New hydrophobic pi* indicators. Solvatochromic properties and interactionsin micellar solutions

Solvent-sensitive probes have wide application in the study and characteriz ation of interfacial systems. This work presents some new, more hydrophobic versions of the solvatochromic pi* indicators of solvent dipolarity-polari zability. The new dyes form a homologous series of N,N-dialkyl-p-nitroanili nes. The indicators are designed to act as improved probes of micelles and lipid bilayers in polar and aqueous solutions. UV-vis studies of the N,N-di pentyl- and N,N-dihexyl-p-nitroanilines show the two indicators to be stron gly solvatochromic; Values of -s estimated from two-point correlation are 3 .081 and 3.110, respectively. W-vis spectra of N,N-dipentyl- and N,N-dihexy l-p-nitroanilines obtained at variable indicator concentration in cyclohexa ne and dimethyl sulfoxide, within the 10(-5) M concentration range, display no concentration-dependent spectral shifting and no evidence of indicator aggregation in accordance with Beer's law. Studies of the indicator series in aqueous solutions of varying concentration of the anionic surfactant sod ium dodecyl sulfate (SDS) suggest that N,N-dipropyl-p-nitroaniline probes m icelles of SDS more deeply than the commonly used N,N-dimethyl- and N,N-die thyl-p-nitroanilines. Of the three indicators in the series, N,N-dimethyl-, N,N-diethyl-, and N,N-dipropyl-p-nitroaniline, the dipropyl species is mos t responsive to the critical micelle concentration of SDS. The longer chain dyes N,N-dibutyl-, N,N-dipentyl-, and N,N-dihexyl-p-nitroaniline are more effective probes of larger micelles of the nonionic surfactant Triton X-114 . UV-vis studies of N,N-dihexyl-p-nitroaniline in micellar solutions of Tri ton X-114 and model systems suggest that the longest chain indicator probes the outer polyoxyethylene portion of the Triton X-114 micelle.