Thermodynamic investigation (volume and compressibility) of the systems β-cyclodextrin plus n-alkyltrimethylammonium bromides plus water

Density and sound velocity data for aqueous solutions at 298 K containing a homologue series of alkyltrymethylammonium bromides (C(n)TAB, n = 10, 12, 14, 16) in the absence and presence of beta-cyclodextrin were analyzed to calculate the molar apparent and partial volumes and adiabatic compressibilities. For the binary systems, the molar partial compressibilities and volumes of the pure surfactants in water have been obtained as a function of the concentration and compared with the literature data, and the methylene group contributions have been deduced. For the ternary systems, a remarkable increase of both the molar partial volume and compressibility of the surfactant at infinite dilution with respect to the value in water is observed. The large values of 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 the balance between the released water from the cavity and the methylene groups of the substrate that enter into the macrocycle is considered. The positive molar compressibility of the surfactant when it is forming the complex, compared to the negative value when it is in pure water, proves the hydrophobic component of the interaction. Both partial molar volumes and compressibilities of the surfactants are the same in the absence and in the presence of beta-CD at high surfactant molalities, indicating the nonparticipation of the complex into the micelles, and the cmcs are displaced in an extension that shows the participation of a 2:1 stoichiometry with the longest homologues (n = 14, 16). The application of Young's rule permits to calculate the reaction parameters from the bibliographic data of the binding constants. The transfer volumes and compressibilities increase with n, indicating that the predominant stoichiometry turns to 2:1 when the hydrocarbon chain is long enough.