Self-organization of a wedge-shaped surfactant in monolayers and multilayers

The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at approximately 10 degrees C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are approximately 0.5 nm thinner than the LC regions. AFM images were also obtained of self-assembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed self-assembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles.     

Cationic Gemini surfactant at the air/water interface

The surface properties and structures of a cationic Gemini surfactant with a rigid spacer, p-xylyl-bis(dimethyloctadecylammonium bromide) ([C(18)H(37)(CH(3))(2)N(+)CH(2)C(6)H(4)CH(2)N(+)(CH(3))(2)C(18)H(37)],2Br(-), abbreviated as 18-Ar-18,2Br(-1)), at the air/water interface were investigated. It is found that the surface pressure-molecular area isotherms observed at different temperatures do not exhibit a plateau region but display an unusual "kink" before collapse. The range of the corresponding minimum compressibility and maximum compressibility modulus indicates that the monolayer is in the liquid-expanded state. The monolayers were transferred onto mica and quartz plates by the Langmuir-Blodgett (LB) technique. The structures of monolayers at various surface pressures were studied by atomic force microscopy (AFM) and UV-vis spectroscopy, respectively. AFM measurements show that at lower surface pressures, unlike the structures of complex or hybrid films formed by Gemini amphiphiles with DNA, dye, or inorganic materials or the Langmuir film formed by the nonionic Gemini surfactant, in this case network-like labyrinthine interconnected ridges are formed. The formation of the structures can be interpreted in terms of the spinodal decomposition mechanism. With the increase of the surface pressure up to 35 mN/m, surface micelles dispersed in the network-like ridges gradually appear which might be caused by both the spinodal decomposition and dewetting. The UV-vis adsorption shows that over the whole range of surface pressures, the molecules form a J-aggregate in LB films, which implies that the spacers construct a pi-pi aromatic stacking. This pi-pi interaction between spacers and the van der Waals interaction between hydrophobic chains lead to the formation of both networks and micelles. The labyrinthine interconnected ridges are formed first because of the rapid evaporation of solvent during the spreading processes; with increasing surface pressure, some of the alkyl chains reorient from tilting to vertical, forming surface micelles dispersed in the network-like ridges due to the strong interaction among film molecules.     

Self-assembled spiral nanoarchitecture and supramolecular chirality in Langmuir-Blodgett films of an achiral amphiphilic barbituric acid

An amphiphilic barbituric acid derivative was found to form stable monolayers showing a clear phase transition at the air/water interface. It is interesting to find that the deposited Langmuir-Blodgett (LB) films of the compound showed circular dichroism (CD) although the molecule itself was achiral. AFM measurements on the transferred one-layer LB film revealed that spiral nanoarchitectures were formed. It was further found that the supramolecular chirality of the LB films was related to symmetry breaking at the interface. Hydrogen bonding and the pi-pi stacking between the neighboring molecules resulted in chiral fibers which formed the spiral structures. To the best of our knowledge, this is the first report on the chirality of the molecular assemblies and spiral nanostructures formed through the air/water interface by achiral molecules.     

Pure and mixed films of a nitrostilbene derivative at the air-water interface, Langmuir-Blodgett multilayer fabrication, and optical characterization

This paper reports the preparation and characterization of pure Langmuir and Langmuir-Blodgett (LB) films of a stilbene derivative containing two alkyl chains, namely 4-dioctadecylamino-4'-nitrostilbene. Mixed films incorporating docosanoic acid and the stilbene derivative are also studied. Brewster angle microscopy (BAM) analysis has revealed the existence of randomly oriented three-dimensional (3D) aggregates, spontaneously formed immediately after the spreading process of the stilbene derivative onto the water surface. These 3D aggregates coexist with a Langmuir film that shows the typical gas, liquid, and solid-like phases in the surface pressure and surface potential vs area per molecule isotherms, indicative of an average preferential orientation of the stilbene compound at the air-water interface, and a gradual molecular arrangement into a defined structure upon compression. A blue shift of 55 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a chloroform solution of the nitrostilbene indicates that two-dimensional (2D) H-aggregates are formed at the air-water interface. The monolayers are transferred undisturbed onto solid substrates with atomic force microscopy (AFM) revealing that the one layer LB films are constituted by a monolayer of the stilbene derivative together with some 3D aggregates. When the nitrostilbene compound is blended with docosanoic acid, the 3D aggregation is avoided in the Langmuir and Langmuir-Blodgett films, but does not limit the formation of 2D H-aggregates, desirable for second-order nonlinear optical response in the blue domain. The AFM images of the mixed LB films show that they are formed by a docosanoic acid monolayer and, on the top of it, a bilayer of the stilbene derivative.     

Mixed monolayers of Gemini surfactants and stearic acid at the air/water interface

The properties of mixed monolayers composed of the cationic Gemini surfactant ([C(18)H(37)(CH(3))(2)N(+)(CH(2))(3)N(+)(CH(3))(2)C(18)H(37)],2Br(+), abbreviated as 18-3-18,2Br(-1)) and stearic acid (SA) at the air/water interface were investigated by using a Langmuir film balance. The excess areas at the different mixed monolayer compositions were obtained and used to evaluate the miscibility and nonideality of mixing. Due to the electrostatic attractive interactions between 18-3-18,2Br(-1) and SA, the excess areas indicated negative deviations from ideal mixing. Moreover, 18-3-18,2Br(-1) and SA were miscible at the air/water interface, as was confirmed by atomic force microscopy (AFM) images of the LB films transferred onto mica substrates. The attenuated total reflectance (ATR) infrared spectra showed that SA in the mixed monolayers was ionized completely at a composition X(SA)=0.67 and formed a "cationic-anionic surfactant," i.e., the carboxylate, with 18-3-18,2Br(-1) owing to the electrostatic interaction between the head groups.     

Immbolization of uricase enzyme in Langmuir and Langmuir-Blodgett films of fatty acids: possible use as a uric acid sensor

Preserving the enzyme structure in solid films is key for producing various bioelectronic devices, including biosensors, which has normally been performed with nanostructured films that allow for control of molecular architectures. In this paper, we investigate the adsorption of uricase onto Langmuir monolayers of stearic acid (SA), and their transfer to solid supports as Langmuir-Blodgett (LB) films. Structuring of the enzyme in β-sheets was preserved in the form of 1-layer LB film, which was corroborated with a higher catalytic activity than for other uricase-containing LB film architectures where the β-sheets structuring was not preserved. The optimized architecture was also used to detect uric acid within a range covering typical concentrations in the human blood. The approach presented here not only allows for an optimized catalytic activity toward uric acid but also permits one to explain why some film architectures exhibit a superior performance.     

Langmuir monolayers based on rigid wedge-shaped dendrons of benzenesulfonic acid

The structure formation of wedge-shaped monodendrons based on symmetric benzenesulfonic acid with different lengths of peripheral alkyl chains was studied in Langmuir monolayers and Langmuir–Blodgett (LB) films. A phase transition from the liquid-expanded state to the liquid-condensed state was observed on compression of the Langmuir monolayers of the dendrons containing dodecyl lateral chains. The transition is accompanied by the formation of star-shaped aggregates visualized by Brewster angle microscopy. The three-layer LB transfer results in the reorganization of the monolayer into regions of bi-, tetra-, and hexalayers on a solid substrate with a low coverage of the surface. Homogeneous liquid-condensed mono layers are formed for the dendrons with hexa- and octadecyl chains, and the film thickness achieved by the LB transfer corresponds to the monolayer alignment of the molecules with the surface coverage up to 90%. It was determined that varying the alkyl length of wedge-shaped dendrones based on symmetric benzenesulfonic acid leads to a change in phase behavior of Langmuir monolayers as well as Langmuir–Blodgett films formed by them.     

Development of fluorescent film sensors for the detection of divalent copper

Monolayers of several peptide lipids at air-water and air-solid interfaces were prepared using Langmuir and Langmuir-Blodgett (LB) film techniques, and tested as fluorescent sensors for copper ions in aqueous phase. In one method, both the ionophore and the fluorophore were in the same molecule (lipid A), so intramolecular interaction was responsible for the fluorescence quenching of monolayers of this lipid. In the other method, ionophore and fluorophore were located on two different molecules (lipids B and C) so the intramolecular coupling does not exist; instead the fluorescence quenching was realized by a through-space interaction mechanism. Several experimental techniques, including pi-A isotherm, epifluorescence microscopy, and absorption and emission spectroscopies were used to study the different characteristics of copper ion effect on the properties of the lipid monolayers. Additionally, the fluorescence quenching properties of the Langmuir monolayers were found to be transferred to the one-layer LB films. On LB films, the fluorescence response presented a clear selectivity for copper ions in comparison with several other transition metal ions. Further, an excellent reversibility was observed: the fluorescence was switched OFF by immersing the solid substrate in copper ion solution and ON by washing with HCl solution. The intermolecular approach used here seems to be a very flexible and general method to design surface-oriented fluorescent sensors to meet different analytic purposes.     

Supramolecular architecture in Langmuir and Langmuir-Blodgett films incorporating a chiral azobenzene

This article describes the synthesis and fabrication of Langmuir and Langmuir-Blodgett (LB) films incorporating a chiral azobenzene derivative, namely, ( S)-4- sec-butyloxy-4'-[5'-(methyloxycarbonyl)pentyl-1'-oxy]azobenzene, abbreviated as AZO-C4(S). Appropriate conditions for the fabrication of monolayers of AZO-C4(S) at the air-water interface have been established, and the resulting Langmuir films have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and UV-vis reflection spectroscopy. The results indicate the formation of an ordered trilayer at the air-water interface with UV-vis reflection spectroscopy showing a new supramolecular architecture for multilayered films as well as the formation of J aggregates. Films were transferred onto solid substrates, with AFM revealing well-ordered multilayered films without 3D defects. Infrared and UV-vis absorption spectroscopy indicate that the supramolecular architecture may be favored by the formation of H bonds between acid groups in neighboring layers and pi-pi intermolecular interactions. Circular dichroism spectra reveal chiro-optical activity in multilayered LB films.     

Molecular Orientation and Structure in LB Films of a Liquid Crystal MPUOPM Investigated by UV-Vis and IR Spectroscopy

"?Langmuir monolayers and LB films of 4-((s)-2-methylbutoxy)phenyl-(4'-(10-undecen-1-oyloxy)phenyl) methylenimine (MPUOPM) were investigated by ultraviolet-visible, polarized infrared spectroscopy. ?-A isotherms showed well-defined Langmuir monolayers were formed at an air/water interface for the MPUOPM and their mixture with SA. An inflection point at 13 mN/m appeared on the isotherm, which was due to the transition from the monolayer to multilayer. The polarized IR spectra of LB films of MPUOPM had provided new insight into the molecular orientation and structure. In LB films, the tilted angle between the alkyl chain and the normal line of the substrate was 48ffi, the tilted angle between the dipole moment of C=N and the normal line of the substrate was 51ffi. The alkyl chains assumed a trans-zigzag conformation but it included a few gauche conformers. The C=N groups were almost in one plane in the LB films. "     

Headgroup effects of template monolayers on the adsorption behavior and conformation of glucose oxidase adsorbed at air/liquid interfaces

Stearic acid (SA) and octadecylamine (ODA) monolayers at the air/liquid interface were used as template layers to adsorb glucose oxidase (GOx) from aqueous solution. The effect of the template monolayers on the adsorption behavior of GOx was studied in terms of the variation of surface pressure, the evolution of surface morphology observed by BAM and AFM, and the conformation of adsorbed GOx. The results show that the presence of a template monolayer can enhance the adsorption rate of GOx; furthermore, ODA has a higher ability, compared to SA, to adsorb GOx, which is attributed to the electrostatic attractive interaction between ODA and GOx. For adsorption performed on a bare surface or on an SA monolayer, the surface pressure approaches an equilibrium value (ca. 8 mN/m) after 2 to 3 h of adsorption and remains nearly constant in the following adsorption process. For the adsorption on an ODA monolayer, the surface pressure will increase further 1 to 2 h after approaching the first equilibrium pressure, which is termed the second adsorption stage. The measurement of circular dichroism (CD) spectroscopy indicates that the Langmuir-Blodgett films of adsorbed GOx transferred at the first equilibrium state (π = 8 mN/m) have mainly a β-sheet conformation, which is independent of the type of template monolayers. However, the ODA/GOx LB film transferred at the second adsorption stage has mainly an α-helix conformation. It is concluded that the specific interaction between ODA and GOx not only leads to a higher adsorption rate and adsorbed amount of GOx but also induces a conformation change in adsorbed GOx from β-sheet to α-helix. The present results indicate that is possible to control the conformation of adsorbed protein by selecting the appropriate template monolayer.     

Adsorption kinetics of a fluorescent dye in a long chain fatty acid matrix

This work reports the adsorption kinetics of a highly fluorescent laser dye rhodamine B (RhB) in a preformed stearic acid (SA) Langmuir monolayer. The reaction kinetics was studied by surface pressure-time (π-t) curve at constant area and in situ fluorescence imaging microscopy (FIM). Increase in surface pressure (at constant area) with time as well as increase in surface coverage of monolayer film at air-water interface provide direct evidence for the interaction. ATR-FTIR spectra also supported the interaction and consequent complexation in the complex films. UV-vis absorption and Fluorescence spectra of the complex Langmuir-Blodgett (LB) films confirm the presence of RhB molecules in the complex films transferred onto solid substrates. The outcome of this work clearly shows successful incorporation of RhB molecules into SA matrix without changing the photophysical characteristics of the dye, thus making the dye material as LB compatible.     

DNA分子在气液界面的组装相变特性及其LB膜结构研究

对十八胺与DNA在气液界面上组装及其相变过程进行了研究，利用AFM观察了不 同压力下转移的DNA复合LB膜结构。发现在低表面压时，DNA复合单分子膜表现为技 术发散的分形结构；随着压力的升高，DNA复合膜逐渐由紧密的网状排布结构变为 团聚的块状和团簇结构。表明通过调节膜压，可使膜内DNA分子的构象发生大的变 化，从而生成具有特定形态的二维纳米图案。这种具有特殊形态和结构的DNA LB膜 可望为合成新型生物纳米结构有序功能体系提供模板。     

Directionally oriented LB films of an OPE derivative: assembly, characterization, and electrical properties

Langmuir films have been fabricated from 4-[4'-(4'-thioacetyl-phenyleneethynylene)-phenyleneethynylene]-aniline (NOPES) after cleavage of the thioacetyl protecting group. Characterization by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveal the formation of a high quality monolayer at the air-water interface. One layer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of the NOPES Langmuir film onto solid substrates. X-ray photoelectron spectroscopy (XPS), surface polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of one layer LB films in which the functional group associated with binding to the substrate can be tailored by the film transfer conditions. Using LB methods this molecule could be transferred to gold samples with either the amine or thiol group attached to the gold surface. The amine group is directly attached to the gold substrate (Au-NH(2)-OPE-SH) when the substrate is initially immersed in the subphase and withdrawn during the transfer process; in contrast, monomolecular films in which the thiolate group is attached to the gold substrate (Au-S-OPE-NH(2)) are obtained when the substrate is initially out of the subphase and immersed during the transfer process. The morphology of these films was analyzed by atomic force microscopy (AFM), showing the formation of homogeneous layers. Film homogeneity was confirmed by cyclic voltammetry, which revealed a large passivation of gold electrodes covered by NOPES monolayers. Electrical properties for both polar orientated junctions have been investigated by scanning tunnelling microscopy (STM), with both orientations featuring a nonrectifying behavior.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

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.     

Role of microenvironment in the mixed Langmuir-Blodgett films

This paper reports the pi-A isotherms and spectroscopic characteristics of mixed Langmuir and Langmuir-Blodgett (LB) films of nonamphiphilic carbazole (CA) molecules mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). pi-A isotherm studies of mixed monolayer as well as the remarkable change in collapse pressure of the mixed monolayer isotherms definitely show that CA is incorporated into PMMA and SA matrices. However, CA is stacked in the PMMA/SA chains and forms microcrystalline aggregates, as is evidenced from the scanning electron micrograph picture. The nature of these aggregated species in the mixed LB films has been revealed by UV-vis absorption and fluorescence spectroscopic studies. The presence of two different kinds of band systems in the fluorescence spectra of the mixed LB films have been observed. This may be due to the formation of low-dimensional aggregates in the mixed LB films. Intensity distribution of different band systems is highly sensitive to the microenvironment of two different matrices as well as also on the film thickness.     

Langmuir-Blodgett films of a Mo-blue nanoring [Mo(142)O(429)H(10)(H(2)O)(49)(CH(3)CO(2))(5)(CH(3)CH(2)CO(2))](30)(-) (Mo(142)) by the semiamphiphilic method

Langmuir monolayers and LB films of the ring-shaped mixed-valence polyoxomolybdate [Mo142O429H10(H2O)49(CH3CO2)5(CH3CH2CO2)](30-) (Mo142) dissolved in the aqueous subphase have been successfully fabricated by using the adsorption properties of a DODA monolayer. Infrared and ultraviolet-visible spectroscopy of the LB films indicates that Mo142 and DODA molecules are incorporated within these LB films. X-ray reflectivity experiments indicate that the LB films exhibit a well-defined lamellar structure formed by bilayers of DODA molecules alternating with monolayers of Mo142. Using behenic acid-modified hydrophobic quartz substrate is critical for the formation of the well-defined lamellar structure. From the values of the periodicity obtained by these experiments it is clear that the Mo142 clusters lie flat along the charged organic layers. AFM images also showed the flat and homogeneous films on the quartz substrates treated with behenic acid. Cyclic voltammograms of Mo142-LB films deposited on ITO substrates showed quasi-reversible oxidation/reduction waves with positive shift of the potential compared to the case of solution.     

Influence of ammonium nitrate in phase transitions of Langmuir and Langmuir-Blodgett films at air/solution and solid/solution interfaces

The effect of ammonium nitrate on the phase transitions in Langmuir films of amphiphiles-stearic acid, stearyl amine (STAM), stearyl alcohol, dihexadecylphosphate, and the quarternized ammonium salt dioctadecyldimethylammonium bromide have been studied at air/water interface and in local ordering of their Langmuir-Blodgett films (LB films). The study shows that except for the stearyl amine (STAM) all other monolayers exhibit a liquid-expanded to liquid-condensed transition with slight expansion in area in the presence of ammonium nitrate. STAM monolayers show a new phase transition, which possibly arises due to the differently ionized amino groups, and change in solvation sheath due to an ion-dipole type interaction between the amino groups and the ammonium ion in the subphase. Mixed films of the amine with the acid and alcohol did not show such intermediate phases indicating that competing H-bonds between polar groups themselves and dipolar couplings between the polar groups and ammonium nitrate play a major role in the organization of the molecules at the interface. The above effect resulting in a change in the local order is borne out by Brewster angle micrographs (BAM) of the Langmuir films of STAM at air/solution interface. Such behavior is also seen at solid/liquid interfaces where the polar component of surface energy undergoes a drastic change for the amine films transferred onto solid substrates from the air/ammonium nitrate solution interface.     

Langmuir monolayers and Langmuir–Blodgett films of ferritin prepared by using a surfactant mixture of eicosylamine (EA) and methyl stearate (SME)

Magnetic Langmuir–Blodgett films of ferritin have been prepared by using the adsorption properties of a 1/4 mixed monolayer of eicosylamine (EA) and methyl stearate (SME). BAM images show that a more homogeneous distribution of ferritin at the air–water interface is achieved by using this mixture of surfactants instead of the DODA/SME mixed matrix of a previous work. Transfer of the monolayer onto different substrates allowed the preparation of multilayer LB films. Infrared and UV–Vis spectroscopies indicate that ferritin molecules are incorporated within the LB films. Furthermore, UV–Vis spectroscopy measurements reveal that the amount of ferritin incorporated into these LB film has been increased with respect to that in the DODA/SME LB films. Finally magnetic measurements confirm that the superparamagnetic properties of this molecule are preserved in the LB films.