Showing papers by "Jyh-Ping Hsu published in 2003"

On the Factors Influencing the Preparation of Nanosized Titania Sols

Abstract: Nanosized titania particles are prepared from titanium isopropoxide dissolved in alcohol and water under acidic conditions. The effects of the key parameters, including (alkoxide/water) ratio, HCl concentration, feed rate, and temperature, on the mean particle size and the standard deviation of size distribution of the final products are investigated through an experimental design technique. This technique can also be used to find the condition for obtaining transparent sols with mean particle size smaller than 50 nm and standard deviation smaller than 5 nm. We show that the main factors influencing the mean particle size and the standard deviation are (water/alkoxide) ratio, HCl concentration, and the interaction between these two factors. The present approach is applicable to finding the optimum conditions for preparation of titania with desired mean particle size and standard deviation.

Drag force on a porous, non-homogeneous spheroidal floc in a uniform flow field.

Abstract: The force acting on a porous spheroidal floc having a nonhomogeneous structure in a uniform flow field is evaluated theoretically. Here, the floc is simulated by an entity having a two-layer type of structure, and its porous nature is mimicked by varying the relative magnitudes of the permeabilities of its inner and outer layers. The results of numerical simulation reveal that, for the same volume-averaged permeability, the drag coefficient of a spheroidal floc with a nonhomogeneous structure is much larger than that of a floc with a homogeneous structure for both prolate and oblate spheroids. This is true regardless of the relative magnitudes of the permeability of the inner layer and that of the outer layer. While the drag coefficient of a homogeneous prolate is the same as that of a homogeneous oblate the drag coefficient of a nonhomogeneous prolate is larger than that of a nonhomogeneous oblate. For the same volume-averaged size, the more nonhomogeneous the structure of a spheroidal floc the easier for the relation between the drag coefficient and the Reynolds number to deviate from a Stokes-law-like relation. For a fixed volume-averaged permeability, the effective drag coefficient increases with the increase in the ratio (polar radius of inner layer/polar radius of floc), regardless of whether its inner layer is less permeable than its outer layer or not.

Estimation of the ionic distribution in a reverse micelle: Effect of ionic size

Abstract: The effect of ionic size on both the degree of dissociation of the surfactant shell of a spherical reverse micelle and the spatial distribution of ionic species inside is examined on the basis of a modified Poisson−Boltzmann equation and a local ideal mixing rule. The dissociation of surfactant molecules is the consequence of the competition between the entropic effect of counterions in the aqueous core, the osmotic effect of the surfactants on the micelle shell, and the effect of electrical interactions. We show that the ion distribution close to the surfactant shell can be altered remarkably, and the saturation adsorption concentration varies with dissociation conditions. The free energy associated with the dissociation of surfactant molecules is studied taking the steric effect of ionic species into account. The presence of a neutral particle in a reverse micelle is found to have a negative effect on the degree of dissociation of the surfactant shell.

Electrophoresis of biological cells: charge-regulation and multivalent counterions association model.

Abstract: The electrophoresis of a biological cell is analyzed theoretically. An entity, which is of amphoteric nature, is used to simulate its electrophoretic behavior. To reflect conditions of practical interest, we assume that the liquid phase contains mixed (a:b)+(c:b) electrolytes, where a and c are the valences of cations, and b is the valence of anions. We consider the case where the surface of a cell contains both bivalent acidic and monovalent basic functional groups, the dissociation/association of them yields fixed surface charge, and the multivalent cations in the liquid phase are allowed to combine with dissociated acidic functional groups, which has the effect of lowering the charge density on cell surface. The electrophoretic behaviors of a cell under various conditions are illustrated. The results obtained can be used to identify the types of functional groups that may be present on cell surface. On the other hand, if the surface functional groups involved in cell electrophoresis are known, then their density and the associated dissociation/association constants can be estimated from experimental data.

Stability of a dispersion of particles covered by a charge-regulated membrane: effect of the sizes of charged species

Abstract: The influence of the sizes of charged species on the stability of a colloidal dispersion is investigated theoretically. We consider the case where a particle comprises a rigid core and an amphoteric, charge-regulated membrane layer, which simulates biocolloids and particles covered by artificial membranes. A modified Poisson-Boltzmann equation, which takes the sizes of all the charged species into account, is adopted to describe the electrical field. The effects of other key parameters such as electrolyte concentration, pH, and the valence of counterions on the behavior of a dispersion are also examined. We show that the larger the effective size of the counterions, the greater the stability ratio, which is consistent with experimental observations in the literature.

Dynamic electrophoretic mobility of a sphere in a spherical cavity

Abstract: Dynamic electrophoresis is a powerful analytical tool for the description of the surface properties of the charged entities in a concentrated dispersion. In our study the boundary effect on this dynamic phenomenon is investigated theoretically for the case, when the surface potential is low. In particular, the dynamic electrophoresis of a sphere in a spherical cavity is discussed as are the effects of the key factors on the phenomenon under consideration, which include the thickness of the double layer, the frequency of the applied electric field, the ratio of particle radius to cavity radius, and the boundary conditions of the surfaces of the particle and the cavity. The results of numerical simulation reveal that these key factors can have both quantitative and qualitative influence on the electrophoretic behavior of the particle. As an example, for the case of a positively charged particle placed in a negatively charged cavity if the double layer surrounding the particle is thin, the magnitude of the electrophoretic mobility of the particle increases with an increase in the frequency of the applied electric field and a phase lead may occur, but the opposite is true if the double layer is thick. These effects are not observed for the case of a positively charged particle placed in an uncharged cavity or for a positively charged particle placed in a positively charged cavity.

Electrophoresis of a Concentrated Dispersion of Nonrigid Particles

Abstract: Electrophoresis is one of the most important analytical tools for the quantification of charged entities in a dispersing medium. Although the electrophoresis of rigid entities has been studied exte...

Electrophoresis of concentrated mercury drops.

Abstract: The electrophoretic behavior of concentrated monodispersed, positively charged mercury drops is investigated theoretically. The present study extends previous analyses by considering arbitrary surface potentials, double-layer polarization, and the interaction between adjacent double layers. The coupled equations describing the spatial variations in the flow field, the electric field, and the concentration field are solved by a pseudo-spectral method. For a low surface potential φr, the mobility increases monotonically with κa; κ and a are respectively the reciprocal Debye length and the radius of a mercury drop. For medium and high φr, the mobility curve has a reflection point, which arises from the interaction of adjacent double layers, for κa. Also, if φr is high, the mobility curve may exhibit a local minimum as κa varies. This phenomenon is pronounced if the concentration of the dispersed phase is high. If the double layer is thick, the mobility increases with φr, and the reverse is true if it is thin. We show that the higher the concentration of the dispersed phase the smaller the mobility, and as κa becomes large the mobility approaches a constant value, which is independent of the concentration of the dispersed phase. The mobility of mercury drops is larger than that of the corresponding rigid particles.

Electrophoresis of a charge-regulated spheroid along the axis of an uncharged cylindrical pore

Abstract: Boundary effects can have a profound influence on the electrophoretic behavior of a charged entity, in particular, when the entity is nonspherical and its surface conditions are dependent upon the nearby environment. In this study, the electrophoresis of a spheroid along the axis of an uncharged cylindrical pore is analyzed for the case where the electrical potential is low and the applied electric field is weak. We consider the case where the surface of a particle contains dissociable acidic and basic functional groups, which simulate biological colloids and entities covered by an artificial membrane. This leads to a mixed-type boundary value problem, which extends the conventional constant-surface-potential and constant-surface-charge-density models to a more general case. The effects of the particle aspect ratio, the relative magnitudes of particle and pore, the thickness of the double layer surrounding a particle, and the pH of the liquid phase on the electrophoretic mobility of a particle are investigated. Several interesting results are observed; for example, if the volume of a particle is fixed, its mobility may have a local maximum as the relative magnitudes of its two axes vary.

Electrical interaction between two planar, parallel dissimilar surfaces in a general electrolytic solution

Abstract: The electrical interaction between two planar, parallel dissimilar surfaces, which may have different charged conditions arising from different ion-adsorption mechanisms, in an arbitrary electrolytic solution is investigated theoretically. The electrical interaction force and the interaction energy between these surfaces are evaluated, and analytical expressions for various charged conditions under the Debye-Huckel condition are derived. In general, assuming constant surface potential and assuming constant surface charge density lead respectively to the lower and the upper bounds in the electrical interaction energy between two surfaces. We show that assuming a linear relation between surface potential and surface charge density under the Debye-Huckel condition, as is often done in the literature, is appropriate for two planar parallel surfaces only but becomes inadequate for other orientations or nonplanar surfaces. The present approach does not have this limitation.

Effect of Ionic Sizes on the Electrokinetic Flow in a Planar Slit Covered by an Ion-Penetrable Charged Membrane

Abstract: The electrokinetic flow in a planar slit covered by an ion-penetrable charged membrane in an a:b electrolyte solution, taking the effect of the sizes of charged species into account, is analyzed theoretically. The electrokinetic properties such as volumetric flow rate, total electric current, and streaming potential of the system under consideration are evaluated. The influences of the key parameters, which include the frictional coefficient of the membrane layer, membrane thickness, ionic strength, valence of counterions, fixed charge concentration, and sizes of cations and anions, are discussed. We demonstrate the importance of the steric ionic effects by showing that a substantial difference between the classic point-charge model and the present model.