Thermodynamic investigation (volume and compressibility) of the systems beta-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 c alculate the molar apparent and partial volumes and adiabatic compressibili ties. For the binary systems, the molar partial compressibilities and volum es of the pure surfactants in water have been obtained as a function of the concentration and compared with the literature data, and the methylene gro up contributions have been deduced. For the ternary systems, a remarkable i ncrease of both the molar partial volume and compressibility of the surfact ant at infinite dilution with respect to the value in water is observed. Th e large values of the transfer properties of the surfactants at infinite di lution, molar partial compressibilities and volumes, can be discussed in te rms 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 ma crocycle is considered. The positive molar compressibility of the surfactan t 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 p artial 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 s toichiometry with the longest homologues (n = 14, 16). The application of Y oung's rule permits to calculate the reaction parameters from the bibliogra phic data of the binding constants. The transfer volumes and compressibilit ies increase with n, indicating that the predominant stoichiometry turns to 2:1 when the hydrocarbon chain is long enough.