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.     

Thin films of discotic liquid crystals and their applications

ABSTRACT

Discotic liquid crystals (DLCs) are considered as fascinating systems due to their unique property of self-assembly to yield different columnar structures. DLCs are organic semiconductors and create pathways for the development of numerous optical and electrical devices. The thin films of DLCs can be considered as low dimensional system which can exhibit remarkable optical and physical properties. In this article, we present a review on ultrathin films of some interesting DLC molecules at air–water and air–solid interfaces. The Langmuir monolayer and Langmuir–Blodgett films of DLC molecules are extensively studied. The ultrathin films of DLC molecules can yield highly anisotropic layer wherein the molecular orientation and aggregation can have large impact on the physicochemical properties of the film. Different surface phases with different molecular orientations as function of surface density and temperature can be obtained by forming the Langmuir monolayer of the DLC molecules at the air–water interface. The Langmuir monolayer in a particular phase can be deposited onto the active area of a device layer-by-layer by employing a highly controlled Langmuir–Blodgett technique. Here, we report some interesting results related on molecular orientation of the DLC molecules at different interfaces. Such aggregation of DLC molecules in ultrathin films may find applications in thin film-based electro-optical devices.     

Langmuir–Blodgett films of stearic acid deposited on substrates at different orientations relative to compression direction: alignment layer for nematic liquid crystal

The Langmuir monolayer at an air–water interface shows remarkably different surface pressure (π)–area (A) isotherm, when measured with the surface normal of a Wilhelmy plate parallel or perpendicular to the direction of compression of the monolayer. Such difference arises due to difference in stress exerted by the monolayer on the plate in different direction. In this article, we report the effect of changing the direction of substrate normal with respect to the compression of the monolayer during Langmuir–Blodgett (LB) film deposition on the morphology of the films. The morphology of the LB film of stearic acid is studied using an atomic force microscope. The morphology of the LB films is found to be different due to difference in the stress in different directions. The role of such surface morphology on the alignment of a nematic liquid crystal (LC) in LC cells is studied. The granular texture of LB films of stearic acid supports the homogeneous alignment of the LC whereas the uniform texture supports the homeotropic alignment of the LC.     

Study on the Film Properties and Recognition for Metal Ions of Calix[4]arenes Derivative by Langmuir Monolayer

Calixarene molecules are very powerful ligand for ions and small molecules, and have been studied with several techniques as models for host‐guest systems. In this approach, the formation of Langmuir monolayer properties of three kinds calix[4]arene derivative were characterized and one of them, p‐tert‐butylthiacalix[4]arene (TCA), was chosen as object to study its Langmuir monolayer affected by different subphase conditions. The purpose of this study is to investigate the molecular recognition ability of TCA for metal ions at the water‐air interface. Changing the composition of aqueous subphase (containing various metal ion solutions respectively) produced strong variations on the monolayer parameters, indicating a different selectivity of the TCA ligand for the different metal cations. In particular, high selectivity for transition metal ions was found. Limiting area values are discussed in relation to the orientation of the cone‐shaped molecules at the water‐air interface.     

Determination of porphyrin dimer in a mixed monolayer of porphyrin/phospholipid transferred on ITO electrodes

The Langmuir–Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air|water interface onto optically transparent indium–tin oxide (ITO) electrodes. The surface concentration of porphyrin, Γ, transferred on the ITO surface, has been obtained by integration of the reduction current from the cyclic voltammograms. The experimental Γ values ranged from Γ_m=5.19×10⁻¹¹ mol cm⁻², and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Γ_d=8.65×10⁻¹¹ mol cm⁻², and equivalent to the total amount of the porphyrin molecules at the air|water interface under a compression of 35 mN m⁻¹. Prior to the electrochemical experiments, the transmission spectrum was recorded. The surface concentration obtained of the porphyrin is not directly proportional to the transmission of the film, ΔT. This phenomenon is assigned to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on an intact ITO electrode is altered with respect to that found at the air|water interface. A simple model has been developed to evaluate the contribution of monomer and dimer phases of the porphyrin in the mixed monolayer. Furthermore, spectroscopic measurements with linearly polarized light under oblique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces.     

下缘芳香取代结构对杯[4]芳烃Langmuir膜的影响

The Langmuir monolayer properties of lower rim aromatically substituted calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis（2-naphth-1＇-ylacetylaminoethoxy）-26,28-dihydroxylcalix[4]arene （BNAEC）, 5,11,17,23-tetra-tert- butyl-25,27-bis（2-benzoylamino ethoxy）-26,28-dihydroxylcalix[4]arene （BBAEC） and 5,11,17,23-tetra-tert-butyl- 25,27-bis（2-cinnamoylaminoethoxy）-26,28-dihydroxylcalix[4]arene （BCAEC）, have been studied. Film balance measurements and Brewster angle microscopy （BAM） observation demonstrate that all the compounds can form Langmuir monolayers with different molecular limiting areas. BNAEC or BBAEC monolayer is able to form condensed domains during compression, while BCAEC monolayer can never form condensed domain. BNAEC monolayer is more readily to form condensed domain than BBAEC monolayer. Moreover, BNAEC monolayer can form the total condensed phase during compression even when T=28℃, while BBAEC monolayer can not when T 〉 10 ℃. The results imply that different lower rim aromatic substitutions affect essentially the intermolecular interaction and molecular packing in the monolayer at air/water interface.     

掺杂态聚苯胺LB膜的制备与性质

掺杂樟脑磺酸的聚苯胺在纯水表面可以形成稳定的单分子层,采用Ｌａｎｇｍｕｉｒ－Ｂｌｏｄｇｅｔｔ技术可以将其较好地转移到氟化 基片上制成薄膜,对其ＬＢ膜的红外和紫外－可见－近红外光谱研究表明,在膜的压缩或转移过程中有少量的樟脑磺酸分子发生脱掺杂的现象,并且该ＬＢ膜在室温下对乙醇有较好的气敏性。     

Effect of the molecular structure on the hierarchical self-assembly of semifluorinated alkanes at the air/water interface

Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.     

Ordered supramolecular assembly of bis[3,4,1 2,13,21,22,30, 31-octa(dodecylthio)-2,3-naphthalocyaninato] erbium at the air/water interface

The supramolecular assembly of bis[3, 4, 12, 13, 21, 22, 30, 31-octa(dodecylthio)-2, 3-naphthalocyaninato] erbium complex (Er[Nc(SC₁₂H25)₈]₂) was fabricated at the air/water interface by Langmuir monolayer technique. Surface pressure-area isotherm indicates that stable monolayer is formed. The monolayer morphology on mica was investigated by atomic force microscopy (AFM). Columnar domains of ca. 100 nm X (20–30) nm appearing in the micrograph have ordered orientation. Polarized UV-vis spectra and small-angle X-ray diffraction pattern show that the macrocycles tilt in the monolayer. The orientation angle of the naphthalocyanine ring with respect to the substrate was found to be ca. 54°. Each monolayer is ca. 3.53 nm thick. The columnar domain comprises 4–6 rows of columnar supramolecular assemblies where the molecules stack face-to-face.     

Ordered supramolecular assembly of bis[3,4,12,13,21,22,30,31-octa(dodecylthio)-2,3-naphthalocyaninato]erbium at the air/water interface

The supramolecular assembly of bis[3, 4, 12, 13, 21, 22, 30, 31-octa(dodecylthio)-2, 3-naphthalocyaninato] erbium complex (Er[Nc(SC12H25)8]2) was fabricated at the air/water interface by Langmuir monolayer technique. Surface pressure-area isotherm indicates that stable monolayer is formed. The monolayer morphology on mica was investigated by atomic force microscopy (AFM). Columnar domains of ca. 100 nm ×(20-30) nm appearing in the micrograph have ordered orientation. Polarized UV-vis spectra and small-angle X-ray diffraction pattern show that the macrocycles tilt in the monolayer. The orientation angle of the naphthalocyanine ring with respect to the substrate was found to be ca. 54°. Each monolayer is ca. 3.53 nm thick. The columnar domain comprises 4-6 rows of columnar supramolecular assemblies where the molecules stack face-to-face.     

Orientation of the alkyl side chains and glucopyranose rings in Langmuir–Blodgett films of a regioselectively substituted cellulose ether

An arrangement for the long alkyl side chains in a Langmuir–Blodgett (LB) film from regioselectively alkylated 2,3-di-O-octadecylcellulose on an Au-coated substrate was investigated using Fourier transform infrared reflection absorbance spectroscopy. The IR results indicated that the hydrophobic long alkyl side chains were forced to be repellent to the surface of water, resulting in deposition on the substrate, with it being tilted in the vertical direction, and further formed both hexagonal and orthorhombic crystalline structures up to 10 layers in the LB film. In addition, molecular modeling with CAChe software indicated that the optimized assembly of the side chains was supposed to own a vertical arrangement against the substrate surface after compression of the monolayer. This means that the conformation of 2₁ screw of the cellulose molecular chain may be altered by the compression to have an unusual conformation by a different φ–ψ dihedral angle such as onefold axis without a symmetry element.     

The Influence of Polysaccharides/TiO2 on the Model Membranes of Dipalmitoylphosphatidylglycerol and Bacterial Lipids

The aim of the study was to determine the bactericidal properties of popular medical, pharmaceutical, and cosmetic ingredients, namely chitosan (Ch), hyaluronic acid (HA), and titanium dioxide (TiO₂). The characteristics presented in this paper are based on the Langmuir monolayer studies of the model biological membranes formed on subphases with these compounds or their mixtures. To prepare the Langmuir film, 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG) phospholipid, which is the component of most bacterial membranes, as well as biological material-lipids isolated from bacteria Escherichia coli and Staphylococcus aureus were used. The analysis of the surface pressure-mean molecular area (π-A) isotherms, compression modulus as a function of surface pressure, C_S⁻¹ = f(π), relative surface pressure as a function of time, π/π₀ = f(t), hysteresis loops, as well as structure visualized using a Brewster angle microscope (BAM) shows clearly that Ch, HA, and TiO₂ have antibacterial properties. Ch and TiO₂ mostly affect S. aureus monolayer structure during compression. They can enhance the permeability of biological membranes leading to the bacteria cell death. In turn, HA has a greater impact on the thickness of E. coli film.     

Supramolecular packing of alkyl substituted Janus face all-cis 2,3,4,5,6-pentafluorocyclohexyl motifs

This study uses X-ray crystallography, theory and Langmuir isotherm analysis to explore the conformations and molecular packing of alkyl all-cis 2,3,4,5,6-pentafluorocyclohexyl motifs, which are prepared by direct aryl hydrogenations from alkyl- or vinyl-pentafluoroaryl benzenes. Favoured conformations retain the more polar triaxial C–F bond arrangement of the all-cis 2,3,4,5,6-pentafluorocyclohexyl ring systems with the alkyl substituent adopting an equatorial orientation, and accommodating strong supramolecular interactions between rings. Langmuir isotherm analysis on a water subphase of a long chain fatty acid and alcohol carrying terminal all-cis 2,3,4,5,6-pentafluorocyclohexyl rings do not show any indication of monolayer assembly relative to their cyclohexane analogues, instead the molecules appear to aggregate and form higher molecular assemblies prior to compression. The study indicates the power and potential of this ring system as a motif for ordering supramolecular assembly.

Theory and Langmuir isotherm analysis was used to explore the conformations and molecular packing of alkyl all-cis 2,3,4,5,6-pentafluorocyclohexyl motifs, which are prepared by direct aryl hydrogenations from alkyl- or vinyl-pentafluoroaryl benzenes.     

Interfacial Water Structure in Langmuir Monolayer and Gibbs Layer Probed by Sum Frequency Generation Vibrational Spectroscopy

Langmuir monolayer and Gibbs layer exhibit surface‐active properties and it can be used as simple model systems to investigate the physicochemical properties of biological membranes. In this report, we presented the OH stretching vibration of H₂O in the 4′′‐n‐pentyl‐4‐cyano‐p‐terphenyl (5CT), nonadecanenitrile (C₁₈CN) Langmuir monolayer and compared them with CH₃CN Gibbs layer at the air/water interface with polarization SFG‐VS. This study demonstrated that the hydrogen bond network is different in the Langmuir monolayer of 5CT, C₁₈CN from CH₃CN Gibbs layer at the air/water interface which showed two different water structures on the different surface layer. The results provided a deeper insight into understanding the hydrogen bond on the interfaces.     

Synthesis of an Open‐Cage Structure POSS Containing Various Functional Groups and Their Effect on the Formation and Properties of Langmuir Monolayers

Recently, silsesquioxanes have been recognized as a new group of film‐forming materials. This study has been aimed at determining the effect of the kind of functional groups present in two different open‐cage structure POSS molecules on the possibility of the formation of Langmuir monolayers and their properties. To achieve this goal, two new POSS derivatives (of open‐cage structures) containing polyether and fluoroalkyl functional groups have been synthesized on the basis of a hydrosilylation process. An optimization of the process was performed, which makes it possible to obtain the above‐mentioned derivatives with high yields. In the next step, the Langmuir technique was applied to measurements of the surface pressure (π) ? the mean molecular area (A) isotherms during the compression of monolayers formed by molecules of the two POSS derivatives considered. Subsequently, the monolayers were transferred onto quartz plates according to the Langmuir–Blodgett technique. Both derivatives are able to form insoluble Langmuir films at the air–water interface, which can be transferred onto a solid substrate and effectively change its wetting properties.     

Structural Modification Influences the Characteristics of Langmuir Monolayers from Aromatic Carboxylic Acids

The molecular organization of purely aromatic, polyphenyl carboxylic acids, as Langmuir monolayers at the air/water interface, has been investigated by means of surface pressure and electric surface potential measurements upon film compression. The monolayer characteristics of the basic compound, a symmetrical triphenylbenzene (5'-phenyl-m-terphenyl) ring with a carboxylic group at the 4 position (namely 5'-phenyl-1,1' : 3',1"-terphenyl-4-carboxylic acid), are compared with those of its derivatives containing hydrophilic (nitro) or hydrophobic (phenyl) substituents. The nature of the substituent as well as its position (2' or 4') has a profound influence on the monolayer properties. The results are discussed in view of molecular orientation deduced from values of effective dipole moments. Copyright 2001 Academic Press.     

Silver nanoparticle monolayer-to-bilayer transition at the air/water interface as studied by the GISAXS technique: application of a new paracrystal model

An original diffraction model for the analysis of grazing-incidence small-angle X-ray scattering (GISAXS) from the nanoparticle Langmuir films was developed. This model relies on the concept of the 2D hexagonal paracrystal and employs the distorted-wave Born approximation that is relevant for GISAXS measurements at the air/water interface when the angle of incidence is close to the critical value. The model comprises the cases of the close-packed nanoparticle monolayer and bilayer with the AB-type layer stacking. In this way, both the lateral (along the interface) and vertical (normal to the interface) correlations of the nanoparticle positions can be analyzed. The model was applied to an in situ GISAXS study of the formation of a silver nanoparticle Langmuir film during compression at the air/water interface in the Langmuir-Blodgett trough. Spherical nanoparticles of 5.8 ± 0.6 nm diameter were employed. Different compression stages starting from the submonolayer up to the monolayer collapse via bilayer formation were analyzed in terms of the mean lateral interparticle distance, degree of paracrystal disorder, interlayer distance, vertical disorder, and layer-stacking type in the bilayer as well as the ratio between the monolayer and bilayer coverage in the final film. The model developed is applicable to any nanoparticle Langmuir film formed at the air/liquid interface to extract structural parameters on the nanoscale. The particular results obtained have direct implications on the preparation of silver plasmonic templates with "hot spots" for surface-enhanced Raman scattering.     

Langmuir monolayer characteristics of a perfluoroaryl surfactant: 10-Perfluorobenzyl-decan-1-ol (PBD)

A surfactant containing the perfluorobenzyl moiety attached at the 10 position of decanol (10-perfluorobenzyldecan-1-ol, PBD) was synthesized and employed for Langmuir monolayer characterization, using surface pressure (π) and electric surface potential (ΔV)-mean molecular area (A) isotherms. Both static and dynamic stability experiments demonstrate that the monolayers formed by PBD are of satisfactory stability. Characteristic dendritic structures were observed with Brewster Angle Microscope (BAM) upon the whole compression up to the vicinity of collapse, where 3D crystallites appeared. The monolayer thickness is found to be comparable to the PBD molecule length. It occurs that ΔV has negative value even at large molecular areas, and decreases gradually along the compression, reaching a minimum of ca. −500 mV. Using the Helmholtz model, the apparent dipole moment was calculated upon film compression. The measurements were complemented with semi-empirical calculations involving molecular geometry optimization and dipole moment estimation.     

Mixed monolayers of alkylated azacrown ethers and palmitic acid at the air-water surface

The Langmuir films of two alkylated azacrown ethers at the air-water surface were characterized using surface pressure-area isotherms, ellipsometry, Brewster angle microscopy, and constant-area surface pressure relaxation. The azacrown ether molecules aggregate in the monolayer, which significantly stabilizes the film against dissolution. Mixed azacrown ether-palmitic acid monolayers were also characterized; results suggest that at high compression the two molecules interact repulsively. The influence of Cu(II) ions present in the aqueous subphase on the single components and mixed monolayer characteristics was also studied.