| Title
| - 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, andTetrabutylammonium Dodecyl Sulfates
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| Abstract
| - The surfactants cesium, tetramethylammonium, tetraethylammonium, tetrapropylammonium and tetrabutylammonium dodecyl sulfates (CsDS, TMADS, TEADS, TPADS, and TBADS) have been synthesized byion-exchange. The critical micellization concentration in the absence of added salt (cmc) has been determinedat 10, 25, and 40 °C using the electrical conductivity method. The cmc was found to decrease in the sequenceCsDS > 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 aggregationnumbers 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, inthe case of TMADS, in the presence of tetramethylammonium chloride. The micelle ionization degree α0 atthe cmc has been determined from the electrical conductivity data and the values of the aggregation numberextrapolated to the cmc. The micelle ionization degree was the largest for SDS (sodium dodecyl sulfate) andthe smallest for TBADS. The micelle micropolarity, as determined by the pyrene polarity ratio I1/I3, was amaximum 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, themicelle aggregation number decreases as the counterion radius increases, contrary to what was expected onthe basis of the cmc values. At 40 °C, the sequence of the aggregation numbers is almost that expected fromthe cmc values. An intermediate result was obtained at 25 °C. The micelle aggregation number increasedlittle with the surfactant concentration. The results are explained on the basis of the fact that theteraalkylammonium ions are so large that steric hindrance may impede their binding to the micelles, therebyrestricting micelle size and limiting micelle growth.
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