Association of Benzene and Alkylbenzenes with Cyclodextrins in Aqueous Medium

Formation constants of the following aromatic hydrocarbons with α-,β-, and γ-cyclodextrin(CyD) were determined in aqueous medium at 25°C in the presence of excess CyD : benzene, toluene, ethylbenzene, propylbenzene, isopropylbenzene, (o-, m-, and p-)xylene, and (1,2,3-, 1,2,4-, and 1,3,5-)trimethylbenzene. The technique used to study association between host(CyD) and guest (hydrocarbon) is based on the facts that the guest molecules can be driven out to gaseous phase by introducing an inert gas at a constant flow rate into the aqueous solution and that the volatilization rate of guest decreases with increasing association with the host in the aqueous solution. The 1 : 1 and 2 : 1 (host : guest) formation constants were evaluated. As a measure of hydrophobicity of guest molecules, the free energy change of dehydration derived from Henry’s law constant was used. Based on hydrophobicity and a host-guest spatial-fitting model, the formation constants have been discussed.