Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of diln. at 25° and osmotic coeffs. at 37° were measured for N-octyl- and N-dodecyl-nicotinamide chlorides in water over an extended concn. region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25° were derived as functions of the surfactant concn. For both surfactants, plots of vols., enthalpies and free energies vs. concn. are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1 m for the octyl and dodecyl compds., resp. Changes in the slope of a plot of osmotic coeffs. times molality vs. molality were also obsd. at these same concns. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/Ccmc. Also, a plot of the nonideal free energy vs. logm/Ccmc is roughly independent of the nature of the surfactant because of the const. activity of surfactants in micellar solns. Nonideal free energies. enthalpies and entropies have been calcd. at 15 and 35°. At each concn., the nonideal free energy is temp. independent as a result of a compensatory effect between enthalpy and entropy. The thermodn. functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.