Supramolecular self-organization in PEO-modified C60 fullerene/water solutions: influence of polymer molecular weight and nanoparticle concentration

Utilizing a first-principles-based coarse-grained implicit solvent model, we have investigated the self-association of C(60) fullerenes that have been symmetrically modified with six grafted poly(ethylene oxide) (PEO) chains in aqueous solution. Despite the highly symmetric nature of the pair interactions between PEO-grafted fullerenes, their supramolecular assemblies are highly anisotropic and resemble the linear clusters formed in Stockmayer fluids. The dipole-like interaction between these symmetrically modified fullerenes results from the shielding of the C(60) fullerenes by PEO, favoring the addition of more PEO-grafted fullerenes to the linear clusters at the relatively unprotected ends. At low nanoparticle concentrations, self-association is dominated by the formation of stable dimers and trimers resulting from fullerene-fullerene contact and favorable PEO-fullerene interactions. With increasing nanoparticle concentration, larger clusters become increasingly probable. The molecular weight of the PEO tethers can be treated as a temperature-like analogue, with a reduction in average cluster size with increasing chain length due to increased steric repulsion, which is qualitatively similar to effects observed in Stockmayer fluids with increasing temperature. The role of PEO in supramolecular self-organization in PEO-modified C(60) fullerene/water solutions is complex, contributing not only to steric stabilization but also to favorable energetic interactions, nanoparticle shielding, and depletion-driven aggregation.     

A Study About Nanocluster Deposition of Thin-film Formation by Molecular Dynamics Simulation

The thin-film growth has been confirmed to be assembled by an enormous number of clusters in ICBD method. In sequence of clusters’ depositions proceeds to form the thin-film to understand quantitatively the interaction mechanisms between the cluster atoms and the substrate atoms, we use molecular dynamics simulation with EAM potential. The quantitative of flatness of deposition and percent of disordered atoms were proposed to evaluate the property of thin-film. In this simulation, three different Co cluster sizes of 55, 70, and 100 atoms with different velocities (100 up to 800 m/s) were deposited on a Al(0 0 1) substrate whose temperatures were set between 300 and 500 K. The simulations begin at specific equilibrium temperature of clusters and the substrate. The simulations are performed at different temperatures of the clusters and substrate and for different sizes of clusters. We showed that the percent of disordered atoms of substrate are affected by the cluster size and velocity of the clusters. Temperature dependence of the number of disordered atoms for different cluster’s velocity was observed. We investigated the effect of cluster size and initial velocity of cluster on the value of flatness.     

Dielectric characteristics of O2(H2O)i and (O2)2(H2O)i clusters. Computer-aided experiment

Absorption of oxygen molecules by water clusters with sizes of 10 ≤ i ≤ 50 is studied by the molecular dynamics method using the modified TIP4P model. It is revealed that the total dipole moment of the clusters nonmonotonically increases with their sizes. Absorption of O₂ molecules tends to raise the static permittivity of the ultradispersed medium formed by the clusters. The real and imaginary parts of the permittivity of water clusters with absorbed O₂ molecules are aperiodic functions of frequency. The permittivity components turn out to be nonmonotonic functions of cluster sizes. The IR absorption and reflectance spectra are calculated for clusters of pure water and aggregates with absorbed O₂ molecules. After the addition of oxygen molecules, the absorption coefficient of the clusters decreases, while the reflection coefficient increases. It is concluded that the capture of oxygen molecules by atmospheric moisture may reduce the greenhouse effect. Original Russian Text ? A.E. Galashev, V.N. Chukanov, O.A. Galasheva, 2006, published in Kolloidnyi Zhurnal, 2006, Vol. 68, No. 2, pp. 155–160.     

Flow field-flow fractionation: a versatile approach for size characterization of α-tocopherol-induced enlargement of gold nanoparticles

Flow field-flow fractionation (FlFFF) was used for size characterization of gold nanoparticles. The measured particle sizes obtained from FlFFF for the commercial 10 nm gold nanoparticle standard and the gold nanoparticles synthesized in the laboratory were in good agreement with those measured by transmission electron microscopy (TEM). Further, the capability of α-tocopherol to induce enlargement of gold nanoparticles by catalysis of the reduction of AuCl₄⁻ by citrate was observed by monitoring the changes in particle size of gold nanoparticles using FlFFF. The effects of α-tocopherol and incubation time on enlargement of the gold nanoparticles were examined. Higher concentrations of α-tocopherol resulted in larger nanoparticles. At fixed α-tocopherol concentration, larger nanoparticles were formed at longer incubation times.     

Metastable liquid clusters in super- and undersaturated protein solutions

Dense liquid phases, metastable with respect to a solid phase, but stable with respect to the solution, have been known to form in solutions of proteins and small-molecule substances. Here, with the protein lumazine synthase as a test system, using dynamic and static light scattering and atomic force microscopy, we demonstrate submicron size clusters of dense liquid. In contrast to the macroscopic dense liquid, these clusters are metastable not only with respect to the crystals, but also with respect to the low-concentration solution: the characteristic cluster lifetime is limited to approximately 10 s, after which they decay. The cluster population is detectable only if they occupy >10(-6) of the solution volume and have a number density >105 cm-3 for 3 to 11% of the monitored time. The cluster volume fraction varies within wide limits and reaches up to 10(-3). Increasing protein concentration increases the frequency of cluster detection but does not affect the ranges of the cluster sizes, suggesting that a preferred cluster size exists. A simple Monte Carlo model with protein-like potentials reproduces the metastable clusters of dense liquid with limited lifetimes and variable sizes and suggests that the mean cluster size is determined by the kinetics of growth and decay and not by thermodynamics.     

Growth of Co clusters on thin films Al2O3NiAl(100)

We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al(2)O(3) thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570 K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6 nm and a height of 0.7 nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3-4-nm-wide crystalline Al(2)O(3) strips where the deposited Co atoms are confined, the same features sustain. Only at 570 K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5 nm and height of 1.4 nm) appears for growth at 450 K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300 K and annealed to 750 K, in which the coalescence is eliminated and the two preferable geometries appear and coexist.     

Mechanism of silver nucleation upon the radiation-induced reduction of its ions in polyphosphate-containing aqueous solutions

Long-lived (hours to days) silver clusters, Ag ₄ ²⁺ , Ag ₄ ⁺ , Ag ₈ ²⁺ , etc., are formed upon the radiation-induced reduction of Ag⁺ ions in aqueous solutions containing sodium polyphosphate. The efficiency of the cluster formation decreases and the stability of the clusters increase with a rise in the concentration of the polymeric stabilizer. In the course of the aggregation of clusters, their sizes increase, quasi-metallic particles emerge, and the process terminates with the formation of silver nanoparticles. The mechanism of silver nucleation upon the radiation-induced reduction of silver ions in aqueous solutions is discussed.     

Effect of the Conditions the Reaction on the Formation of Iron Nanoparticles during the Reduction of Iron(III) Ions with Sodium Borohydride

Abstract—Iron nanoparticles are obtained by reducing iron(III) chloride with sodium borohydride in aqueous solutions at room temperature without using stabilizing agents. The obtained samples are characterized by X-ray diffraction analysis, low-temperature adsorption of argon, and transmission electron microscopy. The effect of the concentration of reagent solutions, the molar ratio of reagents, and exposure to ultrasound and inert atmosphere (Ar) on the size and composition of the resulting particles is found. Depending on the conditions of borohydride reduction of iron salts in an aqueous solution, both agglomerates of iron nanoparticles (5–50 nm) of 200 nm or larger and individual iron nanoparticles of 1 to 20 nm in size can be obtained. The presence and concentration of wustite and magnetite in the composition of the obtained particles mainly depend on the concentration of the reducing agent.     

Unisized two-dimensional platinum clusters on silicon(111)-7x7 surface observed with scanning tunneling microscope

Uni-sized platinum clusters (size range of 5-40) on a silicon(111)-7 x 7 surface were prepared by depositing size-selected platinum cluster ions on the silicon surface at the collision energy of 1.5 eV per atom at room temperature. The surface thus prepared was observed by means of a scanning tunneling microscope (STM) at the temperature of 77 K under an ambient pressure less than 5 x 10(-9) Pa. The STM images observed at different cluster sizes revealed that (1) the clusters are flattened and stuck to the surface with a chemical-bond akin to platinum silicide, (2) every platinum atom occupies preferentially the most reactive sites distributed within a diameter of approximately 2 nm on the silicon surface at a cluster size up to 20, and above this size, the diameter of the cluster increases with the size, and (3) the sticking probability of an incoming cluster ion on the surface increases with the cluster size and reaches nearly unity at a size larger than 20.     

Restricted self-diffusion in an aqueous solution of poly(ethylene oxide) poly(propylene oxide) poly(ethylene oxide) triblock copolymer

 The self-diffusion behavior of a triblock copolymer (PEO–b– PPO–b–PEO) in an aqueous solution of 20% (m/m) was investigated during a temperature-induced phase transition from liquid to gel state using pulsed field gradient NMR and static light scattering. The measured self-diffusivity shows a strong dependence on the observation time in the gel phase indicating the existence of diffusion barriers in the size range of about 0.6 μm. Additional static light-scattering measurements show a structure in the same size range of several hundred nanometers, which is far above molecular or micellar sizes and thus, has to be caused by larger clusters. The similarity in the space scales suggests that the restriction of molecular propagation is correlated with the grain boundaries between the domains of the poly-crystalline structure formed by the arranged micelles. Received: 28 October 1996 Accepted: 21 March 1997     

Electrochemical kinetic study about cobalt electrodeposition onto GCE and HOPG substrates from sulfate sodium solutions

In the present work, we analyze the electrodeposition of cobalt by electrochemical techniques onto GCE (system I) and HOPG (system II) electrodes from sulfate solutions. Cyclic voltammetry and current transient measurements were used to obtain the nucleation and growth mechanism. The results clearly showed that electrodeposition of cobalt is a diffusion-controlled process with a typical 3D nucleation mechanism in both substrates. The average ΔG calculated for the stable nucleus formation was 1.97 × 10⁻²⁰ J nuclei⁻¹ and 3.58 × 10⁻²⁰ J nuclei⁻¹ for system I and system II, respectively. The scanning electron microscope (SEM) images indicated similar nucleation and growth processes on GCE and HOPG substrates at same overpotential with a homogeneous disperse cobalt clusters. X-ray energy-dispersive spectroscopy (EDS) was performed in order to ensure that the clusters formed are cobalt. The nuclei’s size obtained was dependent of the overpotential applied; at lower overpotentials, the growth rate of the cobalt clusters diminishes when their number increases due to the strongly reduced concentration of cobalt ions because of their consumption by a larger number of growing particles. A theoretical quantum study employing PM6 method suggests that Na⁺ adsorbed deactivate the local surface occasionating the formation of disperse cobalt clusters on carbon electrodes.     

Noncontact atomic force microscopy studies of ultrathin films of amorphous solid water deposited on Au(111)

Noncontact atomic force microscopy was used to study the morphological changes of an ultrathin amorphous solid water (ASW) film as a function of deposition temperature, annealing temperature, and annealing time. ASW deposited at 80 or 108 K on Au(111) formed truncated hemispherical clusters of increasing size during annealing at 134 K; these clusters were inferred to be crystalline. The number of nuclei present at the outer surface of the film after deposition was greater for higher deposition temperature. For lower cluster densities, depletion of the ASW film around the clusters was observed when the clusters became larger and dendritic growth was observed when the apparent cluster footprint radius exceeded 100 nm.     

Dispersion of magnetic susceptibility and the microstructure of magnetic fluid

The microstructure of magnetic fluid produced on the basis of kerosene with oleic acid as a stabilizer is studied experimentally. An analytical procedure based on the known dependence of the time of Brownian relaxation of the magnetic moment of the colloidal particle on its size and the expansion of a low-frequency spectrum of dynamic susceptibility into the series of Debye functions is used. Magnetic susceptibility is measured at frequencies from 10 Hz to 100 kHz and temperatures from 225 to 360 K for colloidal solutions with the volume fraction of magnetite from 0.08 to 0.17. The clusters with uncompensated magnetic moments and sizes varying from 50 to 70 nm that are three-or fourfold larger than the mean diameter of a single colloidal particle are found. It is revealed that characteristic sizes of clusters are virtually independent of temperature and concentration of colloidal particles. The contribution of clusters to the equilibrium susceptibility of magnetic fluid grows exponentially with decreasing temperature, being manyfold larger at low temperatures than that of single particles. The obtained temperature dependence of equilibrium susceptibility is compared with that predicted from current theoretical models.     

Structural transition to electrolyte-water solvent and changes in the molecular dynamics of water and properties of solutions

A model of concentration transition “ions and complexes in a water structure → ionic and ionicaqueous clusters → polymer structures of salt and crystal hydrate melts” is suggested. The appearance of cluster nanostructures outside the first zone of a waterlike structure is regarded as a general characteristic of solutions. The characteristics of solutions, phase equilibria, and salts of complex composition are interpreted based on this model. Investigation of the complex dielectric constant of electrolytic solutions in the SHF and EHF bands (7–119 GHz) at high concentrations showed that there are two dispersion regions in which the relaxation times differ by a factor of 5–10. Relaxation processes are separated, the numbers of molecules in hydration shells are calculated, and relaxation times are determined for bulky tetrahedral water with hydration shells of ions, for clusters, and for ionicaqueous polymer chains. It is shown that the two structure subsystems of water molecules in concentrated solutions may be described using the limited rotator/generalized diffusion molecular model. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 5, pp. 851–863, September–October, 1998.     

Mechanism of conductivity in molecular systems and polymer nanocomposites

A model is considered in which the characteristics of polymer nonocomposites based on poly(p-xylylene) are observed due to the formation of large percolation clusters consisting of semiconductor or metal nonoparticles, and to electron transfer from the surfaces of clusters on -C₆H₄- phenyl rings. It is assumed that an electrical double layer is formed near a cluster’s surface, accompanied by the appearance of excess electrons in the polymer matrix as components of -C₆H₄⁻- ion resonance. It is shown that a metallic cluster in the polymer matrix is oxidized following an increase in its electrostatic potential. This could be explained by the notable reduction in the oxidation of nanocomposite metallic clusters by atmospheric oxygen and water molecules. Upon the formation of semiconductor clusters in poly(p-xylylene) nanocomposite, conductivity is observed due to the hopping mechanism. The extra electron of the −C₆H₄⁻- anion resonance in the electric field jumps to another ring. The hopping character of nanocomposite conductivity is explained for the case of small nanoparticle concentrations using the Mott model. Precise equations from percolation cluster theory are used to explain the nonmonotonic dependency of nanocomposite photoconductivity on the concentration of semiconductor nanoparticles.     

Aggregation numbers of SDS micelles formed on EHEC. A steady state fluorescence quenching study

The present investigation proves that in the interaction between an uncharged polymer and a negatively charged amphiphilic ion (surfactant) clusters are actually formed and it provides data for the cluster concentration and the cluster size and their variation with composition. The polymer bound cluster size increases after a certain critical surfactant concentration and passes through a maximum. This maximum cluster size decreases with decreasing polymer concentration and attains a limiting value at infinite dilution. For the highest polymer concentration the cluster size is close to the size of normal surfactant micelles. The cluster concentration was determined by a fluorescence quenching technique and the amount surfactant adsorbed to the polymer by dialysis equilibrium measurements. Combining these independent sets of data permits the cluster aggregation number to be unambiguously determined. Solubilization experiments indicate the possibility to regulate the amount solubilized by varying the polymer concentration. The molecular properties of the system are sensitively monitored by the variation in two vibronic peaks in the pyrene fluorescence emission spectrum which defines a hydrophobic index. Very good agreement is found between all three experimental methods. Finally, the model suggested is analyzed in terms of coil size and cluster-cluster distance. Depending upon the degree of adsorption saturation and the density of polymer segments in solution the interaction may switch from being intramolecular to becoming intermolecular.     

A mass and time-of-flight spectroscopy study of the formation of clusters in free-jet expansions of normal D2

The mass spectra in the range of 2(D+)-38(D19+) amu of clusters formed in a supersonic free-jet expansion of normal D2 are investigated as functions of source temperature in the range of 95-220 K and of source pressure in the range of 10-120 bars. For some of the small ion fragments, time-of-flight distributions are also measured. For large clusters (n > 200) the intensities of the odd-numbered ion fragments exhibit magic numbers at D9+ and D15+ in accordance with previous experiments and calculations. The even-numbered ion fragments have much smaller intensities and exhibit new magic numbers at D10+ and D14+. For source conditions such that large clusters are formed, the intensities of the various different ion fragments are observed to saturate beyond a certain source pressure. At lower source pressures, where only small clusters are formed, the terminal mole fractions of the neutral dimers are analyzed in the light of available theories which take into account both the thermodynamics and the kinetics of the expansion. At higher source pressures and lower temperatures, where larger clusters are formed, the sizes of the neutral clusters are estimated using scaling laws and are found to be consistent with the mass spectra and measured time-of-flight distributions. By using a variety of techniques it has been possible to obtain reliable conclusions about the neutral cluster sizes for the present free-jet expansion conditions.     

Isomerization kinetics of carbon clusters 1. Potential of interatomic interaction

Formation of fullerenes — spherical carbon clusters C₆₀ and clusters of other sizes — during condensation of carbon vapors has not yet received theoretical explanation. Recent experimental works concerned with cluster formation in carbon vapors have established that during condensation carbon atoms form rings and then polycyclic clusters, which are precursors of fullerenes. Theoretical investigation of the spontaneous formation of fullerenes from polycyclic rings calls for a simple model of the potential of interatomic interaction of carbon, which would allow fast calculations of bond energies and statistical sums of the clusters. We use the modified Brenner potential, which was developed for hydrocarbon molecules. The parameters of the potential are refitted according to the results of quantum chemical calculations. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computation Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No.4, pp. 664–670, July–August, 1996. Translated by I. Izvekova