Langmuir monolayers of Co nanoparticles and their patterning by microcontact printing

In this paper, we describe an easy and reliable method for the production of patterned monolayers of Co nanoparticles. A two-dimensional monolayer of Co nanoparticles is fabricated by spreading a nanoparticle solution over an air-water interface and then transferring it to a hydrophobic substrate by using the Langmuir-Blodgett (LB) method. Transmission electron microscopy (TEM) was used to show that, with increasing surface pressure, the Co nanoparticles become well-organized into a Langmuir monolayer with a hexagonal close-packed structure. By controlling the pH of the subphase, it was found that a monolayer of Co nanoparticles with long-range order could be obtained. Further, by transferring the Langmuir monolayer onto a poly(dimethoxysilane) (PDMS) mold, the selective micropatterning of the Co nanoparticles could be achieved on a patterned electronic circuit. The electronic transport properties of the Co nanoparticles showed the ohmic I-V curve.     

Langmuir monolayers of bent-core molecules

A systematic study of five different, symmetric bent-core liquid crystals in Langmuir thin films at the air/water interface is presented. Both the end chains (siloxane vs hydrocarbon) and the core (more or less amphiphilic) are varied, to allow an exploration of different possible layer structures at the interface. The characterization includes systematic surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. The properties of these layers are strongly dependent on the individual type of molecule: the molecules with amphiphilic end chains lie quite flat on the surface, while the molecules with hydrophobic end chains construct multilayer structures. In both cases, the three-dimensional collapse structure is reversible.     

Pressure-area relations for Langmuir monolayers

Pressure-area relations are derived for quasi-static compression of a Langmuir monolayer in the regimes of expanded and partially compressed chains. A monolayer is treated as an ensemble of flexible chains anchored at a flat water-air interface. Formation of surface pressure is attributed to excluded-volume interactions between segments of hydrophobic tail-groups. Close-form expressions are derived for the energy of interactions. Based on this relations, strength of excluded-volume interactions is evaluated by fitting pressure-area isotherms on stearic, behenic and pentacosadiynoic acids. It is found that the dimensionless strength of excluded-volume interactions is weakly affected by chain length and its value is close to .     

Phase transition in Langmuir monolayers

The main features of several theoretical models which describe the main phase transition and correspondingly the non-horizontal shape of the Π-A isotherms of the Langmuir monolayers, are discussed. New equations of state are based on the generalised Volmer's equation and consider the coexistence monomers and large clusters as bimodal distribution. A further generalisation for the case of quasi-bimodal distribution allows the consideration of monomers with small aggregates on one side of the spectrum and large clusters on the other side. The new theoretical model is corroborated by the Π-A isotherms of various amphiphilic monolayers the condensed phases of which have various gradual differences in the crystalinity and packing density data. The large variety of shapes of the Π-A isotherms in the region A<A_c can be determined by a single general equation whose parameters, except A_c, correspond to the isotropic fluid-like monolayer region. The application of the generalised equation of state on the experimental Π-A isotherms indicates the formation of small aggregates in the region A>A_c.     

Langmuir monolayers from perfluorobutyl-n-eicosane

Perfluorobutyl-n-eicosane (abbreviated as F4H20) was spread at the air/water as Langmuir monolayers and studied under different experimental conditions, such as spreading volume, subphase temperature and compression speed. The Langmuir monolayer experiments (π-A isotherms) have been complemented with Brewster angle microscopy results, which enabled direct visualization of the monolayers’ structure and estimation of the film thickness at different stages of compression. It has been found that the molecules are oriented almost vertically (with respect to the interface) in the vicinity of film collapse. The negative sign of the measured surface potential, ΔV, is evidence for the orientation of F4H20 molecules with their perfluorinated parts exposed towards the air. In the case of F4H20 a limited fluorination relative to perfluoroeicosane also results in monolayer formation, in contrast to eicosane itself, which forms lenses.     

Langmuir monolayers as unique physical models

Physico-chemical processes at air/liquid interfaces are of paramount importance in nature. The Langmuir technique offers the possibility of forming a well-defined monolayer of amphiphilic molecules under study at the air/liquid interface, with a unique control of the area per molecule and other experimental conditions. Despite being a traditional technique in Colloid and Interface science, there is an ever growing interest in Langmuir studies. Herein, recent developing fields of research currently taking advantage of the Langmuir technique are reviewed, comprising the interfacial structure of: water, biomolecules and inorganic/organic hybrids. The good state of the Langmuir technique at present and the foreseeable increase of its usage are discussed.     

Modern physicochemical research on Langmuir monolayers

Recent developments in characterising Langmuir monolayers of a variety of film-forming materials and employing several physicochemical techniques are reviewed. The extension of the LB method to non-amphiphilic substances, especially macromolecular systems, has increased the need of a thorough understanding of Langmuir film properties, which requires characterising techniques that provide complementary information. Since there is vast literature in the subject, only selected examples are given of results that illustrate the potential of the techniques discussed.     

Iso-branched semifluorinated alkanes in Langmuir monolayers

A series of semifluorinated n-alkanes (SFAs), of the general formula: (CF3)2CF(CF2)6(CH2)nH (in short iF9Hn), n = 11-20 have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were performed in addition to Brewster angle microscopy results, which enabled both direct visualization of the monolayers structure and estimation of the monolayer thickness at different stages of compression. Our paper was aimed at investigating the influence of the iso-branching of the perfluorinated fragment of the SFA molecule on the surface behavior of these molecules at the air/water interface. It occurred that iF9 SFAs with the number of carbon atoms in the hydrogenated moiety from 11 to 20 are capable of Langmuir monolayer formation. Monolayers from iF9H11 to iF9H13 are instable, whereas those formed by iF9 SFAs with longer hydrogenated chains form stable films at the free surface of water. As compared to SFAs containing perfluorinated chain in a normal arrangement, iso-branched molecules have a greater tendency to aggregate. Lower stability of monolayers formed by iF9 SFAs as compared to F10 SFAs originated from the surface nucleation observed in BAM images, even at the very initial stages of compression. The dipole moment vector for iso-branched SFAs was found to be virtually aligned with the main axis of the molecule, contrary to F10 SFAs, where the dipole moment vector was calculated to be tilted with respect to the main molecular axis. Quantitative Brewster angle microscopy measurements (relative reflectivity experiments) enabled us to monitor the changes of monolayer thickness at different stages of monolayer compression.     

Flow-controlled phase boundaries in Langmuir monolayers

Flow-controlled fingering of the liquid expanded/liquid condensed phase boundary in a 2-d insoluble monolayer is investigated using a laser-induced thermocapillary pump. Spatially periodic perturbations of the initially smooth monolayer phase boundary between a liquid expanded and liquid condensed phase are shown to lead to the development of steady profile of one-dimensional fingers. The steady-state modulation wave vector and the transient growth rate increase with the flow velocity that drives the instability following power scaling laws consistent with a theory of Bruinsma, Rondelez, and Levine (Bruinsma, R.; Rondelez, F.; Levine, A. Eur. Phys. J. E. 2001, 6, 191) on flow rather than diffusion dominated instabilities in monolayers.     

Interfacial polymerisation of anilinium at Langmuir monolayers

Anilinium is strongly adsorbed at monolayers of the phospholipid L-alpha-dimyristoylphosphatidic acid (DMPA) and hexadecanesulfonic acid (HDSA) at the air-water interface, and undergoes chemical polymerisation under conditions where bulk polymerisation does not occur.     

Langmuir monolayers of a photoisomerizable macrocycle surfactant

An amphiphilic photoisomerizable macrocycle has been prepared that forms stable Langmuir monolayers at the air-water interface. The hydrophilic core of the molecule switches between closed and open isomers upon irradiation by the appropriate wavelengths of light. Isotherm measurements, Brewster angle microscope images, and atomic force micrographs (of transferred Langmuir-Blodgett films) suggest a phase transition between a face-on to a tilted edge-on molecular orientation as a function of surface concentration. In the face-on phase, in situ photoisomerization results in a reversible increase in surface pressure due to greater molecular crowding in the open configuration.     

Chiral recognition of dipeptides in Langmuir monolayers

The recognition of the enantiomeric couples of ditryptophan in Langmuir films of N-hexadecanoyl-l-proline was investigated by surface pressure–area (π–A) isotherm measurements and Brewster angle microscopy experiments. The π–A isotherms relative to the films including the enantiomeric dipeptides show small differences whereas an evident enantiodiscrimination is observed by Brewster angle microscopy images.     

Site-selective metal-coordination-based patterning of silane monolayers

Herein, we report a method to pattern a derivatized nifedipine silane monolayer that aromatizes under UV irradiation. Using a functionalized SCS-Pd(II) pincer complex, we demonstrate that a strong metal-coordination complex between the aromatized nifedipine derivative and the pincer complex is formed. This methodology integrates SAM photolithography (top-down) and molecular recognition directed self-assembly (bottom-up) strategies to create simple and rapid synthesizable functional patterned surfaces.     

Thermodynamics of the liquid states of Langmuir monolayers

The liquid states and the liquid-liquid equilibrium of surfactant molecules forming an interphase between air and water have been considered using Monte Carlo computer simulations. Specifically, the expanded and compressed liquid phases observed for surfactant molecules were characterized as a function of pressure and temperature. Simple modified beadlike potentials were implemented in order to describe the interparticle forces between the hydrophobic and hydrophilic portions of surfactant molecules at the air/water interface. A simulation box was defined such that the monolayer was exposed to an externally applied lateral pressure in a modified isothermal-isobaric ensemble, whereas the water bath was modeled in a canonical ensemble. The simulation resembles the experimental setup used to measure lateral pressure (Pi) versus area isotherms obtained with Langmuir troughs. The applied lateral pressure-surface area phase diagram clearly showed the coexistence of the expanded and compressed liquid phases within certain temperature and pressure ranges. Distribution functions of distances and enthalpies for the monolayer were computed to clearly identify each liquid phase and the coexistence region.     

Langmuir monolayers of fractions of cork suberin extract

The wide variability in composition and molecular weight of natural polymers has hampered understanding of their physicochemical properties and ultimately their use in noble applications, especially in the cases where surface properties need to be probed at the molecular level. A useful approach to analyse data from surface monolayers of complex mixtures is to try distinguishing the effects from the distinct fractions in such mixtures. The cork suberin extract investigated here is known to contain aliphatic monomers with terminal carboxylic acid and methyl ester groups as well as long esterified aliphatic chains dispersed in a polymeric aliphatic matrix. The role of such terminal groups was studied and the results showed that depending on the nature of the terminal groups the monolayers present distinct isotherms due to the different interactions with the water subphase. Fractionation strategies based on different solubilities of the cork suberin components in chloroform were also employed to probe their effect on the monolayer characteristics. From the two sets of experiments it is clear that the presence of monomers with terminal carboxylic acids in the suberin extract affects considerably the monolayer-forming ability. This approach may be used as a complementary, relatively simple route to assess suberin genetic engineering strategies towards resistance to environmental stress.     

Pattern formation and morphology evolution in Langmuir monolayers

We present a study of how patterns formed by Langmuir monolayer domains of a stable phase, usually solid or liquid condensed, propagate into a metastable one, usually liquid expanded. During this propagation, the interface between the two phases moves as the metastable phase is transformed into the more stable one. The interface becomes unstable and forms patterns as a result of the competition between a chemical potential gradient that destabilizes the interface on one hand and line tension that stabilizes the interface on the other. During domain growth, we found a morphology transition from tip splitting to side branching; doublons were also found. These morphological features were observed with Brewster angle microscopy in three different monolayers at the water/air interface: dioctadecylamine, ethyl palmitate, and ethyl stearate. In addition, we observed the onset of the instability in round domains when an abrupt lateral pressure jump is made on the monolayer. Frequency histograms of unstable wavelengths are consistent with the linear-instability dispersion relation of classical free-boundary models. For the case of dendritic morphologies, we measured the radius of the dendrite tip as a function of the dendrite length as well as the spacing of the side branches along a dendrite. Finally, a possible explanation of why Langmuir monolayers present this kind of nonequilibrium growth patterns is presented. In the steady state, the growth behavior is determined by Laplace's equation in the particle density with specific boundary conditions. These equations are equivalent to those used in the theory of morphology diagrams for two-dimensional diffusional growth, where morphological transitions of the kind observed here have been predicted.     

Orientational changes in dipalmitoyl phosphatidyl glycerol Langmuir monolayers

Dipalmitoyl phosphatidyl glycerol (DPPG) as Langmuir monolayers at the air/water interface was investigated by means of surface pressure measurements in addition to Brewster angle microscopy (BAM) during film compression/expansion. A characteristic phase transition region appeared in the course of surface pressure-area (pi-A) isotherms for monolayers spread on alkaline water or buffer subphase, while on neutral or acidic water the plateau region was absent. This phase transition region was attributed to the ionization of DPPG monolayer. It has been postulated that the ionization of the phosphatidyl glycerol group leads to its increased solvation, which probably provokes both a change in the orientation of the polar group and its deeper penetration into bulk phase. Film compression along the transition region provokes the dehydration of polar groups and subsequent change of their conformation, thus causing the DPPG molecules to emerge up to the interface. Quantitative Brewster angle microscopy (BAM) measurements revealed that along the liquid-expanded to liquid-condensed phase transition the thickness of the ionized DPPG monolayer increases by 4.2 A as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface.     

Phase equilibria in model surfactants forming Langmuir monolayers

The study of Langmuir monolayers has generated the attention of researchers because of their unique properties and their not well understood phase equilibrium. These monolayers exhibit interesting phase diagrams where the unusual liquid-liquid equilibrium can be observed for a single component monolayer. Monte Carlo computer simulations in the virtual Gibbs ensemble were used to obtain the phase diagram of Langmuir monolayers. The liquid-vapor and liquid-liquid phase equilibria were considered by constructing the Cailletet-Mathias phase diagrams. By using the Ising model and the rectilinear approximations the identification of the critical properties for both equilibria was determined. These critical parameters were calculated as a function of the strength of the interaction between the surfactant molecules and the aqueous subphase. As a result, we have identified the coexistence between a liquid expanded state (LES)-vapor and the liquid condensed state-LES, in agreement with experimental and theoretical evidence in the literature. We obtained a clear separation of phases and a strong dependence on the strength of the solvent used. Namely, as the interaction between the solvent and the head of the surfactant increases, the critical properties also increase. Equilibrium states were characterized by computing thermodynamic quantities as a function of temperature and solvent strength.     

Gold nanoparticle self-assembly in two-component lipid Langmuir monolayers

Self-assembly processes are considered to be fundamental factors in supramolecular chemistry. Langmuir monolayers of surfactants or lipids have been shown to constitute effective 2D "templates" for self-assembled nanoparticles and colloids. Here we show that alkyl-coated gold nanoparticles (Au NPs) adopt distinct configurations when incorporated within Langmuir monolayers comprising two lipid components at different mole ratios. Thermodynamic and microscopy analyses reveal that the organization of the Au NP aggregates is governed by both lipid components. In particular, we show that the configurations of the NP assemblies were significantly affected by the extent of molecular interactions between the two lipid components within the monolayer and the monolayer phases formed by each individual lipid. This study demonstrates that multicomponent Langmuir monolayers significantly modulate the self-assembly properties of embedded Au NPs and that parameters such as the monolayer composition, surface pressure, and temperature significantly affect the 2D nanoparticle organization.     

Miscibility and segregated structures in mixed Langmuir monolayers

The phase behavior and morphology of segregated structures are considered for mixed Langmuir monolayers, which comprise a type of supramolecular polymer having a complex internal structure mixed with a long chain fatty acid. We fabricated two different series of mixed monolayers from a polyglutamate (PG) copolymer having 30% octadecyl ester side chains and 70% methyl ester side chains and fatty acids. These mixed monolayers deposited on a solid substrate were studied by pressure-area diagram measurements, X-ray analysis, and atomic force microscopy. Stearic acid (STA) and hexacosanoic acid (HCA) with alkyl chain lengths of 17 and 25 carbon atoms, respectively, were used as low molecular weight components. For the mixture PG:STA, where the length of the STA molecules is comparable to the length of the PG side chains, we observed the formation of partially miscible monolayers. These mixtures exhibit a nanometer scale domain morphology formed by the STA molecules dissolved in the outer shell of the PG monolayer. In contrast, for the PG:HCA mixture we observed a strong tendency for microphase separation and the formation of well-defined submicron segregated structures in the monolayers. Lateral compression of the mixed monolayers to a point close to the collapse pressure promotes microphase separation in both types of mixed monolayers with the formation of anisotropic surface morphology and oriented domains.