Chapter 10
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Capillary Hydrodynamic Fractionation of Organic Nanopigment Dispersions John Texter College of Technology, Eastern Michigan University, 122 Sill H a l l , Ypsilanti, MI 48197
Capillary hydrodynamic fractionation (CHDF) is generally useful for sizing particles in the range of 30-800 nm in equivalent spherical diameter. This fractionation method uses capillaries of 10-15 μm inside diameters. Such hydrodynamic fractionation of particles according to their equivalent diameter comes about because of Poiseuille flow and the tendency of larger particles to sample higher velocity components of the flow field. This paper describes the application of CHDF to a series of organic nanopigment dispersions used in photographic films and papers to manage light transmission and reflection. The results are benchmarked against analyses of the same dispersions by sedimentation field flow fractionation and by disk centrifugation. Transmission electron micrographs are presented to allow for an independent check of the results. The CHDF and SFFF results are in good agreement and may be viewed as competitive techniques. CHDF measurements are much faster, and SFFF measurements allow for finer resolution.
© 2004 American Chemical Society Provder and Texter; Particle Sizing and Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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Introduction Organic nanopigments are becoming increasingly important because of photographic (7,2), printing (3,4), inkjet (5-7), and other marking and display technologies. Organic nanoparticles are also becoming extremely important in drug delivery and targeting (8,9). Such particulates tend to be made by comminution processing (10-12) or by various forms of condensation (13-16). There are only a few reported examples where organic nanoparticles are being directly precipitated (17), but such reports are likely to grow as volume demand for such nanoparticles increases. Whatever the ultimate application, means are needed to quantify particle size and polydispersity, and in this chapter we focus on the applicability of capillary hydrodynamic fractionation (CHDF) to sizing organic nanopigments that have been demonstrated to be useful in a wide variety of photographic products (18). While CHDF is the focal point of this work, we examine it from a benchmarking perspective in comparison to two competitive techniques, sedimentation field flow fractionation (SFFF) and disk centrifugation (DC).
Elements of CHDF CHDF has largely supplanted hydrodynamic chromatography^ 9-20) as an analytical technique. Hydrodynamic chromatography utilizes a column filled with monodisperse beads, for example, in the size range of 10-250 um. The beads are impervious to the sample solvent and small particulates suspended therein. The multiphase flow is driven through the column in many parallel paths of varying tortuosity, but each path at an instant may be envisioned as a flow through an interstice between three beads in axisymmetric trigonal contact with one another. The flow through the "center" along the symmetry axis will have the greatest velocity component and the flow creeping along the bead surfaces will have the slowest velocity component. Smaller particles may approach die beads more closely than larger particles, and on average will sample a slower hydrodynamic flow field. This is the basis of hydrodynamic chromatography. Larger particles pass through more quickly than smaller particles, and are thereby separated or fractionated. This process is shown schematically in Figure 1. Better separation is obtained with smaller diameter columns. A marker is injected, and retention time calibrated against the marker elution. Particle transport depends on the size of the packing particles, ionic strength, flow velocity, and eluting particle size. Naturally, agglomerates and floes, when present, will be analyzed as i f they are single particles. Thus stability of dispersions may be studied by comparing particle size distributions obtained as a function of time or treatment. CHDF is a single flow-path analogue of hydrodynamic chromatography and was invented by Silebi and Dos Ramos (22). The essential element of CHDF is the hydrodynamic laminar flow of a particle through a very small capillary. Fused silica capillaries with internal diameters as small as 4 μηι are readily commercially available. Hagan-Poiseuille flow in such a microcapillary is }
Provder and Texter; Particle Sizing and Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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Figure 1. Separation of suspended particles according to effective size by hydrodynamic chromatography (21). © 2000 Strider Research Corporation.
shown schematically in Figure 2, for flow from the axisymmetric axis at r = 0 to the capillary wall. The parabolic fluid velocity components are qualitatively illustrated and show how the magnitude of the fluid velocity changes from a minimum at the walls of the capillary, to a maximum along the symmetry axis of the capillary. This fluid velocity profile is given quantitatively by the HaganPoiseuille equation: 2
