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This two-day laboratory experiment describes how the influence of the hydrocarbon chain length and added salt on the micellar aggregation number of ionic surfactants can be investigated by steady-state fluorescence measurements. Emphasis is put on the necessity to validate the models used to evaluate the data, and examples are given when a given model does not work.
For the effect of added salt, sodium dodecylsulphate with and without added sodium chloride was investigated. Both quenchers used, i.e., dodecylpyridinium chloride (DoPyrCl) and 1,3-dicyanobenzene (DCB), yield aggregation numbers in agreement with literature values.
For the effect of surfactant chain length, however, the used model failed under certain circumstances. Three alkyltrimethylammonium halides were investigated (CnTA+; n=12, 14, and 16). For the shortest one, DoPyrCl yields a good aggregation number while DCB fails due to electrostatic interactions. For the tetradecyl surfactant, DoPyrCl does not work well, as the difference between the tail-lengths of the quencher and surfactant causes problems in the determination of the amount of micelle-bound quencher. Tetradecylpyridinium chloride circumvents this problem and a consistent aggregation number is obtained. Finally, for the longest surfactant, not even a quencher with a tail of equal length of the surfactant's worked out well, due to the slow diffusion of the quencher in the micellar aggregate.
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