Effect of the nature of the counterion on the properties of anionic surfactants. 2. Aggregation number-based micelle ionization degrees for micelles of tetraalkylammonium dodecylsulfates

Tetramethyl-, tetraethyl-, tetrapropyl-, and tetrabutylammonium dodecylsulfate micelles in aqueous solution grow as either the surfactant or added electrolyte concentrations are increased. The variation in the aggregation number, N, of the first three surfactants is described as follows: N = N-0(C-aq/cmc(0))(gamma) where N-0 is the aggregation number at the critical micelle concentration in the absence of added electrolyte, cmc(0), C-aq is the concentration of counterions in the aqueous phase, and gamma is a constant. The values of N-0 are rather small, in the range 54 to 64 at 25degrees; 61 to 74 at 10degrees; and 51 to 56 at 40 degreesC. The values of gamma are insensitive to temperature and are rather small; 0.05 to 0.1. In contrast, tetrabutylammonium dodecylsulfate grows nearly linearly with C-aq. The ionization degrees (alpha) of these micelles are studied by applying the hypothesis recently introduced (J. Phys. Chem. B 2001, 105, 6798) that the aggregation number at a given temperature is determined solely by C-aq whether these counterions are supplied by the surfactant alone or by the surfactant plus added salt with a common counterion. In all cases, the values of (x are larger than those obtained at the emc(0) by conductivity. For comparison, cesium dodecylsulfate is also studied, where we find that alpha is equal to its value at the cmc(0).