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

Abstract

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.

Methylene Blue Adsorption on a DMPA Lipid Langmuir Monolayer

“…[9] Even though electrostatic interactions between charged surfaces and MB molecules have already been reported elsewhere, [10][11][12] atomic information on the system is still incomplete. In this regard, this study will provide valuable atomic-scale information concerning thermodynamic parameters and structural rearrangements involved in the transfer of a positively charged MB molecule from the bulk solution to the interior of the negatively charged lipid monolayer.…”

“…This is a fundamental difference from previous experimental work on mixed MB/DMPA Langmuir monolayers, wherein the MB molecules showed collective behaviour in DMPA domains. [11] Molecular dynamics (MD) simulation offers an atomistic description of the mixed Langmuir monolayer which enriches the previous experimental descriptions of the system. Moreover, the free-energy profile along the interface, which can not be accessed directly by experiment, can be calculated.…”

Control of the Lateral Organization in Langmuir Monolayers via Molecular Aggregation of Dyes

“…7 In this case, the domain morphology does not entirely correspond to the molecular lattice structure. 8 To design well-defined structures in which the lateral organization is controlled, we need somehow to compensate the repulsion energy between dipoles. One method to compensate this repulsion energy is to form monolayers containing different components with attractive interactions between them.…”

“…2 This type of bonding leads to the formation of large domains with long-range orientation of the molecules. Another method to compensate the repulsion energy between dipoles, and used recently by us, 8 is the preparation of mixed monolayers of an anionic phospholipid (DMPA) and a water-soluble cationic dye (methylene blue, MB), which is retained at the interface by electrostatic interactions, establishing lateral connections through self-aggregation of the dye. This paper attempts to show that the method used in the DMPA:MB system can be applied in a general manner to other dyes that show self-aggregation, under the condition of a suitable balance between the sizes of the hydrophobic and polar groups.…”

2D Chiral Structures in Quinoline Mixed Langmuir Monolayers

“…[1][2][3][4][5] Formation of infinite lineal aggregates of the HQ polar headgroups is assumed. Within these aggregates, the relative arrangement of two neighbors HQ groups is calculated from the final result of the molecular mechanics simulation.…”