Characterization Of Colloidal Particles Research Articles

Nonparaxial Mie Theory of Image Formation in Optical Microscopes and Characterization of Colloidal Particles

We derive an explicit partial-wave (Mie) series for the image of a dielectric microsphere collected by a typical infinity-corrected microscope. We model the propagation of the illumination and scattered vector fields through the optical components of the microscope by using the angular-spectrum theorem with the help of Wigner rotation matrix elements, allowing us to identify the contribution from spin-orbit helicity reversal. We consider a high numerical aperture objective well beyond the validity range of the paraxial approximation. The spherical aberration introduced by refraction at the planar interface between the sample and the glass slide is fully taken into account. By comparing our theoretical model with images of colloidal particles placed at different positions with respect to the objective focal plane, we characterize their radii and refractive index. We employ polystyrene microspheres with a known refractive index in order to fit the transverse attenuation length describing the transmission loss of the scattered field. As an application, we measure the radius and refractive index of individual silica beads. We compare the result for the radius with an independent measurement using high-resolution scanning electron microscopy. To validate the result for the refractive index, we develop a second method, independent of the theoretical model, based on the image contrast in glycerin-water solutions. In all cases we find very good agreement between our method and the validation procedures. In addition, the nonparaxial theory provides a reliable description of the images found for all focal-plane positions and for both polystyrene and silica microspheres. Our approach allows a common optical microscope to be used to measure the refractive index and radius of spherical particles covering the entire size range from the Rayleigh regime to the ray optics one.

Characterization of colloidal particles formed in plastic coating solution at room temperature

The structure of solution of methyltrimethoxysilane (MTMS), which is used for hard coating for plastic materials through its sol–gel reactions, was investigated by light scattering, atomic force microscopy, infrared spectroscopy, and X-ray diffraction. Although hard films can be formed only after heating the MTMS solution and no significant increase in viscosity can be found at room temperature, colloidal particles were detected in the solution by light scattering immediately after adding the catalyst. Particles with a size comparable to that determined by light scattering were observed by atomic force microscopy. The change induced by the growth of the particles was measured for 4 weeks. The infrared spectroscopy and X-ray diffraction for the solutions changed gradually, which supports the formation and growth of the colloidal particles. These results indicate a very slow reaction of MTMS and structural change in the solution even at room temperature, which can affect successful coating of plastic materials using MTMS.

Characterization of Colloidal Particles Using Electrical Impedance Spectroscopy in Two-electrode System with Carbon Probe

The colloidal particles have an electrical double layer associated with their surfaces when suspended in an aqueous medium. Under the influence of an alternating electric field, an induced electrical dipole moment can be formed due to the polarization of the electrical double layer. The electrical impedance spectroscopy (EIS) measurement can record the complex impedance, conductivity, relaxation frequency and phase angle caused by the polarization of the electrical double layer. These impedance parameters are in relation to particle characteristics, for example, the particle size. The research about particle size effect on electrical impedance spectra was carried out in a four-electrode system and the result indicated that impedance parameters shows a capability for characterizing the particle size. This paper reports the experimental results from electrical impedance spectroscopy measurements on silica suspensions in a two-electrode system with carbon probe. The main aim is to study the particle size effect on impedance parameters, especially the relaxation frequency and phase angle, to compare the data with those obtained from a four-electrode system with stainless steel electrodes and verify the capability for characterizing colloidal particles in different electrode systems. The particle size effect on the relaxation frequency and impedance phase angle was studied in two different electrode systems and a similar tendency can be observed. It indicates that the capability of impedance parameters for particle characterization is not limited in a four-electrode system, but commonly applicable in different electrode systems.

POTENTIAL-BARRIER FIELD-FLOW FRACTIONATION: POTENTIAL CURVES AND INTERACTIVE FORCES

ABSTRACT Potential-barrier field-flow fractionation (PBFFF), a tool for the separation and size characterization of colloidal particles is based on the variation of the surface forces in the FFF process, which control particle adsorption–desorption on the channel wall. The latter phenomena depend on the total potential energy of interaction between the colloidal particles and the channel wall. Our results suggest, that at high ionic strengths of the suspending medium, the colloidal particles used (submicron monodisperse spherical particles of hematite and titanium dioxide) are retained within the secondary minimum even though the energy barrier is sufficiently high to inhibit attachment in the primary minimum of the interaction energy curve. This hypothesis is further supported by the particle detachment experiments at low ionic strengths of the suspension in which the total number of adhered particles was revealed when the secondary minimum was eliminated. The secondary minimum energies, φmn1, necessary for the separation of the colloidal particles used in PBFFF, which were computed using the electrical double layers and van der Waals expressions were found, approximately, in the range −4.7<φmn1<−3.2 kT, even though the energy maxima were much greater (φmax≈109 kT). As the secondary minimum energies depend not only on the size of the particles, but also on their nature, one could separate by PBFFF particles of different size, as well as of the same size but with different chemical constitution.

Thermal Behavior Characterization of Colloidal Particles of Nickel Hydroxide and its Connection with the Grain-Size Effect

Thermal Behavior Characterization of Colloidal Particles of Nickel Hydroxide and its Connection with the Grain-Size Effect

“Figure/Ground” Study of Colloidal Silica Nanoparticles and Corresponding Microporous Xerogels

A critical hurdle in the development of sol−gel techniques for the preparation of microporous inorganic materials is elucidating the effect of key synthesis and processing parameters on the structure of the resulting sols. Characterization of colloidal particles with diameters of less than approximately 10 nm is problematic using conventional techniques such as dynamic light scattering. Similarly, quantitative determination of pore size distribution in the resulting materials using gas adsorption isotherms is difficult in the microporous regime. The present work extends these traditional methods with the objective of obtaining information both on the particle size in an inorganic sol and on the pore size distribution in the resulting microporous xerogel. Silica sols synthesized at pH 3 and an H2O/TEOS ratio of r = 83 were shown by atomic force microscopy to have a distribution of particle diameters centered around approximately 4−6 nm. Nitrogen and molecular probe adsorption studies on xerogels derived fr...

Characterization of colloidal particles by laser-induced plasma spectroscopy (LIPS)

To study the nature and abundance of colloids in aquifer systems Laser-Induced Plasma Spectroscopy (LIPS) was used for elemental analysis of heavy metal colloids with particle diameters between 0.1 and 1 μm. Different experimental approaches to the qualitative and quantitative characterization of inorganic colloids are examined. Some of the inherent problems associated with a direct detection are discussed. A direct detection of colloids in water or a liquid jet without separation between solvent and particles yielded only inadequate spectral resolution and detection limits in the order of mg l −1. Utilization of a miniaturized ultrafiltration system with a 0.1 μm membrane filter gave absolute limits of detection in the ng-range with good reproducibility. Figures of merit are given for representative heavy metal colloids. The filtration device used in this work is suitable for subsurface sampling under flow conditions, which minimizes sampling artifacts. A slightly modified LIPS system will be used for field screening and monitoring.

Potential barrier field-flow fractionation for the separation and characterization of colloidal particles

The reversibility of adsorption of colloidal particles on the channel wall in Sedimentation Field-Flow Fractionation (SFFF), which is based on the variation of the ionic strength of the carrier solution, suggests a new method, for the separation and characterization of colloidal materials. This new method has been called “Potential Barrier Field Flow Fractionation” (PBFFF).