Thermodynamic evidence of cyclodextrin-micelle interactions

The enthalpy of transfer (DeltaH(t)) of hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD), and beta-cyclodextrin (beta-CD) from water to the aqueous C6F13CO2Na and C7F15CO2Na solutions were determined in the pre- and post-micellar regions. The behavior of the macrocycles is system specific. Generally, the magnitude of the enthalpy is influenced by several factors: (1) the alkyl chain length of the surfactant, (2) the cyclodextrin cavity and its alkylation, (3) the interactions between the free cyclodextrin and the free surfactant, (4) the host-guest equilibrium constant, (5) the host/guest stoichiometry, and (6) the micelle-cyclodextrin (free and/or complexed) interactions. As far as the premicellar region is concerned, HP-alpha-CD does not form the host-guest complexes. beta-CD and, HP-beta-CD in the aqueous C7F15CO2Na solutions form host-guest complexes of 1:1 stoichiometry; beta-CD shows a larger binding affinity toward the surfactant as a compensative effect between the more negative enthalpy and entropy. Besides 1:1 complexes, HP-beta-CD in aqueous C6F13CO2Na solutions forms complexes of 1:2 stoichiometry (I cyclodextrin:2 surfactants). Their presence was evidenced by the minimum in the DeltaH(t) vs the surfactant concentration (f(S)m(S)) trend. The equation derived to take into account both 1:1 and 1:2 complexes equilibria was successfully applied to the present data and those of HP-alpha-CD/sodium alkanoate systems previously studied by us. As far as the postmicellar region is concerned, HP-alpha-CD was treated like an additive, which distributes between the aqueous and the micellar phases. An equation was proposed to rationalize the enthalpy data dealing with the cyclodextrins exhibiting inclusion complex formation. It was based on the following phenomena: (1) formation of 1:1 and 1:2 complexes in the aqueous phase, (2) distribution of free cyclodextrin, 1:1 complex, and 1:2 complex between the aqueous and the micellar phases, and (3) shift of the micellization equilibrium induced by the cyclodextrin. As a general feature, cyclodextrin (free and/or complexed) shows affinity toward the micelles because of the favorable interactions between the carboxylate head in the hydrophilic shell and the hydroxyl groups of the cyclodextrin. C6F13CO2Na micelles compared to C7F15CO2Na exhibit a slightly larger affinity toward HP-alpha-CD controlled by more negative enthalpy and entropy changes. A single mechanism governs the interaction between the C7F15CO2Na micelles and the 1:1 complexes of HP-beta-CD/surfactant and beta-CD/surfactant, as the standard free energy, enthalpy, and entropy of transfer of the two complexes from the aqueous to the micellar phases are identical. The 1,:2 complex (1 HP-beta-CD:2 C6F13CO2Na) weakly binds to the micelles according to the unfavorable interactions between the micellar surface and the doubly charged complex.