The aggregation behavior of amphiphilic pyrene chromophore in solutions and langmuir monolayers

The aggregation behavior of 12-(1-pyrene)dodecanoic acid (PDDA) in chloroform and ethanol solutions and water-ethanol mixtures, as well as in monolayers at the air-water interface, has been investigated. It has been established that the bathochromic shift in electron absorption spectra (EAS) for the solutions observed during the transition from a more polar solvent (ethanol) to a less polar one (chloroform) is not related to the chromophore aggregation, but is caused by specific interactions of chloroform with the pyrene fragment of the amphiphilic molecule. These data were corroborated by the method of NMR spectroscopy (diffusion ordered spectroscopy (DOSY)), the results of which demonstrated that the diffusion coefficients and diffusing particles size in two organic solvents under study corresponded to the monomer form of PDDA. Inducing the excimer aggregation of the chromophore was carried out through an increase of the water content in the mixed water-ethanol solvent, in which, as was found, the solvating (shielding) ethanol capacity preventing pyrene aggregation with formation of excimers is preserved up to high concentrations of the 'poor' solvent: the excimer emerges at a water content in the mixture exceeding 65%. The sequence of structural transformations in PDDA films on the water surface was suggested on the basis of the monolayers' compression isotherms and absorption and fluorescence spectra. Using the PDDA monolayer, the polarity of the surface of the water subphase was estimated by juxtaposition of PA parameters of this 2D system and water-ethanol solutions of different polarities (component ratios) and it was demonstrated that the surface polarity corresponded to that of water-ethanol solutions containing sufficiently large amounts of the organic solvent (more than 20 vol % ethanol). The interaction of the PDDA monolayer with beta-cyclodextrine (beta-CD) added into the subphase has been examined.