Thermodynamic Properties of N-octyl-, N-decyl- and N-dodecylpyridinium Chlorides in Water

Article abstract of DOI:10.1007/BF00649096

Densities, heat capacities and enthalpies of dilution at 25 deg C and osmotic coefficients at 37 deg C were measured for N-octyl-, N-decyl- and N-dodecylpyridinium chlorides in water over a wide concentration region.Conductivity measurements were performed in order to evaluate the cmc and the degree of counterion dissociation.Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25 deg C were derived from the experimental data as functions of the surfactant concentration.The changes with concentration of these properties are quite regular with the exception of the heat capacities which display anomalies at about 0.9, 0.25 and 0.12 mol-kg^-1 for the octyl, decyl and dodecyl compounds, respectively.At these concentrations there were also changes in the slopes of the specific conductivity and of the product of the osmotic coefficients and the molality vs. concentration.These pecularities can be ascribed to micelle structural transitions.The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model.These data have been compared to those for alkyltrimethylammonium bromides and alkylnicotinamide chlorides.It is shown that the introduction of the hydrophilic CONH2 group lowers the hydrophilic character of the pyridinium ring. KEY WORDS: N-octylpyridinium chloride; N-decylpyridinium chloride; N-dodecylpyridinium chloride; densities; heat capacities; enthalpies of dilution; osmotic coefficients; activity coefficients; partial molar volumes; partial molar heat capacities; partial molar relative enthalpies; nonideal free energies; nonideal entropies; thermodynamics of micellization