Mean amplitudes of vibration and thermodynamic properties of methyl cyanide and methyl isocyanide

The mean amplitudes of vibration of methyl cyanide and methyl isocyanide have been calculated at 300°K, using the secular equation method based on symmetry coordinates. The thermodynamic properties of these molecules are also estimated for temperatures ranging from 200°K to 1000°K at 100° intervals.     

Two-dimensional self-assembly of esters with different configurations at the liquid-solid interface

Self-assembled monolayers of hexadecyl palmitate (HP) and 3,3′-thiodipropionic acid di-n-octadecyl ester (TADE) physisorbed on highly oriented pyrolytic graphite (HOPG) are investigated using scanning tunneling microscope (STM) and computer simulation. Both molecules form alkane-like linear shapes to maximize the interactions with substrate when they adsorb on HOPG surface. The HP molecules self-assemble into lamellae with the chain-trough angle of 48°, which is the result of a shifting 3/2 units from the adjacent molecule in a lamella. Based on the simulation insights combined with STM images, we confirm that a perpendicular orientation appears in which the HP molecular backbone is rotated 90° with respect to the substrate such that the carbonyl points away from the HOPG surface. TADE molecules form three kinds of configurations with chain-trough angles of 90°, 72° and 60° by shifting 0, 1/2 and 1 units from their adjacent molecules, respectively. The bright stripes in STM images reveal the electron density distribution of the part between two ester groups. The energy differences of three TADE adsorption configurations by molecular mechanics (MM) simulation are used to explain the structural coexistence phenomenon. It is also shown that lattice match between alkyl chain of molecules and HOPG substrate could change molecular conformation upon self-assembly.     

Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

Molecular interaction of oxazine dyes in aqueous solution: Temperature dependent molecular disposition of the aggregates

Temperature dependent molecular interaction of oxazine dyes, viz., brilliant cresyl blue (C.I. Basic dye), cresyl violet (C.I. Basic violet 3) and nile blue (C.I. Basic blue 12) are studied in aqueous media within a concentration range of 5.0 × 10^− 6 M to 8.0 × 10^− 4 M by UV-visible absorption spectroscopy. The effect of temperature on the geometrical structure of the dimer in solution along with the dimerization equilibria is explained in terms of electrostatic and hydrophobic interactions. Modified non-covalent interaction between two monomer molecules in a dimer as a function of temperature affects the extinction coefficient as well as the geometrical disposition of the dimers and this is well manifested in the exciton splitting of the dimer spectra. The angle θ between main oscillators of the two monomer molecules in a dimer increases by 1.94° for brilliant cresyl blue, whereas an increase of 4.32° and 1.73° were observed for cresyl violet and nile blue respectively due to the increase of temperature from 20 °C to 60 °C.     

Configuration of pentacene (C22H14) films on Si(1 0 0)-2 × 1 studied by NEXAFS

Pentacene films on Si(1 0 0)-(2 × 1) surface at 300 K were investigated using near edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. NEXAFS spectra show that pentacene molecules are chemisorbed on the Si(1 0 0)-(2 × 1) surface for monolayer with flat-laying and predominantly physisorbed on the Si(1 0 0)-(2 × 1) surface for multilayer films with an upright molecular orientation. Absorption angle of pentacene molecules were measured through π^∗ transition. The angles between the double bond and the silicon surface were 35-55°, 65° and 76° at monolayer, 24 and 48 nm pentacene deposited on the Si(1 0 0) surface, respectively. We observed that the intermediate flat-laying phase is favored for monolayer coverage, while the films of molecules standing perpendicular to the Si(1 0 0) surface are favored for multilayer coverage.     

Water adsorption on hydrogenated Si(1 1 1) surfaces

The adsorption of water on the hydrogen terminated Si(1 1 1) surface is studied by means of first-principles calculations as well as contact angle measurements. Possible initial adsorption configurations for single water molecules and the potential energy surface are calculated. Only small adsorption energies of the order of meV are predicted. Calculations for higher coverage show that the water-water interactions are stronger than the water-surface bonding. The contact angle formed between a water droplet on the surface approximated from the total-energy calculations amounts to 88°, while our measured value is 91°.     

Effect of annealing on phase composition,structural and magnetic properties of Sm-Co based nanomagnetic material synthesized by sol-gel process

Sm-Co based nanomagnetic material was synthesized by means of a Pechini-type sol-gel process. In this method, a suitable gel-precursor was prepared using respective metal salts and complexing agent such as citric acid. The gel-precursor was dried at 300 °C and then subjected to various reductive annealing temperatures: 350, 500 and 600 °C. The nanopowders so obtained were characterized for their structure, phase composition and magnetic properties. FT-IR studies on the gel-precursor showed the binding of metal cations with the citrate molecules in the form of metal-citrate complex. The gel-precursor, which was annealed at 350 °C showed the presence of both meta-stable cobalt carbide (Co₂C, Co₃C) and Co₃O₄ phases; while the sample annealed at 500 °C indicated the sign of SmCo₅ phase. Upon increasing the reductive annealing temperature to 600 °C, crystalline phase such as fcc-Co and Sm₂C₃ were formed prominently. FE-SEM analysis revealed the change in sample morphology from spherical to oblate spheres upon increasing the annealing temperature. VSM measurements demonstrated ferromagnetic nature at room temperature for all the nanopowders obtained irrespective of their after reductive annealing temperature.     

Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

Contact angles in controlled atmospheres: Some theoretical considerations

Contact angle measurements in controlled atmospheres must take into account the effect of residual gas molecules in the less-than-perfect vacuum that is encountered experimentally and of the gas molecules formed by vaporization of the liquid component. The possibility of measuring the contact angle of a non-vaporizing liquid has been considered. In an experiment with a low-melting lead bismuth alloy on glass, the contact angle at 142°C was 136°. Calculations based on the Fowkes theory of dispersion forces indicate that the contact angle should not exceed 141°. In contrast to the reasonable agreement with the Fowkes theory, the observed angle is lower than would be expected from the Yarnold-Mason theory as it would apply to a competition between residual gas molecules and metal atoms for the solid surface and the effect of this competition on the contact angle. Finally, the theory of works of adhesion and cohesion is applied to the known variability of the hydrophilicity of graphite surfaces. The known reactivity of adsorbed water with graphite makes it impossible to apply an exact theory, but a limiting value of 53 % hydrophilic surface is indicated for graphite at room temperature.     

α-Sexithiophene on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O: An STM and NEXAFS study

The adsorption of α-sexithiophene (6T) on Cu(1 1 0), Cu(1 1 0)-(2 × 1)O and the mesoscopically patterned Cu-O striped surface have been studied by STM (scanning tunnelling microscopy), XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure). The molecular resolution of the STM allowed to determine the orientation and local order of the molecules in the submonolayer and monolayer regime. It is shown that the 6T molecules align with their long molecular axis along the densely packed copper rows on Cu(1 1 0) and along the Cu-O rows on the Cu(1 1 0)-(2 × 1)O surface. On the striped phase with alternating copper and Cu-O regions the molecules adsorb first on the Cu regions and after complete filling of these regions, on the Cu-O. The orientation is the same on both areas as on the respective pristine surfaces with the only exception that the molecules reorient by 90° if the width of the copper regions is smaller than the molecular length. The NEXAFS measurements allowed for a determination of the adsorption geometry of the molecules: while 6T lies flat on the surface on clean copper, the molecular planes are inclined with an angle as high as 39° with respect to the substrate on (2 × 1)O. For the latter, this inclination angle is 4° higher than in the bulk crystal structure of 6T observed for thicker films to release stress and allow commensurability with the substrate lattice, while for the former it is a result of the aromatic system bonding to the Cu(1 1 0) surface, as confirmed by XPS.     

Specific heat of a synthetic zeolite and the heat of fusion of its absorbed water

Curves of specific heat plotted against temperature are presented for a synthetic zeolite (Linde 5A) at a number of water contents. Above a threshold water content of 0·233 g of water per gram of zeolite the curves showed a peak, centred about −30°C, that is ascribed to the fusion of sorbed water in the zeolite cavities. Above a second threshold near 0·28 g/g, a second peak was found between −5°C and 0°C. This peak arises from the fusion of water on the external surface of the zeolite granules. All the water in the system was distinguishable from ordinary water up to the content 0·306 g/g, which is therefore saturation water content at 0°C. Up to the first threshold there is no freezable water and no clustering of water molecules. At higher water contents clustering begins, but the water in the zeolite cavities always has a much lower temperature of fusion and heat of fusion than bulk water. Heats of fusion of internal and external water are given. Values of apparent specific heat of sorbed water are calculated, and shown to be consistent with the nature of sorbed water deduced from the fusion behaviour.     

Kinetic limitations to surface segregation during MBE growth of III–V compounds: Sn in GaAs

The incorporation of Sn as a dopant in GaAs has been studied in the temperature range of 500°–650° C, over a wide range of Ga and As fluxes, the latter being incident as either As₄ or As₂ molecules. The results are explained in terms of a surface segregation model in which the behaviour at high growth temperatures (above 600 °C) approaches thermal equilibrium, but growth at lower temperatures involves a kinetic limitation to the segregation process.     

Electronic and structural studies of immobilized thiol-derivatized cobalt porphyrins on gold surfaces

The immobilisation of thiol-derivatized cobalt porphyrins on gold surfaces has been studied in detail by means of combined scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). S-thioacetyl has been used as a protective group for the thiol. Different routes for deprotection of the acetyl groups were performed in acidic and in basic conditions. The results show the formation of monolayer films for the different preparation schemes. The immobilisation of the molecules on the gold surface takes place through the thiol-linkers by the formation of multiple thiolate bonds. In the case of layers formed with protected porphyrins approximately 60% of the linkers are bonded to the gold surface whereas for deprotected layers the amount of bonded linkers is increased up to about 80%. STM measurements revealed that the molecules arrange in a disordered overlayer and do not exhibit mobility on the gold surface. Annealing experiments have been performed in order to test the stability of the porphyrin layers. Disordered patterns have been observed in the STM images after annealing at T = 400 °C. XPS revealed that the sulphur content disappeared completely after annealing at T = 180 °C and that the molecules did undergo significant modifications.     

Microwave structural studies of organoselenium compounds 1. Microwave spectra, molecular structure, and methyl barrier to internal rotation of dimethyl diselenide

The rotational spectra of nine isotopomers of dimethyl diselenide, CH₃SeSeCH₃, have been measured with a molecular-beam Fourier transform microwave spectrometer. The spectra were complex due to the presence of many isotopomers in natural abundance and the splitting caused by the interactions with two methyl internal rotors. The spectra were assigned and fit to experimental precision to an effective rotational Hamiltonian for molecules with two periodic internal motions. The spectra of the symmetric isotopomers are consistent with a C₂ equilibrium structure. The rotational constants were used to determine the r_s structure of the C-Se-Se-C frame with the results r(SeSe)=2.306(3) Å, r(SeC)=1.954(6) Å, ?(CSeSe)=99.8(2)°, ?(CSeSeC)=85.2(1)°. A barrier to internal rotation of the methyl groups of 395 ± 2 cm⁻¹ was derived from the internal rotation splittings.     

Scanning tunneling microscopy of surface-adsorbed fullerenes: C60, C70, and C84

Scanning tunneling microscopy (STM) is used to characterize partial monolayers of C₆₀, C₇₀, and C₈₄ adsorbed on the Au(1 1 1) surface at room temperature and under ambient conditions. A high degree of structural polymorphism is observed for monolayers of each of these fullerenes. For C₆₀, three lattice packings are observed, including a previously unreported 7 × 7 R21.8° structure that is stabilized by adjacent surface step defects. For C₇₀, two lattice packings are observed, and analysis of molecular features in STM images allows molecular binding geometry to be determined. In one of the two observed lattice structures, C₇₀ molecules align their long axis along the surface normal, while in the other, molecules align parallel to the surface and along a gold lattice direction. The parallel geometry is also preferred for isolated and loosely packed molecules on the surface. C₈₄ exhibits a large number of lattice orientations and no long-range order, and likely binds incommensurately on Au(1 1 1). Time series of images of partial C₇₀ monolayers show progressive surface modification as a result of perturbation by the STM tip; this is in contrast to the behavior of C₆₀, where alterations in surface structure at room temperature are thermally driven.     

Fragrance release profile from sonochemically prepared protein microsphere containers

Protein microspheres have been prepared by sonicating a mixture of pure fragrant oil (amyl acetate (AA)) with an aqueous protein (bovine serum albumin) solution. The prepared protein spheres are nano- to micrometer sized with an encapsulation efficiency of approx. 97% for the AA present on the surface and inside the BSA capsule. Containers were found stable for more than 6 months when stored sealed at 4 °C and 20 °C. For the release profile measurements, we used a simple, automated and direct method. We continuously weighed the encapsulated microspheres and measured the evaporation rates. The release profiles at 15 °C and 25 °C display two different evaporation rates. The higher rate is the sum of a few evaporation rates, including water molecules, while the slower rate is due to the evaporation of pure AA. The changes in the evaporation rates occur upon the collapse of the container. This event coincides with the full evaporation of water. For morphological characterization we dyed the AA with Nile red, and used SEM, ESEM, Cryo-SEM, light microscopy, and confocal laser scanning microscopy measurements.     

Preparation of Cd3As2 and CdAs2 crystals by transport reaction in vapour phase

A method of preparation of Cd₃As₂ and CdAs₂ single crystals employing transport reaction in vapour phase without transport gas is described. Optimum conditions for growth of the crystals are determined. In the case of Cd₃As₂ the average temperature and the temperature gradient are 535 °C and 3·3 °C/cm respectively, in the case of CdAs₂ the corresponding values are 550 °C and 4 °C/cm. Bulk crystals as large as a few cm³ can be obtained within several days using evacuated quartz tubes 150 mm in length and 20 mm in diameter. Furthermore, thermodynamical arguments indicating possible reaction mechanisms are presented. Sublimation appears to be the most probable mechanism of transport in the case of Cd₃As₂, whereas the process is more complicated in the case of involving the decay of CdAs₂ molecules.     

Attenuated total reflectance spectroscopy of coumarin organosilane molecules adsorbed on a fused silica surface

Attenuated total reflectance (ATR) spectroscopy was used to investigate the adsorption of coumarin organosilane molecules onto a fused silica surface. The difference between the absorption spectra of the molecules on the surface and in solution was explained by the interaction of the adsorbed coumarin organosilane molecules with the hydroxyl groups on the fused silica surface. This interaction produces a perturbation of the π electron distribution and the electronic transitions of the coumarin chromophore of the organosilane molecules adsorbed on the surface. From the kinetics adsorption curves, the calculated enthalpy values of 74.8 ± 5.2 kJ mol⁻¹ and free energy of −38.22 ± 0.70 kJ mol⁻¹ at 23 °C indicates a chemisorption process. The high sensitivity of ATR spectroscopy allows the detection of a monolayer formed by a 10 nM concentration of coumarin organosilane molecules, which covers more than half of the maximum surface coverage at 60 °C.     

Conformation transitions of blood proteins under influence of physical factors on microwave dielectric method

In this article, the influence of γ-irradiation and temperature on albumin and fibrinogen conformation and dielectric properties of protein solutions have been studied by the microwave dielectric method. Both the values of the real part ε′ (dielectric permittivity) and the imaginary part ε″ (dielectric losses) of the complex dielectric permittivity of the aqueous solution of bovine serum albumin and human fibrinogen as functions of temperature and γ-irradiation dose have been obtained. The time of dielectric relaxation of water molecules in the protein solutions was calculated. The hydration of the albumin and fibrinogen molecules was determined. The temperature dependencies of hydration are non-monotonous and have a number of characteristic features at the temperatures 30-34 and 44-47 °C for serum albumin, and 24 and 32 °C for fibrinogen.     

Understanding the relationship between the structure of an organofunctional methoxysilane bilayer and its properties as adhesion primer for Zn-epoxidized rubber system

The structure of a new organofunctional methoxysilane bilayer system, which was found to improve the adhesion of epoxidized natural rubber (ENR) to zinc, is analyzed by High Resolution Electron Energy Loss and Infrared Reflection Absorption Spectroscopy. This bilayer is formed when zinc plate is successively dip-coated in water-alcohol solutions of γ-mercaptopropyltrimethoxysilane and aminopropyltrimethoxysilane. The optimal adhesion, obtained when both silane layers are cured at 110 °C, is due to the specific structure of this bilayer, which depends dramatically on the cure temperature of the first layer. From HREELS results under the conditions of an impact mechanism, it is shown that the surface density of NH₂ groups (which react with the epoxy groups of rubber and provide the adhesion of the whole system) is much higher when the γ-MPS layer is cured at 110 °C instead of 70 °C or room temperature. This high NH₂ group surface density is also confirmed by wettability measurements, which indicate a marked increase in the basic component of the surface energy at 110 °C. An IRRAS study of the γ-MPS layer deposited on zinc shows that up to 70 °C hydrolysis of the SiOCH₃ groups is poor, and becomes effective only after the zinc/γ-MPS layer is dipped in the basic APS solution. This makes the penetration of APS molecules across the γ-MPS layer easier and, consequently, amino groups can interact with zinc. Therefore, they are unavailable for a reaction with epoxy groups. In contrast, when the γ-MPS layer is cured at 110 °C or higher, hydrolysis of its SiOCH₃ groups begins and, simultaneously, lateral cross-linking occurs between SiOH groups. Under these conditions, penetration of the γ-MPS layer by APS molecules is hindered and lateral mixed cross-linking occurs at the interface γ-MPS/APS via the SiOH groups. This entails a greater density of pendant amino groups directed towards the surface, which enhances the practical adhesion of rubber to zinc.