Thermodynamics and Fluorescence Spectra of 1,10-Phenanthroline in Micelles of Poly (Ethylene Oxide)-Type Nonionic Surfactant

Abstract

The distribution of 1,10-phenanthroline (phen) in micellar solutions of the nonionic surfactants Triton X and C12E n with varying poly (ethylene oxide) chain lengths has been studied by potentiometry, calorimetry, and fluorometry at 298 K. Micelles accommodate 1,10-phenanthroline according to the reaction, phen + Ym = Ym(phen), where Ym denotes a surfactant molecule aggregated in micelles. The constant K m for the reaction of Triton X increases as a linear function of n*, the number of ethylene oxide (EO) groups, as K m = KEO n* + Kc. Nonzero K EO and K c values suggest a heterogeneous inner structure of the micelle, i.e., the hydrophobic core surrounded by a hydrophilic poly (ethylene oxide) (PEO) shell. On the basis of molar volumes, the intrinsic thermodynamic parameters of transfer of 1,10-phenantholine were extracted. The enthalpy and entropy of transfer of 1,10-phenanthroline from the PEO shell to the core are found to be small and negative. By using K EO and K c values for C12E n obtained by fluorometry, individual fluorescence spectra of 1,10-phenanthroline in the PEO shell and core were extracted. The fluorescence intensity of 1,10-phenathroline accommodated in the core, like in organic solvents, is significantly reduced relative to that in water. These facts indicate that the aromatic rings of 1,10-phenanthroline penetrate into the hydrophobic core, while its hydrophilic N site is still hydrated in the PEO shell.