置換ニトロフェノール類の可視スペクトル及び酸性度に及ぼす界面活性剤の効果
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The effects of surfactants and substituents on the visible spectra and the values of pKa of twenty one substituted nitrophenols were studied in 3%, aqueous ethanol at 30°C by means of the spectrophotometric method. The used surfactants were CTAB, CPC, LBS, and B-35. The addition of the cationic surfactant over cmc caused marked effects on the spectra of the nitrophenolate ion (NPO<SUP>-</SUP>), that is, the hyperchromic effect and the bathochromic shift were observed. Moreover, these spectral changes were more significant in <I>o</I>-nitro-isomers rather than <I>p</I>-nitro-isomers. LBS and B-35 had no effect on the spectra. These results were discussed mainly from both the changes in the apparent values of pKa of nitrophenols and the electrostatic interactions of NPO<SUP>-</SUP> with micelles. In order to clarify this discussion, the measurements of pKa of substituted nitrophenols were carried out. The cationic surfactants caused the marked lowering of the apparent values of pKa, and the anionic one did a little increase. However, non-ionic one had no effect. It was found that these pKa changes depended on the extents of electrostatic, and hydrophobic interactions of the substituents (including the hydroxy and the nitro groups) with the micelles as well as the ionic properties of the surfactants. The extents of the hydrophobic interaction of the alkyl substituents with micelles were discussed by the thermodynamic parameter, -δ<I>ΔG</I> (Y-X) which implied that the substituent effect on the stabilization energy of NPOH (or NPO<SUP>-</SUP>) due to the hydrophobic interaction with the micelles.<BR>A significant deviation from Hammett relationship between pKa of <I>p</I>-substituted <I>o</I>-nitrophenols and σ<SUP>-</SUP> was observed in the presence of CTAB, although this relationship showed a fairly straight line in the absence of the surfactant. This observation also suggested that the substituent present influenced the value of pKa of nitrophenol in the micelle solution not only by the electronic effects but also by above described interactions.
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