Formation Of Two-dimensional Clusters Research Articles

Molecular Dynamics Simulation of Surface Nucleation during Growth of an Alkane Crystal

Crystal growth from the melt of n-pentacontane (C50) was studied by molecular dynamics simulation. Quenching below the melting temperature gives rise to propagation of the crystal growth front into the C50 melt from a crystalline polyethylene surface. By tracking the location of the crystal–melt interface, crystal growth rates between 0.02 and 0.05 m/s were observed, for quench depths of 10–70 K below the melting point. These growth rates compare favorably with those from a previous study by Waheed et al. [Polymer 2005, 46, 8689−8702]. Next, surface nucleation was identified with the formation of two-dimensional clusters of crystalline sites within layers parallel to the propagating growth front. Critical nucleus sizes, waiting times, and rates for surface nucleation were estimated by a mean first passage time analysis. A surface nucleation rate of ∼0.05 nm–2 ns–1 was observed, and it was nearly temperature-independent. Postcritical “spreading” of the surface nuclei to form a completely crystallized layer...

Surface induced nucleation of a Lennard-Jones system on an implicit surface at sub-freezing temperatures: A comparison with the classical nucleation theory

The aggregation-volume-bias Monte Carlo method was employed to study surface-induced nucleation of Lennard-Jonesium on an implicit surface below the melting point. It was found that surfaces catalyze not only the formation of the droplets (where the nucleation free energy barriers were shown to decrease with increasing surface interaction strength), but also the transition of these droplets into crystal structures due to the surface-induced layering effects. However, this only occurs under suitable interaction strength. When surface attraction is too strong, crystallization is actually inhibited due to the spread of the particles across the surface and corresponding formation of two-dimensional clusters. The simulation results were also used to examine the bulk-droplet based classical nucleation theory for surface-induced nucleation, particularly the additional contact angle term used to describe both the nucleation free energy barrier heights and the critical cluster sizes compared to its homogeneous nucleation formalism. Similar to what has been found previously for homogeneous nucleation, the theory does poorly toward the high-supersaturation region when the critical clusters are small and fractal, but the theoretical predictions on both barrier heights and critical cluster sizes improve rapidly with the decrease of the supersaturation.

Quantum-Chemical Analysis of Thermodynamics of Two-Dimensional Cluster Formation of Racemic α-Amino Acids at the Air/Water Interface

The quantum-chemical semiempiric PM3 method is used to calculate the thermodynamic parameters of clusterization for the racemic α-amino acids C(n)H(2n+1)CHNH(2)COOH with n=5-15 at 278 and 298 K. Possible relative orientations of the monomers in the heterochiral clusters are considered. It is shown that, for the racemic mixtures of α-amino acids, the formation of heterochiral 2D films is most energetically preferable with the alternating (rather than "checkered") packing of the enantiomers with opposite specific rotation. The two enantiomeric forms of α-amino acids in the heterochiral 2D clusters are tilted with respect to the normal to the q direction at angles of φ(1)=20° and φ(2) = 33°, whereas the single enantiomeric forms are oriented at an angle of δ=9° with respect to the normal to the p direction. It is shown that the heterochiral 2D film based on the α-amino acid structures oriented at the angle φ(2)=33° with respect to the normal to the q direction possesses a rectangular unit cell with the geometric parameters a = 4.62 Å and b = 10.70 Å and the tilt angle of the alkyl chain of the molecule with respect to the interface t(2)=35°, which is in good agreement with the X-ray structural data a=4.80 Å, b=9.67 Å, and t(2)=37°. The parameters of the lattice structure of monolayers formed by amphiphilic amino acids are shown to be determined by the "a" type of the intermolecular H-H interactions, whereas the tilt angle of the molecules with respect to the interface depends on the volume and the structure of the functional groups involved in the hydrophilic part of the molecule. Spontaneous clusterization of the racemic form of α-amino acids at the air/water interface at 278 K takes place if the alkyl chain length is equal or higher than 12-13 carbon atoms, whereas for 298 K this clusterization threshold corresponds to 14 carbon atoms in the hydrocarbon chain. These values agree with the experimental data.

Adsorption of thiophene on silica-supported Mo clusters

The adsorption/decomposition kinetics/dynamics of thiophene has been studied on silica-supported Mo and MoS x clusters. Two-dimensional cluster formation at small Mo exposures and three-dimensional cluster growth at larger exposures would be consistent with the Auger electron spectroscopy (AES) data. Thermal desorption spectroscopy (TDS) indicates two reaction pathways. H 4C 4S desorbs molecularly at 190–400 K. Two TDS features were evident and could be assigned to molecularly on Mo sites, and S sites adsorbed thiophene. Assuming a standard preexponential factor (ν = 1 × 10 13/s) for first-order kinetics, the binding energies for adsorption on Mo (sulfur) sites amount to 90 (65) kJ/mol for 0.4 ML Mo exposure and 76 (63) kJ/mol for 2 ML Mo. Thus, smaller clusters are more reactive than larger clusters for molecular adsorption of H 4C 4S. The second reaction pathway, the decomposition of thiophene, starts at 250 K. Utilizing multimass TDS, H 2, H 2S, and mostly alkynes are detected in the gas phase as decomposition products. H 4C 4S bond activation results in partially sulfided Mo clusters as well as S and C residuals on the surface. S and C poison the catalyst. As a result, with an increasing number of H 4C 4S adsorption/desorption cycles, the uptake of molecular thiophene decreases as well as the H 2 and H 2S production ceases. Thus, silica-supported sulfided Mo clusters are less reactive than metallic clusters. The poisoned catalyst can be partially reactivated by annealing in O 2. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H 2, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S 0, of C 4H 4S has been determined. At thermal impact energies ( E i = 0.04 eV), S 0 for molecular adsorption amounts to 0.43 ± 0.03 for a surface temperature of 200 K. S 0 increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S 0( T s) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S 0( T s) curves allows one to determine the bond activation energy for the first elementary decomposition step of C 4H 4S, which amounts to (79 ± 2) kJ/mol and (52 ± 4) kJ/mol for small and large Mo clusters, respectively. Thus, larger clusters are more active for decomposing C 4H 4S than are smaller clusters.

Quantum-Chemical Analysis of Thermodynamics of Two-Dimensional Cluster Formation of α-Amino Acids at the Air/Water Interface

The semiempirical quantum-chemical PM3 method is used to calculate the thermodynamic parameters of clusterization of the S-form of alpha-amino acids with the general composition C(n)H(2n+1)CHNH(2)COOH (n = 5-15) at 278 and 298 K. It is shown that six stable conformations of monomers exist, for which the thermodynamic parameters (enthalpy and Gibbs' energy) of the formation and absolute entropy are calculated. The correlation dependencies of the calculated parameters on the alkyl chain length are found to be linear. The structures of the monomers are used to build larger clusters (dimers, tetramers, hexamers). For all small clusters (comprised of two to six molecules), the thermodynamic parameters of formation and clusterization are calculated. It is shown that for tetramers and hexamers the enthalpy, entropy, and Gibbs' energy of clusterization are linearly dependent on the alkyl chain length, whereas for the dimers these dependencies are stepwise. The thermodynamic characteristics of clusterization of associates tilted by angles of 9 and 30 degrees with respect to the normal to the interface are calculated. It is shown that the 30 degrees angle orientation is more energetically advantageous for this class of compounds. The geometric parameters of the unit cell characteristic for the infinite 2D film which corresponds to the most advantageous conformation of the monomer were calculated using the PM3 parametrization to be a = 4.57-4.71 A and b = 5.67-5.75 A, with the angle between the axes theta = 100-103 degrees . These values agree well with the available experimental data. Spontaneous clusterization of alpha-amino acids at the air/water interface at 278 K takes place if the alkyl chain length exceeds 11-12 carbon atoms, whereas for 298 K this clusterization threshold corresponds to 13-14 carbon atoms in the alkyl chain, also in agreement with the experimental data.

Two-Dimensional Clustering of Nanoparticles on the Surface of Cellulose Fibers

Density of surface charges of cellulose fibers and ξ-potential of TiO 2 nanoparticles in the solution were measure< and controlled by pH. The adsorption and clustering of TiO 2 nanoparticles on the surface of cellulose fiber; were studied using scanning electron microscopy (SEM) and a scale in different situations of repulsion and attraction between the particles and surface. The experiments show formation of two-dimensional clusters o nanoparticles on the surface. Weight measurement of the adsorbed particles and clusters via the adsorption time results in that there are three stages containing nucleation, two-dimensional growth and saturation fo clustering of nanoparticles on the surface. Smoluchowski theory was used to propose an evolutional mode to explain three stages of clustering.

Dressed particle simulation of dusty plasmas

A dressed particle approach to numerical simulation of dusty plasmas is presented. Since the dust grains in the laboratory are usually immersed in a streaming plasma, their interaction potential has a complex spatial structure which can be computed within the framework of linear response theory. For typical experimental parameters the two-body force is sufficiently short-ranged to make molecular dynamics techniques feasible. As an application of this approach, the formation of two-dimensional clusters is investigated and compared with experimental results.

Transition State for Aggregation and Reorganization of Normal Fatty Alcohols at the Air/Water Interface

The transition state between the initial and final clustering products is analyzed with special reference to the two-dimensional cluster formation and reclustering in the monolayers of normal fatty alcohols (n = 8-16). The calculations were performed using the PM3 molecular orbital approximation implemented in the Mopac 2000 software. It is shown that taking into account the enthalpy factor only does not lead to any saddle point at the potential energy surface. This point is reproduced if the entropy factor is assumed. A linear dependence of enthalpy, entropy, and Gibbs free energy on the number of the carbon atoms n is shown to exist for any positions of the system at the reaction coordinate, including the positions of the transition state. This fact was used to construct an additive scheme for the description of transition states in clustering and reclustering reactions in fatty alcohol monolayers for any arbitrary size of the initial cluster. It is shown that in the kinetic formalism the formation of linear clusters is more probable as compared with the formation of star clusters, whereas the attachment of any linear cluster to a star cluster is preferential both from the kinetic and thermodynamic considerations. The dependence of the parameters of the activated complex, corresponding to the dimerization reaction of n-alcohols, on alkyl chain length and temperature is analyzed. The results of the theoretical calculations are in qualitative agreement with experimental data obtained for tetradecanol monolayers.

Aggregation and re-organization of normal fatty alcohols at the air/water interfacePM3 molecular orbital approximation

The transition state of dimerization at the interface and between the initial and final clusterization products is analyzed. The calculations were performed using the PM3 molecular orbital approximation, implemented in the MOPAC 2000 software. It is shown that the account of the Gibbs energy Δ G leads to a saddle point at the potential energy surface for the two-dimensional cluster formation and re-clusterization in monolayers of normal fatty alcohols ( n =8, 12, and 16). The re-organization of a linear trimer and monomer into two dimers was considered. The maximum value Δ G =110 kJ/mol for n -tetradecanol is observed for a distance of 10.5 Å which corresponds to the transition state of re-clusterization reaction. If the distance between the n -tetradecanol trimer and monomer is lower than 10.5 Å, then tetrameric clusters are formed again. For higher distances the monomer is detached from the trimer. The dependencies of the parameters of the activated complex corresponding to the alcohol dimerization reaction on the chain length are analyzed. The position of the maximum on the Gibbs energy curve for dimerization at 298 K corresponds to an intermolecular distances for the dimers in the transition state of 6.31–6.69 Å.

Aggregation and re-organization of normal fatty alcohols at the air/water interface: PM3 molecular orbital approximation

The transition state of dimerization at the interface and between the initial and final clusterization products is analyzed. The calculations were performed using the PM3 molecular orbital approximation, implemented in the MOPAC 2000 software. It is shown that the account of the Gibbs energy Δ G leads to a saddle point at the potential energy surface for the two-dimensional cluster formation and re-clusterization in monolayers of normal fatty alcohols ( n=8, 12, and 16). The re-organization of a linear trimer and monomer into two dimers was considered. The maximum value Δ G=110 kJ/mol for n-tetradecanol is observed for a distance of 10.5 Å which corresponds to the transition state of re-clusterization reaction. If the distance between the n-tetradecanol trimer and monomer is lower than 10.5 Å, then tetrameric clusters are formed again. For higher distances the monomer is detached from the trimer. The dependencies of the parameters of the activated complex corresponding to the alcohol dimerization reaction on the chain length are analyzed. The position of the maximum on the Gibbs energy curve for dimerization at 298 K corresponds to an intermolecular distances for the dimers in the transition state of 6.31–6.69 Å.

Electrically Controlled Two-Dimensional Cluster Formation of Polystyrene Particles and Cluster Fixation with Adsorbed Polyelectrolyte

An attempt was made to orderly arrange a large number of polystyrene particles dispersed in aqueous medium on the electrode surface and fix the formed particle arrangement. The measuring cell used consisted of two transparent indium tin electrodes 500 μm apart from each other in the vertical direction. The gap between the electrodes was filled with a dispersion of polystyrene particles (5 μm size) and the behavior of the particles in electric fields applied between the electrodes was on-site observed with an optical microscope. Simultaneous application of DC and AC fields caused polystyrene particles to form single-layered clusters each consisting of several tens particles on the lower electrode. Densely arranged clusters were formed for the particles with polyallylamine hydrochloride (PAH), a cationic polyelectrolyte, adsorbed on their surface upon application of DC and AC fields and the clusters formed remained unbroken after removal of the fields. No cluster formation was observed, however, for the particles after adsorbing the cationic polyelectrolyte and then poly (sodium styrenesulfonate) (PSS), an anionic polyelectrolyte. These findings suggested that particle arrangement and its fixation of polystyrene particles are appreciably affected by polyelectrolyte adsorption.

Quantum Chemical Analysis of Thermodynamics of the Two-Dimensional Cluster Formation at the Air/Water Interface

The thermodynamics of the two-dimensional cluster formation of normal fatty alcohols (n ) 8-16) andthat of 2-methylhexadecanolat in the air/water interface is quantum chemically analyzed. The calculation provides reasonable values for the thermodynamic characteristics for the formation of alcohol clusters ( m ) 2-7) with various structures at the air/water interface. The calculated values of enthalpy ¢Hm , entropy ¢Sm , and Gibbs energy ¢G m for the formation of a definite cluster structure can be satisfactory represented by a linear dependence on the number of CH2 groups in the alcohol molecule. The absolute terms and coefficients of these equations can be characterized in form of the dependencies on the number of bonds formed between the alkyl groups (one to four bonds) and the contributions to the interactions from hydrogen bonds and lone pairs of oxygen atoms. The enthalpy and entropy of the cluster formation can be estimated from the molecular geometry of the clusters (relative positions of methylene and hydroxyl groups), the number of carbon atoms in the monomer and the number of molecules in the cluster. Reliable estimates predict plane clusters with tetragonal or hexagonal structure, and linear clusters with an arbitrary number of CH 2 groups and an arbitrary (up to infinite) number of monomers in the cluster. The calculations show that stable tetramers are formed by n-decanol, whereas n-dodecanol and the higher homologues cannot only form tetramers but also infinite clusters. These results are in agreement with the existence of a first-order phase transition in the experimental surface pressure-area isotherms of n-dodecanol, n-tetradecanol, n-hexadecanol, and 2-methylhexadecanol monolayers that does not occur in n-decanol monolayers. The thermodynamic model which assumes equilibrium between the oligomers and clusters within the monolayer agrees well with the experimental results, and suggests that in the fluid state the monolayers are comprised of monomers and oligomers (dimers to tetramers), the aggregation degree of which increases with the increase of the alkyl chain length. The data obtained by the thermodynamic model agree qualitatively, and also quantitatively (especially for Gibbs energy) with the quantum chemical calculations.

Crystalline architectures at the air-liquid interface: From nucleation to engineering

The air-liquid interface is a convenient medium to obtain monolayers or multilayers. The molecular structure of such crystalline films can be established by grazing incidence X-ray diffraction (GIXD), complemented by spectroscopic techniques and lattice energy calculations. Formation of two-dimensional clusters using GIXD may provide insight into the first stages of crystal nucleation. Langmuir monolayer can serve as a template for induced nucleation of bulk crystals or to be a model system of natural membranes. A host of architectures composed of water-insoluble molecules can be formed on liquid surfaces. The number of layers and the polymorphism can be controlled. The concept of supramolecular design was applied in two-dimensions with the use of water-insoluble and water-soluble components.

Aggregation of phospholipid vesicles by a chimeric protein with the N-terminus of annexin I and the core of annexin V.

A chimeric protein was produced with the N-terminal domain (amino acids 1-45) of annexin I and the core of annexin V (amino acids 19-320). This protein, annexin IN-VC, has a similar Ca2+ requirement for binding to phospholipid bilayers of 20% phosphatidylserine (PS)/80% phosphatidylcholine (PC) as annexin V. In contrast to annexin V, this protein has a strong potency to aggregate phospholipid vesicles as is shown by turbidimetric measurements and cryo-electron microscopy. Ellipsometry was employed to study quantitatively the phenomenon of phospholipid vesicle adhesion to annexin IN-VC bound to a planar phospholipid bilayer. The amount of phospholipid vesicles bound by annexin IN-VC on the planar bilayer is proportional to its surface coverage and can be inhibited by coadsorption of annexin V on the planar bilayer or by shielding the phospholipid surface of the vesicles with blood coagulation factor Va. Annexin IN-VC, like annexin V, does not bind to pure PC bilayers, but its adsorption on anionic phospholipid bilayers brings about the capacity to bind pure PC vesicles. This suggests that annexin IN-VC generates or exposes after binding to anionic phospholipids another phospholipid binding site, that differs from the annexin V phospholipid binding site. Collectively, the data suggest that two-dimensional cluster formation of annexin IN-VC on a bilayer with anionic phospholipids is involved in vesicle adherence.