Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH3+. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD3+ and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH4 (formal hydride ion addition) and C2H6 (formal methyl anion addition) from a hydrocarbon surface and CH3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH3CF3 (formal addition of trifluoromethyl anion).     

Immobilized hyperbranched glycoacrylate films as bioactive supports

[Image: see text] We report on the low-pressure plasma immobilization, characterization and application of thin films of hyperbranched glycoacrylates, poly(3-O-acryloyl-alpha,beta-D-glucopyranoside) (AGlc), on PTFE-like fluorocarbon surfaces. This method is an efficient and versatile way to immobilize sugar-carrying branched acrylates as thin films of approximately 5 nm thickness on polymeric substrates while the functional groups and properties of the immobilized molecules are largely retained. The extent of poly(AGlc) degradation during plasma immobilization was investigated using FTIR-ATR spectroscopy and XPS. The thickness and topography of the immobilized films were characterized using spectroscopic ellipsometry and SFM, respectively. Studies of protein adsorption, as well as cell adhesion and proliferation on the poly(AGlc) surfaces, showed that these materials are suitable for the control of biointerfacial phenomena. Fluorescence images of fibronectin adsorbed on to the branched glycoacrylate with a mask.     

Prediction of conformational characteristics and micellar solution properties of fluorocarbon surfactants

A molecular-thermodynamic theory is developed to model the micellization of fluorocarbon surfactants in aqueous solutions, by combining a molecular model that evaluates the free energy of micellization of fluorocarbon surfactant micelles with a previously developed thermodynamic framework describing the free energy of the micellar solution. In the molecular model of micellization developed, a single-chain mean-field theory is combined with an appropriate rotational isomeric state model of fluorocarbon chains to describe the packing of the fluorocarbon surfactant tails inside the micelle core. Utilizing this single-chain mean-field theory, the packing free energies of fluorocarbon surfactants are evaluated and compared with those of their hydrocarbon analogues. We find that the greater rigidity of the fluorocarbon chain promotes its packing in micellar aggregates of low curvatures, such as bilayers. In addition, the mean-field approach is utilized to predict the average conformational characteristics (specifically, the bond order parameters) of fluorocarbon and hydrocarbon surfactant tails within the micelle core, and the predictions are found to agree well with the available experimental results. The electrostatic effects in fluorocarbon ionic surfactant micelles are modeled by allowing for counterion binding onto the charged micelle surface, which accounts explicitly for the effect of the counterion type on the micellar solution properties. In addition, a theoretical formulation is developed to evaluate the free energy of micellization and the size distribution of finite disklike micelles, which often form in the case of fluorocarbon surfactants. We find that, compared to their hydrocarbon analogues, fluorocarbon surfactants exhibit a greater tendency to form cylindrical or disklike micelles, as a result of their larger molecular volume as well as due to the greater conformational rigidity of the fluorocarbon tails. The molecular-thermodynamic theory developed is then applied to several ionic fluorocarbon surfactant-electrolyte systems, including perfluoroalkanoates and perfluorosulfonates with added LiCl or NH(4)Cl, and various micellar solution properties, including critical micelle concentrations (cmc's), optimal micelle shapes, and average micelle aggregation numbers, are predicted. The predicted micellar solution properties agree reasonably well with the available experimental results.     

Synthesis and solid state structure of fluorous probe molecules for fluorous separation applications

A series of colored hydrocarbon and fluorocarbon tagged 1-fluoro-4-alkylamino-anthraquinones and 1,4-bis-alkylamino-anthraquinone probe molecules were synthesized from a (fluorinated) alkyl amine and 1,4-difluoroanthraquinone to aid in the development of fluorous separation applications. The anthraquinones displayed stacking of the anthraquinone tricycle and interdigitation of the (fluorinated) alkyl chains in the solid state. Furthermore, intramolecular N-H?O hydrogen bonds forced the hydrocarbon and fluorocarbon tags into a conformation pointing away from the anthraquinone tricycle, with the angle of the tricycle plane normal and the main (fluorinated) alkyl vector ranging from 1° to 39°. Separation of the probe molecules on fluorous silica gel showed that the degree of fluorination of the probe molecules plays only a minor role with most eluents (e.g., hexane/ethyl acetate and methyl nonafluorobutyl ethers/ethyl acetate). However, toluene as eluent caused a pronounced separation by degree of fluorination for fluorocarbon, but not hydrocarbon tagged probe molecules on both silica gel and fluorous silica gel. These studies suggest that hydrocarbon and fluorocarbon tagged anthraquinones are useful probe molecules for the development of laboratory scale fluorous separation applications.     

Quantum chemistry studies of unbranched fluoropolymers

The results of quantum chemistry calculations of energy and topology parameters, vibration and NMR spectra of model fluorocarbon and unbranched hydrocarbon molecules are presented in this work. The formation of radicals and branches in fluorocarbon molecules and mechanisms of hydrogen substitution by fluorine at fluorination of hydrocarbon paraffins and polymers are discussed based on obtained results.     

Surface anchoring of rodlike molecules on corrugated substrates

We studied the mechanism of surface anchoring of rodlike molecules on substrates with the surfaces corrugated at molecular scale by molecular-dynamics simulation. We constructed a model for substrates that can have anisotoropic topographical patterns such as corrugation. The structural and thermodynamic properties of rodlike molecules on the corrugated surfaces, including the elastic and anchoring properties, were calculated and the influence of the surface structure on the anchoring was discussed. We found that the rodlike molecules are aligned along the grooves of the corrugated surfaces guided by the anisotropic molecular interaction between the molecules and the corrugated surface. The strength of anchoring was found to be increased when the period of corrugation is decreased at molecular level.     

From superamphiphobic to amphiphilic polymeric surfaces with ordered hierarchical roughness fabricated with colloidal lithography and plasma nanotexturing

Ordered, hierarchical (triple-scale), superhydrophobic, oleophobic, superoleophobic, and amphiphilic surfaces on poly(methyl methacrylate) PMMA polymer substrates are fabricated using polystyrene (PS) microparticle colloidal lithography, followed by oxygen plasma etching-nanotexturing (for amphiphilic surfaces) and optional subsequent fluorocarbon plasma deposition (for amphiphobic surfaces). The PS colloidal microparticles were assembled by spin-coating. After etching/nanotexturing, the PMMA plates are amphiphilic and exhibit hierarchical (triple-scale) roughness with microscale ordered columns, and dual-scale (hundred nano/ten nano meter) nanoscale texture on the particles (top of the column) and on the etched PMMA surface. The spacing, diameter, height, and reentrant profile of the microcolumns are controlled with the etching process. Following the design requirements for superamphiphobic surfaces, we demonstrate enhancement of both hydrophobicity and oleophobicity as a result of hierarchical (triple-scale) and re-entrant topography. After fluorocarbon film deposition, we demonstrate superhydrophobic surfaces (contact angle for water 168°, compared to 110° for a flat surface), as well as superoleophobic surfaces (153° for diiodomethane, compared to 80° for a flat surface).     

Orientational orders of small anisotropic molecules confined in slit pores

Based on a constant-pressure Monte Carlo molecular simulation, we have studied orientationally ordered transitions of small anisotropic molecules confined in two parallel hard walls. These molecules are modeled by the hard Gaussian overlap model. The molecular elongations of the chosen molecules are so small that the molecules cannot form stable liquid-crystal (LC) phases in the bulk. But in the slit pores, we found, while the distance between two walls of the pores decreases to the molecular scale, an orientationally ordered phase can form. It shows that even hard confining surfaces favor the alignment of the small anisotropic molecules. Thus we conclude that the required molecular elongation for forming LC phases will decrease in confinement. Our results indicate that some non-LC small molecules may form stable LC phases due to the inducement of confining surfaces.     

Dipole and quadrupole moments of molecules in crystals: a novel approach based on integration over Hirshfeld surfaces

Elegant expressions are derived for the computation of dipole and quadrupole moments of molecules using the electrostatic potential and electric field evaluated on an oriented molecular surface. These expressions are implemented for Hirshfeld surfaces, applied to various molecular crystals, and compared with the results from the quantum theory of atoms in molecules. The effect of intermolecular interactions is also explored by examining the differences between electrostatic moments derived from a periodic Hartree-Fock electron density and an electron density resulting from a superposition of noninteracting molecules. The enhancement of the dipole moment for hydrogen bonded molecular crystals is typically 30%-40% and shown to be largely independent of the partitioning scheme. Dipole moments calculated from Hirshfeld surfaces systematically underestimate those from zero-flux surfaces, a result attributed to the translation of the Hirshfeld surface relative to the zero-flux surfaces for these molecules. For acetylene and benzene, the differences between a crystal calculation and the sum of noninteracting molecules are small, and both partitioning schemes yield quadrupole and second moment results in close agreement.     

Properties of large organic molecules on metal surfaces

The adsorption of large organic molecules on surfaces has recently been the subject of intensive investigation, both because of the molecules’ intrinsic physical and chemical properties, and for prospective applications in the emerging field of nanotechnology. Certain complex molecules are considered good candidates as basic building blocks for molecular electronics and nanomechanical devices. In general, molecular ordering on a surface is controlled by a delicate balance between intermolecular forces and molecule–substrate interactions. Under certain conditions, these interactions can be controlled to some extent, and sometimes even tuned by the appropriate choice of substrate material and symmetry. Several studies have indicated that, upon molecular adsorption, surfaces do not always behave as static templates, but may rearrange dramatically to accommodate different molecular species. In this context, it has been demonstrated that the scanning tunnelling microscope (STM) is a very powerful tool for exploring the atomic-scale realm of surfaces, and for investigating adsorbate–surface interactions. By means of high-resolution, fast-scanning STM unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of largish molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces, and molecular self-assembly. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom–up fashion, collectively or one at a time. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. Here we will present a short review of some recent results, several of which were obtained by our group, in which several features of the complex interaction between large organic molecules and metal surfaces were revealed. The focus is on experiments performed using STM and other complementary surface-sensitive techniques.     

DYNAMICS OF SINGLE-MOLECULE ROTATIONS ON SURFACES DEPEND ON SYMMETRY, INTERACTIONS AND MOLECULAR SIZES

Rotating surface-mounted molecules have attracted attention of many research groups as a way to develop new nanoscale devices and materials. However, mechanisms of motion of these rotors at the single-molecule level are still not well understood. Theoretical and experimental studies on thioether molecular rotors on gold surfaces suggest that the size of the molecules, their flexibility and steric repulsions with the surface are important for dynamics of the system. A complex combination of these factors leads to the observation that the rotation speeds have not been hindered by increasing the length of the alkyl chains. However, experiments on diferrocene derivatives indicated that a significant increase in the rotational barriers for longer molecules. We present here a comprehensive theoretical study that combines molecular dynamics simulations and simple models to investigate what factors influence single-molecule rotations on the surfaces. Our results suggest that rotational dynamics is determined by the size and by the symmetry of the molecules and surfaces, and by interactions with surfaces. Our theoretical predictions are in excellent agreement with current experimental observations.     

Fluorocarbon Surfactant Polymers: Effect of Perfluorocarbon Branch Density on Surface Active Properties

We describe a series of fluorocarbon surfactant polymers designed for modifying fluorocarbon surfaces such as poly(tetrafluoroethylene). Novel fluorocarbon surfactant polymers poly(N-vinyldextranaldonamide-co-N-vinylperfluoroundecanamide), in which hydrophilic dextran oligosaccharides and hydrophobic perfluoroundecanoyl groups were incorporated sequentially onto a poly(vinylamine) backbone, were synthesized and characterized by FT-IR, NMR, and XPS spectroscopy. By adjusting the feed ratio of dextran to fluorocarbon branches, surfactant polymers with different hydrophilic/hydrophobic balances were prepared. The surface activity of the surfactants at the air/water interface was demonstrated by significant reductions in water surface tension. Surfactant adsorption and adhesion at the solid PTFE/aqueous interface were examined under well-defined dynamic flow conditions, using a rotating disk system. The surface activity at the air/water interface and adhesion stability on PTFE under an applied shear stress both increase with increasing density of fluorocarbon branches on the polymer backbone. The results show that stable surfactant adhesion on PTFE can be achieved by adjusting the hydrophilic dextran to hydrophobic fluorocarbon branch ratio.     

Electrochemically fabricated molecule–electrode contacts for molecular electronics

Connecting molecules to electrodes is key for a range of applications. Conventional methods typically involve a spontaneous reaction of thiol/disulfide-terminated molecules with metal surfaces. Although modifying metal surfaces with thiol chemistry is simple, it is limited to forming a specific S–metal bonding, which is labile and hence there are concerns regarding its mechanical instability. In addition, spontaneous grafting requires long processing times to achieve high molecular coverages on the surface, which adds challenges for manufacturing devices comprising molecular films. Electrochemical methods for forming molecular films on surfaces offer powerful advantages over traditional methods, including reaction acceleration, molecular coverage control, and guiding the chemical bonding at the molecule?electrode interface. Electrochemical grafting enables connecting molecules to various types of electrodes including those that cannot be functionalized by other methods. More recently, electrochemical approaches were expanded to enable connecting 2D materials to electrodes, opening a realm of possibilities for hybrid technologies. In this opinion, we survey the recent progress in electrochemical methods for connecting (bio) molecules to electrodes for advancing molecular and bioelectronics.     

Adsorption of chain molecules into a thin film structure and solvation interaction versus molecular flexibility

The influence of molecular flexibility on the properties of thin fluid films formed by linear chain molecules is studied by means of a singlet level of inhomogeneous integral equation theory. The considered m-mer chain molecules are formed through the polymerization of m hard-sphere beads with two sticky bonds randomly placed inside each bead core. Different molecular flexibility, from totally flexible up to almost completely rigid is reached by varying the interbead bonding length. The homogeneous properties of the same model that is necessary input to the singlet approach are extracted from the Wertheim’s theory of polymerization. The adsorption, local density distribution, disjoining pressure and solvation force of the chain molecule films confined by attractive and repulsive surfaces are analyzed. The obtained results indicate significant influence of the molecular flexibility on the film layering that is the origin of oscillations of solvation interaction arising between film surfaces. The oscillations of solvation pressure and force become more pronounced with restriction of molecular flexibility and with increase of bulk volume fraction of chain molecules. The decay of the oscillations across the film depends on the chain length and on the physical nature of the film surfaces, i.e. whether they are lyophilic or lyophobic. The partitioning of chain molecules from the bulk into the film strongly depends on the chain flexibility and this effect is more pronounced for the lyophilic surfaces.     

Effect of molecular weight on reaction-induced phase separation of epoxy resin modified with fluorocarbon chain terminated polyetherimide

Epoxy resins are currently used for many important applications such as adhesives, encapsulates and ad-vanced composite matrixes. However, the further use of epoxies is limited because of their inherent brittle-ness. Thus, the modifications of epoxy resin…     

Influence of the nature of non-polar phase on the mechanical stability of adsorption layers of hydrocarbon and fluorocarbon surfactants at the interface between their aqueous solutions and non-polar media

A variety of experimental approaches has been used for companson of the stabilizing effect with respect to droplets coalescence caused by the interfacial adsorption layers (IAL) of a nunber of hydrocarbon and fluorocarbon surfactants at the boundary between their aqueous solutions and various non-polar hydrocarbon and fluorocarbon liquids: (I) compression of two individual droplets in surfactant solution up to their coalescence and consequent tension and rupture of a newly formed drop; (II) evaluation of the free energy of interaction between non-polar surfaces by measuring the contact rupture force for smooth spherical particles; (III) rheological study of IAL by torque pendulum method; (IV) SEM observation of the IAL morphology; (V) study of the stability with respect to the Ostwald ripening. These observations reveal the predominant role of the lyophilic structure-mechanical barrier formed by the IAL as a factor of strong stabilization with respect to coalescence and particular dependence of the mechanical strength of such layer on the nature of the non-polar liquid and on the interaction between this liquid phase and hydrophobic parts of the surfactant molecules.     

Water at surfaces and interfaces: From molecules to ice and bulk liquid

The structure and growth of water films on surfaces is reviewed, starting from single molecules to two-dimensional wetting layers, and liquid interfaces. This progression follows the increase in temperature and vapor pressure from a few degrees Kelvin in ultra-high vacuum, where Scanning Tunneling and Atomic Force Microscopies (STM and AFM) provide crystallographic information at the molecular level, to ambient conditions where surface sensitive spectroscopic techniques provide electronic structure information. We show how single molecules bind to metal and non-metal surfaces, their diffusion and aggregation. We examine how water molecules can be manipulated by the STM tip via excitation of vibrational and electronic modes, which trigger molecular diffusion and dissociation. We review also the adsorption and structure of water on non-metal substrates including mica, alkali halides, and others under ambient humid conditions. We finally discuss recent progress in the exploration of the molecular level structure of solid-liquid interfaces, which impact our fundamental understanding of corrosion and electrochemical processes.     

A mild photoactivated hydrophilic/hydrophobic switch

Surface modification using light is one of the most powerful methods for controlling the physical and chemical properties offunctionalized surfaces. In this paper, we report on systems where soft UV irradiation (lambda = 365 nm) converts a "low" activity fluorocarbon to a "high" activity amine-functionalized surface. An amine-functionalized SAM (self-assembled monolayer) is first masked using a tertiary amine catalyzed reaction with an N-hydroxysuccinimidyl carbonyl reagent. This mild, room-temperature reaction introduces a hydrophobic photocleavable nitrobenzyl "protecting group" terminated with a fluorocarbon end-chain. UV irradiation (lambda = 365 nm) of this hydrophobic/fluorocarbon surface cleaves the nitrobenzyl residue, returning the surface to the original hydrophilic/amine-functionalized state. This provides a mild, generic method of producing surfaces with hydrophilic/hydrophobic patterns or patterned with amine functional residues. Two different protecting groups, one terminated with a single and the other with three fluorocarbon end chains, are compared. In the case of the more bulky protecting group, only a small proportion of the amine residues react, but the surface is equally hydrophobic and the amine residues equally well shielded from further reaction. Surfaces are characterized by X-ray photoelectron spectroscopy, ellipsometry, surface potential, and contact angle measurements. Images of the photopatterned SAMs were obtained using scanning electron microscopy.     

Roughness assessment and wetting behavior of fluorocarbon surfaces

The wetting behavior of fluorocarbon materials has been studied with the aim of assessing the influence of the surface chemical composition and surface roughness on the water advancing and receding contact angles. Diamond like carbon and two fluorocarbon materials with different fluorine content have been prepared by plasma enhanced chemical vapor deposition and characterized by X-ray photoemission, Raman and FT-IR spectroscopies. Very rough surfaces have been obtained by deposition of thin films of these materials on polymer substrates previously subjected to plasma etching to increase their roughness. A direct correlation has been found between roughness and water contact angles while a superhydrophobic behavior (i.e., water contact angles higher than 150° and relatively low adhesion energy) was found for the films with the highest fluorine content deposited on very rough substrates. A critical evaluation of the methods currently used to assess the roughness of these surfaces by atomic force microscopy (AFM) has evidenced that calculated RMS roughness values and actual surface areas are quite dependent on both the scale of observation and image resolution. A critical discussion is carried out about the application of the Wenzel model to account for the wetting behavior of this type of surfaces.