Jürgen Reiche

Calcium Phosphate Mineralization beneath a Polycationic Monolayer at the Air-Water Interface

The self-assembly of the amphiphilic block copolymer poly(n-butyl methacrylate)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface has been investigated at different pH values. Similar to Rehfeldt et al. (J. Phys. Chem. B 2006, 110, 9171), the subphase pH strongly affects the monolayer properties. The formation of calcium phosphate beneath the monolayer can be tuned by the subphase pH and hence the monolayer charge. After 12 h of mineralization at pH 5, the polymer monolayers are still transparent, but transmission electron microscopy (TEM) shows that very thin calcium phosphate fibers form, which aggregate into cotton ball-like features with diameters of 20 to 50 nm. In contrast, after 12 h of mineralization at pH 8, the polymer film is very slightly turbid and TEM shows dense aggregates with sizes between 200 and 700 nm. The formation of calcium phosphate is further confirmed by Raman and energy dispersive X-ray spectroscopy. The calcium phosphate architectures can be assigned to the monolayer charge, which is high at low pH and low at high pH. The study demonstrates that the effects of polycations should not be ignored if attempting to understand the colloid chemistry of biomimetic mineralization. It also shows that basic block copolymers are useful complementary systems to the much more commonly studied acidic block copolymer templates.

Control of the structure of Langmuir-Blodgett films of a discotic liquid crystalline compound via the subphase composition and the adjacent molecular environment

Abstract Changes of the ,olecular arrangement that can be induced by means of the LB technique in the multilayers of a disc-shaped multialkynyl amphiphile are monitored by means of small angle X-ray diffraction. Studies of the monolayers at the air-water interface reveal “edge-on” orientation of the discs. Specific effects of the counter-ions (Na + , Cd 2+ , Pb 2+ and Ba 2+ ) and sub-solution pH on the monolayer collapse pressure, transfer efficiency and molecular order in the multilayers are found. A correlation between the monolayer properties and the ability for formation of periodic discotic structures in the presence of divalent counterions is established. The discotic molecules retain their “edge-on” arrangement in the highly compressed transferred films with slight irregular interdigitation of the flexible wings and inclination to the substrate normal. The tilt and the interdigitation are reduced when the discotic monolayers are deposited in alternating LB films with barium arachidate spacer layers.

Edge-on Langmuir-Blodgett multilayers derived from disc-shaped multiyne mesogens☆

Abstract Langmuir-Blodgett (LB) multilayers were prepared from disc-shaped multiyne mesogens based on amphiphilic alkyl pentakis (aryl-ethynyl) benzene ethers. The two compounds used are characterized by five hydrophobic flexible chains and one hydrophilic substituent at the terminal position of the alkoxy chain. The LB films were analyzed by X-ray scattering and spectroscopic measurements. An edge-on arrangement of the two discotic pentaalkynes within Y -type bilayers with a different packing density proved to be possible for the LB films of both compounds.

Molecular organization of amphiphilic disc-shaped penta-alkynes in LB-mono- and multilayers☆

Abstract Amphiphilic disc-shaped penta-alkynes were studied with regard to their molecular organization in Langmuir-Blodgett (LB) mono- and multilayers. It was found that each compound investigated forms edge-on arraged stable monolayers at the air-water interface. LB-multilayers derived from fivefold pentyl-substituted pentaynes are characterized by an edge-on ordering of the molecules within Y-type bilayers. One of these compounds, containing a hydroxy substituent as hydrophilic head group, is exemplified and two possible rectangular molecular assemblies perpendicular to the substrate, each with a columnar in-plane packing, will be discussed as a result of molecular modelling. Based on the experimental results, hexagonal layer packing in the LB-film of a disc-shaped penta-alkynyl carboxylic acid without lateral substituents proved to be possible, which, furthermore, could be confirmed by molecular mechanics simulation.

Calcium phosphate growth beneath a polycationic monolayer at the air–water interface: effects of oscillating surface pressure on mineralization

The self-assembly of the amphiphilic block copolymer poly(butadiene)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface and the mineralization of the monolayers with calcium phosphate was investigated at different pH values. As expected for polyelectrolytes, the subphase pH strongly affects the monolayer properties. The focus of the current study, however, is on the effect of an oscillating (instead of a static) polymer monolayer on calcium phosphate mineralization. Monitoring of the surface pressure vs. mineralization time shows that the monolayer is quite stable if the mineralization is performed at pH 8. In contrast, the monolayer at pH 5 shows a measurable decrease of the surface pressure already after ca. 2 h of mineralization. Transmission electron microscopy reveals that mineralization at low pH under constant oscillation leads to small particles, which are arranged in circular features and larger entities with holes of ca. 200 nm. The larger features with the holes disappear as the mineralization is continued in favor of the smaller particles. These grow with time and form necklace-like architectures of spherical particles with a uniform diameter. In contrast, mineralization at pH 8 leads to very uniform particle morphologies already after 2 h. The mineralization products consist of a circular feature with a dark dot in the center. The increasing contrast of the precipitates in the electron micrographs with mineralization time indicates an increasing degree of mineralization vs. reaction time. The study therefore shows that mechanical effects on mineralization at interfaces are quite complex.

Zeolite films prepared via the Langmuir–Blodgett technique

Abstract Micrometer sized zeolite particles were found to form relatively stable particle monolayers at the air–water interface when spread from a chloroform dispersion. For these layers compression isotherms were recorded. The presence of polyelectrolytes in the aqueous subphase was found to have a significant influence on the compression behaviour of the films and on the degree of orientation of the zeolite particles in the film. The transfer of the floating thin film onto a solid substrate via the Langmuir–Blodgett (LB) technique proved to be possible. The parameters influencing the deposition of zeolite particles on different solid substrates are discussed. The resulting films are analysed with respect to the orientation of the individual zeolite particles by means of optical microscopy. The LB films are characterised by a dense packing of zeolite particles with a predominant ‘ side-on ’ orientation of the zeolite crystals. The adhesion of the zeolites to solid substrates is improved by a plasma pre-treatment of the substrate and by the presence of polyelectrolytes in the subphase.

Synthesis, Langmuir and Langmuir–Blodgett film behaviour of new dendritic amphiphiles

New amphiphilic compounds 1–9 that feature a construction with dendronized hydrophilic and hydrophobic segment groups connected to a specific aromatic or aliphatic spacer unit have been synthesized, following a modular building block strategy. The hydrophilic dendrons are typically branched elements with peripheral carboxylic groups, unlike the hydrophobic dendrons that contain peripheral alkyl chains as part of respective amide functions. The hydrophilic dendrons are in different generations of branching, while the hydrophobic dendrons are all in the first generation of branching (three terminal branching), but differ in the length of the alkyl chains, thus giving rise to designed structure and amphiphilic properties in the new compounds. The resulting surfactants are capable of forming well-defined Langmuir films of remarkable stability when spread from a solution onto an aqueous subphase. Nevertheless, specific packing behaviour and orientation of the amphiphilic molecules were found, depending on the molecular structure, as determined using analysis of the surface pressure–area (π–A) isotherms. Langmuir–Blodgett transfer of the first monolayer from a pure water subphase to a clean silicon wafer proved possible for the amphiphiles of peripheral alkyl chain length C12, while the amphiphiles with the longer alkyl chains failed, possibly due to the more rigid monolayers they form, impeding the transfer.

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.

Head-to tail or head-to-head structure of Langmuir-Blodgett long chain ester multilayers?

Abstract The effect of the deposition velocity on the type of transfer (X-, Y- or XY-) of methylarachidate and methylbehenate monolayers is investigated. To provide conditions for deposition of monolayers, but not of three-dimensional aggregates, the effect of the temperature on monolayer stability at constant surface pressure is followed at first. It is found that the gradual increase of the deposition velocity results in a transition from X- to Y-type transfer of the layers. Transition from X- to Y-type deposition is also observed on increase of the number of deposited monolayers. Small angle X-ray diffraction investigation reveals repeated spacings of a bilayer (head-to-head) structure for Y-type deposition. Structural similarity is found for multilayers built up by either XY- or X-type transfer. The specific electron density profile of the methyl ester films might result in a misleading conclusion about head-to-tail molecular arrangement in the multilayers.

Impact of temperature on the LB patterning of DPPC on mica.

The influence of the subphase temperature on the stripe pattern formation during Langmuir-Blodgett transfer (LB patterning) is investigated in a combined experimental and theoretical study. According to our experiments on the LB transfer of dipalmitoylphosphatidylcholine (DPPC) on planar mica substrates, even small temperature changes between 21.5 and 24.5 °C lead to significant changes in the monolayer patterns. For a constant surface pressure and dipper speed, the width of the stripes and the overall spatial period of the patterns increase with increasing subphase temperature. Because the stripe patterns are ascribed to alternating monolayer domains in the liquid-expanded and the liquid-condensed phases, the working regime for the formation of stripes is found to depend strongly on the respective surface pressure-area isotherm. These experimental findings are in accordance with the results of a theoretical investigation based on a model that takes hydrodynamics and the monolayer thermodynamics into account.