Michael Z. Hu

Size, Volume Fraction and Nucleation of Stober Silica Nanoparticles

29Si NMR, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) are used to monitor the synthesis of silica nanoparticles from the base-catalyzed hydrolysis of TEOS in methanol and ethanol. The reactions are conducted at a [TEOS] =0.5 M, low concentrations of ammonia ([NH(3)] =0.01-0.1 M), and [H(2)O] =1.1-4.4 M to resolve the initial size of the first nuclei and to follow their structural evolution. It is found that after an induction period where there is a buildup of singly hydrolyzed monomer, the first nuclei are fractal and open in structure. Interestingly, the nuclei are twice as large in ethanol (R(g) approximately 8 nm) as those in methanol (R(g) approximately 4 nm). The data suggest that the difference in primary particle size is possibly caused by a higher supersaturation ratio of the singly hydrolyzed monomer in methanol than in ethanol if it is assumed that the surface energy of the first nuclei is the same in methanol and ethanol. The particle number concentration and the volume fraction of the silica particles are calculated independently from the SAXS, DLS, and 29Si NMR results. Finally, the rate of nucleation is obtained from the particle number concentrations.

Electrocoagulation for magnetic seeding of colloidal particles

Abstract Electrocoagulation (EC) was investigated as a magnetic seeding method to be used prior to a high-gradient magnetic separation (HGMS) process. Experimental results showed that particulates with significant magnetic susceptibility were formed by EC with steel electrodes. Magnetite was specifically formed most readily in a sodium chloride solution, in the absence of other ions, but also in surrogate wastewater under controlled conditions. These results suggest that EC could be employed in a magnetic seeding process in which paramagnetic particles are formed and then removed through high-gradient magnetic filtration (HGMF). The effects of applied potential and current on the magnetic susceptibility and composition of particles formed by EC in a pure sodium chloride solution and in a surrogate wastewater solution were investigated by X-ray diffraction analysis, scanning electron microscopy (SEM), and magnetic susceptibility measurements. Results showed that pure magnetite particles of 100-nm diameter were formed in the sodium chloride solution. Various compounds were present in particles produced by the surrogate wastewater solution, indicating that the formation of magnetite particles was hindered by the presence of some cations and anions. Although these particles had a much lower magnetic susceptibility than magnetite particles, their magnetic susceptibility was relatively high and permitted good removal in HGMF. The removal of copper ions from both deionized water and surrogate wastewater by EC followed by HGMF was also investigated. Results obtained at pH 10 and 12 showed a higher copper ion removal from the deionized water solution than from the surrogate wastewater.

Homogeneous (co)precipitation of inorganic salts for synthesis of monodispersed barium titanate particles

Various processes of coprecipitation or crystallization of inorganicsalts of barium and titanium from homogeneous solutions were studiedin this work. In particular, barium hydroxide and barium chloridesalt as well as titanium tetrachloride were used as the startingmaterials for dielectric-tuning homogeneous precipitation in mixedsolvents of isopropanol and water. Hydroxypropylcellulose was used asa steric dispersant. Evaluations of size, shape, and composition ofsynthesized particles were made using scanning electron microscopy,high-temperature X-ray diffractometry, and differential thermalanalysis. Results show that salt concentration, pH, and reaction timeare important in determining the morphology and composition of thefinal powder. The titania particles from dielectric-tuningprecipitation are perfect microspheres with narrow size distribution(near monodispersed), while the particles from barium salts areflake-like, irregular in shape and size. Instead of particlescontaining uniform compositions of barium and titanium compounds,dielectric-tuning coprecipitation yielded powders of two separatedphases, i.e., monodispersed titania microspheres (∼1 μm) coated onbarium chloride salt flakes. Titanium-rich barium titanate wasobtained after calcination of coprecipitated powders. However,preliminary results show that the titania particles obtained bydielectric-tuning precipitation can be hydrothermally converted toBaTiO3 particles that are fully crystallized after calcination above950°C.

Mechanism of nanocrystalline BaTiO3 particle formation by hydrothermal refluxing synthesis

The nanocrystal-formation mechanisms of barium titanate (BaTiO3) particles in a hydrothermal refluxing process have been studied. The refluxing method allows in situ sampling and requires ambient conditions (i.e., temperature below the boiling point of solvent and atmospheric pressure). It was found that the formation of BaTiO3 from the amorphous precursor was a very rapid process. BaTiO3 nuclei were formed at around 75 °C, 15 min after the beginning of the hydrothermal reaction. Individual BaTiO3 particles with dimensions in the range of 70–130 nm were formed 5 min after the nucleation of nanocrystals. Further reaction for crystal growth proceeded by consumption of the remaining precursors and possible aggregation of BaTiO3 particles via Ostwald ripening. It is proposed on the basis of the microscopic observations that the in situ transformation mechanism, rather than dissolution–precipitation mechanism, is dominant in the early stage of BaTiO3 formation.

Probing the early-stage/rapid processes in hydrolysis and condensation of metal alkoxides

Understanding and control of the early-stage sol-gel reaction processes involving metal alkoxides are important to many advanced materials development and applications. In this work, Fourier transform infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS) were coupled with a specially designed, rapid flow-through mixing cell for monitoring such processes. The rapid, early-stage hydrolysis and condensation of zirconium n-butoxide in ethanol were chosen as the basis for a model system. FTIR was used to study soluble-species reaction kinetics, while the SAXS technique monitored in situ the solid-phase particle formation/growth (i.e., the nucleation and aggregation kinetics of polymeric clusters/particles). Monitoring the reactions and cluster/particle growth within a millisecond time regime was achieved. In addition, key parameters such as reaction time, concentration of base (i.e., quarternary tetramethyl ammonium hydroxide), temperature, water concentration, and alkoxide concentration were fully investigated in a continuous flow-through reactor.

Synthesis of MFI-type zeolite membranes on porous α-alumina supports by wet gel crystallization in the vapor phase

A new method has been developed for synthesis of MFI-type zeolite membranes on porous α-alumina supports. Using this method, a thin layer of wet-gel precursor precoated from a synthesis sol containing SiO2, tetrapropylammonium hydroxide (TPAOH), NaOH, and H2O was converted to an MFI zeolite film by vapor-phase treatment at elevated temperatures. MFI zeolite films were formed on the α-alumina substrates, respectively, in the vapor phases of TPAOH solution and an ethylenediamine(EDA)/triethylamine (TEA)/water mixture, but an amorphous material was obtained in the vapor phase of pure water. Scanning electron microscopy and helium permeation examinations before calcination showed that the MFI membrane obtained in the vapor phase of the TPAOH solution was of higher quality than that synthesized in the vapor phase of the EDA/TEA/water mixture in terms of zeolite film integrity and compactness. It was also found that the existence of the template TPA+ in the parental synthesis sol was critical to the formation of MFI zeolite under the investigated conditions. When precursor layers coated from a template-free colloidal silica system were used, the vapor-phase treatment resulted in formation of Na-P1 and ANA zeolite films, respectively, in the vapors of TPAOH solution and the EDA/TEA/water mixture. The method developed in this work has the advantages of improved controllability, minimal waste generation, and reduced chemical consumption that are desirable for large-scale production of zeolite membranes.

Ionic strength effects on hexadecane contact angles on a gold-coated glass surface in ionic surfactant solutions

Abstract As aqueous surfactant cleaning becomes increasingly prevalent in industry, an understanding behind surfactant cleaning behavior is needed to improve the systems. Improvements in these aqueous surfactant solution systems can reduce the impact of the industrial cleaning systems on the environment and improve their efficiency. Studying various parameters of the cleaning systems can provide useful insight into surfactant behavior. The contact angle of the soil to the solid surface is strongly related to cleaning system performance and is a relatively simple parameter to measure. For this study, a simplified cleaning system was studied to investigate the effects of modifications to the surfactant system. The contact angle of hexadecane on a gold-coated glass surface in aqueous surfactant solutions of sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) were determined up to and above the critical micelle concentrations. The effects of modifying the ionic strength of the aqueous surfactant solution by the addition of sodium chloride (NaCl) have been studied. Similar trends in contact angle behavior of the oil droplet to the gold-coated surface were found for both the anionic (SDS) and cationic (CTAB) surfactant systems. The addition of NaCl to the aqueous surfactant systems resulted in a significant increase in the contact angles over the surfactants concentration ranges. The increasing trend in the contact angles was observed for NaCl concentrations ranging from 0.05 to 0.5 mM, while little effect was noted for the higher NaCl concentrations. The observed effect is consistent with literature on electrostatic-charge-induced changes in surfactant adsorption at the solid surface.

Oil Droplet Detachment from Metal Surfaces as Affected by an Applied Potential

Removal of organic contaminants from metal surfaces submerged in an aqueous surfactant solution can be improved by application of an applied potential. Earlier work has shown that the detachment of organic droplets is noticeably affected by variations in the pH of the solution and charge of the surfactant. Modifications in solution pH result in modifications of the electrostatic charge of the solid surface, which, depending on the charge and nature of the surfactant, will improve or hinder adsorption of surfactant at the solid–aqueous interface. This adsorption of surfactant molecules is of great importance to the detachment of organic droplets and, consequently, on the determination of cleaning efficacy. It is proposed that modification of the surface charge through application of potential will also result in changes in the adsorption of surfactant and, as a result, will alter drop detachment. Experimental results from a battery of tests for the removal of industrial quench oil from a stainless steel surface with different applied potentials are shown. Additionally, several different surfactants were examined, and drop detachment times and cleaning efficiency by ultrasonication were measured. A mechanistic model describing the affect of potential on surfactant adsorption was proposed, and a correlation between detachment time and cleaning was developed for some of the surfactants.