ENERGETICS OF WATER-DODECYL SURFACTANT-MACROCYCLIC COMPOUND TERNARY-SYSTEMS

Enthalpies of dilution and osmotic coefficients of sodium dodecyl sulf ate (NaDS) and dodecyltrimethylammonium bromide (DTAB) in water + 18-c rown-6 ether (CR) and water + beta-cyclodextrin (CD) at a fixed cosolv ent concentration were measured at 298 and 310 K, respectively, as fun ctions 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 aqueo us 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. Despi te CR forms inclusion complexes with the anionic surfactant only, the L(2,S) vs m(S) profiles are similar and the enthalpies of micellizatio n 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 pre micellar one. The trends in the premicellar region are discussed in te rms of different solute-solute hydrophilic interactions other than enc apsulation while those in the postmicellar region are discussed in ter ms of the micellization process. The enthalpies of micellization are v ery large because of the complexed monomers contribution. Delta H (W - -> W + S) data for CR in DTAB micellar solutions were fitted through a n equation previously reported which permits simultaneously obtaining the distribution constant of the uncomplexed CR and its enthalpy of tr ansfer from the aqueous to the micellar phases. The equations were rev iewed for CR in NaDS micellar solutions to account for the CR complexa tion and for the distribution of both the complexed and uncomplexed CR between the aqueous and the micellar phases. The derived properties a re briefly discussed. The osmotic coefficient (Phi(s)) vs m(S) curve o f both surfactants in W + CD shows a minimum at ms equal to the CD con centration (m(CD)) and a maximum at m(S) = m(CD) + cmc. These peculiar ities are ascribed to the inclusion complex formation between the macr ocyclic compound and the apolar chain of the surfactant. The addition of CR to water leads to the shift of the osmotic coefficient toward lo wer values. This shift is not very important for DTAB while it is for NaDS for which negative Phi(S) values were obtained. Sodium perfluoroo ctanoate 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.