Thermodynamic analysis of inclusion complexation between alpha-cyclodextrin-based molecular tube and sodium alkyl sulfonate

Thermodynamics on inclusion complexation of a-cyclodextrin-based molecular tube (MT) with sodium octyl-, decyl-, and dodecylsulfonates (C-8-, C-10-, a nd C12SO3Na) were measured by isothermal titration calorimetry (ITC) to eva luate the effects of alkyl chain length of CnSO3Na and temperature. It was found that CnSO3Na with a longer alkyl chain formed more stable inclusion c omplex with MT. The association constant of inclusion complexation between MT and C12SO3Na at 298 K was found to be on the order of 10(5). The stoichi ometric number ([host]/[guest] ratio) was found to be 0.50 +/- 0.05. This r esult indicates that one MT forms an inclusion complex with two CnSO3Na mol ecules, regardless of the alkyl chain length of CnSO3Na. Enthalpy-entropy c ompensation was also observed in MT/CnSO3Na systems, and it was confirmed t hat the free energy change was not affected by the change in temperature (2 88-308 K). From the thermodynamic parameters, it was considered that both h ydrophobic interaction and van der Waals interaction were mainly contribute d to inclusion complexation of MT with CnSO3Na. The large and negative entr opy change (T DeltaS) attributed to the strong binding of MT was observed. The strong T DeltaS dependence on the alkyl chain length of CnSO3Na was fou nd to be a characteristic for inclusion complexation of MT.