Solvation forces between silica bodies in supercritical carbon dioxide

We report Monte Carlo simulations of the solvation pressure between two planar surfaces, which represent the interface of spherical silica nanoparticles in supercritical carbon dioxide. Carbon dioxide (CO2) was modeled as an atomistic dumbbell or a spherical Lennard-Jones particle. The interaction between CO2 molecules and silica surfaces was characterized by the standard Steele potential with energetic heterogeneities representing the hydrogen bonds. The parameters for the solid-fluid interaction potentials were obtained by fitting our simulations to the experimental isotherms of CO2 sorption on mesoporous siliceous materials. We studied the dependence of the solvation force on the distance between planar silica surfaces at T = 318 K, at equilibrium bulk pressures p(bulk) ranging from 69 to 200 atm. At 69 atm, we observed a long-range attraction between the two surfaces, and it vanished when the pressure was increased to 102 and then 200 atm. The results obtained with different fluid models were consistent with each other. According to our observations, energetic heterogeneities of the surface have negligible influence on the solvation pressure. Using the Derjaguin approximation, we calculated the solvation forces between spherical silica nanoparticles in supercritical CO2 from the solvation pressures between the planar surfaces.     

Hydrogen bond strength and network structure effects on hydration of non-polar molecules

We measure the solvation free energy, Δμ*, for hard spheres and Lennard-Jones particles in a number of artificial liquids made from modified water models. These liquids have reduced hydrogen bond strengths or altered bond angles. By measuring Δμ* for a number of state points at P = 1 bar and different temperatures, we obtain solvation entropies and enthalpies, which are related to the temperature dependence of the solubilities. By resolving the solvation entropy into the sum of the direct solute-solvent interaction and a term depending on the solvent reorganisation enthalpy we show that, although the hydrophobic effect in water at 300 K arises mainly from the small molecular size, its temperature dependence is anomalously low because the reorganisation enthalpy of liquid water is unusually small. We attribute this to the strong tetrahedral network which results from both the molecular geometry and the hydrogen bond strength.     

AFM colloidal forces measured between microscopic probes and flat substrates in nanoparticle suspensions

Colloidal forces between atomic force microscopy probes of 0.12 and 0.58 N/m spring constant and flat substrates in nanoparticle suspensions were measured. Silicon nitride tips and glass spheres with a diameter of 5 and 15 mum were used as the probes whereas mica and silicon wafer were used as substrates. Aqueous suspensions were made of 5-80 nm alumina and 10 nm silica particles. Oscillatory force profiles were obtained using atomic force microscope. This finding indicates that the nanoparticles remain to be stratified in the intervening liquid films between the probe and substrate during the force measurements. Such structural effects were manifested for systems featuring attractive and weak repulsive interactions of nanoparticles with the probe and substrate. Oscillation of the structural forces shows a periodicity close to the size of nanoparticles in the suspension. When the nanoparticles are oppositely charged to the probes, they tend to coat the probes and hinder probe-substrate contact.     

Nanoparticle Size and Concentration Dependence of the Electroactive Phase Content and Electrical and Optical Properties of Ag/Poly(vinylidene fluoride) Composites

This paper describes the processing of silver‐nanoparticle‐doped poly(vinylidene fluoride). The effects of the concentration and size of the filler on the electroactive phase of the polymer and the optical and electrical properties are discussed. Spherical silver nanoparticles incorporated into the poly(vinylidene fluoride) polymeric matrix induce nucleation of the electroactive γ phase. The electroactive phase content strongly depends on the content and size of the nanoparticles. In particular, there is a critical nanoparticle size, below which the filler losses its nucleation efficiency due to its small size relative to that of the polymer macromolecules. Furthermore, the presence of surface plasmon resonance absorption in the composites is observed, which once again shows a strong dependence on the concentration and size of the particles. The absorption is larger for higher concentrations, and for a given concentration increases with particle size. This behavior is correlated to the electrical response and is related to the extra bands and electrons provided by the nanoparticles in the large energy band gap of the polymer.     

Shape effect on nanoparticle solvation: a comparison of morphometric thermodynamics and microscopic theories

Conventional wisdom for controlling the nanoparticle size and shape during synthesis is that particle growth favors the direction of a facet with the highest surface energy. However, the particle solvation free energy, which dictates the particle stability and growth, depends not only on the surface area and surface free energy but also on other geometric measures such as the solvent excluded volume and the surface curvature and their affiliated thermodynamic properties. In this work, we study the geometrical effects on the solvation free energies of nonspherical nanoparticles using morphometric thermodynamics and density functional theories. For idealized systems that account for only molecular excluded-volume interactions, morphometric thermodynamics yields a reliable solvation free energy when the particle size is significantly larger than the solvent correlation length. However, noticeable deviations can be identified in comparison to the microscopic theories for predicting the solvation free energies of small nanoparticles. This conclusion also holds for predicting the potential of mean force underlying the colloidal "key-and-lock" interactions. Complementary to the microscopic theories, morphometric thermodynamics requires negligible computational cost, therefore making it very appealing for a broad range of practical applications.     

Ionic solvation at the solid/electrolyte interface: A statistical-mechanical approach

We have studied studied the influence of the size of ions on their adsorbability at a solid surface in the presence of a molecular solvent. Ions and molecules are represented respectively by charged hard spheres and dipolar hard spheres and the surface is just a neutral hard wall. We have found that the electrostatic interaction between ions and molecules can induce the exclusion of small ions from the surface. A pure MSA (mean spherical approximation) calculation would not give any effect of the solvation on the ionic density profile. The present calculation is limited to the case of infinite ionic dilution.     

Molecular dynamics simulation of nanocolloidal amorphous silica particles: Part I

Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles in aqueous solution, with diameter of 4.4 nm and with four different background electrolyte concentrations, to extract the mean force acting between the two silica nanoparticles. Dependences of the interparticle forces on the separation and the background electrolyte concentration were demonstrated. The nature of the interaction of the counterions with charged silica surface sites (deprotonated silanols) was investigated. A "patchy" double layer of adsorbed sodium counterions was observed. Dependences of the interparticle potential of mean force on the separation and the background electrolyte concentration were demonstrated. Direct evidence of the solvation forces is presented in terms of changes of the water ordering at the surfaces of the isolated and double nanoparticles. The nature of the interaction of the counterions with charged silica surface sites (deprotonated silanols) was investigated in terms of quantifying the effects of the number of water molecules separately inside each pair of nanoparticles by defining an impermeability measure. A direct correlation was found between the impermeability (related to the silica surface "hairiness") and the disruption of water ordering. Differences in the impermeability between the two nanoparticles are attributed to differences in the calculated electric dipole moment.     

Ab initio and DFT studies on tautomerism of indazole in gaseous and aqueous phases

Electronic structure of optimized Ge₅, Ge₁₇, Ge₅–O and Ge₅ embedded in SiO₂ nanoparticles have been studied by density functional theory to find out the effect of cluster size and Ge–O bond(s) on the optical energy gap between LUMO and HOMO. It was found that the optical energy gap depends on both cluster size and the number of Ge–O bonds nonlinearly. The optical energy gap was found to be in visible light range when the Ge₅ nanoparticle has been embedded in SiO₂ matrix.     

Binary nanoparticle superlattices in the semiconductor-semiconductor system: CdTe and CdSe

We report binary nanoparticle superlattices obtained by self-assembly of two different semiconductor quantum dots. Such a system is a means to include two discretized, quantum-confined, and complimentary semiconductor units in close proximity, for purposes of band gap matching and/or energy transfer. From a range of possible structures predicted, we observe an exclusive preference for the formation of Cuboctahedral AB13 and AB5 (isostructural with CaCu5) obtained in the system of 8.1 nm CdTe and 4.4 nm CdSe nanoparticles. For this system, a possible ionic origin for the formation of structures with lower packing densities was ruled out on the basis of electrophoretic mobility measurements. To understand further the principles of superlattice formation, we constructed space-filling curves for binary component hard spheres over the full range of radius ratio. In addition, the pair interaction energies due to core-core and ligand-ligand van der Waals (VDW) forces are estimated. The real structures are believed to form under a combined influence of entropic driving forces (following hard-sphere space filling principles) and the surface (due to ligand-ligand VDW).     

Ultra small Gd(2)O(3) nanoparticles: Absorption and emission properties

In this paper, we report a facile method for synthesis of ultra small (1-3nm) gadolinium oxide (Gd(2)O(3)) nanoparticles using citric acid (CA) as a capping agent. The dependence of nanoparticle (NP) size on the ratio between CA and gadolinium (Gd) is investigated. Absorption properties of the ultra small Gd(2)O(3) NPs in UV region have four characteristic peaks at 312nm, 274nm, 253nm and 228nm. Finally, we show that the Gd(2)O(3) nanoparticles synthesized by this method induce triplet emission (phosphorescence) from CA and EG in the NIR region.     

SERS from molecules adsorbed on small Ag nanoparticles: A microscopic model

A microscopic approach to surface-enhanced Raman scattering (SERS) from molecules adsorbed on noble-metal nanoparticles is developed. For nanoparticle sizes smaller than 10 nm, the classical electromagnetic enhancement mechanism is modified by quantum-size effects. Using time-dependent local field approximation, we perform systematic analysis of SERS in nanometer-sized Ag nanoparticles. We find that, in small nanoparticles, Raman cross-section enhancement is governed by the interplay between Landau damping of the surface plasmon and interband screening in the nanoparticle surface layer.     

General principles of the crystallization of nanostructured disperse systems

Regularities of the self-organization of ensembles of nanoparticles into crystalline structures in metal lyosols are studied by the Brownian dynamics. Van der Waals, elastic, and electrostatic interparticle interactions, as well as the action of dissipative and fluctuation forces are taken into account in the model. Pair interaction potentials for the polydisperse ensembles of particles are analyzed and the correlations between the pattern of potential curves and the type of formed structures (from crystalline to fractal) are revealed. The effect of interparticle tangential friction on the degree of ordering of formed structures is studied. It is demonstrated that this factor can determine the degree of ordering of nanoparticle aggregates. An essentially new approach to describing elastic interactions of nanoparticles of polymer-containing sols is implemented.     

Raman scattering of 4-aminobenzenethiol sandwiched between Ag nanoparticle and macroscopically smooth Au substrate: effects of size of Ag nanoparticles and the excitation wavelength

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.     

Attachment of nanoparticles to the AFM tips for direct measurements of interaction between a single nanoparticle and surfaces

Here we report a universal method of attachment/functionalization of tips for atomic force microscope (AFM) with nanoparticles. The particles of interest are glued to the AFM tip with epoxy. While the gluing of micron size particles with epoxy has been known, attachment of nanoparticles was a problem. The suggested method can be used for attachment of virtually any solid nanoparticles. Approximately every other tip prepared with this method has a single nanoparticle terminated apex. We demonstrate the force measurements between a single approximately 50 nm ceria nanoparticle and flat silica surface in aqueous media of different acidity (pH 4-9). Comparing forces measured with larger ceria particles ( approximately 500 nm), we show that the interaction with nanoparticles is qualitatively different from the interaction with larger particles.     

Optical gap and excitation energies of small Ge nanocrystals

Using the density functional theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the optical gap and the oscillator strengths for several of the lowest, spin and symmetry allowed, electronic transitions of small Ge nanocrystals passivated by hydrogen. The largest nanoparticle has an approximate diameter of 2 nm. Our results show that the optical gap exhibits size dependence (due to quantum confinement) roughly similar to silicon nanoparticles. However, for this range of diameters, there is an indirect-to-direct transition in the spectra of Ge as the size of the nanocrystals decrease. The first allowed excitation (fundamental optical gap) of each germanium nanoparticle has relatively larger oscillator strengths compared to silicon. The diameter of the smallest Ge nanocrystal capable to emit in the visible region of the spectrum, is approximately 1.9 nm, compared to 2.2 nm for silicon nanocrystals.     

Distance dependent quenching effect in nanoparticle dimers

In this paper, we investigate the emission characteristics of a molecule placed in the gap of a nanoparticle dimer configuration. The emission process is described in terms of a local field enhancement factor and the overall quantum yield of the system. The molecule is represented as a dipolar source, with fixed length and fed by a constant current. We first describe the coupled dimer-molecule system and compare these results to a single sphere-molecule system. Next, the effect of dimer size is investigated by changing the radius of the nanoparticles. We find that when the radius increases, a saturation effect occurs that trends towards the case of a radiating dipole between two flat interfaces, which we refer to as a parallel plate waveguide geometry. An analytical solution for the parallel plate waveguide geometry is presented and compared to the results for the spherical dimer configuration. We use this approximation as a reference solution, and also, it provides useful guidelines to understand the physical mechanism behind the energy transfer between the molecule and the dimer. We find that the emission intensity undergoes a quenching effect only when the inter-nanoparticle gap distance of the dimer is very small, meaning that strong coupling prevails over energy engaged in the heating process unless the molecule is extremely close to the metal surface.     

Nanoparticle formation in water-in-oil microemulsions: experiments, mechanism, and Monte Carlo simulation

The dynamics of nanoparticle formation in water-in-oil microemulsions via temporal size evolution has been followed from UV-visible absorption spectra of CdS nanoparticles. Existing Monte Carlo (MC) simulations of nanoparticle formation are primarily based on the mechanism of nuclei formation and their growth by coalescence-exchange of drops, which alone do not predict particles of large size as observed in some experiments. Hence, we have included an additional size enlargement process, namely coagulation of nanoparticles during drop coalescence. We find that particle coagulation, constrained by microemulsion drop size, shows very good agreement with our experimental data on CdS nanoparticle size evolution, for different drop sizes. Thus a combined approach of spectroscopy and MC simulation is helpful in elucidating the mechanism of nanoparticle formation in these confined systems, leading to prediction of size-controlled nanoparticle synthesis.     

Effect of catalysis on the stability of metallic nanoparticles: Suzuki reaction catalyzed by PVP-palladium nanoparticles

The small size of nanoparticles makes them attractive in catalysis due to their large surface-to-volume ratio. However, being small raises questions about their stability in the harsh chemical environment in which these nanoparticles find themselves during their catalytic function. In the present work, we studied the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by PVP-Pd nanoparticles to investigate the effect of catalysis, recycling, and the different individual chemicals on the stability and catalytic activity of the nanoparticles during this harsh reaction. The stability of the nanoparticles to the different perturbations is assessed using TEM, and the changes in the catalytic activity are assessed using HPLC analysis of the product yield. It was found that the process of refluxing the nanoparticles for 12 h during the Suzuki catalytic reaction increases the average size and the width of the distribution of the nanoparticles. This was attributed to Ostwald ripening in which the small nanoparticles dissolve to form larger nanoparticles. The kinetics of the change in the nanoparticle size during the 12 h period show that the nanoparticles increase in size during the beginning of the reaction and level off toward the end of the first cycle. When the nanoparticles are recycled for the second cycle, the average size decreases. This could be due to the larger nanoparticles aggregating and precipitating out of solution. This process could also explain the observed loss of the catalytic efficiency of the nanoparticles during the second cycle. It is also found that the addition of biphenyl to the reaction mixture results in it poisoning the active sites and giving rise to a low product yield. The addition of excess PVP stabilizer to the reaction mixture seems to lead to the stability of the nanoparticle surface and size, perhaps due to the inhibition of the Ostwald ripening process. This also decreases the catalytic efficiency of the nanoparticles due to capping of the nanoparticle surface. The addition of phenylboronic acid is found to lead to the stability of the size distribution as it binds to the particle surface through the O(-) of the OH group and acts as a stabilizer. Iodobenzene is found to have no effect and thus probably does not bind strongly to the surface during the catalytic process. These two results might have an implication on the catalytic mechanism of this reaction.     

Controlled nanoparticle assembly by dewetting of charged polymer solutions

In this paper, we present an alternative approach for controlled nanoparticle organization on a solid substrate by applying dewetting patterns of charged polymer solutions as a templating system. Thin films of charged polymer solutions dewet a solid substrate to form complex dewetting patterns that depend on the polymer charge density. These patterns, ranging from polygonal networks to elongated structures that are stabilized by viscous forces during dewetting, serve as potential templates for two-dimensional nanoparticle organization on a solid substrate. Thus, while nanoparticles dried in pure water undergo self-assembly to form close-packed arrays, addition of charged polymer in the dispersion leads to the formation of open structures that are directed by the dewetting patterns of the polymer solution. In this study, we focus on the application of elongated structures resulting from dewetting of high-charge-density polymer solutions to align nanoparticles of silica and gold into long chains that are several micrometers in length. The particle ordering process is a two-step mechanism: an initial confinement of the nanoparticles in the dewetting structures and self-assembly of the particles within these structures upon further drying by lateral capillary attractions.     

Capillary forces between two spheres with a fixed volume liquid bridge: theory and experiment

Capillary forces are commonly encountered in nature because of the spontaneous condensation of liquid from surrounding vapor, leading to the formation of a liquid bridge. In most cases, the advent of capillary forces by condensation leads to undesirable events such as an increase in the strength of granules, which leads to flow problems and/or caking of powder samples. The prediction and control of the magnitude of capillary forces is necessary for eliminating or minimizing these undesirable events. The capillary force as a function of the separation distance, for a liquid bridge with a fixed volume in a sphere/plate geometry, was calculated using different expressions reported previously. These relationships were developed earlier, either on the basis of the total energy of two solid surfaces interacting through the liquid and the ambient vapor or by direct calculation of the force as a result of the differential gas pressure across the liquid bridge. It is shown that the results obtained using these methodologies (total energy or differential pressure) agree, confirming that a total-energy-based approach is applicable, despite the thermodynamic nonequilibrium conditions of a fixed volume bridge rupture process. On the basis of the formulas for the capillary force between a sphere and a plane surface, equations for the calculation of the capillary force between two spheres are derived in this study. Experimental measurements using an atomic force microscope (AFM) validate the formulas developed. The most common approach for transforming interaction force or energy from that of sphere/plate geometry to that of sphere/sphere geometry is the Derjaguin approximation. However, a comparison of the theoretical formulas derived in this study for the interaction of two spheres with those for sphere/plate geometry shows that the Derjaguin approximation is only valid at zero separation distance. This study attempts to explain the inapplicability of the Derjaguin approximation at larger separation distances. In particular, the area of a liquid bridge changes with the separation distance, H, and thereby does not permit the application of the "integral method," as used in the Derjaguin approximation.