The Importance of Hydrophobic-Hydrophilic Factors in Binding of Charged Substrates to Micelles: The Use of Extramicellar Probe Luminescence to Monitor Association of Cations to the Micelle

Article abstract of DOI:10.1021/ja00408a009

The anionic metal complexes 1, RuL3^4-, where L = 4,4'-dicarboxy-2,2'-bipyridine, has been used as an extramicellar lumeniscent probe to monitor the binding capacity of anionic sodium dodecylsulfate micelles for the quencher Cu²⁺ and several organic cations.Addition of surfactant attenuates the quenching activity of the cations toward 1 in each case.For Cu²⁺ the binding capacity, as monitored by the quenching of the extramicellar probe, is in good agreement with those determined for Cu²⁺ and other divalent metal ions, using quite different methods; curvature in the quenching plot as 2+> increases is attributed to screening of the micellar charge by bound Cu²⁺.For MV²⁺ quite different behavior is observed; the Stern-Volmer type quenching plot shows two good linear portions at any surfactant concentration.The portion with low slope at low 2+> is attributed to MV²⁺ binding to the organized surfactant; at higher 2+> the steeper plot has a slope essentially the same as that obtained in water in the absence of micelles.The absence of curvature suggests association of several MV²⁺ molecules does not inhibit binding of additional quenchers until nearly all the binding sites are occupied.This finding, combined with the relatively high binding capacity in terms of cation/SDS for MV²⁺ compared to metal ions, suggests that MV²⁺ binds at different sites and that other factors control its binding.Similar results heve been obtained with other organic cations.The results are most consistent with a model whereby these bind at hydrocarbon-water interfaces and not primarily with the micelle head groups.