In Russian

Abstract

This paper summarizes the cycle of authors’ studies devoted to polyamic acid-based Langmuir–Blodgett (LBF) functionalized by organic dyes. Previously reported results are supplemented with new experimental data. The monolayers of the polyamic acid under study, poly(4, 4'-diphenyl oxide)-2-carboxyisophthalamide, on water surface, which contains 110 –5 М Pb 2+ , at pH 5.8–6.0 the limiting area per repeating unit is S m = (0.52±0.02) nm 2 . The character of the compressing isotherms depends on the pH of the subphase because of the presence of the carboxylic group in the polymer unit. The monolayers and corresponding LBF obtained by the Schaefer method on solid support were modified via n-octadecyl alcohol, N-n-octadecylpyridinium bromide, and acid-base indicators and luminophores. For this purpose, n-decyl esters of fluorescein and eosin, rose Bengal B, n-heptadecyl ester of rhodamine B, N,N'-di-n-octadecylrhodamine, bromothymol blue, quinaldine red, and duplex quinaldine red are used. As a rule, the LBF contained 30–60 monolayers and 2 to 23 molar percent of dyes. The character of the emission and absorption spectra of rhodamine and hydroxyxanthene compounds in the mixed LBF as well as in multilayers of N-n-octadecylpyridinium bromide and stearic acid indicates the weakening of the dye dimerization and further aggregation of these dyes by the matrix. The procedure of determination of the apparent ionization constants of the dyes, pK a app , consisted in immersing of the LBF during 1–5 min into the aqueous solutions with different pH, air drying during several min, and measuring the absorption spectra. Experiments are performed as a rule at ionic strength of the subphase 0.05 М maintained by NaCl additives and 20 °С. Analysis of thus obtained pK a app (≡ – lgK a app ) values allows concluding that they are in outline close to the corresponding values obtained in micellar solutions of surfactants. These pK a app values may be divided into three groups. In the LBF consisting of entire N-n-octadecylpyridinium bromide, without polyamic acid, the pK a app values are generally close to those obtained for the same indicators in micelles of cationic surfactants and in droplets of cationic N-cetylpyridinium chloride-based microemulsions. An expressed differentiating action of this kind of BLF pseudophase is observed. The value of the electrostatic potential in the indicator locus is estimated as + 107 mV. The second group is represented by the pK a app values of indicators in polyamic acid-based LBF modified either by N-n-octadecylpyridinium bromide or n-octadecyl alcohol in the acidic region of pH. Under these conditions, the COOH groups of the polyamic acid are non-ionized, and the pK a app values are also close to those in cationic surfactant micelles. Finally, the third group of pK a app values corresponds to the pH region of 4–10. Here, the ionization of carboxylic groups of polyamic acid takes place. As a result, the transition range of indicators is anomalously expanded. It is these indicator equilibria that can be used in pH-sensor devices. The obtained results are compared with those reported by other authors. In addition, the peculiarity of dissociation of four-charged and triple-charged cations of the duplex quinaldine red is considered and compared with the dissociation of double-charged quinaldine red.