Molar Partial Compressibilities and Volumes, ¹H NMR, and Molecular Modeling Studies of the Ternary Systems β-Cyclodextrin + Sodium Octanoate/Sodium Decanoate + Water

The thermodynamic behavior of the ternary systems β-cyclodextrin (β-CD) + sodium octanoate (NaO) or sodium decanoate (NaD) + water has been studied from density and speed of sound measurements in a broad concentration range at 298 K and at natural pH. The molar partial compressibilities and volumes of the pure surfactants in water as a function of concentration have been obtained and compared with the literature data. For the ternary systems, a remarkable increase of the molar partial compressibility of the surfactant at infinite dilution with respect to the value of the surfactant in water is observed, whereas it does not change in the micelle region, and the same behavior is found with the partial volume. The changes in the transfer properties of the surfactants at infinite dilution, molar partial compressibilities, and volumes can be discussed in terms of a simple model in which it is considered the balance between the released water from the cavity and the methylene groups of the substrate that enter into the macrocycle. The positive molar compressibility of the surfactant when it is forming the complex, as a difference with the negative value when it is in pure water, prove the hydrophobic component of the interaction and permits estimating from this property the binding constants by application of Young's rule. 1 H NMR studies on the systems permit us to elucidate the complex structure and corroborate the thermodynamic data. The association constants and stoichiometry have been deduced from volumes, compressibilities, and 1 H NMR data, yielding consistent values that agree with other literature results obtained at fixed pH. Molecular mechanics calculations have been performed to shed light on the structure of the complex in solution. The results confirm the NMR data and indicate that the polar head in the complex is at the wider rim of the macrocycle, protruding in the cavity, with the surfactant tilted within the β-CD.