Cesar A. Silebi

Separation of submicrometer particles by capillary hydrodynamic fractionation (CHDF)

Abstract This paper shows, for the first time, that an analytical separation of submicrometer colloidal particles can be achieved by flowthrough small-bore open capillary tubes. When dispersed colloidal particles of different sizes are carried through the open capillary tube by a fluid, they fractionate, emerging in order of decreasing diameter. We describe this technique as capillary hydrodynamic fractionation because the separation results solely from the parabolic velocity profile in the microcapillary tube. Relative elution times of particles are dependent on particle size, tube diameter, surfactant species and concentration, and the average velocity of the mobile phase. In principle, any pair of particle sizes can be resolved using this technique. Maximum resolution is attained when the eluant velocity is increased if the mixture contains micrometer-sized and submicrometer-sized particles. If all the particles in the mixture are submicrometer sized, resolution is best when the residence time of the eluant in the capillary is increased.

The determination of particle size distribution of submicrometer particles by capillary hydrodynamic fractionation (CHDF)

Abstract Capillary tubes have been used to separate submicrometer colloidal particles by size. When dispersed colloidal particles of different sizes are transported through the open capillary tube by a fluid, they fractionate, emerging in order of decreasing diameter. This paper shows, for the first time, that the particle size distribution of polydisperse mixtures of submicrometer colloidal particles can be obtained from the separation achieved by flow through small-bore open capillary tubes. The integrated, computerized capillary hydrodynamic fractionation (CHDF) described here determines the particle size distribution of the sample containing submicrometer particles in suspension. An iterative numerical method was used to obtain the particle size distributions from the fractograms obtained. Several mixtures of well-characterized standards were injected into the CHDF system in order to illustrate the resolution of the separation. The high resolution of the separation achieved in a 4-μm-diameter capillary tube allows analytical fractionation of mixtures of particles with diameters of 234 and 357 nm in less than 200 s; the difference between their peak elution times is only 8 s. Particle size distributions of multimodal distributions obtained by CHDF were compared to those obtained by transmission electron microscopy (TEM) in order to determine the accuracy of the calculated particle size distributions. Our results demonstrate that CHDF can be used to obtain particle size distributions of dispersions, regardless of the breadth of the distribution of submicrometer particles.

An analysis of the separation of submicron particles by capillary hydrodynamic fractionation (CHDF)

Particle dispersions pumped under laminar flow conditions through narrow bore capillaries separate by size, with larger particles eluting before smaller ones. This technique is called capillary hydrodynamic flow fractionation (CHDF). The separation is due to both the size exclusion of the particles from the slow moving streamlines and the colloidal and inertial forces acting on the particles. The purpose of this paper is to present a detailed, and we believe, complete analysis of the separation process. This analysis is based on a fundamental model for the convected Brownian motion of colloidal particles in a capillary tube. A parameter sensitivity study suggests that the separation factor (a parameter equal to the ratio of the mean displacement velocities of particle and eluant) is nearly independent of the ionic strength of the eluant and the chemical nature of the particle (i.e., Hamaker constant and surface potential) when the product of particle Reynolds number and Peclet number is greater than 80 (RepPe > 80). On the other hand, at RepPe < 3, the inertial forces are negligible and the colloidal forces along with the size exclusion effect become the determining factor in the separation process. The particles chemical nature becomes important at high eluant ionic strengths and RepPe < 3. Plots of the separation factor, under different eluant and flow conditions, are provided and some implications of the theory of CHDF are noted. Results from previous experimental studies are briefly summarized along with more recent data; these have been used to develop a quantitative model of the flow-induced separation, which is described in detail in this paper.

Kinetics of agitation-induced coagulation of high-solid latexes

Abstract the rate of mechanical coagulation of a high-solids latex, in a stirred tank (∼35%), was measured experimentally. Elimination of the air-liquid interface by conducting coagulation experiments in a filled tank reduced the rate of coagulation as compared to that of an unfilled tank with an air-liquid interface. Also, a semi-theoretical equation was developed to better describe the experimental coagulation rate, which showed acceleration in the later stages of coagulation. The equation which combines the effects of shear and surface coagulation has the form dc dt = P 1 + P 2 C − P 3 C where C is particles concentration and t is time. The parameters P1, P2, and P3 are related to the different aspects of the mechanical coagulation process.

Rheology and particle interaction of thickened latex

Abstract The pseudoplastic rheological properties of concentrated monodisperse polystyrene latexes with known sodium lauryl sulfate and methylcellulose surface coverages have been studied. It was assumed that the flow units of a concentrated thickened latex subjected to mechanical shear are “flocs” which comprise many particles with immobilized medium in the interstices. During shearing, the particle-particle bonds within the flocs undergo compression and stretching, sometimes breaking and reforming, causing the energy dissipation measured as the yield stress. A model was developed to calculate the average number of bonds per floc and this model was applied to an empirical modification of Firth and Hunters elastic floc model to correlate the yield stress with the particle-particle separation pressure (defined as a measure of the interaction strength). It was found that the yield stress of a thickened latex is affected by the particle-particle interaction and the morphology of the particle flocs. The particle-particle interaction is affected by the surface coverage of thickener and emulsifier, and their concentrations in the aqueous phase, as well as other factors. The morphology of the particle flocs is affected by the particle interaction and the mechanical treatment. The adsorption of emulsifier and thickener, the rheology of the thickened latexes, the morphology of the particle flocs, and the particle-particle interactions, as well as their interrelationships, are described.

Latex Particle Size Analysis by Chromatographic Methods: Porous Packed Systems and Detection of Polystyrene

Experimental results are presented for the size separation of polystyrene latexes using a porous packed system of a single large pore size. Parameters such as separation factor, ionic strength, flow rate, axial dispersion, and resolution are considered, and compared to results obtained using nonporous hydrodynamic chromatography (HDC). The porous system shows improved peak separation, however, overall resolution is decreased due to increased axial dispersion. A parallel flow-through bank model is presented to account for the ionic strength behavior of the separation factor in a porous system. Resolution in terms of optimum signal detection is discussed to account for absorption and scattering effects of polystyrene latexes. Overall improvement in HDC signal resolution resulting from optical density measurements in the absorbing region is shown to occur.

HYDRODYNAMIC CHROMATOGRAPHY OF LATEX PARTICLES

ABSTRACT Hydrodynamic chromatography (HDC) is a method developed by Small (1) for the measurement of the particle sizes of colloidal sols. The diluted colloidal sol is injected into an eluant stream, which is pumped through a series of beds packed with solid spheres. The larger colloidal particles pass through the beds faster than the smaller particles, and the separation achieved is detected turbidimetrically. This paper describes a mathematical model of the flow dynamics of the colloidal particles within the packed beds. This model is based on the analysis of flow of colloidal particles through a collection of equal-size parallel capillaries which are connected in series by mixing regions. The model takes into account the disturbance of the eluant flow profile by the particles and the forces (electrostatic repulsion and London-van der Waals attraction) between the colloidal particles and the bed packing. The predictions of the model are in excellent agreement with data for monodisperse polystyrene latexes reported by Small (1) and obtained in our laboratory, except for large particles and eluants of low ionic strength. The reasons for these discrepancies are outlined.

Monomer Distribution and Transport in Miniemulsion Copolymerization

Miniemulsions are oil-in water emulsions prepared using a mixed emulsifier system comprised by an ionic surfactant and a cosurfactant, such as a fatty alcohol or a long chain alkane1. The two main characteristics of the miniemulsions are their good stability and droplet size, ranging from 50 to 400 nm in diameter. From the latter characteristic arises the term miniemulsion, to distinguish them from the conventional emulsions or macroemulsions with droplets larger than 1 µm in diameter and from the microemulsions with droplets less than 0.1 µm in diameter.