Apparent and partial molar enthalpies of potassium 4-((hexylcarbonyl)amino)-2-hydroxybenzoate aqueous micellar solutions

Enthalpies of diln. at 25° of potassium 4-((hexylcarbonyl)amino)-2-hydroxybenzoate (KPAS-C7) were measured as functions of the pH and of the ionic strength by using an LKB 2107 flow microcalorimeter. From the exptl. data, apparent and partial molar relative enthalpies as functions of the molality of the surfactant were derived. Enthalpic properties are strongly dependent on the pH, probably because the dissocn. of the salicylic OH affects the hydrophilic-lipophilic balance of the mol. Around a surfactant concn. of 0.13 mol/kg, the plot of apparent and partial molar enthalpies of the solns. in KOH (0.02 mol/kg) shows an abrupt change in slope: this is attributed to a transition in the micelle structure. The presence of max. in the partial molar enthalpy vs. molality curves at the highest KOH concns. could confirm this hypothesis. An increase in the ionic strength in the KOH solns. (0.02 mol0kg) causes the transition to shift to a lower concn., without a strong effect on the overall trend of the enthalpic curves. The thermodn. functions of micellization were evaluated on the basis of the pseudo-phase-transition model by graphical extrapolation: in the limits of exptl. error, they vary almost linearly with pH.