AQUEOUS MICELLAR SOLUTIONS OF SOME N-ALKYLNICOTINAMIDE AND N-ALKYLPYRIDINIUM HALIDE SURFACTANTS - APPARENT AND PARTIAL MOLAR ENTHALPIES

In order to acquire thermodynamic data for aqueous solutions of functionalized surfactants, and to study the effect of the amidic group in the polar head and of the counterion, the enthalpies of dilution at 25°C of N-alkylnieotinamide chlorides (alkyl chain length 7, 9 or 10 carbon atoms), N-alkylpyridinium chlorides (alkyl chain length 8, 10 or 12 carbon atoms), N-octylpyridinium bromide and N-decylpyridinium bromide have been measured by means of 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. The amidic group in the nicotinamide compounds greatly enhances the hydrophobicity relative to the pyridinium compounds. For cationic surfactants with the same alkyl chain, the trends of the molar enthalpies vs. concentration can be explained in terms of charge localization on the polar head. For the compounds under investigation, enthalpic data are not able to show phase transitions in the micellar region, but they confirm the role of the counterion in the energetics of micellar solution. The thermodynamic functions of micellization have been evaluated on the basis of the pseudo-phase transition model by graphical extrapolation. © 1990.