., & Yliruusi, J. (2001). Characterization of wet massing behavior of silicified microcrystalline cellulose and α-Lactose Monohydrate using near-infrared spectroscopy. Pharmaceutical Development and Technology, 6(1), 1-9. https://doi.org/10.1081/PDT-100000007