Characterization of crystalline and spray-dried amorphous α-cyclodextrin powders

• Spray-dried α-CD powder was in amorphous structure with Tg ~ 83.4 °C.
• Water adsorption ability of spray-dried α-CD was higher than that of crystalline one.
• Water adsorption data of spray-dried α-CD were fitted well with GAB and BET models.
• Spray-dried α-CD powder crystallized as kept at above 65.25% RH condition for 3 weeks.

Alpha-cyclodextrin (α-CD) powder has been used to adsorb organic compounds for various applications. In order to study the complexation of α-CD powder with guest compounds, an understanding of physical properties α-CD powder including molecular structure and water adsorption behavior is essential. Two known types of α-CD powder structure are crystalline and amorphous. However, commercial α-CD powder exists only in crystalline form and there is no reported information on properties of amorphous α-CD powder. This study aims at characterization and comparison of those properties of commercial (crystalline structure) and spray-dried α-CD powders. The results of SEM, X-ray, FTIR, DSC, PSD, TGA and 13C solid-state NMR analyses indicated that spray-dried α-CD powder was in an amorphous form with spherical shape particles (mean diameter ~ 13.28 μm) and its glass transition temperature (Tg) was about 83.4 °C. The spray-dried α-CD powder crystallized when kept above 65.30% Equilibrium Relative Humidity (ERH), corresponding to 13.70–14.97 g of moisture 100 g− 1 of dry solids. In moisture adsorption study, at the same ERH levels, amorphous α-CD powder adsorbed moisture much higher than crystalline α-CD powder did, and its adsorption isotherm data were fitted well with both BET and GAB models at a low RH range (< 65.30% ERH). A poor fitting to a wider RH range (ERH = 11.12–97.30%) of the GAB model was due to the crystallization of amorphous α-CD powders equilibrated at above 65% RH.

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