Seong-Geun Oh

Preparation and antibacterial effects of Ag–SiO2 thin films by sol–gel method

In the present study, silver-doped silica thin films were successfully prepared by sol-gel method to apply for antibacterial materials. The starting solution was prepared from 1:0.24:3.75:2.2 molar ratios of Si(OC2H5)4):AgNO3:H2O:C2H5OCH2CH2OH and then the pH value controlled at 3 with 0.5 N HNO3 solution. The formation of silver-doped glassy silica thin films at various temperatures was investigated through infrared spectroscopy, ultraviolet-visible, scanning electron microscopy and X-ray diffraction. From these analysis data, it was found that silver ions were completely trapped in the silica matrix and their reduction could be achieved at 600 degrees C annealing temperature. The antibacterial effects of silica thin films against Escherichia coli and Staphylococcus aureus were examined by film attachment method. The coating films had an excellent antibacterial performance.

Hollow ZnO Nanofibers Fabricated Using Electrospun Polymer Templates and Their Electronic Transport Properties

Thin (0.5 to 1 microm) layers of nonaligned or quasi-aligned hollow ZnO fibers were prepared by sputtering ZnO onto sacrificial templates comprising polyvinyl-acetate (PVAc) fibers deposited by electrospinning on silicon or alumina substrates. Subsequently, the ZnO/PVAc composite fibers were calcined to remove the organic components and crystallize the ZnO overlayer, resulting in hollow fibers comprising nanocrystalline ZnO shells with an average grain size of 23 nm. The inner diameter of the hollow fibers ranged between 100 and 400 nm and their wall thickness varied from 100 to 40 nm from top to bottom. The electronic transport and gas sensing properties were examined using DC conductivity and AC impedance spectroscopy measurements under exposure to residual concentrations (2-10 ppm) of NO(2) in air at elevated temperatures (200-400 degrees C). The inner and outer surface regions of the hollow ZnO fibers were depleted of mobile charge carriers, presumably due to electron localization at O(-) adions, constricting the current to flow through their less resistive cores. The overall impedance comprised interfacial and bulk contributions. Both contributions increased upon exposure to electronegative gases such as NO(2) but the bulk contribution was more sensitive than the interfacial one. The hollow ZnO fibers were much more sensitive compared to reference ZnO thin film specimens, displaying even larger sensitivity enhancement than the 2-fold increase in their surface to volume ratio. The quasi-aligned fibers were more sensitive than their nonaligned counterparts.

Preparation of hollow silica microspheres in W/O emulsions with polymers.

Micrometer-sized hollow silica particles were synthesized by sol-gel reaction in water-in-oil emulsion. To obtain hollow structures in silica particles, the viscosity of water droplets in W/O emulsion was controlled with polyethylene glycol (PEG) or polyvinylpyrrolidone (PVP). To stabilize the emulsion structure, hydroxypropyl cellulose (HPC) was added to the oil phase. Without HPC, the particles have an irregular shape and hardly have a particulate form. As the concentration of HPC increased from 0.8 to 1.4 wt%, the size of silica particles decreased from 10 to 1 microm. But above 1.4 wt%, the solution was very viscous, so that it was difficult to handle. Especially, the role of PEG or PVP in the water phase was very important, not only because it stabilized the W/O emulsion structure, but also because it influenced the formation of hollow structure. Interestingly, the hollow silica particles were formed when the molar ratio of water to TEOS (Rw) was 4 and the concentrations of PEG and HPC were 6 and 1.4 wt%, respectively. Also, when PEG was replaced with polyvinylpyrrolidone (PVP), hollow silica particles ranging from 3 to 7 microm were formed.

Kinetics of micellization: its significance to technological processes

The association of many classes of surface active molecules into micellar aggregates is a well-known phenomenon. Micelles are often drawn as static structures of spherical aggregates of oriented molecules. However, micelles are in dynamic equilibrium with surfactant monomers in the bulk solution constantly being exchanged with the surfactant molecules in the micelles. Additionally, the micelles themselves are continuously disintegrating and reforming. The first process is a fast relaxation process typically referred to as t1. The latter is a slow relaxation process with relaxation time t2. Thus, t2 represents the entire process of the formation or disintegration of a micelle. The slow relaxation time is directly correlated with the average life-time of a micelle, and hence the molecular packing in the micelle, which in turn relates to the stability of a micelle. It was shown earlier by Shah and coworkers that the stability of sodium dodecyl sulfate (SDS) micelles plays an important role in various technological processes involving an increase in interfacial area, such as foaming, wetting, emulsification, solubilization and detergency. The slow relaxation time of SDS micelles, as measured by pressure-jump and temperature-jump techniques was in the range of 10 4 ‐10 1 s depending on the surfactant concentration. A maximum relaxation time and thus a maximum micellar stability was found at 200 mM SDS, corresponding to the least foaming, largest bubble size, longest wetting time of textile, largest emulsion droplet size and the most rapid solubilization of oil. These results are explained in terms of the flux of surfactant monomers from the bulk to the interface, which determines the dynamic surface tension. The more stable micelles lead to less monomer flux and hence to a higher dynamic surface tension. As the SDS concentration increases, the micelles become more rigid and stable as a result of the decrease in intermicellar distance. The smaller the intermicellar distance, the larger the Coulombic repulsive forces between the micelles leading to enhanced stability of micelles (presumably by increased counterion binding to the micelles). The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped-flow and pressure-jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results show relaxation times t2 in the range of seconds for Triton X-100 to minutes for polyoxyethylene alkyl ethers. The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation time t2 was related to dynamic surface tension and foaming www.elsevier.nl:locate:colsurfa

Analysis of trihalomethanes in drinking water using headspace-SPME technique with gas chromatography.

In many drinking water treatment plants, the chlorination process is one of the main techniques used for the disinfection of water. This disinfecting treatment leads to the formation of trihalomethanes (THMs) such as chloroform, dichlorobromomethane, chlorodibromomethane and bromoform. In this study, headspace-solid-phase microextraction (HS-SPME, 85 microm carboxen/polydimethylsiloxane fiber) technique was applied for the analysis of THMs in drinking water. The effects of experimental parameters such as kinds of SPME fiber, the volume ratio of sample to headspace, the addition of salts, magnetic stirring, extraction temperature, extraction time and desorption time on the analysis were investigated. Analytical parameters such as linearity, repeatability and limit of detection were also evaluated. The results of THMs from the survey of Seongnam (Korea) drinking water samples showed that the highest total trihalomethane and chloroform were 24.03 and 13.34 microg/l, which were well within the Korean drinking water quality standard of 100 and 80 microg/l, respectively.

Preparation of silver nanoparticles in hexagonal phase formed by nonionic Triton X-100 surfactant

Abstract Silver nanoparticles were prepared by reducing silver ions in the hexagonal phase formed by Triton X-100 in aqueous solution. Triton X-100 molecules have been used to form the hexagonal phase in aqueous solution as well as to reduce the silver ions into silver atoms. The hexagonal phase hindered the growth and aggregation of particles. The microstructure of the hexagonal phase was investigated by polarizing microscopy, 2 H NMR spectroscopy and small-angle X-ray diffraction. At the initial stage of the reaction, silver particles prepared in the hexagonal phase exhibited a size of 1–7 nm. As the reaction proceeded, particles grew up to about 30 nm as determined by transmission electron microscopy (TEM). The formation rate of silver particles was investigated by UV–Visible spectroscopy. It was found that the reaction temperature was an important factor for the rate of particle formation. With TEM, it was confirmed that surfactant aggregates, which have flexible structures, could not absolutely prevent particles from growing and aggregating. But, the results of polarizing microscopy, 2 H NMR spectroscopy and X-ray diffraction exhibited that the growth of particles could not cause a deformation of the original structure (hexagonal phase), which was employed as the reaction medium.

Micelle-Mediated Synthesis of Single-Crystalline β(3C)-SiC Fibers via Emulsion Electrospinning

Submicroscale SiC fiber mats were prepared by the electrospinning of an oil-in-water(O/W) precursor emulsion, a subsequent thermal curing treatment, and calcination at 1600 °C. Low-molecular-weight PCS micelles entrapped within an aqueous PVP matrix played an important role in forming the continuous and dense core structure, resulting in pure SiC fibers. The manipulation of SiC fiber diameters could be obtained via control of the micellar PCS concentration (10-30 wt %), enabling the production of dense and highly crystallized SiC fiber architectures with diameters ranging from 200 to 350 nm.

Effects of amine and amine oxide compounds on the zeta-potential of emulsion droplets stabilized by phosphatidylcholine

Abstract A positively charged submicron emulsion based on phospholipid was suggested and their surface electrical properties were investigated at various pHs. Additives used to change the zeta potential of emulsion were decyldimethylamine oxide (DeDAO), dodecyldimethylamine oxide (DDAO) stearylamine (SA) and dodecylamine (DA). Also the emulsion droplet size and size distribution were measured to study the effects of additives on the emulsion properties. By adding additives, the zeta potential of emulsion droplet has been significantly changed but there were little effects on emulsion size and size distribution. In a wide range of pH (≈8.0), all additives yielded the positive zeta potential values. But at pH 10, amine compounds generated the negative-charged emulsion droplets while amine oxide compounds generated the neutral emulsion droplet. From this study, it was found that amine oxide compounds as well as amine compounds can be used to modify the zeta potential of emulsion droplets from negatively charged to positively charged below pH 8.0 for the effective drug delivery vehicles.

Preparation of aluminum oxide particles using ammonium acetate as precipitating agent

The effect of ammonium acetate on the preparation of alumina particles as precipitating agent was investigated. Alumina particles were prepared by precipitation of aqueous aluminum chloride solution by using ammonium acetate and ammonium hydroxide. The thermal behaviors, morphologies and surface area were studied for alumina particles formed by ammonium acetate and compared with those of particles formed by ammonium hydroxide. The particles formed by ammonium acetate showed a narrower size distribution, spherical shape, larger surface area and lower crystallinity. These results were explained based on the chelating effects of acetate ions.

Microemulsions as reaction media for the synthesis of sodium decyl sulfonate 1: Role of microemulsion composition

Abstract Reaction kinetics for the formation of sodium decyl sulfonate from alkyl halide and sodium sulfite in microemulsions based on nonionic surfactant were investigated and compared with those in oil-water two-phase systems. Reactions were run at room temperature at various oil-water ratios. Whereas at room temperature almost no reaction occurred in the two-phase systems, all microemulsion-based reactions proceeded at a fair rate. No clear relationship between microemulsion structure and reaction rate could be seen. An equation describing reaction kinetics in the microemulsion system has been derived based on the pseudophase model.