Chemistry Reference
In-Depth Information
Figure 1.20.
Water vapor
absorption by amorphous (squares)
and crystalline (circles)
indomethacin at 30
°
C as a function
of relative humidity (reproduced
with permission from Ref. 52.
Copyright 1997, John Wiley &
Sons, Inc.).
solid. This is in contrast to water uptake by crystalline solids where water molecules are
generally con
ned to the surface, rarely reaching more than 0.1% w/w adsorbed, or the
equivalent of 3
4 molecular layers, even for very polar solids such as NaCl [51]. The
relationship between the amount of water vapor absorbed and RH at a particular
temperature can be analyzed by determining a water sorption isotherm, as shown for
amorphous indomethacin in Figure 1.20 [52], and for three widely used polymers PVP,
HPMC, and HPMCAS in Figure 1.21 [53]. Note that the amount of water absorbed at
30
-
°
C by the relatively hydrophobic indomethacin molecule is quite small, for example,
Figure 1.21.
Water vapor absorption by three amorphous polymers varying in polarity
(reproduced with permission from Ref. 53. Copyright 2008, American Chemical Society).
