Chiral interfaces are of capital importance for biorecognition processes and nanotechnology. In this work, a mixed Langmuir monolayer was built using a surface-active dye and a phospholipid. The monolayer displayed optical activity. The driving force for the formation of the supramolecular chirality is the self-assembly of the polar headgroups of the dye. The existence of supramolecular chirality inside nonchirally-shaped domains is shown.

The surface active derivative of the organic dye Acridine Orange (N-10-dodecyl-acridine orange (DAO)) has been included in mixed Langmuir monolayers with stearic acid (SA). The maximum relative content on DAO for a stable mixed monolayer is a molar ratio of X(DAO) = 0.5. Brewster angle microscopy (BAM) reveals a high homogeneity at the micrometer level for the mixed monolayer in equimolar proportion (X(DAO) = 0.5), whereas the appearance of domains occurs for lower content of DAO, i.e., X(DAO) = 0.2 and 0.1. The aggregation of the DAO headgroup leads to well-defined H-aggregates at the air/water interface for those mixed monolayers with a low content of DAO. However, for the mixed monolayers enriched in DAO, e.g., X(DAO) = 0.5, the molecular crowding prevents the formation of defined supramolecular structures. Molecular organization and tilting of the DAO headgroup is quantitatively analyzed by in situ UV-visible reflection spectroscopy. The formation of H-aggregates of the DAO headgroup can be reversibly tuned with the applied surface pressure. A molecular mechanism for the conformational rearrangement of the DAO molecule is proposed using RM1 quantum semiempirical calculations.

The molecular arrangement in the Langmuir monolayers is determined by the interplay of the hydrophilic and hydrophobic interactions between the components. Herein, the competition between the interactions of the components of the organic dye:phospholipid HSP:DMPA mixed monolayer has been studied. The HSP:DMPA monolayer gives rise to chiral domains at the air/solution interface. Brewster angle microscopy (BAM) has been used for the recording of the micrometric structure of the chiral domains. HSP absorb radiation at the wavelength of the laser used in BAM, providing information on the organization of the polar groups. The crystalline structure of the alkyl chains of the HSP:DMPA monolayer has been described by synchrotron-based grazing incidence X-ray diffraction. The interaction between alkyl chains dominates over the aggregation of the polar headgroup in the case of a pure water subphase. By simply adding ions in the subphase, this tendency is reversed, leading to a predominance of the aggregation of the polar headgroup, and a modification of the micrometric domains. This tuning of the interplay between the different molecular regions in a monolayer might be extended to different supramolecular systems, allowing the adjustment of the molecular arrangement.

The formation of well-defined supramolecular structures on the nanoscopic scale is a fundamental step in nanotechnology. The fine control of the layer-by-layer growth of the supramolecular assemblies at interfaces is most desirable. The collapse of a mixed monolayer composed of two surfactants in an equimolar ratio (the organic dye N-10-dodecyl acridine (DAO) and stearic acid (SA)) is analyzed herein. The collapse process of the DAO/SA mixed monolayer has been monitored using surface pressure-molecular area (π-A) and surface potential isotherms, UV-visible reflection spectroscopy, polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), Brewster angle microscopy (BAM), and synchrotron-based in situ X-ray reflectivity (XRR) measurements. The collapse of the DAO/SA mixed monolayer leads to an ordered trilayer. The growth of anisotropic 2D domains of micrometric size is observed during the formation of the trilayer, related to the ordering of the acridine polar headgroups. The trilayer is organized with the first and third monolayers displaying the polar headgroups pointing to the aqueous subphase, whereas the intermediate layer displays the polar headgroups pointing to the air. The trilayer is stabilized by the strong self-aggregation acridine dye group of the DAO molecule. The controlled transition from a monolayer to a trilayer described herein is proposed as a model for further interfacial supramolecular structures of tunable thickness comprising organic dyes.