Partial molar enthalpies of the components of a quaternary water/oil microemulsion system

The molar enthalpy of mixing has been determined calorimetrically for a large number of solutions of the quaternary w/o microemulsion system n-decanol-n-octane-water-sodium n-octanoate at 298.15 K. The system is of fundamental interest since it allows a direct comparison of the thermal properties of complex quaternary microemulsions with the simpler ternary and binary reference mixtures being a part of the same system. Such systems are the inverted micellar system water + sodium octanoate in decanol, the detergentless microemulsion water-decanol-octane, and the binary mixtures water in decanol and decanol-octane. When analyzing the thermal properties of complex multicomponent systems where all the components participate in the association process solution properties such as the apparent molar enthalpy are difficult to interpret. A method is therefore suggested to derive the partial molar enthalpies when mixing the components in order to successfully highlight the thermal interactions. In order to test the precision of the polynomial representation of the experimental material different sets applicable to each system are used in the partial derivation procedure and the resulting partial molar enthalpies are illustrated in the figures. The partial molar enthalpies indicate that the interaction between the water and the ionic groups primarily determines the capacity of the microemulsion to incorporate octane. Octane seems to influence the association equilibria by shifting the distribution of decanol between the micellar interface and the solvent. Despite the different properties of the water-rich (low octane solubility) and water-poor (high octane solubility) phase regions a variation in the degree of aggregation seems sufficient to explain the results qualitatively. © 1988.