Effect of the Molecular Methylene Blue Aggregation on the Mesoscopic Domain Morphology in Mixed Monolayers with Dimyristoyl−Phosphatidic Acid

• Published in: Journal of physical chemistry. C Vol. 113; no. 14; pp. 5711 - 5720
• Main Authors: Giner-Casares, Juan J., de Miguel, Gustavo, Pérez-Morales, Marta, Martín-Romero, María T., Camacho, Luis, Muñoz, Eulogia
• Format: Journal Article
• Language: English
• Published: American Chemical Society 09.04.2009
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp810935x

The molecular organization and the domain morphology of an anionic matrix dimyristoyl−phosphatidic acid (DMPA) as influenced by the presence of a cationic water-soluble methylene blue (MB) have been studied by using Brewster angle microscopy and reflection spectroscopy at the air−water interface. Mixed monolayers of DMPA:MB, molar ratios 5:1 and 1:1, were formed by the cospreading method. BAM images show particular domain morphologies as a function of the MB density in the mixed film: circular domains for DMPA:MB = 5:1, and domains with hexagonal shape for DMPA:MB = 1:1. The reflection measurements during the compression process of the mixed films reveal a blue shift of the visible band with respect to that of the MB monomer. This blue shift is larger for the DMPA:MB = 5:1 in comparison with that for the 1:1 monolayer, despite lower MB density. By using the extended dipole model, we demonstrate that such blue shift is due to the formation of a 2D hexagonal network of MB molecules underneath the DMPA monolayer in the mixed 1:1 film, while it is due to infinite linear aggregates in the 5:1 monolayer. BAM domain textures have been simulated by using the Fresnel equations for biaxial anisotropic materials. The results fit correctly the predictions of the extended dipole model.