Adsorption of inorganic polyanions along a Langmuir film: A Brewster angle microscopic study

Interactions of large inorganic polyanions along a positively charged Langmuir film of dimethyldioctadecylammonium bromide were studied by Brewster angle microscopy (BAM). Because of their large size (>10 Angstrom) and high refractive index, their adsorption along the organic monolayer induces an increase of the interface reflectivity and can be directly monitored by BAM. Furthermore, they strongly modify the morphology of the film and a coexistence of three phases (gaseous, liquid expanded, and liquid condensed) is easily obtained at a temperature close to 20 degrees C. Around this "triple point", nucleation of liquid-condensed domains is observed along the gaseous/liquid-expanded interface, inducing unusual structures such as strings of liquid-expanded domains. Those experiments demonstrate that inorganic/organic Langmuir films based on the semiamphiphilic ionic association could present a variety of morphologies as rich as those of classical amphiphilic molecules at the gas/water interface.