Host–Guest Complexation of Amphiphilic Molecules at the Air–Water Interface Prevents Oxidation by Hydroxyl Radicals and Singlet Oxygen

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host–guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A‐12C) and negatively charged oleic acid (OA), a well‐known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air–water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field‐induced droplet ionization mass spectrometry results have demonstrated that the host–guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and “sleeved in” physical arrangement, rather than the chemical reactivity, of the complexes.     

pH‐Modulated Molecular Assemblies and Surface Properties of Metal–Organic Supercontainers at the Air–Water Interface

The orientation of metal–organic supercontainer (MOSC) molecules in Langmuir films was systematically studied at the air–water interface. The acidity of the aqueous subphases plays a significant role in tuning the orientation of MOSC molecules in the Langmuir films. Furthermore, Langmuir–Blodgett films of MOSCs were prepared and the uniform multilayer structures demonstrated various surface properties, depending on their conditions of fabrication. Our use of Langmuir films provides a novel approach to access tunable assemblies of MOSC molecules in two‐dimensional thin films.     

Molecular Structure and Dynamics of Water at the Water–Air Interface Studied with Surface‐Specific Vibrational Spectroscopy

Water interfaces provide the platform for many important biological, chemical, and physical processes. The water–air interface is the most common and simple aqueous interface and serves as a model system for water at a hydrophobic surface. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water at the water–vapor interface is essential for understanding the processes occurring on the water surface. At the water interface the network of very strong intermolecular interactions, hydrogen‐bonds, is interrupted and the density of water is reduced. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen‐bonded network and thus differ from those of bulk water. Herein, we discuss recent advances in the study of interfacial water at the water–air interface using laser‐based surface‐specific vibrational spectroscopy.     

Vesicle Formation from an Amphiphilic Porphyrin Derivative at the Air–Water Interface

A tetraphenyl porphyrin derivative with two C₁₆ alkyl chains covalently bound to each of the four peripheral phenyl rings through ether linkages formed multilayer clusters or vesicles at the air–water surface. More interestingly, spherical vesicles were also formed when deposited on appropriate solid surfaces, and these vesicles were stable even in dry conditions. Various microscopic images of the cast film deposited on a mica surface confirmed closed‐ended nanotube/nanorod‐type formation with necking and bulging. These narrow tubes are proposed to be intermediates for the formation of vesicles by fission at either side of the bulge. Such vesicular formation is not common when either cast or Langmuir–Blodgett films were deposited on a solid surface.     

Real‐Time Probing of Nanowire Assembly Kinetics at the Air–Water Interface by In Situ Synchrotron X‐Ray Scattering

Although many assembly strategies have been used to successfully construct well‐aligned nanowire (NW) assemblies, the understanding of their assembly kinetics has remained elusive, which restricts the development of NW‐based device and circuit fabrication. Now a versatile strategy that combines interfacial assembly and synchrotron‐based grazing‐incidence small‐angle X‐ray scattering (GISAXS) is presented to track the assembly evolution of the NWs in real time. During the interface assembly process, the randomly dispersed NWs gradually aggregate to form small ordered NW‐blocks and finally are constructed into well‐defined NW monolayer driven by the conformation entropy. The NW assembly mechanism can be well revealed by the thermodynamic analysis and large‐scale molecular dynamics theoretical evaluation. These findings point to new opportunities for understanding NW assembly kinetics and manipulating NW assembled structures by bottom‐up strategy.     

Kinetics of Oleate Adsorption at a Nigerian Hematite-Water Interface

The rate of adsorption of oleate soap onto a Nigerian hematite in an aqueous medium was determined from 29 to 60°C using the differential analysis method. The activation energy and frequency factor were determined at 57.1 kJ mol⁻¹ K⁻¹ and 4.0 × 10³ liter mol⁻¹ min⁻¹, respectively, indicating that the chemical processes are the slow, rate-determining step and that the reaction proceeds relatively fast. The adsorption isotherm was the Langmuir type: chemisorption was considered the dominant mode of adsorption. The desorption isotherm indicated a minor contribution of physical adsorption to the overall adsorption process.     

Selective adsorption behavior of polymer at the polymer–nanoparticle interface

Coarse‐grained molecular dynamics simulations are used to investigate the adsorption behavior of monodisperse and bidisperse polymer chains on the nanoparticle (NP) surface at various polymer–NP interactions, chain lengths, and stiffness. At a strong polymer–NP interaction, long chains preferentially occupy interfacial region and squeeze short chains out of the interfacial region. Semiflexible chains with proper stiffness wrap NPs dominantly in a helical fashion, whereas fully flexible chains constitute the surrounding matrix. As chain stiffness increases, the results of the preferential adsorption are the opposite. The chain‐length or chain‐stiffness‐induced selective adsorption behavior of polymer chains in the polymer–NP interfacial region relies on a delicate competition between entropic and enthalpic contributions to the total free energy. These results could provide insights into polymer–NP interfacial adsorption behavior and guide the design of high‐performance nanocomposites. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1829–1837     

Intramolecular Hydrogen Bonding Restricts Gd–Aqua‐Ligand Dynamics

Aqua ligands can undergo rapid internal rotation about the M−O bond. For magnetic resonance contrast agents, this rotation results in diminished relaxivity. Herein, we show that an intramolecular hydrogen bond to the aqua ligand can reduce this internal rotation and increase relaxivity. Molecular modeling was used to design a series of four Gd complexes capable of forming an intramolecular H‐bond to the coordinated water ligand, and these complexes had anomalously high relaxivities compared to similar complexes lacking a H‐bond acceptor. Molecular dynamics simulations supported the formation of a stable intramolecular H‐bond, while alternative hypotheses that could explain the higher relaxivity were systematically ruled out. Intramolecular H‐bonding represents a useful strategy to limit internal water rotational motion and increase relaxivity of Gd complexes.     

Nanostructure Synthesis at the Solid–Water Interface: Spontaneous Assembly and Chemical Transformations of Tellurium Nanorods

Bottom‐up synthesis offers novel routes to obtain nanostructures for nanotechnology applications. Most self‐assembly processes are carried out in three dimensions (i.e. solutions); however, the large majority of nanostructure‐based devices function in two dimensions (i.e. on surfaces). Accordingly, an essential and often cumbersome step in bottom‐up applications involves harvesting and transferring the synthesized nanostructures from the solution onto target surfaces. We demonstrate a simple strategy for the synthesis and chemical transformation of tellurium nanorods, which is carried out directly at the solid–solution interface. The technique involves binding the nanorod precursors onto amine‐functionalized surfaces, followed by in situ crystallization/oxidation. We show that the surface‐anchored tellurium nanorods can be further transformed in situ into Ag₂Te, Cu₂Te, and SERS‐active Au–Te nanorods. This new approach offers a way to construct functional nanostructures directly on surfaces.     

Investigation of Thermodynamic,Dynamic, and Structural Properties of H2 Adsorption on a Ag–Au Nanoalloy with a Carbon Nanotube Support

We have performed MD simulations to investigate H₂ adsorption on Ag–Au nanoclusters with the different Au mole fractions supported on the carbon nanotubes with the different diameters. Our thermodynamic results shown that the saturation value of coverage and the enthalpy of adsorption increases as the mole fraction of Au is increased. Our structural results showed that the presence of the H₂ gas exerts a significant effect on the nanocluster surface atoms and tends to stabilize the surface atoms on the nanocluster. Also, the structural changes are irreversible in such a way that by gradually decreasing the pressure to zero, the nanocluster geometry is not reversed to its initial structure in vacuum conditions. We have also shown that the nanoclusters have smaller values of the self‐diffusion coefficients in presence of H₂ molecules than those values in the initial state (vacuum), which is due to the increasing of the interface structure between the nanocluster and the nanotube.     

Zinc Porphyrin Metal‐Center Exchange at the Solid–Liquid Interface

Demetalation of zinc 5,10,15,20‐tetraphenylporphyrin (ZnTPP) under acidic conditions and ion exchange with Cu²⁺ ions at neutral pH are both rapid reactions in the liquid medium. However, for ZnTPP monolayers adsorbed on a Au(111) surface exposed to aqueous solution, we find that, although ion exchange takes place rapidly as expected, demetalation does not occur, even at pH values as low as 0. Based on this, we conclude that metal center exchange on the surface does not proceed through a free‐base porphyrin as an intermediate. Furthermore, once formed, CuTPP is stable on the surface and the reverse exchange from CuTPP to ZnTPP in the presence of Zn²⁺ ions could not be achieved. The preference for copper is so strong that even an attempt to exchange adsorbed ZnTPP with Ni²⁺ ions in the presence of traces of Cu²⁺ yielded CuTPP rather than NiTPP.     

Synthesis of Monodisperse Bi‐Compartmentalized Amphiphilic Janus Microparticles for Tailored Assembly at the Oil–Water Interface

Janus particles endowed with controlled anisotropies represent promising building blocks and assembly materials because of their asymmetric functionalities. Herein, we show that using the seeded monomer swelling and polymerization technique allows us to obtain bi‐compartmentalized Janus microparticles that are generated depending on the phase miscibility of the poly (alkyl acrylate) chains against the polystyrene seed, thus minimizing the interfacial free energy. When tetradecyl acrylate is used, complete compartmentalization into two distinct bulbs can be achieved, while tuning the relative dimension ratio of compartmented bulb against the whole particle. Finally, we have demonstrated that selectively patching the silica nanoparticles onto one of the bulb surfaces gives amphiphilicity to the particles that can assemble at the oil–water interface with a designated level of adhesion, thus leading to development of a highly stable Pickering emulsion system.     

Molecular dynamics simulations of aqueous ions at the liquid–vapor interface accelerated using graphics processors

Molecular dynamics (MD) simulations are a vital tool in chemical research, as they are able to provide an atomistic view of chemical systems and processes that is not obtainable through experiment. However, large‐scale MD simulations require access to multicore clusters or supercomputers that are not always available to all researchers. Recently, scientists have returned to exploring the power of graphics processing units (GPUs) for various applications, such as MD, enabled by the recent advances in hardware and integrated programming interfaces such as NVIDIA's CUDA platform. One area of particular interest within the context of chemical applications is that of aqueous interfaces, the salt solutions of which have found application as model systems for studying atmospheric process as well as physical behaviors such as the Hoffmeister effect. Here, we present results of GPU‐accelerated simulations of the liquid–vapor interface of aqueous sodium iodide solutions. Analysis of various properties, such as density and surface tension, demonstrates that our model is consistent with previous studies of similar systems. In particular, we find that the current combination of water and ion force fields coupled with the ability to simulate surfaces of differing area enabled by GPU hardware is able to reproduce the experimental trend of increasing salt solution surface tension relative to pure water. In terms of performance, our GPU implementation performs equivalent to CHARMM running on 21 CPUs. Finally, we address possible issues with the accuracy of MD simulaions caused by nonstandard single‐precision arithmetic implemented on current GPUs. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Two-Dimensional Crystallography of Amphiphilic Molecules at the Air–Water Interface

The advent of well-collimated, high-intensity synchrotron X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air–liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, α-amino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occurring by phase transitions. Furthermore, the surface X-ray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.     

Emission Control by Molecular Manipulation of Double‐Paddled Binuclear PtII Complexes at the Air‐Water Interface

Molecular functions depend on conformations and motions of the corresponding molecular species. An air–water interface is a suitable asymmetric field for the control of molecular conformations and motions under a small applied force. In this work, double‐paddled binuclear Pt^II complexes containing pyrazole rings linked by alkyl spacers were synthesized and their orientations and emission properties dynamically manipulated at the air–water interface. The complexes emerge from water with concurrent variation of interface orientation of the planes of the Pt^II complexes from perpendicular to parallel during mechanical compression suggesting a unique ‘submarine emission‘. Phosphorescence of the complexes is quenched at the air–water interface prior to monolayer formation with intensities subsequently rapidly increasing during monolayer compression. These results indicate that asymmetric reactions and motions might be controlled by applying mechanical force at the air–water interface.     

Direct Structural Identification of Gas Induced Gate‐Opening Coupled with Commensurate Adsorption in a Microporous Metal–Organic Framework

Gate‐opening is a unique and interesting phenomenon commonly observed in flexible porous frameworks, where the pore characteristics and/or crystal structures change in response to external stimuli such as adding or removing guest molecules. For gate‐opening that is induced by gas adsorption, the pore‐opening pressure often varies for different adsorbate molecules and, thus, can be applied to selectively separate a gas mixture. The detailed understanding of this phenomenon is of fundamental importance to the design of industrially applicable gas‐selective sorbents, which remains under investigated due to the lack of direct structural evidence for such systems. We report a mechanistic study of gas‐induced gate‐opening process of a microporous metal–organic framework, [Mn(ina)₂] (ina=isonicotinate) associated with commensurate adsorption, by a combination of several analytical techniques including single crystal X‐ray diffraction, in situ powder X‐ray diffraction coupled with differential scanning calorimetry (XRD‐DSC), and gas adsorption–desorption methods. Our study reveals that the pronounced and reversible gate opening/closing phenomena observed in [Mn(ina)₂] are coupled with a structural transition that involves rotation of the organic linker molecules as a result of interaction of the framework with adsorbed gas molecules including carbon dioxide and propane. The onset pressure to open the gate correlates with the extent of such interaction.     

Mechanochemical Tuning of the Binaphthyl Conformation at the Air–Water Interface

Gradual and reversible tuning of the torsion angle of an amphiphilic chiral binaphthyl, from ?90° to ?80°, was achieved by application of a mechanical force to its molecular monolayer at the air–water interface. This 2D interface was an ideal location for mechanochemistry for molecular tuning and its experimental and theoretical analysis, since this lowered dimension enables high orientation of molecules and large variation in the area. A small mechanical energy (<1 kcal mol^?1) was applied to the monolayer, causing a large variation (>50 %) in the area of the monolayer and modification of binaphthyl conformation. Single‐molecule simulations revealed that mechanical energy was converted proportionally to torsional energy. Molecular dynamics simulations of the monolayer indicated that the global average torsion angle of a monolayer was gradually shifted.