Aqueous micellar solutions of some N-alkylnicotinamide and N-alkylpyridinium halide surfactants: apparent and partial molar enthalpies

In order to acquire thermodn. data for aq. solns. of functionalized surfactants, and to study the effect of the amidic group in the polar head and of the counterion, the enthalpies of diln. at 25° of N-alkylnicotinamide chlorides (alkyl chain length 7.9 or 10 carbon atoms), N-alkylpyridinium chlorides (alkyl chain length 8, 10, or 12 atoms), N-octylpyridinium bromide and N-decylpyridinium bromide were measured by means of a flow microcalorimeter. From the exptl. data, apparent and partial molar relative enthalpies as function of the molaity of the surfactant were derived. The amidic group in the nicotinamide compds. greatly enhances the hydrophobicity relative to the pyridinium compds. For cationic surfactants with the same alkyl chain, the trends of the molar enthalpies vs. concn. can be explained in terms of charge localization on the polar head. For the compds. 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 soln. The thermodn. functions of micellization were evaluated on the basis of the pseudo-phase transition model by graphical extrapolation.