TA338
Characterization of Water Adsorption and
Absorption in Pharmaceuticals
Introduction
Dynamic vapor sorption (DVS) analysis can characterize material response to humidity changes. Typical responses are surface adsorption/
desorption, bulk absorption, hydration / dehydration, and deliquescence. In surface adsorption, water can be weakly held to the surface by
van der Waals forces (physisorption) or more strongly held (chemisorption). Physisorption is reversible by decreasing humidity or increasing
temperature. Chemisorption is generally considered as irreversible. In bulk absorption, water is attracted deep into the internal structure of the
material. Bulk absorption is reversible, but the kinetics are slower than for surface adsorption.
Experimental
Adsorption and absorption are evaluated by increasing humidity stepwise over a broad range, then decreasing it to the starting level and
finally increasing it again to higher levels. The profiles are generated at constant temperature. The experiments were performed on a Sorption
Analyzer from TA Instruments (e.g., Discovery SA).
Results and Discussion
Surface adsorption is characterized by increasing / decreasing humidity curves, which coincide (i.e., overlap), as well as by small total weight changes
over a broad humidity range. Crystalline materials generally exhibit this type of profile.
Figure 1 (carbamazepine) illustrates a typical surface adsorption profile. Bulk absorption profiles are shown in Figures 2 and 3. Choline
(Figure 2) exhibits overlapping adsorption/desorption curves similar to that observed for surface adsorption. However, since the total water uptake is
too large (150 % from 0-90 % RH) to be a surface phenomenon, bulk absorption into the material is occurring. As there is no evidence that the structure
of choline changes as water enters and exits the material’s internal structure, that structure must be very open
2.5
200
Adsorption
2.0
Adsorption
150
Desorption
Desorption
1.5
100
1.0
50
0.5
0
0.0
-0.5
-50
0
20
40
60
80
100
0
20
40
60
80
100
Relative Humidity (%)
Relative Humidity (%)
Figure 1 Carbamazepine Surface Adsorptio
Figure 2. Choline Bulk Absorption
© 2023 TA Instruments. A division of Waters Corporation. All rights reserved.
1
Weight Change (%)
Weight Change (%)
