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 = N0(Caq/cmc0)γ where N0 is the aggregation number at the critical micelle concentration in the absence of added electrolyte, cmc0, Caq is the concentration of counterions in the aqueous phase, and γ is a constant. The values of N0 are rather small, in the range 54 to 64 at 25°; 61 to 74 at 10°; and 51 to 56 at 40 °C. The values of γ are insensitive to temperature and are rather small; 0.05 to 0.1. In contrast, tetrabutylammonium dodecylsulfate grows nearly linearly with Caq. The ionization degrees (α) 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 Caq, 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 α are larger than those obtained at the cmc0 by conductivity. For comparison, cesium dodecylsulfate is also studied, where we find that α is equal to its value at the cmc0.