VOLUMES, HEAT-CAPACITIES, AND CONDUCTIVITIES OF WATER SURFACTANT-18-CROWN-6 ETHER SYSTEMS

Volumes, heat capacities, and conductivities of water-18-crown-6 ether (CR)-surfactant ternary systems were measured at 25-degrees-C as functions of the surfactant (m(S)) and the CR (m(CR)) concentrations and at fixed CR/surfactant (R) ratios. The surfactants studied are sodium dodecyl sulfate (NaDS) and dodecyltrimethylammonium bromide (DTAB). From conductivity data the cmc and the degree of counterion dissociation (beta) were evaluated. The increase of beta with m(CR) is essentially the same for the two surfactants while it is not so for the cmc. In fact, the cmc always increase with m(CR) for DTAB while it is a concave curve for NaDS. The apparent molar volume of transfer (DELTAV(PHI,S)(W-->W+CR)) of DTAB from water to water-CR mixtures indicates the presence of CR-DTAB interactions in the aqueous phase and the lack of specific interactions in the micellar phase. A plot of DELTAV(PHI,S)(W-->W+CR) of NaDS shows a maximum just beyond the cmc and then decreases with the increase in ms. The C(PHI,S) vs ms curves are similar to those in water and slightly affected by the presence of CR. The apparent molar volumes and heat capacities of the composite mixtures, at fixed R, are higher and smaller, respectively, than those calculated on the basis of the additivity of both binary mixtures. At a given total concentration, the excess volumes (V(exc)) and heat capacities (obtained as a difference between the experimental and calculated properties) present a maximum and minimum, respectively, vs the mole fraction. V(exc) is slightly negative for CR-DTAB and positive for CR-NaDS. Since positive V(exc) values have been observed for CR-NaCl and pentanol-NaDS mixtures, therefore, we cannot ascertain if hydrophilic and hydrophobic interactions are involved between NaDS micelles and CR. The apparent molar volumes (V(PHI,CR)) and heat capacities (C(PHI,CR)) of CR at 0.04 m in micellar solutions of the two surfactants were also determined. For NaDS, the profiles of the two properties are similar to those observed for other additives which distribute between the aqueous and the micellar phases; for DTAB they essentially do not depend on m(S) as it was observed for those additives which do not solubilize in the micelles. V(PHI,CR) was rationalized as a function of m(S) by using a previously reported approach for the distribution between the aqueous and micellar phases where the CR complexation with Na+ ions in the aqueous phase was taken into account. From the resulting equation the distribution constant of the complexed CR between the two phases and its partial molar volume in the micellar phase were derived. These properties indicate that strong interactions between NaDS micelles and CR are present. The site of solubilization of CR is the micellar surface where its complex is the counterion of the micelle.