Dense Monolayer Films Research Articles

Molecular Dynamics Simulations of the Short-Chain Fluorocarbon Surfactant PFHXA and the Anionic Surfactant SDS at the Air/Water Interface.

The research and development of alternatives to long-chain fluorocarbon surfactants are desperately needed because they are extremely toxic, difficult to break down, seriously harm the environment, and limit the use of conventional aqueous film-forming foam fire extinguishing agents. In this study, mixed surfactant systems containing the short-chain fluorocarbon surfactant perfluorohexanoic acid (PFHXA) and the hydrocarbon surfactant sodium dodecyl sulfate (SDS) were investigated by molecular dynamics simulation to investigate the microscopic properties at the air/water interface at different molar ratios. Some representative parameters, such as surface tension, degree of order, density distribution, radial distribution function, number of hydrogen bonds, and solvent-accessible surface area, were calculated. Molecular dynamics simulations show that compared with a single type of surfactant, mixtures of surfactants provide superior performance in improving the interfacial properties of the gas-liquid interface. A dense monolayer film is formed by the strong synergistic impact of the two surfactants. Compared to the pure SDS system, the addition of PFHXA caused SDS to be more vertically oriented at the air/water interface with a reduced tilt angle, and a more ordered structure of the mixed surfactants was observed. Hydrogen bonding between SDS headgroups and water molecules is enhanced with the increasing PFHXA. The surface activity is arranged in the following order: PFHXA/SDS = 1:1 > PFHXA/SDS = 3:1 > PFHXA/SDS = 1:3. These results indicate that a degree of synergistic relationship exists between PFHXA and SDS at the air/water interface.

Synergistic Adsorption and Molecular Arrangement of Mixed Surfactants at the Air/Water Interface

The self-assembly of mixed surfactants at the air/water interface plays a crucial role in mineral flotation separation. This study aimed to investigate the molecular arrangement and synergistic adsorption of mixed surfactants at the air/water interface using molecular dynamics (MD) simulation and surface tension measurement. The stability of mixed surfactant solutions was also measured using dynamic light scattering and ζ potential. We investigated the mixture of octylhydroxamic acid (OHA) and dodecanol (DOD) surfactants at different weight ratios. The surface activity of the OHA/DOD mixture was characterized in detail, including the relative concentration distribution, structural properties, and radial distribution function. The MD simulations revealed that OHA/DOD was inserted into the water as a headgroup at the air/water interface, and the carbon chain extended into the air. A strong synergistic effect was observed between the two surfactants, and a dense monolayer film was formed at the air/water interface, demonstrating higher surface activity. With an increase in the DOD ratio, the hydrogen bond between the OHA headgroup and water molecules became stronger. The order of surface activity of the mixture was OHA:DOD = 1:1 > OHA:DOD = 2:1 > OHA:DOD = 4:1. In the temperature range from 24 to 30 °C, the surface tension decreased and the critical micelle concentration increased with increasing temperature. The addition of DOD changed the conformation of the mixed surfactants, which was beneficial to the aggregation of OHA/DOD complexes. When OHA:DOD = 1:1, the average particle size was the largest and the formed micelles were the most stable. The experimental results are consistent with the surface tension and simulation results.

Influence of the Surface Chemistry of Metal-Organic Polyhedra in Their Assembly into Ultrathin Films for Gas Separation.

The formation of ultrathin films of Rh-based porous metal–organic polyhedra (Rh-MOPs) by the Langmuir–Blodgett method has been explored. Homogeneous and dense monolayer films were formed at the air–water interface either using two different coordinatively alkyl-functionalized Rh-MOPs (HRhMOP(diz)12 and HRhMOP(oiz)12) or by in situ incorporation of aliphatic chains to the axial sites of dirhodium paddlewheels of another Rh-MOP (OHRhMOP) at the air–liquid interface. All these Rh-MOP monolayers were successively deposited onto different substrates in order to obtain multilayer films with controllable thicknesses. Aliphatic chains were partially removed from HRhMOP(diz)12 films post-synthetically by a simple acid treatment, resulting in a relevant modification of the film hydrophobicity. Moreover, the CO2/N2 separation performance of Rh-MOP-supported membranes was also evaluated, proving that they can be used as selective layers for efficient CO2 separation.

Highly reproducible SERS sensor based on self-assembled Au nanocubic monolayer film for sensitive and quantitative detection of glutathione

The detection of glutathione (GSH) in human serum is significant for the clinical diagnosis of various diseases, such as Alzheimer's disease and cancer. A sensitive surface enhanced Raman scattering (SERS) sensor is proposed to detect low-concentration biomolecule GSH. The sensor is built based on a dense monolayer film of self-assembly Au Nano cubes. The film has an excellent enhancement effect on the probe molecule DTNB. After the GSH molecule is continuously added, the disulfide bond in DTNB breaks into TNB molecule. In a neutral or alkaline environment, TNB will be hydrolyzed into anions, and bind to the surface of the positively charged Au NCs film through electrostatic interaction, thereby enhancing the Raman spectroscopy of TNB. As a result, the proposed sensor can detect low concentrations GSH. The detection limit is 50 nM, and the relative spectral intensity of TNB characteristic peak existed good linear relationship with the concentration of GSH between 50 and 750 nM. The SERS substrate used in the sensor is easy to prepare, with high uniformity and reproducibility, the sample does not require complicated pre-processing and preparation. The SERS sensor provides a novel strategy for sensitive and quantitative detection of the trace analytes and has potential for clinical applications.

Fabrication of ultrathin MIL-96(Al) films and study of CO2 adsorption/desorption processes using quartz crystal microbalance

This contribution reports the fabrication and characterization of ultrathin films of nanoparticles of the water stable microporous Al tricarboxylate metal organic framework MIL-96(Al). The preparation of MOF dispersions in chloroform has been optimized to obtain dense monolayer films of good quality, without nanoparticle agglomeration, at the air-water interface that can be deposited onto solid substrates of different nature without any previous substrate functionalization. The MOF studied shows great interest for CO2 capture because it presents Al3+ Lewis centers and hydroxyl groups that strongly interact with CO2 molecules. A comparative CO2 adsorption study on drop-cast, Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films using a Quartz Crystal Microbalance-based setup (QCM) has revealed that the CO2 uptake depends strongly on the film fabrication procedure and the storage conditions. Noteworthy the CO2 adsorption capacity of LB films is increased by 30% using a simple and green treatment (immersion of the film into water during 12 h just after film preparation). Finally, the stability of LB MOF monolayers upon several CO2 adsorption/desorption cycles has been demonstrated, showing that CO2 can be easily desorbed from the films at 303 K by flowing an inert gas (He). These results show that MOF LB monolayers can be of great interest for the development of MOF-based devices that require the use of very small MOF quantities, especially gas sensors.

Dense monolayer films of atomically precise graphene nanoribbons on metallic substrates enabled by direct contact transfer of molecular precursors.

Atomically precise graphene nanoribbons (GNRs) of two types, chevron GNRs and N = 7 straight armchair GNRs (7-AGNRs), have been synthesized through a direct contact transfer (DCT) of molecular precursors on Au(111) and gradual annealing. This method provides an alternative to the conventional approach for the deposition of molecules on surfaces by sublimation and simplifies preparation of dense monolayer films of GNRs. The DCT method allows deposition of molecules on a surface in their original state and then studying their gradual transformation to polymers to GNRs by scanning tunneling microscopy (STM) upon annealing. We performed STM characterization of the precursors of chevron GNRs and 7-AGNRs, and demonstrate that the assemblies of the intermediates of the GNR synthesis are stabilized by π-π interactions. This conclusion was supported by the density functional theory calculations. The resulting monolayer films of GNRs have sufficient coverage and density of nanoribbons for ex situ characterization by spectroscopic methods, such as Raman spectroscopy, and may prove useful for the future GNR device studies.

Adsorption and lubricating properties of HFBII hydrophobins and diblock copolymer poly(methyl methacrylate-b-sodium acrylate) additives in water-lubricated copper vs. a-C:H contacts

In a metal forming process the adhesion between the workpiece and the tool needs to be minimised, which can be achieved by use of lubricants and coatings. Here adsorption and lubrication properties of HFBII hydrophobins and diblock copolymer poly(methyl methacrylate-b-sodium acrylate) in water-lubricated copper vs. a-C:H coating contacts were studied by Surface Plasmon Resonance (SPR) and by a pin-on-disc (POD) tribometer. Hydrophobins formed a dense monolayer film on a-C:H surface and reduced friction by 13–30% but increased the wear of copper compared to pure water lubrication. Poly(methyl methacrylate-b-sodium acrylate) formed a sparse lubricating layer compared to HFBII lubricated contacts, but the friction coefficient was lower. HFBII molecules prevented copper oxide tribofilm formation on the copper pin.

Studying electron transfer reaction at the Au/n-decanethiol/aqueous solution of NaNO3 interface by electrochemical impedance spectroscopy

The electrochemical behavior of the gold/electrolyte interface in aqueous 1 M NaNO3 solutions in the presence of an organic monolayer of n-decanethiol (CH3(CH2)9S) is studied by electrochemical impedance spectroscopy in the frequency range of 10–105 Hz and also by cyclic voltammetry. It is experimentally shown that in the potential interval from 0 to −0.5 V (vs. SCE), the dense monolayer film decreases the measured current density approximately 40-fold. The measured capacitance falls down to 1–2 μF/cm2. Based on the analysis of impedance characteristics acquired with the use of empirical equivalent circuits comprising ideal and nonideal analogues of electric circuits, the tentative estimates of the thickness of organic monolayers formed on Au electrodes with various roughness factors are obtained. Using the complex nonlinear regression (CNLS) method and a model of microarray electrode, the porous structure of adsorbed monolayers is revealed and the transition frequency of interfaces under study is determined. The degree of inhibition of the electron transfer across the Au/n-decanethiol/solution interface is determined by comparing the rate constants for the Ru[(NH3)6]3+/2+ redox process on clean and modified electrodes. The acquired results are compared with available literature data.

Individual single-walled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes.

Single-walled carbon nanotubes were oxidized by a technique previously developed for the oxidation of graphite to graphite oxide (GO). This process involves treatment with concentrated H(2)SO(4) containing (NH(4))(2)S(2)O(8) and P(2)O(5), followed by H(2)SO(4) and KMnO(4). Oxidation results in complete exfoliation of nanotube ropes to yield individual oxidized tubes that are 40-500 nm long. The C:O:H atomic ratio of vacuum-dried oxidized nanotubes is approximately 2.7:1.0:1.2. XPS and IR spectra show evidence for surface O-H, C=O, and COOH groups. The oxidized nanotubes slowly form viscous hydrogels at unusually low concentration (>or=0.3 wt %), and this behavior is attributed to the formation of a hydrogen-bonded nanotube network. The oxidized tubes bind readily to amine-coated surfaces, on which they adsorb as smooth and dense monolayer films. Thin films of the oxidized nanotubes show ohmic current-voltage behavior, with resistivities in the range of 0.2-0.5 Omega-cm.

Self-assembled monolayer of a redox-active calix[4]arene: voltammetric recognition of the Ba2+ ion in aqueous media.

The Redox-active monolayer of a novel calix[4]arene recognizing redox-inactive ionic species by voltammetry is reported. Calix[4]arene-disulfide-diquinone, which is not only redox-active but is also a highly selective ionophore for the Ba2+ ion, spontaneously forms a stable and dense monolayer film on gold. The redox-active calixarene monolayer selectively recognizes Ba2+ ion in aqueous media, and the voltammetric signals are proportional to the ionic concentration. A new voltammetric peak can be detected by square-wave voltammetry upon adding a dilute solution containing Ba2+ ion having a concentration as low as 1.0 x 10(-6) M. The Langmuir plot (1/ip vs 1/[Ba2+]) shows a linear slope in the range from 1.0 x 10(-6) M to 1.0 x 10(-4) M. This modified electrode does not show any significant interference from alkali and alkaline earth metal ions except for Sr2+ and Ca2+. Only 100- and 500-fold concentrations of Sr2+ and Ca2+ ions, respectively, can lead to voltammetric responses comparable to that of Ba2+.

Phase Transitions in Adsorbed Layers Formed on the (100) Plane of Face Centered Cubic Crystals

The Monte Carlo simulation method is used to study the structure and phase transitions in adsorbed films formed on the (100) plane of the model face-centered cubic crystals characterized by the different corrugation of the surface potential. The systems consisting of the adsorbate atoms that tend to form the c(2 × 2) registered phase are studied. It is shown that the actual structure of the dense monolayer films depends on the corrugation of the surface potential and on the density of the adsorbed layer and the temperature. The mechanism of melting transition in submonolayer, monolayer, and bilayer films is discussed. In particular, it is demonstrated that the submonolayer (incommensurate and registered) films retain monolayer character upon melting, whereas melting of a dense (incommensurate) two-dimensional solid is accompanied by the promotion of the second layer. The phase diagrams for a series of systems are determined. It is shown that the location of the critical point for the first-layer condensation is affected considerably by the surface corrugation. In the second layer formed on weakly corrugated surfaces and built on top of the dense incommensurate first layer, the effects due to the surface potential corrugation seem to be negligible. One observes the strong influence of the first-layer roughness (induced by the surface potential corrugation) on the critical temperature of the second-layer condensation only in the strongly corrugated surfaces.

The second‐generation polysulfone gas‐separation membrane. I. The use of lewis acid: Base complexes as transient templates to increase free volume

AbstractThe second‐generation polysulfone (PSU) gas‐separation membrane is seen as a trilayer that is considerably more permeable and at least as selective as the first‐generation bilayer that it has replaced. In air separation, a fourfold increase in oxygen permeabiliy has been obtained with no loss in oxygen/nitrogen selectivity. The enhanced performance is the result of a membrane skin that is not only thinner, but also exhibits increased free volume and a graded density. The key to the emergency of the trilayer morphology was the discovery of a hitherto unsuspected relationship between the size of solvent molecules within a sol and the free volume and permeability in the resultant gel! Solvent molecules with a molar volume V > ˜ 147 cc/mol function as transient templates (spacers) that decrease macromolecular packing density. As a practical matter, the low diffusivity (difficult extractibility) of large solvent molecules is circumvented by the use of 1:1 Lewis acid: base (A:B) complexes such as propionic acid: N‐methyl pyrrolidone instead of neat solvents. Complexes whose acid and base strengths, respectively, lie between (Gutmann 47 < AN < 53 and 27 < DN < 28) are sufficiently stable to function as templates, while at the same time exhibiting the hydrolytic instability that leads to their ready disassociation and extraction by water. Selectivity is maintained by the use of A:B complexes whose Hildebrand solubility parameters differ from that of PSU by less than ˜ 1.3 (cal/cc)½. The emergence of the trilayer membrane is considered to be the second decoupling of permeability from selectivity. By the formation of an anisotropic (graded density) skin, permeability has been increased and selectivity maintained. This is analogous to the first decoupling by Loeb and Sourirajan who essentially replaced a thick dense monolayer film with a bilayer consisting of a thin skin of uniform density in series with a thick porous substructure.

Bilayer lipid membrane permeation and rupture due to hole formation

A theory is developed for the permeation and rupture of bilayer lipid membranes due to fluctuation formation of holes (or pores) in them. The two monolayers of the bilayer lipid membrane are considered as mutually adsorbed on each other and the bilayer lipid membrane equilibrium is described by an adsorption isotherm in mean field approximation. The theory of nucleation is used for determination of the work for hole formation and the hole equilibrium size distribution as functions of the concentration C of monomer lipid in the solution. The bilayer lipid membrane permeation and rupture are analyzed from a unified point of view and expressions are derived for the dependence of the bilayer lipid membrane diffusion permeability coefficient and lifetime on C. The effect of foreign bodies (e.g., proteins) on the bilayer lipid membrane permeation and rupture is considered and a possible experimental application of the theory is discussed. The results obtained are directly applicable to dense monolayer films on liquid surfaces.