Fluorescence analysis of single and mixed micelle systems of SDS and DTAB

Structural features of the single and mixed micellar systems of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) were characterized using the fluorescence probe 6-propionyl-2-(dimethylamino)naphthalene (Prodan). In particular, our investigations capitalized on the spectral sensitivity of Prodan to its environment as well as the extensive solubility of Prodan in solvents of varying polarity and/or hydrophobicity to effectively use a three-mode factor analysis technique to resolve the coincident emission from Prodan in multiple microenvironments of single and mixed micelle systems. Our investigations reveal parameters of Prodan fluorescence that reflect the relative polarities of the surfaces of SDS and DTAB micellar cores, the permeability of the SDS micelle interface, and the heterogeneity of SDS-DTAB mixed micellar systems. In particular, we observe a strong affinity of Prodan for both SDS and DTAB micelles at the water-surfactant interface with the emission lambda(max) of Prodan consistent with greater water accessibility in the SDS interfacial region. Reduction in SDS head-group repulsion upon the addition of both an alkali metal series of counterions (Li+ --> Na+ --> K+) and a tetrasubstituted ammonium series of counterions (NH4+ --> N(CH3)(4)(+) --> N(CH3CH2)(4)(+) --> N(CH3CH2CH2CH2)(4)(+)) appears to induce a more nonpolar environment for Prodan. Each one-phase and two-phase region of the dilute aqueous binary SDS-DTAB pseudoternary diagram is identified by distinct Prodan lambda(max) values. Evidence is presented for the presence of SDS-DTAB mixed micelle systems.