Solvatochromism in Cationic Micellar Solutions:  Effects of the Molecular Structures of the Solvatochromic Probe and the Surfactant Headgroup

Abstract

The solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB), 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (WB), 1-methyl-8-oxyquinolinium betaine (QB), and sodium 1-methyl-8-oxyquinolinium betaine 5-sulfonate (QBS) has been studied as a function of increasing the length of R in the series C 12 H 25 N + R 3 Br - (R = methyl, ethyl, n -propyl, and n -butyl). The microscopic polarity of water at the solubilization site of the micelle-bound probe, E T in kcal/mol, has been calculated from the position of its intramolecular charge-transfer band in the UV−vis region. Calculated polarities depend on the length of R and the probe structure and charge. This is attributed to gradual “dehydration” of the interfacial region as a function of the increasing length of R, and different (average) solubilization sites of the probes. Thus, hydrophobic RB and WB are located in a less polar environment than hydrophilic QB and QBS. These conclusions have been confirmed by measuring 1 H NMR chemical shifts of the discrete protons 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. Both reference binary mixtures gave consistent [water] interfacial; our data also agree with those based on the use of a micelle-bound arenediazonium ion.