Effect of the Nature of the Counterion on the Properties of Anionic Surfactants. 1. Cmc, Ionization Degree at the Cmc and Aggregation Number of Micelles of Sodium, Cesium, Tetramethylammonium, Tetraethylammonium, Tetrapropylammonium, and Tetrabutylammonium Dodecyl Sulfates

The surfactants cesium, tetramethylammonium, tetraethylammonium, tetrapropylammonium and tetrabutylammonium dodecyl sulfates (CsDS, TMADS, TEADS, TPADS, and TBADS) have been synthesized by ion-exchange. The critical micellization concentration in the absence of added salt (cmc) has been determined at 10, 25, and 40 °C using the electrical conductivity method. The cmc was found to decrease in the sequence CsDS > TMADS > TEADS > TPADS > TBADS. The value of the cmc depends very little on temperature, going through a shallow minimum around 25 °C for most surfactants investigated. The micelle aggregation numbers have been determined using the time-resolved fluorescence quenching method, with the pyrene/ dodecylpyridinium chloride as fluorescent probe/quencher pair, at various surfactant concentrations and, in the case of TMADS, in the presence of tetramethylammonium chloride. The micelle ionization degree R0 at the cmc has been determined from the electrical conductivity data and the values of the aggregation number extrapolated to the cmc. The micelle ionization degree was the largest for SDS (sodium dodecyl sulfate) and the smallest for TBADS. The micelle micropolarity, as determined by the pyrene polarity ratio I1/I3, was a maximum for TEADS. The micelle microviscosity, investigated using the fluorescent probe 1,3-dipyrenylpropane, increased in the sequence CsDS < SDS < TMADS < TEADS ≈ TBADS ≈ TPADS. At 10 °C, the micelle aggregation number decreases as the counterion radius increases, contrary to what was expected on the basis of the cmc values. At 40 °C, the sequence of the aggregation numbers is almost that expected from the cmc values. An intermediate result was obtained at 25 °C. The micelle aggregation number increased little with the surfactant concentration. The results are explained on the basis of the fact that the teraalkylammonium ions are so large that steric hindrance may impede their binding to the micelles, thereby restricting micelle size and limiting micelle growth.