Chain orientation and headgroup structure in Langmuir monolayers of stearic acid and metal stearate (Ag, Co, Zn, and Pb) studied by infrared reflection-absorption spectroscopy

Electron transport through a (terpyridine)ruthenium metallo-surfactant containing a redox-active aminocatechol derivative

Aiming to develop a new class of metallosurfactants with unidirectional electron transfer properties, a (terpyridine) ruthenium complex containing a semiquinone derivative L², namely [Ru^III(L^terpy)(L²)Cl]PF₆ (1), was synthesized and structurally characterized as a solid and in solution. The electronic and redox behaviour of 1 was studied experimentally as well as by means of DFT methods, and is indicative of significant orbital mixing and overlap between metal and ligands. The complex forms stable Pockels-Langmuir films at the air-water interface and allows for the formation of thin films onto gold electrodes to prepare nanoscale Au|LB 1|Au junctions for current-voltage (I/V) analysis. Complex 1 shows asymmetric electron transfer with a maximum rectification ratio of 32 based on tunnelling through MOs of the aminocatechol derivative.

From Low to High Saturation Magnetization in Magnetite Nanoparticles: The Crucial Role of the Molar Ratios Between the Chemicals

In this study, a comprehensive characterization of iron oxide nanoparticles synthesized by using a simple one-pot thermal decomposition route is presented. In order to obtain monodisperse magnetite nanoparticles with high saturation magnetization, close to the bulk material, the molar ratios between the starting materials (solvents, reducing agents, and surfactants) were varied. Two out of nine conditions investigated in this study resulted in monodisperse iron oxide nanoparticles with high saturation magnetization (90 and 93% of bulk magnetite). The X-ray diffraction analyses along with the inspection of the lattice structure through transmission electron micrographs revealed that the main cause of the reduced magnetization in the other seven samples is likely due to the presence of distortion and microstrain in the particles. Although the thermogravimetric analysis, Raman and Fourier transform infrared spectroscopies confirmed the presence of covalently bonded oleic acid on the surface of all the samples, the particles with higher polydispersity and the lowest surface coating molecules showed the lowest saturation magnetization. Based on the observed results, it could be speculated that the changes in the kinetics of the reactions, induced by varying the molar ratio of the starting chemicals, can lead to the production of the particles with higher polydispersity and/or lattice deformation in their crystal structures. Finally, it was concluded that the experimental conditions for obtaining high-quality iron oxide nanoparticles, particularly the molar ratios and the heating profile, should not be chosen independently; for any specific molar ratio, there may exist a specific heating profile or vice versa. Because this synthetic consideration has rarely been reported in the literature, our results can give insights into the design of iron oxide nanoparticles with high saturation magnetization for different applications.

Infrared spectroscopy of 2-oxo-octanoic acid in multiple phases

Alpha-keto acids are environmentally and biologically relevant species whose chemistry has been shown to be influenced by their local environment. Vibrational spectroscopy provides useful ways to probe the potential inter- and intramolecular interactions available to them in several phases. We measure and compare the IR spectra of 2-oxo-octanoic acid (2OOA) in the gas phase, solid phase, and at the air-water interface. With theoretical support, we assign many of the vibrational modes in each of the spectra. In the gas phase, two types of conformers are identified and distinguished, with the intramolecularly H-bonded form being the dominant type, while the second conformer type identified does not have an intramolecular hydrogen bond. The van der Waals interactions between molecules in solid 2OOA manifest C-H and CO vibrations lower in energy than in the gas phase and we propose an intermolecular hydrogen bonding scheme for the solid phase. At the air-water interface the hydrocarbon tails of 2OOA do interact with each other while the carbonyls appear to interact with water in the subphase, but not with neighboring 2OOA as might be expected of a closely packed surfactant film.

One-step fabrication of soft calcium superhydrophobic surfaces by a simple electrodeposition process

A simple, one-step electrodeposition process was rapidly performed on a metal substrate to fabricate calcium superhydrophobic surfaces in an electrolyte containing calcium chloride (CaCl₂), myristic acid (CH₃(CH₂)₁₂COOH), and ethanol, which can avoid the intricate post-processing of surface treatment. The morphology and surface chemical compositions of the fabricated superhydrophobic surfaces were systematically examined by means of SEM, XRD, and FTIR, respectively. The results indicate that the deposited surfaces were mainly composed of calcium myristate, which can dramatically lower surface free energy. The shortest process for constructing a superhydrophobic surface is about 0.5 min, and the maximum contact angle of the as-prepared surfaces can reach as high as 166°, showing excellent superhydrophobicity. By adjusting the electrodeposition time, the structure of the cathodic surface transforms from the turfgrass structure, loose flower structures, larger and dense flower structures, secondary flower structures, and then into tertiary or more flower structures. The superhydrophobic surfaces showed excellent rebound performance with a high-speed camera. After a pressing force, their hardness increases, but the superhydrophobic performance is not weakened. Inversely, the bouncing performance is enhanced. This electrodeposition process offers a promising approach for large areas of superhydrophobic surfaces on conductive metals and strongly impacts the dynamics of water droplets.

We investigated a one-step method for calcium superhydrophobic surface preparation and researched the formation process of loose, flower-like microstructures. Also, we found that the pressing force strongly impacts the dynamics of water droplets.

Studies of Langmuir and Langmuir-Schaefer Films of Poly(3-Hexylthiophene) and Poly(Vinylidene Fluoride)

The synergistic use of blends of regioregular poly(3-hexylthiophene) (P3HT) and poly(vinylidene fluoride) (PVDF) or poly((vinylidene fluoride)-block-(methyl methacrylate)) (PVDF-PMMA) to form Langmuir and Langmuir-Schaefer (LS) films is reported. P3HT has wide applications in sensor devices because of its properties such as conductivity, luminescence, and chromism; however, the stiffness of the films and the difficulty in organizing the molecules may pose a problem in these applications. In this context, polymers based on PVDF can be used in the formation of thin P3HT films and present an alternative to improve the organization of P3HT molecules. In addition, PVDF acts as a plasticizer, making the film less rigid. The films were obtained from the blends of P3HT/PVDF and P3HT/PVDF-PMMA in a solution containing chloroform and DMAc (N,N-dimethylacetamide). Surface pressure isotherms, in situ ultraviolet-visible (UV-vis) spectroscopy, polarization-modulation infrared reflection-absorption spectroscopy, and Brewster angle microscopy techniques were used to analyze Langmuir films. The surface morphology of LS films was characterized by atomic force microscopy and UV-vis spectroscopy, and their degradation was analyzed by UV-vis spectroscopy after exposure to natural light under atmospheric conditions. The Langmuir films containing PVDF indicate a direct formation of the ferroelectric β phase, with dipoles parallel to the water surface. The Langmuir films formed by P3HT presented dipoles of side chains parallel and aromatic groups perpendicular to the water surface. P3HT and PVDF or PVDF-PMMA films show high molecular organization compared with pure P3HT films. The results suggest that these films could be used to improve the properties of P3HT in several device applications, such as in optical and electrical sensors.

Salting Up of Proteins at the Air/Water Interface

Vibrational sum frequency generation (VSFG) spectroscopy and surface pressure measurements are used to investigate the adsorption of a globular protein, bovine serum albumin (BSA), at the air/water interface with and without the presence of salts. We find at low (2 to 5 ppm) protein concentrations, which is relevant to environmental conditions, both VSFG and surface pressure measurements of BSA behave drastically different from at higher concentrations. Instead of emerging to the surface immediately, as observed at 1000 ppm, protein adsorption kinetics is on the order of tens of minutes at lower concentrations. Most importantly, salts strongly enhance the presence of BSA at the interface. This "salting up" effect differs from the well-known "salting out" effect as it occurs at protein concentrations well-below where "salting out" occurs. The dependence on salt concentration suggests this effect relates to a large extent electrostatic interactions and volume exclusion. Additionally, results from other proteins and the pH dependence of the kinetics indicate that salting up depends on the flexibility of proteins. This initial report demonstrates "salting up" as a new type of salt-driven interfacial phenomenon, which is worthy of continued investigation given the importance of salts in biological and environmental aqueous systems.

Hybrid Oleate-Iodide Ligand Shell for Air-Stable PbSe Nanocrystals and Superstructures

A postsynthetic treatment is presented to improve the air stability of PbSe nanocrystals (NCs) and PbSe square superstructures. The addition of z-type Pb(oleate)₂ ligands together with x-type iodide ligands creates a hybrid ligand shell containing both ligands. The air stability of the PbSe NCs is checked by enduring absorption spectroscopy under ambient conditions. With a combined NaI + Pb(oleate)₂ treatment, the absorption spectrum remains unchanged for several days under ambient conditions. Fourier transform infrared spectroscopy shows that the surface coordination of the oleate ligands changes by the chemical treatment: from mixed chelating bidentate + bridging to Pb for the pristine nanocrystals to almost exclusive chelating bidentate coordination after chemical passivation. The shift of the C-H stretching vibration shows that the oleate hydrocarbon layer is in a more liquidlike state after the chemical treatment, suggesting that oleate and iodide ligands are often present on adjacent surface positions.

Molecular rectifiers based on five-coordinate iron(iii)-containing surfactants

The state-of-the-art of metallorganic-based molecular rectification is reviewed with an emphasis on asymmetric five-coordinate Fe^III-containing surfactants in electrode|LB film|electrode assemblies.

Aqueous-phase aerosols on the air-water interface: Response of fatty acid Langmuir monolayers to atmospheric inorganic ions

Atmospheric aerosol particles composed of a mixture of organic and inorganic compounds are common and constitute an important fraction of air pollutants. In this study, the activities of common atmospheric inorganic ions (Ag⁺, Zn²⁺, Fe³⁺, Fe²⁺, Ca²⁺ and Al³⁺) and fatty acid molecules (stearic acid and arachidic acid) at air-aqueous interface were investigated by Langmuir methods and infrared reflection-absorption spectroscopy (IRRAS). In the presence of different inorganic ions, surface pressure-area isotherms of the Langmuir films showed compressed or expanded characteristics. IRRAS spectra confirmed that the existence of inorganic ions in the fatty acid monolayer changes the surface properties of aqueous-phase aerosols. Formation of different coordination types of carboxylates at the air-water interface alters the dissolution and partitioning behavior, which has significant influence of Raoult effect on nucleating cloud droplets. Our work displays the relationship between structure and surface properties for aqueous-phase aerosols and implies an efficient method for further understanding of their formation mechanism and potential atmospheric implications.

Monolayer effect of a gemini surfactant with a rigid biphenyl spacer on its self-crystallization at the air/liquid interface

A gemini surfactant with a biphenyl spacer can spontaneously generate crystals at the air/solution interface. X-ray crystallography reveals that surfactant molecules exhibit an almost fully extended conformation with interdigitating alkyl chains, together with an approximate co-planarity of two C—C—C planes in two alkyl chains of one gemini molecule, and a prominent dihedral angle between the benzene rings and C—C—C planes of the alkyl chains. Infrared reflection–absorption spectroscopy shows that the gemini surfactant was stretched at the air/water interface, with the hydrocarbon chains oriented at a tilt angle of ∼75° with respect to the surface normal. In particular, the biphenyl group is more or less perpendicular to the water surface, and the C—C—C plane of the alkyl chain tends to be parallel to the water surface. Both results point out a remarkable similarity in the molecular conformation between the crystal and the monolayer. Meanwhile, dynamic light scattering and transmission electron microscopy results indicate that the crystallization of such gemini surfactants at the interface is contrary to the crystallization behavior in the bulk phase, meaning that the surfactant solution can only form a supersaturated solution as it is cooled, though the crystallization temperature of 296 K is lower than the Krafft temperature (∼303 K). Therefore, our findings indicate that the Gibbs monolayer of the gemini surfactant plays a critical role in its interfacial crystallization. Additionally, multiple weak intermolecular interactions, involving van der Waals interaction, π–π stacking and cationic–π interactions, as well as the hydrophobic effect during the aggregation of the gemini molecule in solution, are responsible for the formation of the interfacial crystal.

Study of the aggregation of human insulin Langmuir monolayer

The human insulin (HI) Langmuir monolayer at the air-water interface was systematically investigated in the presence and absence of Zn(II) ions in the subphase. HI samples were dissolved in acidic (pH 2) and basic (pH 9) aqueous solutions and then spread at the air-water interface. Spectroscopic data of aqueous solutions of HI show a difference in HI conformation at different pH values. Moreover, the dynamics of the insulin protein showed a dependence on the concentration of Zn(II) ions. In the absence of Zn(II) ions in the subphase, the acidic and basic solutions showed similar behavior at the air-water interface. In the presence of Zn(II) ions in the subphase, the surface pressure-area and surface potential-area isotherms suggest that HI may aggregate at the air-water interface. It was observed that increasing the concentration of Zn(II) ions in the acidic (pH 2) aqueous solution of HI led to an increase of the area at a specific surface pressure. It was also seen that the conformation of HI in the basic (pH 9) medium had a reverse effect (decrease in the surface area) with the increase of the concentration of Zn(II) ions in solution. From the compression-decompression cycles we can conclude that the aggregated HI film at air-water interface is not stable and tends to restore a monolayer of monomers. These results were confirmed from UV-vis and fluorescence spectroscopy analysis. Infrared reflection-absorption and circular dichroism spectroscopy techniques were used to determine the secondary structure and orientation changes of HI by zinc ions. Generally, the aggregation process leads to a conformation change from α-helix to β-strand and β-turn, and at the air-water interface, the aggregation process was likewise seen to induce specific orientations for HI in the acidic and basic media. A proposed surface orientation model is presented here as an explanation to the experimental data, shedding light for further research on the behavior of insulin as a Langmuir monolayer.

In situ studies of metal coordinations and molecular orientations in monolayers of amino-acid-derived Schiff bases at the air-water interface

The surface behaviors of monolayers of amino-acid-derived Schiff bases, namely, 4-(4-(hexadecyloxy)benzylideneamino)benzoic acid (HBA), at the air-water interface on pure water and ion-containing subphases (Cu2+, Ca2+, and Ba2+) have been clarified by a combination of surface pressure-area isotherms and surface plasmon resonance (SPR) technique, and the metal coordinations and molecular orientations in the monolayers have been investigated using in situ infrared reflection absorption spectroscopy (IRRAS). The presence of metal ions gives rise to condensation of the monolayers (Cu2+, pH 6.1; Ca2+, pH 11; Ba2+, pH 10), even leading to the formation of three-dimensional structures of the compressed monolayer in the case of Ba2+ (pH 12). The metal coordinations with the carboxyl groups at the interface depend on the type of metal ions and pH of the aqueous subphase. The orientations of the aromatic Schiff base segments with surface pressure are elaborately described. The spectral behaviors of the Schiff base segments with incidence angle in the case of Ba2+ (pH 12) have so far presented an excellent example for the selection rule of IRRAS at the air-water interface for p-polarization with vibrational transition moments perpendicular to the water surface. The chain orientations in the monolayers are quantitatively determined on the assumption that the thicknesses of the HBA monolayers at the air-water interface are composed of the sublayers of alkyl chains and Schiff base segments.

Interfacial polygonal nanopatterning of stable microbubbles

Micrometer-sized bubbles are unstable and therefore difficult to make and store for substantial lengths of time. Short-term stabilization is achieved by the addition of amphiphilic molecules, which reduce the driving force for dissolution. When these molecules crystallize on the air/liquid interface, the lifetime of individual bubbles may extend over a few months. We demonstrated low gas-fraction dispersions with mean bubble radii of less than 1 micrometer and stability lasting more than a year. An insoluble, self-assembled surfactant layer covers the surface of the microbubbles, which can result in nanometer-scale hexagonal patterning that we explain with thermodynamic and molecular models. The elastic response of the interface arrests the shrinkage of the bubbles. Our study identifies a route to fabricate highly stable dispersions of microbubbles.

Infrared reflection-absorption spectroscopy and polarization-modulated infrared reflection-absorption spectroscopy studies of the organophosphorus acid anhydrolase langmuir monolayer

The secondary structure of the organophosphorus acid anhydrolase (OPAA) Langmuir monolayer in the absence and presence of diisopropylfluorophosphate (DFP) in the subphase was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS (PM-IRRAS). The results of both the IRRAS and the PM-IRRAS indicated that the alpha-helix and the beta-sheet conformations in OPAA were parallel to the air-water interface at a surface pressure of 0 mN.m-1 in the absence of DFP in the subphase. When the surface pressure increased, the alpha-helix and the beta-sheet conformations became tilted. When DFP was added to the subphase at a concentration of 1.1 x 10(-5) M, the alpha-helix conformation of OPAA was still parallel to the air-water interface, whereas the beta-sheet conformation was perpendicular at 0 mN.m-1. The orientations of both the alpha-helix and the beta-sheet conformations did not change with the increase of surface pressure. The shape of OPAA molecules is supposed to be elliptic, and the long axis of OPAA was parallel to the air-water interface in the absence of DFP in the subphase, whereas the long axis became perpendicular in the presence of DFP. This result explains the decrease of the limiting molecular area of the OPAA Langmuir monolayer when DFP was dissolved in the subphase.

Infrared reflection-absorption spectroscopy and polarization-modulated infrared reflection-absorption spectroscopy studies of the aequorin langmuir monolayer

The Langmuir monolayer of aequorin and apoaequorin was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS techniques. The alpha-helices in the aequorin Langmuir monolayer were parallel to the air-water interface at zero surface pressure. When the surface pressure increased to 15 mN.(m-1), the alpha-helices became tilted and the turns became parallel to the air-water interface. As for apoaequorin, the alpha-helices were also parallel to the air-water interface at 0 mN.m(-1). However, the alpha-helix became tilted and the turns became parallel to the air-water interface quickly at 5 mN.m(-1). With further compression of the apoaequorin Langmuir monolayer, the orientation remained the same. The different behaviors of aequorin and apoaequorin at the air-water interface were explained by the fact that aequorin formed dimers at the air-water interface but apoaequorin was a monomer. It is more difficult for a dimer to be tilted by the compression of the Langmuir monolayer.

Determination of chain orientation in the monolayers of amino-acid-derived schiff base at the air-water interface using in situ infrared reflection absorption spectroscopy

The chain orientation in the monolayers of amino-acid-derived Schiff base, 4-(4-dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA), at the air-water interface has been determined using infrared reflection absorption spectroscopy (IRRAS). On pure water, a condensed monolayer is formed with the long axes of Schiff base segments almost perpendicular to the water surface. In the presence of metal ions (Ca2+, Co2+, Zn2+, Ni2+, and Cu2+) in the subphase, the monolayer is expanded and the long axes of the Schiff base segments are inclined with respect to the monolayer normal depending on metal ion. The monolayer thickness, which is an important parameter for quantitative determination of orientation of hydrocarbon chains, is composed of alkyl chains and salicylideneaniline portions for the DSA monolayers. The effective thickness of the Schiff base portions is roughly estimated in the combination of the IRRAS results and surface pressure-area isotherms for computer simulation, since the only two observable p- and s-polarized reflectance-absorbance (RA) values can be obtained. The alkyl chains with almost all-trans conformations are oriented at an angle of about 10 degrees for H2O, 15 degrees for Ca2+, 30 degrees for Co2+, 35 degrees -40 degrees for Zn2+, and 35 degrees -40 degrees for Ni2+ with respect to the monolayer normal. The chain segments linked with gauche conformers in the case of Cu2+ are estimated to be 40 degrees -50 degrees away from the normal.

Protein-directed assembly of binary monolayers at the interface and surface patterns of protein on the monolayers

Ferritin-directed assembly of binary monolayers of zwitterionic dipalmitoylphosphatidylcholine and cationic dioctadecyldimethylammonium bromide (DOMA) at the interface and surface patterns of ferritin on the monolayers have been investigated using a combination of infrared reflection absorption spectroscopy, surface plasmon resonance, and atomic force microscopy. Ferritin binding to the binary monolayers at the air-water interface at the surface pressure 30 mN/m, primarily driven by the electrostatic interaction, gives rise to a change in tilt angle of hydrocarbon chains from 15 degrees +/- 1 degrees to 10 degrees +/- 1 degrees with respect to the normal of the monolayer at the mole fraction of DOMA (XDOMA) of 0.1. The chains at XDOMA = 0.3 are oriented vertical to the water surface before and after protein binding. A new mechanism for protein binding to the binary monolayers is proposed. The secondary structures of the adsorbed ferritin are prevented from changing to some extent due to the existence of the monolayers. The amounts of the bound protein on the monolayers at the air-water interface are increased in comparison with those on the pre-immobilized monolayers at low XDOMA. The increased amounts and different patterns of the adsorbed protein at the monolayers are mostly attributed to the formation of multiple binding sites available for ferritin, which is due to the lateral reorganization of the lipid components in the monolayers induced by the protein in the subphase. The created multiple binding sites on the monolayer surfaces through the protein-directed assembly can be preserved for subsequent protein binding.