Energetics of water-dodecyl surfactant-macrocyclic compound ternary systems

Enthalpies of dilution and osmotic coefficients of sodium dodecyl sulfate (NaDS) and dodecyltrimethylammonium bromide (DTAB) in water + 18-crown-6 ether (CR) and water + beta-cyclodextrin (CD) at a fixed cosolvent concentration were measured at 298 and 310 K, respectively, as functions of the surfactant concentration (m(S)). Enthalpies of transfer Delta H (W --> W + S) of CR (0.03 m) from water to NaDS and DTAB aqueous solutions as functions of m(S) were also determined at 298 K. From the enthalpies of dilution the apparent (L(Phi,S)) and partial (L(2,S)) molar relative enthalpies of both surfactants were calculated. Despite CR forms inclusion complexes with the anionic surfactant only, the L(2,S) vs m(S) profiles are similar and the enthalpies of micellization are lower than those in water by about -5 kJ mol(-1). In the case of CD as a cosolvent, the L(2,S) vs m(S) profile for DTAB is similar to that for NaDS in the postmicellar region but very different in the premicellar one. The trends in the premicellar region are discussed in terms of different solute-solute hydrophilic interactions other than encapsulation while those in the postmicellar region are discussed in terms of the micellization process. The enthalpies of micellization are very large because of the complexed monomers contribution. Delta H (W --> W + S) data for CR in DTAB micellar solutions were fitted through an equation previously reported which permits simultaneously obtaining the distribution constant of the uncomplexed CR and its enthalpy of transfer from the aqueous to the micellar phases. The equations were reviewed for CR in NaDS micellar solutions to account for the CR complexation and for the distribution of both the complexed and uncomplexed CR between the aqueous and the micellar phases. The derived properties are briefly discussed. The osmotic coefficient (Phi(s)) vs m(S) curve of both surfactants in W + CD shows a minimum at ms equal to the CD concentration (m(CD)) and a maximum at m(S) = m(CD) + cmc. These peculiarities are ascribed to the inclusion complex formation between the macrocyclic compound and the apolar chain of the surfactant. The addition of CR to water leads to the shift of the osmotic coefficient toward lower values. This shift is not very important for DTAB while it is for NaDS for which negative Phi(S) values were obtained. Sodium perfluorooctanoate behaves like NaDS. Since the osmotic coefficients for NaCl in W + CR are close to those in pure water, the results are interpreted in terms of complexed CR solubilization in the micellar phase.