Equilibrium and transport properties of alkylpyridinium bromides

The osmotic coefficient, the apparent molar volume, and the electrical conductivity of aqueous solutions of the homologous series of decyl-, undecyl-, dodecyl-, tridecyl-, tetradecyl-, pentadecyl-, and hexadecylpyridinium bromides (CnPyBr) have been determined at 25 degrees C. From these data the variation of the critical micelle concentration (cmc) with the chain length of the surfactant has been obtained. Measurements were performed usually at concentrations from well below to well above the cmc. From osmotic coefficient and conductivity measurements, the degree of counterion binding to micelles, beta, has been calculated. It has been found that the values of beta are rather insensitive to the chain length, and as expected, the values determined from transport measurement are lower than those from the osmotic pressure measurements. From the apparent molar volumes the aggregation numbers of alkylpyridinium bromides have been estimated, following simple geometrical considerations suggested by Tartar and Tanford.