Solvatochromism in CationicMicellar Soutions: Effects of the Molecualr Structures of the Solvatochromic Probe and the Surfactant Headgroup

摘要

The solvatochromicbehaviorof2, 6-diphenyl-4-(2,4,6-triphenyl-l-pyridinio )-l-ph 4-(2.4,6-triphenyl-l-pyridinio)-I-phenolate (WE), I-methyl-8-oxyquinolinium betaine (QE), and sodium I-methyl-8-oxyquinoliniumbetaine 5-sulfonate (QES) has been studied as a function of increasing the length of R in the series C12H25N+R3Er~ (R = methyl, ethyl, n-propyl, arld n-butyl). The microscopic polarity ofater at thesolubi~ization site of the micelle-bound probe, ET in kcaVmol, has been calculated from the position ofits intramolecular charge-transfer band in the UV-vis region. Calculated polarities depend on the length ofR and the probe structure and charge. This is attributed to gradual "dehydration" of the interfacial region as a function of the increasing length ofR, and different (average) solubilization sites of the probes. Thus, hydrophobic RE and WE are located in a less polar environment than hydrophilic QE and QES. These conclusions have been confirmed by measuring lH NMR chemical shifts of the discrete protons of both surfactant and probes. The "effective" water concentration at the probe solubilization site, [wat~r]jnterfacjal,has been cal~ulated ~rom solvatochro?1ic data in bulk aqueous I-propanol and .aqueous 1,4-dIoxane. Both reference bInary mIxtures gave consIstent [water]intert"acial; our data also agree wIth those based on the use of a micelle-bound arenediazonium ion.