The water vapor sorption isotherms have been established for α-, β-, and γ-cyclodextrin at 40°C. The powder X-ray diffraction patterns of crystal forms of cyclodextrins stored at various levels of relative humidity (RH) were examined and the results are discussed in terms of the hydration numbers. α-Cyclodextrin hydrate was stable at RH as low as 11%. The water vapor desorption isotherm of β-cyclodextrin showed pronounced hysteresis, which extended to lower RH.The hydrate form of β-cyclodextrin was stable over a wide range of RH in the desorption process, like that of α-cyclodextrin. The crystal form of γ-cyclodextrin 7H2O was obtaiend in addition to the dehydrate form and the 17H2O hydrate form.
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Nakai et al. (1986) studied this question.