SOLUBILIZATION AND AGGREGATION NUMBERS IN MICELLAR MIXTURES OF ANIONIC AND CATIONIC SURFACTANTS WITH TETRAETHYLENE GLYCOL AND TETRAETHYLENE GLYCOL DIMETHYL ETHER

Critical micelle concentrations, apparent degrees of counterion binding, additive distribution coefficients, and surfactant and additive aggregation numbers are reported for aqueous mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB), with tetraethylene glycol (TEG) and tetraethylene glycol dimethyl ether (TEGDM). Critical micelle concentration (cmc) values and apparent degrees of counterion binding (beta) were determined via a conductance method. The mole fraction of the solubilizate (either TEG or TEGDM) in the micelle was determined using the NMR paramagnetic relaxation method. Surfactant aggregation numbers were obtained from static fluorescence quenching experiments, using pyrene as the fluorescent probe and cetylpyridinium ion (CP+) as the quencher. The results show a remarkable difference in the solubilization behavior in SDS and DTAB micelles for these two ethoxylates. TEG is not incorporated appreciably in either SDS or DTAB micelles, while TEGDM is solubilized easily by SDS micelles (p = 0.44) but hardly incorporated in DTAB micelles (p = 0.05). Addition of TEGDM to a SDS micellar solution decreases both the cmc and beta values of the micelles. When TEGDM is added to a 0.05 m SDS solution, the surfactant aggregation number, N(s), decreases from 66 to 40, while the additive (TEGDM) aggregation number, N(a), increases rapidly from 0 to 37, as the total TEGDM concentration is increased to 0.30 m. These results are discussed in terms of the difference in the solubilization behavior of these ethoxylates in anionic and cationic micelles.