Heat capacity study to evidence the interactions between cyclodextrin and surfactant in the monomeric and micellized states

The heat capacities of transfer (DeltaCp(t)) of hydroxypropyl-alpha-cyclodextrin and hydroxypropyl-gamma-cyclodextrin (0.05 mol kg(-1)) from water to aqueous solutions of sodium hexanoate, sodium decanoate, and sodium dodecanoate were determined at 298 K. The measurements were extended to both the pre- and the post-micellar regions. The shape of the DeltaCp(t) as a function of the surfactant concentration curve is system specific. As well, the DeltaCp(t) magnitude depends on the macrocycle cavity being largely negative for HP-gamma-CD. The qualitative analysis of the experimental data highlights that the features of the heat capacity are different from those of the enthalpy due to the important effect of temperature on the equilibria in solution. The experimental points were treated by means of a new equation based on the following contributions: (i) the formation of host-guest complexes in the aqueous phase, (ii) the shift of the micellization equilibrium induced by the cyclodextrin, and (iii) the interaction between micelle and cyclodextrin (free and complexed). The resulting equation is involved since it contains not only the terms related to the various equilibria in solution but also those relative to their shift with temperature. Despite its complexity, the equation fitted very well the experimental points in the absence and the presence of micelles.