Langmuir Monolayers of Polyion Complexes of Soluble Quaternary Ammonium-Functionalized Azobenzene Chromophores

Polyion complex formation at the air-water interface between carboxymethylcellulose and a series of triethylammonium-functionalized amphiphiles containing the 4-nitro-4´-alkoxy azobenzene chromophore has been investigated. Surface pressure-area isotherms are found to exhibit a systematic dependence on subphase concentration up to a certain critical concentration, which depends on the chromophore spacer length. However,UV-visible, polarized FT-IR, and X-ray reflectivity measurements indicate no significant differences in molecular density, molecular orientation, or film thickness for Langmuir-Blodgett (LB) films transferred at a given surface pressure from different polyion subphase concentrations. LB films prepared at different polyion concentrations thus appear to have essentially identical structures, despite the differences in the recorded surface pressure-area isotherms. Isotherm dependence on subphase concentration is therefore attributed to partial loss of the soluble amphiphile into the water subphase at lower polymer concentrations. These results imply that isotherms obtained below the critical subphase concentration give false (or apparent) molecular areas, which may lead to erroneous conclusions.