Effect of metal ions on monolayer collapses

Nanopattern formation in self-assembled monolayers of thiol-capped Au nanocrystals

The structure and the stability of the transferred monolayers of gold-thiol nanoparticles, formed at air-water interface at different surface pressure, on to silicon surface have been studied using two complementary techniques, x-ray reflectivity and atomic force microscopy (AFM). Networklike nanopatterns, observed through AFM, of the in-plane aggregated nanoparticles can be attributed to the late stage drying of the liquid trapped in the islands formed by nanoparticles. During drying process the trapped liquid leaves pinholes in the islands which by the process of nucleation and growth carry the mobile nanoparticles on their advancing fronts such that the nanoparticles are trapped at the boundaries of similar adjacent holes. This process continues bringing about in-plane as well as out-of-plane restructuring in the monolayer until the liquid evaporates completely rendering a patterned structure to the islands and instability in the monolayer is then stabilized.

Chemistry at air/water interface versus reaction in a flask: tuning molecular conformation in thin films

Atomic force microscopy and X-ray reflectivity studies of cobalt stearate Langmuir-Blodgett (LB) films (CoStp) deposited from a preformed bulk sample on quartz substrates showed formation of a Volmer-Weber type monolayer but no multilayers as compared to the excellent multilayers of cobalt stearate films (CoStn) deposited at the air/water interface by the usual LB technique, in spite of both showing bidentate bridging type coordination of cobalt ions with the carboxylate group. The difference is attributed to existence of different headgroup conformers, observed from Fourier transform infrared (FTIR) studies. The CoStp films had a higher energy 'boat' conformation with linear O-Co-O linkage, whereas CoStn formed a low energy conformer with a bent O-Co-O configuration (bond angle of 105 degrees). Present results support the necessity of bidentate bridging coordination in multilayer deposition, but rejects its sufficiency by bringing out the crucial role played by air/water interface. Differences in surface pressure-molecular area isotherms and hydrocarbon tail-tail interactions (evident from FTIR spectra) of the films support the above statement. Methyl-methyl interactions observed in CoStn samples suggest hierarchy of supramolecular chemistry at the air/water interface in tuning the C-O-Co bond angle essential to satisfy the wetting condition with the substrate and subsequently form LB multilayers.

Relaxation of bimolecular layer films on water surfaces

Ferric stearate, a three-tailed amphiphile, forms bimolecular layers on water surfaces. Molecules in the lower layer are in an "asymmetric" configuration, Fe-containing heads touching water and three hydrocarbon tails in air, while molecules in the upper layer are in a "symmetric" configuration, in pairs of "Y and inverted Y" disposition of tails about the Fe-bearing head. Pressure relaxation at constant area (pi- t curves) and area relaxation at constant pressure ( A- t curves) of this bimolecular layer can be modeled as a sum of three exponential decay terms with distinct time constants and weight factors. Relating the long-term decay with desorption of the total film thus indicates a remarkable long-term stability of the bimolecular layer film. An X-ray reflectivity study of the bimolecular films deposited horizontally on Si(001) at various conditions of relaxation shows no further growth along the vertical of any other layer. Under pressure relaxation molecules are transferred from the upper layer to the lower layer with a change from symmetric to asymmetric configuration, while under area relaxation the transfer is from the lower layer to the upper layer with a configurational change from symmetric to asymmetric.

Irreversible Collapse of Poly(vinyl stearate) Monolayers at the Air-Water Interface

The collapse of Langmuir monolayers of poly(vinyl stearate) (PVS) at the air-water interface has been investigated by combined measurements of the surface pressure-area isotherms and Brewster angle microscopy (BAM). Atomic force microscopy (AFM) has been used to gain out-of-plane structural information on collapsed films transferred onto a solid substrate by a modified version of the inverse Langmuir-Schaefer deposition method. At high areas per monomer repeat unit, BAM imaging revealed that the films are heterogeneous, with large solidlike domains (25-200 mum in diameter) coexisting with liquidlike domains. Upon film compression, the domains coalesced to form a homogeneous monolayer before the film collapsed at constant pressure, forming irreversible three-dimensional (3D) structures. BAM images showed that two 3D structures coexisted: buckles of varying width extending across the surface and perpendicular to the direction of the compression and dotted islandlike structures. Upon expansion, the film fractured and both 3D protrusions persisted, explaining the marked hysteresis recorded in the Langmuir isotherms. Experiments with AFM confirmed the 3D nature of both protrusions and revealed that many buckles contain substructures corresponding to narrow buckles whose heights are a multiple of a single bilayer. Additionally, many multilayer islands with diameters spanning from 0.2 mum to over 3.5 mum were characterized by varying heights between 2 nm and up to over 50 nm. The key to the formation of the irreversible 3D structures is the presence of large inhomogeneities in the PVS monolayer, and a generalized phenomenological model is proposed to explain the collapse observed.

Ordering by collapse: formation of bilayer and trilayer crystals by folding Langmuir monolayers

Neutron and synchrotron X-ray studies of arachidic-acid monolayers compressed to the collapse region, beyond their densely packed molecular area, reveal that the resulting structures exhibit a surprising degree of reproducibility and of order. The structure of the collapsed monolayers differs for films that are spread on pure water or on CaCl2 solutions. On pure water, the collapsed monolayer forms a stable crystalline trilayer structure, with acyl-chain in-plane packing practically identical to the three-dimensional (3D) crystal structure of fatty acids. For monolayers spread on Ca2+ solutions, the collapsed film consists of a bi- and trilayer mixture with a ratio that changes by the collapse protocol. Our analysis suggests that the bilayer structure is inverted, i.e., with the hydrophobic tails in contact with the water surface and the calcium ions bridging the polar heads. The inverted bilayer structure possesses a well-ordered crystalline slab of calcium oxalate monohydrate intercalated between two acyl chains. We provide theoretical arguments rationalizing that the observed structures have lower free energies compared with other possible structures and contend that the collapsed structures may, under certain circumstances, form spontaneously.

Atomistic simulations of langmuir monolayer collapse

Monolayers at the vapor/water interface collapse by exploring the third dimension at sufficient lateral compression, either by forming three-dimensional structures or by solubilization into the aqueous solution. In this paper, we provide an atomistic description of collapse from molecular dynamics (MD) simulations. More specifically, we investigate monolayers of arachidic acids spread on pure water and in an aqueous solution with Ca2+ ions in the subphase. In both cases, it is found that the collapsed systems generally lead to the formation of multilayer structures, which in the system with Ca2+ ions, proceeds by an intermediate regime where the monolayer exhibits significant roughness (of the order of 4 A). If no roughness is present, the system forms collapsed structures into the aqueous solution. The computational cost of atomic MD limits our simulations to relatively small system sizes, fast compression rates, and temporal scales on the order of a nanosecond. We discuss the issues caused by these limitations and present a detailed discussion of how the collapse regime proceeds at long time scales. We conclude with a summary of the implications of our results for further theoretical and experimental studies.

Infrared imaging of a solid phase surfactant monolayer

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.

Self-assembled molecular patterns of fatty acid on graphite in the presence of metal ions

The stripe phase formed by long-chain alkane derivatives on the graphite lattice provides a unique opportunity for the study of molecular adsorption, aggregation, and reaction on patterns. Fatty acids, such as arachidic acid (AA), self-assemble on graphite into a sheet of parallel stripes with a periodicity of twice its molecular chain length. The molecular pattern is thus defined precisely by the size and functionality of the headgroup and tailgroup of the amphiphile. Complexation of metal ions to AA fixes the number and location of the ion, which can serve as a precursor to semiconductor nanocrystal arrays. In order to understand the effect of the ion complexation, we carry out atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) investigations of AA self-assembly in the presence of various metal ions. While the stripe orientation is dictated by the graphite lattice and the stripe periodicity is determined by the AA chain length, the size, shape, and degree of order of the stripe crystalline domain are influenced by the metal ion bond strength to the carboxylic ligand. The change of morphology in the self-assembled pattern shows a trend along the Irving-Williams series.

Growth of a collapsing Langmuir monolayer

Langmuir monolayers of stearic acid on Co ions in the aqueous subphase have been deposited at different stages of constant pressure collapse, on hydrophilic Si(001) using a modified version of the inverse Langmuir-Schaefer method of horizontal deposition. The electron density profiles (EDPs) along the depth of the deposited films, extracted from the x-ray reflectivity data, show that a monolayer to bi-molecular layer transformation takes place after collapse. The molecules in the lower monolayer have asymmetric configurations with head groups touching water and tails in air, whereas molecules in the upper layer are in symmetric configurations with tails on both sides of the heads. Atomic force microscopy images of the deposited films after collapse, however, show nearly circular islands of height more than that of the bimolecular layer observed in the EDP. As pressure increases, ridges are seen to coexist with these islands. Although the coverage of such islands and ridges is low, they play an important role in determining the growth mode. The growth of the wetting and island layers, taken together, has a striking similarity with the Stranski-Krastanow mode, observed usually for heteroepitaxial growth.