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

Abstract

Enthalpies of dilution at 25° C of potassium 4-((hexylcarbonyl)amino)-2-hydroxybenzoate (KPAS-C 7) have been measured as a function of the pH and of the ionic strength using an LKB 2107 flow microcalorimeter. From the experimental data, apparent and partial molar relative enthalpies as a function of the molality of the surfactant have been derived. Enthalpic properties are strongly dependent on the pH, probably because the dissociation of the salicylic OH affects the hydrophilic-lipophilic balance of the molecule. Around a surfactant concentration of 0.13 mol kg −1, the plot of apparent and partial molar enthalpies of the solutions in KOH (0.02 mol kg −1) shows an abrupt change in slope: this is attributed to a transition in the micelle structure. The presence of maxima in the partial molar enthalpy versus molality curves at the highest KOH concentrations could confirm this hypothesis. An increase in the ionic strength in the KOH solutions (0.02 mol kg −1) causes the transition to shift to a lower concentration, without a strong effect on the overall trend of the enthalpic curves. The thermodynamic functions of micellisation have been evaluated on the basis of the pseudo-phase-transition model by graphical extrapolation: in the limits of experimental error, they vary almost linearly with pH.