Kinetics of a structural phase transition in Langmuir monolayers studied using x‐ray diffraction

Langmuir films of low-dimensional nanomaterials

A large number of low-dimensional nanomaterials having different shapes and being dispersible in solvents open a fundamental question if there is a universal deposition technique for the monolayer formation. A monolayer formation of various nanomaterials at the air-water interface, also known as a Langmuir film, is a well-established technique even for the large group of the recently developed low-dimensional nanomaterials. In this review, we cover the monolayer formation of the zero-dimensional, one-dimensional and two-dimensional nanomaterials. Thanks to the formation of a Langmuir layer at the thermodynamic equilibrium, by using a suitable nanomaterial dispersion and subphase, the monolayers can be formed from all kinds of materials, ranging from the graphene oxide to the semiconducting quantum dots. In this review, we will discuss the basic requirements for the successful formation of monolayers and summarize the recent scientific advances in the field of Langmuir films.

Formation of Two-Dimensional Network of Organic Charge-Transfer Complexes at the Air-Water Interface

The air-water interface is an ideal platform to produce two-dimensional (2D) structures involving anything from simple organic molecules to supramolecular moieties by exploiting hydrophobic-hydrophilic interactions. Here, we show, using grazing incidence X-ray scattering, the formation of a 2D ordered structure of a charge-transfer (C-T) complex, namely, dodecyl methyl viologen (DMV) as acceptor and coronene tetracarboxylate potassium salt (CS) as donor, at the air-water interface. We have observed a phase transition in the 2D ordered structure as the area per molecule is decreased with increasing surface pressure in a Langmuir trough. The high-pressure ordering of the hydrocarbon chains associated with DMV destroys long-range C-T conjugation of DMV and CS at the air-water interface. Our results also explain the formation of DMV-CS cylindrical reverse micelles and eventually long nanowires that get formed in the self-assembly process in the bulk medium to preserve both the C-T conjugation and the organic tail-tail organization.

Evolution of langmuir film of nanoparticles through successive compression cycles

The success of nanoparticle-based devices will need reliable fabrication methods for well-defined, defect-free self-assembled structures of nanoparticles (NPs). NP self-assembly is a process governed by an interplay between many inter-particle interactions, such as core-core van der Waals and dipolar attractions, as well as interactions of the ligand shells, which, when generating an interpenetration, is called interdigitation. The result of this delicate balance between strong interactions is often kinetically hindered; hence, the reaching and recognition of an equilibrium structure becomes challenging. Here, a systematic study is presented that aims at approaching equilibrium 2D NP assemblies (i.e., NP monolayers) based on cyclic compression and relaxation in Langmuir NP monolayers. Cyclic isotherm curves are taken at various surface pressures together with a complete characterization of the corresponding structural evolution of the NP structures. Results are obtained through the analysis of the images, using in-house software that enables the analysis and visualization of the degree of ligand interdigitation as well as the NPs short- and long-range order. It is found that the initial structures obtained through Langmuir assembly can be quite far from equilibrium, mostly due to inhomogeneities in the ligand shell interdigitation and interaction. A few compression cycles render the whole assembly more homogeneous and significantly increases the long-range order. Finally, this structural analysis shows that the ligand shell can act a buffer to the size polydispersity in the particle cores.

Domain Structures and Phase Transitions in Langmuir Monolayers

A variety of recent measurements has shown that the phase diagrams of Langmuir monolayers of relatively simple amphiphiles such as fatty acids and their methyl and ethyl esters are remarkably complex. Nine condensed phases have been identified; their structures can be related to those of known smectic phases. A key distinction between phases is the orientation of the molecular tilt azimuth with respect to the local hexagonal order of the head groups. When monolayers are examined by fluorescence microscopy, regions of uniform tilt can be observed if the exciting radiation is polarized with respect to the surface normal. The tilt regions form patterns similar to those observed in freely suspended films of smectic liquid crystals. The patterns can be changed by compressing the film and by changing the temperature. Transitions between different phases can be observed.

Symmetry, structural phase transitions and phase diagram of Langmuir monolayers

The Landau theory of phase transitions is developed for transitions between condensed phases of langmuir monolayers. The phase diagram is explained by the coupling between the order parameters describing collective tilt of the molecules and ordering of their backbone (zigzag) planes. if the latter ordering is considered as occurring between hexatic phases, then the predictions do not agree with X-ray data on the structure of phases of either fatty acid or long-chain monolayers on the water surface. However this transition can be explained as translational ordering with doubling of the unit cell realized either by herringbone ordering or by alternating orientations of the heads of the molecules in adjacent rows.