Photoinduced molecular reorientation dynamics in confined domains of Langmuir monolayers

In a photoresponsive Langmuir monolayer comprised of smectic-C-like domains of mesogenic trans-azobenzene derivative embedded within an isotropic matrix of its cis isomer counterpart, several structurally differing circular droplets were irradiated with linearly polarized light. This report describes the structural rearrangements that occurred in these droplets upon illumination followed by Brewster angle microscopy analysis. Starting from initial well-characterized and symmetric states, final photoaligned situations were reached in which the azimuth angles of the rod-shaped elongated molecules were found to be perpendicular to the electric component of the excitation light. The dynamical aspects of the photoalignments, including their transient patterns, are captured by a theoretical model that couples a relaxational principle incorporating long-range elastic forces with a kinetic formalism presenting an anisotropic rate law.     

Texture changes inside smectic-C droplets in azobenzene Langmuir monolayers

Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.     

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.     

Photoswitchable orientational patterns of confined domains in monolayers

We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.     

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.     

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.     

Flow-induced patterning of Langmuir monolayers

Insoluble monolayers on water have been patterned at the macroscopic scale (i.e., at the centimeter scale of the flow apparatus) as well as the mesoscopic scale (i.e., down to the micron scale resolvable via optical microscopy). The macroscopic patterning at the air/water interface results from a hydrodynamic instability leading to a steadily precessing flow pattern. The velocity field is measured, and the associated shear stress at the interface is shown to be locally amplified by the flow pattern. The resulting hydrodynamic effects on two different monolayer systems are explored: (1) the pattern in a model monolayer consisting of micron-size, surface-bound particles is visualized to show that the particles are concentrated into isolated regions of converging flow with high shear, and (2) Brewster angle microscopy of a Langmuir monolayer (vitamin K1) shows not only that the monolayer is patterned at the macroscopic scale but also that the localized high-shear flow further patterns the monolayer at the mesoscale.     

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.     

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.     

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 .     

Faceted structures in Langmuir monolayers of diethylene glycol mono-n-octadecyl ether at the air--water interface

We have concurrently studied the surface pressure (pi) versus area (A) isotherms and microscopic surface morphological features of Langmuir monolayers of diethylene glycol mono-n-octadecyl ether (C18E2) by film balance and Brewster angle microscopy (BAM) over a wide range of temperature. At temperatures < or =10 degrees C, the monolayers exist in the form of condensed phase even just after the evaporation of the spreading solvent, suggesting that the melting point of the condensed phase is above this temperature. At > or =15 degrees C, the monolayers can exist as gas (G), liquid expanded (LE), and liquid condensed (LC) phases and undergo a pressure-induced first-order phase transition between LE and LC phases showing a sharp cusp point followed by a plateau region in the pi-A isotherms. A variety of 2-D structures, depending on the subphase temperature, are observed by BAM just after the appearance of the cusp point. It is interesting to note here that the domains attain increasingly large and compact shape as the subphase temperature increases and finally give faceted structures with sharp edges and corners at > or =30 degrees C. The BAM observations were coupled with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to gain better understanding regarding the conformational order and subcell packing of the molecules. The constancy of the methylene stretching modes over the studied temperature range suggests that the hydrocarbon chains do not undergo any conformational changes upon compression of the monolayer. However, the full width at half-maximum (fwhm) values of the asymmetric methylene stretching mode (nu(as)(CH(2))) are found to respond differently with changes in temperature. It is concluded that even though the trans/gauche ratio of the hydrocarbon chains remains virtually constant, the LE-LC phase transition upon compression of the monolayer is accompanied by a loss of the rotational freedom of the molecules.     

Dangling OD confined in a Langmuir monolayer

Novel structural features of water confined in lipid Langmuir monolayers are revealed. Using vibrational sum frequency generation spectroscopy at the air/D2O/monolayer interface, dangling OD bonds were investigated. Upon increasing the monolayer surface coverage, the dangling OD stretching mode showed a marked frequency red-shift as well as spectral structure. Furthermore, the dangling OD was found to exist even when a D2O surface was fully covered by the lipid molecules. This phenomenon was observed in monolayers formed with dipalmitoylphosphatidylcholine and with palmitic acid. The frequency red-shift of the dangling OD is interpreted to be due to the perturbation imposed by the lipid hydrophobic tail groups. The observed persistence of the dangling OD at full surface coverage is related to hydrophobicity-induced drying.     

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.     

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.     

The use of size distributions of circular domains in Langmuir monolayers for determining physical parameters of surfactants

An analysis of equilibrium size distributions of circular liquid-condensed (LC) domains in amphiphilic monolayers at the air/water interface, gleaned from microscopic images, is presented in terms of Gaussians fitted to a theoretical expression derived previously. It is demonstrated how, in principle, important properties of the monolayer, such as molecular dipole moments of surfactant head groups and line tension of the liquid-condensed/liquid-expanded phase boundary, can be obtained by combining the results of this simplified analysis with information from X-ray diffraction or surface pressure measurements.     

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.     

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.     

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.