Solvent effects on the complexation of 1-alkanols by parent and modiefied cyclodextrins. Caloimetric studies at 298K

The formation of complexes of parent and alkylated cyclodextrins (CDs) with 1-heptanol and 1-octanol has been studied calorimetrically at 298 K in water and in concentrated aqueous solutions of urea. The forces involved in the association process are discussed in the light of the signs and values of the thermodynamic parameters obtained: association enthalpy, binding constant, Gibbs free energy, and entropy. It was inferred that: (i) in water, the formation of complexes for parent and substituted a-cyclodextrins (aCDs) is determined by enthalpy. For parent and substituted b-cyclodextrins (bCDs), instead, hydrophobic interactions are the prevailing forces determining complexation, as indicated by the small and negative or positive enthalpies and by the high and positive entropies. (ii) In urea, hydrophilic interactions are attenuated. The formation of complexes with alkylated CDs does not occur. (iii) The analysis of the thermodynamic properties confirms that inclusion is a process dominated by hydration phenomena. Modifications experienced by the solvent water in the hydration shells of the interacting substances upon association determine the formation of the complexes.