Surface characterization of aspirin crystal planes using molecular dynamics simulations

Molecular dynamics simulations have been implemented to study the effect of polar and non-polar solvents on the different faces of the aspirin crystal and the possible mechanisms of surface growth. We focus on the role of the H-bond in stabilizing the crystal faces and do a systematic study on the effect of the solvents at intermediate stages of the process of growth. Our results show that for both polar and non-polar solvents, the (0 1 0) surface of aspirin maintains its robustness and lattice order after equilibration, which allows for the fast deposition of additional aspirin molecules onto the existing surface. We find that the growth on this face can be mostly accomplished in a molecule-by-molecule manner, which is favored both thermodynamically and kinetically. The (0 0 1) surface shows that the molecules lose their lattice structure completely if they are not paired with other aspirin molecules before they are in contact with any solvent. For this reason, growth in this direction can only be in a layer-by-layer manner if there is any at all. Our results also show that as the unpaired (1 0 0) surface of aspirin is populated with-OH groups, their hydrophilic nature favors the interaction with a polar solvent and thus growth along this face is enhanced when contacting with ethanol. The methyl and carboxyl groups covering the unpaired (0 0 1) plane cause unfavorable interaction with the polar solvent. On the (0 1 0) plane, the interlaced arrangement of hydrophilic hydroxyl groups and hydrophobic phenyl groups stabilizes the surface structure in both polar and non-polar solvents.     

Investigation of water diffusion in poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) by generalized two-dimensional correlation ATR-FTIR spectroscopy

Water diffusion process in biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx, HHx = 12 mol%) was investigated by generalized 2D correlation time-resolved ATR-FTIR spectroscopy based on the analysis of v(OH) stretching and δ(OH) bending bands of water as well as v(CO) and v(C-O-C) stretching bands of PHBHHx. Three states of water were figured out during water diffusion process. They are bulk water, bound water and free water. The water diffusion mechanism was suggested as: water molecules firstly diffuse into the micro-voids in bulk water form or are dispersed on the surface in free water form, and then penetrate into the polymer matrix in hydrogen bound water with the hydrophilic groups of PHBHHx. Moreover, water molecules diffuse into the loose amorphous phase and then into compact crystalline phase. Water diffusion coefficient in PHBHHx was thus evaluated as 7.8 ± 0.7 × 10⁻⁸ cm² s⁻¹ for the PHBHHx with crystallinity of 16.2 ± 0.3% at 293 K.     

Phase transformation in tungsten carbide–cobalt composite during high temperature treatment in microwave hydrogen plasma

To study phase formation up to 2300 K, tungsten carbide–cobalt (WC–Co) samples were exposed to concentrated solar radiation in hydrogen plasma. The reducing atmosphere was a non-equilibrium hydrogen plasma created in a microwave cavity at the nominal power of 1000 W. The dissociation fraction of hydrogen molecules in plasma was of the order of 10% assuring for almost optimal reduction of any oxide that could be formed on the surface of samples due to the residual atmosphere. Micro-structural characterization was performed by XRD and AES depth profiling. The results showed the appearance of Co₆W₆C phase at the temperature of 1050±50 K. This phase almost vanished at the temperature of 1300±90 K and was replaced by the Co₃W₃C phase. This phase vanished at 1690±150 K where only WC and Co peaks were detected by XRD. The AES depth profiles showed enrichment of the surface film with Co at elevated temperatures. At extremely high temperature of 2300 K, the Co vanished from the surface layer but remained in crystalline form in the bulk material. SEM imaging showed an evolution of the material crystallinity up to perfectly recognizable crystals of the dimension of approx. 1 μm at the maximum temperature. The results are explained by mobility of Co atoms, surface segregation and sublimation at very high temperature.     

Molecular dynamics simulation of oxygen diffusion in dry and water-containing poly(vinyl alcohol)

The kinetics and mechanisms of diffusion of oxygen and water in dry and water-containing amorphous syndiotactic poly(vinyl alcohol) were studied at 502 K and normal pressure by molecular dynamics simulation. Penetrant molecule trajectories were obtained in a system with 600 repeating units of poly(vinyl alcohol) and 0, 40 (2.6 wt%) and 80 (5.2 wt%) water molecules. Under dry conditions, oxygen molecules jumped in a cage-like fashion. The oxygen molecule diffused in a liquid-like fashion while water diffusion was cage-like in the system with 5.2 wt% water. The hydrogen bond lifetimes among the water molecules were significantly shorter than those formed between water and the polymer and between different polymer segments. The hydrogen bond lifetimes among all species were, within experimental error, unaffected by the content of water, even though the oxygen diffusivity increased exponentially and the water diffusivity increased to some extent with increasing water content. It seemed that the diffusivity was sensitive primarily to the decrease in concentration of polymer-polymer hydrogen bonds, which followed from the increase in water content. This finding was consonant with the analysis of the oxygen molecule motion relative to the nearest polymer backbone, which revealed that it jumped preferentially along the polymer chain and towards the backbone. This behavior was more pronounced when the dynamics were analyzed over longer distances (5 Å) and it was less pronounced in the water-rich systems. The simulations indicated that water clustering was absent and consequently that water was homogeneously distributed in the polymer systems.     

First-principles study of carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers

Ab initio plane wave density functional theory calculations are performed to investigate the carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers (CNFs). Results indicate that the octahedral interstitial sites are preferred for C dissolution relative to the tetrahedral sites. And the heat of solution of C in paramagnetic (PM) Ni is larger than that in ferromagnetic (FM) Ni because the induced C atom quenches the magnetic moments of neighboring Ni atoms. The bulk diffusion has been successfully described under two different C concentrations. At the initial CNF growth stage, the C concentration in bulk Ni is low and the calculated energy barriers for the diffusion of an isolated C atom are 1.641 eV and 1.678 eV in the Ni FM and PM state, respectively. When the C content is increased to 20 at.%, two models are established. In one case, it is assumed that all C atoms hop in the same direction at the same time, and the calculated activation energies are 1.137 eV and 1.126 eV. In the other case, only one C atom is permitted to move with the neighboring C atoms fixed at the octahedral sites and the corresponding barriers are decreased to 0.972 eV in the Ni FM state. Through these calculations, it is concluded that the magnetic state has a minor effect on the diffusion energy barrier which can be substantially lowered by the increase of C concentration in bulk Ni. Comparing the activation energy for bulk diffusion with the surface diffusion results, the reason for the formation of different CNF morphologies has been revealed.     

Kinetics and thermodynamics of carbon segregation and graphene growth on Ru(0 0 0 1)

We measure the concentration of carbon adatoms on the Ru(0 0 0 1) surface that are in equilibrium with C atoms in the crystal’s bulk by monitoring the electron reflectivity of the surface while imaging. During cooling from high temperature, C atoms segregate to the Ru surface, causing graphene islands to nucleate. Using low-energy electron microscopy (LEEM), we measure the growth rate of individual graphene islands and, simultaneously, the local concentration of C adatoms on the surface. We find that graphene growth is fed by the supersaturated, two-dimensional gas of C adatoms rather than by direct exchange between the bulk C and the graphene. At long times, the rate at which C diffuses from the bulk to the surface controls the graphene growth rate. The competition among C in three states - dissolved in Ru, as an adatom, and in graphene - is quantified and discussed. The adatom segregation enthalpy determined by applying the simple Langmuir-McLean model to the temperature-dependent equilibrium concentration seriously disagrees with the value calculated from first-principles. This discrepancy suggests that the assumption in the model of non-interacting C is not valid.     

Mechanism of corrosion protection of aluminum alloy substrate by hybrid polymer nanocomposite coatings

The corrosion protection of polymer clay nanocomposite, PCN coatings consisting of polyurea, siloxanes, epoxy ester and montmorillonite clay was determined. Corrosion resistance of the coating, was assessed by monitoring the polarization resistance and impedance of coated aluminum alloy, Al 2024-T3, coupons immersed in 3.5 wt.% of sodium chloride, NaCl, solution. Direct current polarization and electrochemical impedance spectroscopic techniques were used to measure polarization resistance and impedance of the samples, respectively. Diffusion of saturated salt solution into free-standing PCN films was measured gravimetrically and diffusivity of the nanocomposites was determined. The presence of clay decreases diffusivity and increases corrosion resistance of the non-scribed coatings containing up to 10 wt.% of clay. A correlation between polarization resistance and diffusivity was made. It was shown that for non-scribed coatings, polarization resistance increases with decreasing diffusivity. A relationship between coating's diffusivity and weight fraction of clay was established. Increasing clay concentration also resulted in decreasing diffusivity. The scribed nanocomposite coatings show slightly decreasing polarization resistance with increasing weight fraction, however, the polarization resistance of scribed coatings containing low clay weight fraction in the range between 0.5 and 2.0 wt.% was higher than that for the matrix. A barrier mechanism of corrosion prevention of the coated substrate is proposed for non-scribed coatings. The viscoelastic property of the nanocomposites was determined by using dynamic mechanical spectrometer. A correlation between polarization resistance of the coatings and the rubbery plateau modulus on the one hand and polarization resistance and tan δ peak area for α-transition of the nanocomposites is made. Decreasing tan δ peak area for α-transition and increasing rubbery plateau modulus resulted in increasing coatings polarization resistance.     

Monte Carlo study of the electrode|solvent primitive model electrolyte interface

Results of a Monte Carlo simulation of the electrode|electrolyte interface with and without solvent molecules are reported. The solvent molecules are modelled by neutral hard spheres immersed in a homogeneous dielectric medium. Calculations have been performed for 1:1 and 1:2 electrolytes at c = 1 M, with packing fraction η = 0.3 when the solvent molecules were present, and at a wide range of electrode charge. Insertion of the solvent molecules induces a layering of ion and solvent molecules in the vicinity of the electrode surface. The presence of the solvent molecules reduces the thickness of the electric double layer, lowers the value of the mean electrostatic potential and raises capacitance. The differential capacitance results are compared with the MPB theory predictions.     

Diffusivity of n-hexane in poly(ethylene-stat-octene)s assessed by molecular dynamics simulation

Molecular dynamics simulations have been used to study diffusion of n-hexane in wholly amorphous poly(ethylene-stat-octene)s with comonomer contents ranging from 0 to 11.5 mol%. The branches in the polymer increased the specific volume by affecting the packing of the chains in the rubbery state in accordance with experimental data. The diffusion of n-hexane at penetrant concentrations between 0.5 and 9.1 wt% was simulated within time-scales between 0.1 and 0.2 μs. The penetrant diffusivity unexpectedly decreased with increasing comonomer content. The penetrant molecule motion statistics showed that systems with high comonomer content showed a greater tendency for short distance motion (over a sampling period of 3 ps) whereas the systems with lower comonomer content showed penetrant motion over longer distances. It seems that the branches retarded local chain mobility of the polymer thereby trapping the penetrant molecules. All systems studied showed a minimum in penetrant diffusivity at ca. 1 wt% n-hexane and a marked increase in diffusivity at higher penetrant concentrations. The volumetric data for the different polymer-penetrant systems were consonant with additional volumes of the different components. Comparison between simulated diffusivities (for a wholly amorphous polymer) and experimentally obtained diffusivity data for semicrystalline polymers showed that constraining effect of the crystals were substantial for the highly crystalline systems and that it gradually decreased with decreasing crystallinity.     

Molecular dynamics simulation of diffusion of O2 and CO2 in blends of amorphous poly(ethylene terephthalate) and related polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular dynamics simulation techniques have been applied to study the diffusion of oxygen and carbon dioxide as small molecule penetrants in models polyester blends of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions was generated into a unit cell whose dimensions were determined for each of the simulated polyester blends in the cell having the experimental density. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 40 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume distribution was found.     

Polystyrene freeze-dried from dilute solution: Tg depression and residual solvent effects

The calorimetric glass transition temperature, T_g, was measured for both linear and cyclic polystyrenes freeze-dried from dilute solutions of 0.10, 0.05, and 0.02% of polymer by weight in benzene. Upon freeze-drying, T_g was found to be depressed by 4-15 K depending on the sample, solvent concentration, and freezing conditions. Annealing under vacuum at moderate temperatures, from 40 to 140 °C and 0.05 Torr, resulted in the shift of T_g back towards its bulk value and was accompanied by a decrease in sample weight. The data is consistent with the observed weight loss being due to residual solvent. The amount of residual solvent is a strong function of the annealing temperature and the initial freeze-drying solution concentration; exposure to vacuum at temperatures far below T_g is generally insufficient for residual solvent removal.     

Structure–activity studies of dodecatungstophosphoric acid impregnated bentonite clay catalyst in hydroxyalkylation of p-cresol

Bentonite clay impregnated with dodecatungstophosphoric acid (20% DTP/BNT) showed an excellent activity, selectivity and stability [95% product yield with 94% selectivity to 2, 2′-methylenebis (4-methylphenol), DAM] for the hydroxyalkylation of p-cresol with formaldehyde at 353 K and for a mole ratio of 5. Ammonia-TPD results showed that an increase in total concentration of acid sites from 4.9 of parent bentonite to 11.6 micromoles per surface area NH₃ (μmolS^− 1 NH₃) of 20% DTP/BNT was due to a strong interaction of protons of bulk DTP with surface hydroxyl groups of BNT as evidenced by ³¹P NMR studies.     

Intermolecular segregation of siloxane in P3HT: surface quantification and molecular surface-structure

Surfaces of spin-coated and solution-cast poly(3-hexylthiophene) (P3HT) films are analysed by X-ray Photoelectron Spectroscopy and Low Energy Ion Scattering. Here, we use the P3HT-siloxane system with only 2% siloxane monomers in the bulk as a model system to study segregation and surface orientation of molecules in polymers. The surfaces are enriched in siloxane due to the intermolecular segregation of the siloxanes present in P3HT. The siloxane coverage fraction was found to depend on the preparation parameters such as spinspeed and solution-concentration, and ranges from 25 to 100%.The molecular orientation of segregated siloxanes on P3HT was found to resemble that on pure PDMS. Furthermore, siloxane molecules prefer specific sites on P3HT, such that sulphur atoms are screened from being at the outermost surface to lower the surface free energy. The results presented here demonstrate clearly the unique ability of LEIS to quantify the composition of the outermost atomic layer, and to obtain detailed information on the surface structure.     

Hydrogen adsorption in different carbon nanostructures

Hydrogen adsorption in different carbonaceous materials with optimized structure was investigated at room temperature and 77 K. Activated carbon, amorphous carbon nanotubes, SWCNTs and porous carbon samples all show the same adsorption properties. The fast kinetics and complete reversibility of the process indicate that the interaction between hydrogen molecules and the carbon nanostructure is due to physisorption. At 77 K the adsorption isotherm of all samples can be explained with the Langmuir model, while at room temperature the storage capacity is a linear function of the pressure. The surface area and pore size of the carbon materials were characterized by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. A linear relation between hydrogen uptake and specific surface area (SSA) is obtained for all samples independent of the nature of the carbon material. The best material with a SSA of 2560 m²/g shows a storage capacity of 4.5 wt% at 77 K.     

Determination of permeability and diffusivity of oxygen in polymers by polarographic method with inert gas

A modified polarographic method with inert gas for determination of the oxygen permeability in polymers immersed in liquids is described. Owing to the stream of an inert gas towards polymer from the cathode side, lateral oxygen diffusion (edge effects) is minimized. Unlike the standard Fatt method, the method with inert gas is suitable also for thick samples and, therefore, for high-permeable materials. The method was tested for prediction of oxygen permeability in poly(1-vinyl 2-pyrrolidone) (PVP) and poly(2-hydroxyethyl methacrylate) (PHEMA). As an electrolyte, solution of potassium chloride was used. The effect of additional resistances and small lateral diffusion was taken into account. Unexpectedly, oxygen permeability in both polymers was greater for 0.1 M KCl than for 0.5 M KCl. The experimental setup was also used for diffusivity estimation in thick samples of PHEMA and PVP. Here, the oxygen flux response at one sample surface to the stepwise change in oxygen concentration at the other surface is measured and evaluated. The effect of the additional boundary layer on the oxygen transport is taken into account. A simple procedure for the diffusivity determination from the characteristic time of response as a function of the sample thickness is given. Solubility of oxygen in polymer is calculated from the obtained permeability and diffusivity.     

Estimation of mass transfer parameters during dehydroxylation in a large ceramic body by inverse methods

An inverse problem of the parameter estimation for the water transport during dehydroxylation in a large cylindrical ceramic body is solved. The transport is described by a standard type of non-stationary diffusion equation with a volumetric generation. The inverse problem is solved by the Levenberg-Marquardt method, with the estimated parameters being the water transfer diffusivity and water generation per unit volume. Their temperature dependencies are obtained within the dehydroxylation regime for 450 °C, 500 °C, 550 °C, and 600 °C, using experimental data from isothermal heating measurements. The activation energy of the diffusion process is estimated from the temperature dependence of the diffusivity, yielding 72.9 kJ mol⁻¹.     

The rotational and translational dynamics of molecular hydrogen physisorbed in activated carbon: A direct probe of microporosity and hydrogen storage performance

We have measured Incoherent Inelastic Neutron Scattering (IINS) spectra of H₂ physisorbed in high purity chemically activated carbon (AC) at different surface coverage and at temperatures near the triple point of bulk hydrogen. Our experimental results and DFT calculations show that at low surface coverage, due to the very low corrugation of the adsorption potential, and in the absence of H₂-H₂ lateral interactions, the adsorbed molecules are practically free to translate in the 2D plane parallel to the surface. Model calculations show that a complete mixing between the sub-states of the J = 1 manifold occurs on the free surface. The J = 0-to-1 rotational transition should split if the H₂ molecule is adsorbed in a slit type pore. Rotational splitting of up to 13 meV is found in the narrowest pores of around 6 Å investigated. The calculated isosteric heat of adsorption for molecules adsorbed on the free surface, at different sites and molecule orientations, range between −39 and −42 meV/H₂ at 77 K. In the optimum size slit pores, these numbers double up. Micropore volume of 0.34-0.45 ml/g carbon, and an upper limit of 4 wt% hydrogen storage is anticipated for the investigated material.     

The study of dielectric relaxation in propylene glycol-poly(propylene glycol) mixtures

Dielectric complex permittivity of propylene glycol (PG), poly(propylene glycol) (PPG-2000) and their mixtures with concentration of 25, 50 and 75 vol% of PG were measured in the frequency range 10 MHz-4 GHz at 25°C using time domain reflectometry (TDR). For these molecules and their mixtures, only one frequency independent dielectric loss peak was observed. The relaxation for these systems is described by a single relaxation time using Debye model. The large value of observed relaxation time for PG molecules shows the formation of molecular clusters. It is found that the relaxation time for PG-PPG mixtures is smaller in comparison to the relaxation times of PG and PPG molecules, and it linearly increases with the concentration of the PG in the mixtures. The values of relaxation times of PG-PPG mixtures are discussed particularly with respect to the solvent (PG) behaviour, which can be assigned to unaffected, loosely affected and tightly bound solvent and also with respect to the PPG chain coiling. As a peculiar feature the observed relaxation time is direct evidence of the interchange of solvent-solvent to solvent-polymer interaction.     

Availability of biotin incorporated in electrospun PLA fibers for streptavidin binding

Distribution of biotin at a depth of 3-10 nm from the surface of electrospun polylactic acid (PLA) fibers has been assessed by X-ray photoelectron spectroscopy (XPS) and compared to the distribution predicted by bulk calculations. Biotin concentration in the outer 3-10 nm of the fibers is greater than the predicted if biotin was distributed uniformly within the fiber. Availability of biotin for streptavidin binding at the surface of the fibers has been determined via a competitive colorimetric assay. Availability of biotin at the fiber surface was also determined to be greater than predicted by calculations assuming uniform biotin distribution. Additionally, the segregation of biotin to the exterior of the fiber increases disproportionately with increasing overall biotin concentration in the fibers. Confocal microscopy has been used to confirm capture of streptavidin, primary antibodies and fluorescence labeled secondary antibodies on PLA/biotin fibers.     

A tunable mixture solvent for poly(?-caprolactone): Acetone + CO2

We report on a tunable, versatile solvent system for poly(?-caprolactone) (PCL). It is shown that acetone + carbon dioxide mixtures are efficient solvents for this biodegradable polymer. Phase behavior and volumetric properties of PCL + acetone + CO₂ mixtures were determined in a variable-volume view cell. Effect of temperature (323-398 K), pressure (0.1-50 MPa), polymer concentration (1-20 wt%), polymer molecular weight (14k and 65k) and carbon dioxide concentration (0-60 wt%) on the liquid-liquid (L-L) phase boundaries and the densities was explored. Complete miscibility of mixtures with polymer concentrations up to 20 wt% could be achieved in the fluid mixtures containing up to 50 wt% carbon dioxide at modest pressures (5-40 MPa). The solutions all showed LCST-type phase behavior. Comparisons with literature data on the miscibility pressures in other solvent mixtures such as dimethyl ether + carbon dioxide or chlorodifluoromethane + carbon dioxide show that complete miscibilities of PCL in acetone + carbon dioxide mixtures are achievable at much lower pressures.The mixture densities were in the range 0.58-1.20 g/cm³. Mixtures with carbon dioxide content more than 20 wt% showed higher sensitivity and larger change in density with pressure. Densities of the polymer solutions were found to increase significantly with PCL concentration. The densities of solutions with different polymer molecular weights were close to each other, with the lower molecular weight polymer samples showing slightly higher densities.A unique contribution of the present paper is the comparison of compressibility and expansivity of the solutions with the corresponding properties of the solvents. Analysis of the data shows that the compressibilities of PCL solutions are lower than that of the acetone + carbon dioxide solvent mixture at temperatures lower than 373 K. At around 373 K, compressibilities become equal to each other and a switchover is observed at higher temperatures. The difference in the isothermal compressibility of the polymer solution and the solvent decreases with pressure, but reaches a plateau value for pressures greater than 25 MPa. Compared to their solvent mixture, the polymer solutions display a higher isobaric expansivity at the same pressure.