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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 R 0 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 I 1 /I 3 , 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.

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The surfactants tetrabutylammonium dodecyl sulfate (TBADS) and tetradecyl sulfate (TBATS) have been synthesized by ion-exchange, and their self-association behavior has been investigated. The solutions of these surfactants show clouding and phase separation as the temperature is increased. The clouding temperature T C of TBADS solutions has been found to be about 4-5 °C higher than for TBATS solutions in the whole range of composition up to a surfactant content of 64 wt %. The critical micellization concentration (cmc) of TBATS has been determined using the electrical conductivity method. The micelle aggregation numbers (N) have been determined using the time-resolved fluorescence quenching (TRFQ) method, with pyrene/dodecylpyridinium chloride as fluorescent probe/quencher pair. For the two surfactants, N increases with the surfactant concentration and above a threshold concentration with the temperature. This latter increase as well as the phase separation observed for TBADS and TBATS are very unusual features for ionic surfactants. At temperatures approaching T C from below, the TRFQ data show evidence of quencher/probe migration between micelles. This process is shown to occur via collisions between micelles. Approximate calculations indicate that the maximum number of TBA + ions that can be packed at the TBADS micelle surface is smaller than the experimentally determined value of the number of bound TBA + ions. This result suggests that some bound TBA + ions must be located in an outer and probably incomplete second layer of bound TBA + ions. The origin of the attractive intermicellar interaction responsible for the observed phase separation is discussed on the basis of the existence of the second layer of bound TBA + ions and the capacity of TBA + ions to self-associate in water.