This study reports a new strategy for stabilizing palladized iron (Fe-Pd) nanoparticles with sodium carboxymethyl cellulose (CMC) as a stabilizer. Compared to nonstabilized Fe-Pd particles, the CMC-stabilized nanoparticles displayed markedly improved stability against aggregation, chemical reactivity, and soil transport. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analyses indicated that the CMC-stabilized nanoparticles with a diameter <17.2 nm are highly dispersed in water. Fourier transform infrared (FTIR) spectroscopy results suggested that CMC molecules were adsorbed to iron nanoparticles primarily through the carboxylate groups through monodentate complexation. In addition, -OH groups in CMC were also involved in interactions with iron particles. Batch dechlorination tests demonstrated that the CMC-stabilized nanoparticles degraded trichloroethene (TCE) 17 times faster than their nonstabilized counterparts based on the initial pseudo-first-order rate constant. Last, column tests showed that the stabilized nanoparticles can be readily transported in a loamy-sand soil and then eluted completely with three bed volumes of deionized (DI) water.

In this paper, we present the results of variations in the bulk solvent type and the addition of cosolvents on the growth rate of metallic copper nanoparticles produced through a reduction reaction within AOT reverse micelles. The metallic copper particles were characterized using both UV−vis spectroscopy and TEM microscopy. Growth rates are discussed in terms of an absorbance ratio, which provides a correlation between UV−vis absorption spectra and particle size. Time-resolved measurements showed that the intermicellar exchange rate and subsequent particle growth rate is a function of water content, bulk solvent, and the addition of cosolvents and cosurfactants. The copper particle growth rate was found to increase with increasing water content, but essentially the same particle size was eventually approached at all water contents. Copper particle growth was faster in isooctane than in cyclohexane because of the structure of the bulk solvent and the resulting degree of solvation into the micellar tail region. The cosolvent 1-octanol was found to decrease the copper particle growth rate when added in small amounts (up to 3 mol %) at two different water contents, but larger amounts of the cosolvent destabilized the micelle, increasing the growth rate and decreasing the terminal particle size. The cosolvent benzyl alcohol had the opposite effect, increasing the copper particle growth rate and decreasing the terminal particle size at all of the concentrations studied.

This paper demonstrates the rapid and precise size selection of nanoparticle populations using the pressure tunable solvent properties of CO2-expanded liquids. Specifically, by pressurizing and expanding a single organic solution with carbon dioxide gas, ligand-stabilized silver particles of desired mean size were size selectively precipitated at desired locations. Compared to current techniques, this CO2-expanded liquid approach provides for faster and more efficient particle size separation, reduction in organic solvent usage, and pressure tunable size selection.

Organic and inorganic reactions within the aqueous cores of water-in-oil AOT reverse micelle systems are viable methods for the production of nanomaterials of controllable composition and geometry while maintaining narrow size distributions. Considerable research has been done in order to better understand the governing features which comprise the AOT reverse micellar system, particularly stability and the intermicellar exchange of the contents within the aqueous core. The intermicellar exchange rate is affected by the bulk solvent type, the contents dissolved within the core, and the size of the reverse micelle or the water content, referred to as the W value, where W is the molar ratio of the water to AOT surfactant concentrations. Synthesis of nanomaterials within the AOT reverse micelle system is a strong function of the intermicellar exchange process and the factors mentioned previously. This study examines the effects of varying the bulk liquid solvent and the W value on the growth rate and ultimate particle size of copper nanoparticles produced via reduction of CuAOT2 within the micelle core. Particle growth is measured in-situ using time-resolved UV−Vis absorbance spectroscopy, and the particle size is determined by both UV−Vis measurements and TEM analysis. A total interaction energy model is implemented to represent the attractive van der Waals forces acting between the metallic particles and the repulsive osmotic and elastic forces which result from the surfactant tail−tail and solvent−tail interactions responsible for the steric stabilization of the metallic particles within the microemulsion. The model is able to predict the ultimate particle sizes obtained experimentally for copper and silver nanoparticles synthesized using a variety of bulk liquid solvents including isooctane, cyclohexane, and n-alkanes ranging from pentane to dodecane.

This article presents the stabilization of silver nanoparticle intermediates synthesized in ammonium perfluoropolyether (PFPE-NH 4 ) reverse micelles with supercritical fluid (SCF) carbon dioxide solvent as the continuous phase. Specifically, the intermediates were formed by the reduction of silver nitrate salt (AgNO 3 ) encapsulated within PFPE-NH 4 reverse micelles. The effect of reducing agent type, reverse micelle water content, water core buffering, and bulk solvent type were all investigated as factors affecting stabilization of the silver nanoparticle intermediates. Particles were characterized by in situ UV-visible spectroscopy and transmission electron microscopy (TEM). The UV-vis spectrum of these nanosized silver particles is sensitive to particle size, and thus time-resolved spectral measurements were utilized as a means of monitoring both intermediate growth and persistence. The silver intermediates were stabilized in PFPE-NH 4 reverse micelles as indicated by multiple UV-vis absorption bands that persist for periods of time measured to greater than 9 h. Intermediate stabilization is facilitated by a unique environment existing specifically as a result of PFPE-NH 4 surfactant presence and its local water environment in the reverse micelle rather than any effects arising from the carbon dioxide solvent.

Several oxygenated hydrocarbons, including acetylated sugars, poly(propylene glycol), and oligo(vinyl acetate), have been used to generate CO2-soluble ionic surfactants. Surfactants with vinyl acetate tails yielded the most promising results, exhibiting levels of CO2 solubility comparable to those associated with fluorinated ionic surfactants. For example, a sodium sulfate with single, oligomeric vinyl acetate (VAc) tails consisting of 10 VAc repeat units was 7 wt % soluble in CO2 at 25 degrees C and 48 MPa. Upon introduction of water to these systems, only surfactants with the oligomeric vinyl acetate tails exhibited spectroscopic evidence of a polar environment that was capable of solubilizing the methyl orange into the CO2-rich phase. For example, a single-phase solution of CO2, 0.15 wt % sodium bis(vinyl acetate)8 sulfosuccinate, and water, at water loading (W) values ranging from 10 to 40 at 25 degrees C and 34.5 MPa, exhibited a methyl orange peak at 423 nm. This result indicated that the core of a reverse micelle provided a microenvironment with a polarity similar to that of methanol. Quantum chemical calculations indicate that the acetylated sugars may be too hydrophilic to readily form reverse micelles, whereas the VAc-based surfactants appear to have the correct balance of hydrophilic and hydrophobic forces necessary to form reverse micelles.

In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temperature TB≃50K in D≃5nm NPs but only diamagnetism in 12nm NPs. For T&lt;TB=50K, the strong temperature dependence of coercivity Hc, saturation magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc≃250Oe, He≃−40Oe, and Ms≃10−2emu∕g at 5K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ≃0.006μB per Au atom attached to DT is determined using a model which yields Ms varying as 1∕D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.

A thermodynamic model was developed for the size-selective fractionation of ligand-stabilized nanoparticles by a CO2 gas-expanded liquid precipitation process. The tunable solvent strength of gas-expanded liquids, via CO2 pressurization, results in an effective method to fractionate nanoparticles, based on the size-dependent dispersibility of the particles. Specifically, the thermodynamic model is used to estimate the size of dodecanethiol-capped Ag nanoparticles that can be dispersed at a given CO2 pressure by equating the total interparticle interaction energy to the Boltzmann threshold stabilization energy (−3/2 k B T). The ligand−solvent interaction is found to have the greatest impact on the total interaction energy. This model illustrates that the entire length of the ligand is not accessible to the solvent, and three phenomenological model variations were developed to vary the ligand−solvent interaction.