Solvatochromism in cationic micellar solutions: Effects of the molecular structures of the solvatochromic probe and the surfactant headgroup

The solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio) -1-phenolate (RB), 2,6-dickloro-4-(2,4,6-triphenyl-1-pyridinio)-1-phenol (W B), 1-methyl-8-oxyquinolinium betaine (QB), and sodium 1-methyl-8-oxyquinol inium betaine 5-sulfonate (QBS) has been studied as a function of increasin g the length of R in the series C12H25N+R3Br- (R = methyl, ethyl, n-propyl, and n-butyl). The microscopic polarity of water at the solubilization site of the micelle-bound probe, ET in kcal/mol, has been calculated from the p osition of its intramolecular charge-transfer band in the W-vis region. Cal culated polarities depend on the length bf R and the probe structure and ch arge. This is attributed to gradual "dehydration" of the interfacial region as a function of the increasing length of R, and different (average) solub ilization sites of the probes. Thus, hydrophobic RE and WE are located in a less polar environment than hydrophilic QB and QBS. These conclusions have been confirmed by measuring H-1 NMR chemical shifts of the discrete proton s of both surfactant and probes. The "effective" water concentration at the probe solubilization site, [Water](interfacial), has been calculated from solvatochromic data in bulk aqueous 1-propanol and aqueous 1,4-dioxane. Bot h reference binary mixtures gave consistent [water] interfacial; our data a lso agree with those based on the use of a micelle-bound arenediazonium ion .