Determination of the Thickness and Optical Properties of a Langmuir Film from the Domain Morphology by Brewster Angle Microscopy

A method employing Brewster angle microscopy was developed to measure the relative thickness of monolayers and multilayers at the air−water interface. Results proved that multilayers in ultrathin films of 8CB (4‘-n-alkyl[1,1‘-biphenyl]-4-carbonitrile; nCB, n = 8) are built up from one or two stacked bilayers on top of a monolayer at the air−water interface covered by a continuous bilayer. In addition, the fan texture of the mixed splay-bend disclination defects, which are formed in 10CB monolayers on reverse collapse of a continuous trilayer, was used to obtain estimates for the components of the anisotropic dielectric constant of the monomolecular film. This estimate was made by comparing experimental images with computationally generated model images based on an assumed director distribution around the defect. The results support the validity of the folding mechanism of multilayer generation and reverse collapse as opposed to the condensation mechanism. Finally, comparison of the experimental and theoretical values of the ratio of the multilayer thickness and the monolayer thickness indicates that at this level the water−film interface cannot be assumed to be sharp or well defined.