Association of C5–C9 Normal Alkanes with Cyclodextrins in Aqueous Medium

The complex formation of cyclodextrin (CyD, host) with C5 to C9 normal alkanes (guests) has been studied in aqueous medium at 25°C by making use of the volatilization rate of alkane molecules from aqueous into gaseous phase. In the excess of the host, β- and γ-CyDs form 1:1 complexes, while α-CyD forms 1:1 and 2:1 (host: guest) complexes. With an increase in the chain length of alkane, association constants for three CyDs increased; in the case of α-CyD, the 1:1 association constant for C9 was identical to that for C8, while the 2:1 association constant continued to increase. The cavity size of CyD, the alkane chain length, and hydrophobicity and surface area of alkane molecules were taken into account to discuss the host-guest inclusion mechanism.