Formation Of Langmuir Monolayers Research Articles

Crystalline Mono‐ and Multilayer Self‐Assemblies of Oligothiophenes at the Air–Water Interface

AbstractThe formation of Langmuir monolayers at the air–water interface has long been believed to be limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup. Here we report the formation of crystalline mono‐ and multilayer self‐assemblies of oligothiophenes, a class of aromatic nonamphiphilic molecules, self‐aggregated at the air–water interface. As model systems we have examined the deposition of quaterthiophene (S4), quin‐quethiophene (S5), and sexithiophene (S6) from chloroform solutions on the water surface. The structures of the films were determined by surface pressure–area isotherms, by scanning force microscopy (SFM) after transfer of the films onto atomically smooth mica, by cryo‐transmission electron microscopy (Cryo‐TEM) on vitreous ice, and by grazing incidence synchrotron X‐ray diffraction (GID) directly on the water surface. S4 forms two polymorphic crystalline multilayers. In polymorph α, of structure very similar to that of the three‐dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23° from the normal to the water surface. In polymorph β the long molecular axis is perpendicular to the water surface. S5 self‐aggregates at the water surface to form mixtures of monolayers and bilayers of the β polymorph; S6 forms primarily crystalline monolayers of both α and β forms. The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports. The relevance of the present results for the understanding of the early stages of crystal nucleation is presented.

Supramolecular structures formed from heterocyclic aromatic molecules

This paper describes the formation and structure investigation of Langmuir monolayers and Langmuir-Blodgett multilayers formed from amphiphilic derivatives of 2,5-diphenyl-1,3,4-oxadiazole. The 2,5-diphenyl-1,3,4-oxadiazole group as a functional unit with interesting physical and chemical properties is maintained, while the head group, the length of the alkyl chain and the structure of the coupling unit between aromatic and aliphatic part of these linear short-chain amphiphiles is systematically varied in order to explore the influence of this change on the film forming properties and the stability of Langmuir and Langmuir-Blodgett films. Molecular mechanics simulations are shown by these systematic variations to be suitable for the prediction of optimal chemical structures allowing for a stable stratified molecular packing. The combination of a detailed structure investigation of the multilayers based on scanning force microscopy and X-ray data with molecular mechanics simulations yields an insight into the packing of the molecules and the intermolecular interactions.

Preparation and deposition of stable monolayers of fullerene derivatives

Two new fullerene derivatives have been synthesized, which tend to form Langmuir monolayers. Robust monomolecular layers are formed at the air/water interface, that can be transferred onto hydrophilic substrates by Langmuir–Blodgett or Langmuir–Schaefer techniques. The measured areas per molecule (105 Å2 for 1a dropped from a deuteriochloroform solution at 20 °C) reach those expected for a true monolayer of C60 at the air/water interface.

New disc-shaped triarylamino-1,3,5-triazines with heteroaromatic central cores

Abstract New disc-shaped triarylamino-1,3,5-triazines were prepared by reaction of cyanuric chloride with two-fold alkoxy substituted anilines. Each of the triarylmelamines exhibits an enanti-otropic mesophase. X-ray investigations reveal the formation of columnar liquid crystalline structures with either a hexagonal or a rectangular lattice. The lattice parameters, as well as the intracolumnar ordering, depend on the length of the flexible lateral side chains. The new melamines form Langmuir monolayers at the air/water interface due to their amphiphilic nature. The monolayer organization is characterized by a side-on arrangement of the molecules. The polar central molecular parts lie flat on the water, whereas the lateral alkyl chains are oriented perpendicular to the water surface.

Characterization of Langmuir-Blodgett films of parent polyaniline

Conducting Langmuir-Blodgett (LB) films have been fabricated from parent polyaniline (PAni) which was doped with functionalized acids. In order to optimize experimental conditions for the formation of stable Langmuir monolayers and their subsequent transfer onto solid substrates, PAni was dissolved in ten different combinations of chloroform solutions. Use was made of camphor sulfonic acid, dodecyl benzene sulfonic acid, and toluenesulfonic acid, and of the solvents N-methyl pyrrolidine and m-cresol as processing agents. Because acidic subphases have been employed, as-deposited LB films were already doped, which was confirmed by the appearance of a polaronic band in the UV-Vis absorption spectra. The absorbance peak increases with the number of deposited layers indicating that a suitable multilayer buildup is accomplished. When analysed by atomic force microscopy, PAni LB films show a fibrillar structure with the fibril width ranging from ≈60 to 160 nm.

Langmuir−Blodgett Mono- and Multilayers of Rodlike Oligo(cyclohexylidene) Derivatives Bearing an Oxime Headgroup

The Langmuir film-forming properties of rodlike oligo(cyclohexylidene) oximes and derivatives 1(n)-8 have been studied. It is shown that subtle structural modifications of their hydrocarbon framework, such as end-functionalization by a tert-butyl group or an n-alkyl group or (selective) hydrogenation of double bonds, markedly affect their propensity toward Langmuir monolayer formation and in particular monolayer film stability. Mean molecular areas (π-A isotherms) show that the oximes orient nearly perpendicular on the air-water interface. Film transfer of 5(2) onto moderately hydrophilic silicon (100) wafers gave predominantly Z-type deposition ; for a monolayer a tilt angle of 26° was derived (atomic force microscopy). The quantitative elemental composition ofa transferred multilayer assembly of 5(2) was determined using elastic recoil detection.

Synthesis of a Fullerene[60] Cryptate and Systematic Langmuir‐Blodgett and Thin‐Film Investigations of Amphiphilic Fullerene Derivatives

AbstractThe synthesis of the first fullerene cryptate 7 with a sodium ion bound to a benzo[2.2.2]cryptand covalently attached to a methanofullerene[60] is described. The amphiphilic properties of 7 as well as of a variety of other covalent fullerene derivatives with polar functional groups and the ability of these compounds to form Langmuir monolayers at the air‐water interface were investigated in a systematic study. Among these derivatives are Diels‐Alder adducts of C60 and methanofullerenes, four of which are fullerene C‐glycosides. The films at the water surface were characterized by their surface pressure versus molecular area isotherms, compression and expansion cycles, and optical light microscopy. UV/Vis spectroscopy and small‐angle X‐ray diffraction (SAXS) were employed for LB film characterization on solid substrates. Parameters influencing the spreading and monolayer character include (a) polarity, (b) balance of hydrophobicity to hydrophilicity, (c) size and bulkiness of the polar groups attached to the fullerene, and (d) presence of aromatic residues in these groups.

Gas-surface interactions with Langmuir-Blodgett films of new derivatives of dibenzo-tetra-aza-dihydro-(14)-annulene

The authors new derivatives of the parent macrocyclic ligand dibenzo-(b,i)(1,4,8,11)tetra-aza-(4,11)dihydro-(14)-annulene (TTA) can be divided into two classes on the basis of their behaviour concerning the formation of (floating) Langmuir monolayers and solid-supported Langmuir-Blodgett (LB) monolayers and multilayers. Class I consists of molecules that are highly hydrophobic but do not contain a formal hydrophilic centre whereas the class II molecules are truly amphiphilic. The class I compounds all formed stable Langmuir monolayers at surface pressures of 10-15 mN m-1 for prolonged periods and, for the C(6) and C(14) compounds, similar stability was also observed at up to 40 mN m-1. Although the C(14) material failed to deposit as an LB film, limited multilayer deposition (X or Z-type) was displayed by the other members in class I. In class II, only the di-cumylphenoxy-derivative of the cobalt(II) complex of TAA (CPTAACo, TAA as defined previously) was found to form satisfactory LB multilayers for which a surface pressure in the range 16-20 mNm-1 was employed. The Langmuir monolayers of the other members of this class were too rigid for LB deposition. Some observations concerning the effect of NOX upon an LB film of CPTAACo are described.

New amphiphilic terphenyl liquid crystals for the preparation of highly ordered ultrathin films

AbstractFour new amphiphilic liquid crystals have been synthesized, in which terphenyl was used as the mesogenic unit. In order to enable the formation of Langmuir Monolayers at the air/water‐interface, the molecules were equipped with slightly polar headgroups such as esters or a carboxylic acid group. All compounds can be transferred onto solid substrates. In addition, it is possible to prepare freely suspended films of at least one compound in the temperature range of the smectic phases. The phases of the different states, bulk, monolayer, freely suspended film, and Langmuir‐Blodgett multilayers have been investigated by means of monolayer isotherms, optical textures, differential scanning calorimetry (DSC) and small angle X‐ray scattering.

Monolayers of macrocyclic polyamides at the air–water interface

The poly-N-acylated derivatives (1b), (1c), (1d), and (2d) of the macrocyclic polyamines [18]-N6(1a) and [30]-N10(2a) form Langmuir monolayers in which the macrocycle lies flat on the water surface while the alkane chains tilt upright in the compressed film.