Adsorption of Inorganic Polyanions along a Langmuir Film:  A Brewster Angle Microscopic Study

Abstract

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 Å) 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 °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.

Characterization limits of a polymer adsorbed under a monolayer by GIXD measurements

“…The sloping plateau of the LE/LC transition in the isotherm may be induced by large kinetic effects related to the strong repulsion forces between polar heads. Other authors have already observed such effect by BAM experiments [20,21]. They reported that a DODA monolayer at low surface pressure was in a homogeneous LE phase.…”

“…The DODA monolayer has already been characterized at the air-water interface by ellipsometry [17], attenuated total reflection [18], reflection spectroscopy [19], and Brewster angle microscopy [20,21], and recently, some grazing incidence Xray diffraction experiments have been reported [22]. Initially, the DODA monolayer is characterized on water in order to establish a reference under the same experimental conditions as those used for the spontaneous polymerization.…”

“…The difference between the two results can be explained by different compression rates and/or time intervals before compression of the monolayer. However, the BAM microscopy did not show a homogeneous phase whatever the surface pressure [20,21], indicating that all molecules are not in an organized state even for high surface pressures.…”

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air–water interface

“…3). The coexistence of LC and LE phases in a DODAB monolayer over a wide range of surface pressure is well known in the literature [16]. The LC microdomains, disk-like or irregularshaped, appear dispersed in a homogeneous and isotropic LE phase.…”

“…The shape and size of these domains as well as their textural features depend on the chemical structure of the amphiphiles and the monolayer state is determined by the surface pressure and temperature. The observation of DODAB monolayers by BAM has shown the bright domains dispersed in a black matrix forming from low surface pressures, as well as a variety of morphologies, induced by adsorption of inorganic polyanions from the subphase [16].…”