Capillary colloidal filtration

Particles from a dispersion can be convected to the inner or outer surface of a porous substrate in contact with the dispersion due to fluid flow through the porous support. Also body forces due to centrifugal or electric fields can, in principle, be used to assist the particle transport towards the substrate. When the support is not permeable for the particles in the dispersion, the particle transport results in a more or less dense particle compact. The gravitational force on the particles can also contribute to the particle packing process when the gravitational force is in the same direction as the fluid flow. 

The driving force behind the fluid flow can be the capillary suction pressure of the support or an external applied pressure. In the former case the process is equivalent to the slip casting process in pljister moulds well-known in ceramics. 

10 From a topologiccil point of view an open porous medium has only one very complicated surface. Here we mean by outer or inner surface tiie geometrical surfaces on a macroscade of the outside and inside of single or multihole tubes, respectively. The definition of these surfaces on a microscale is arbitrary to some extent and depends on the yardstick used. 

11 We use the term compact in the broad sense. Both a concentrated dispersion near close packing but with overall repulsion between the particles is called a wet compact as well as the case were the particles are trapped in an energy minimum. In the latter case the relaxation time for particle breakup from the compact, i.e. due to diffusion over an energy barrier (activated diffusion), can be much longer than the time-scale of practical interest and the particle compact is in fact consolidated. 

6 — PREPARATION OF ASYMMETRIC CERAMIC MEMBRANE SUPPORTS BY DIP-COATING 

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Fig. 6.8. Summary of principle and important parameters in the capillary colloidal filtration mode...

Capillary colloidal filtration occurs when the dry substrate comes into contact with a dispersion and the pore surface is wetted by the dispersion liquid. The capillary suction of the substrate which occurs, in effect drives particles to the interface. If the surface is not permeable for the particles, the particles concentrate at the substrate-dispersion boundary and a compact layer is formed. The thickness of the compact layer grows with time according to the well-known square root time law (see Section 6.3.1), until the substrate is saturated with dispersion liquid or, in case of a dispersion of small colloidal particles, a stationary state due to back-diffusion occurs. Figure 6.8 summcirises the capillary filtration mode of dip-coating. 

Fig. 6.26. Attempt to apply a mesoporous Y-AI2O3 coating on a layer 2 substrate with cracks by capillary colloidal filtration of a boehmite sol without macromolecular additives. In the layer 2 crack regions no boehmite coating develops. The layer 3 coating shows pinholes (SEM picture).

Fig. 6.31. Unidirectional compact growth in capillary colloidal filtration.

Fig. 6.32. Capillary colloidal filtration (after Tiller and Tsai [52]).

Figure 15.7 Starling principle a summary of forces determining the bulk flow of fluid across the wall of a capillary. Hydrostatic forces include capillary pressure (Pc) and interstitial fluid pressure (PJ. Capillary pressure pushes fluid out of the capillary. Interstitial fluid pressure is negative and acts as a suction pulling fluid out of the capillary. Osmotic forces include plasma colloid osmotic pressure (np) and interstitial fluid colloid osmotic pressure (n,). These forces are caused by proteins that pull fluid toward them. The sum of these four forces results in net filtration of fluid at the arteriolar end of the capillary (where Pc is high) and net reabsorption of fluid at the venular end of the capillary (where Pc is low).

To determine the protein concentration Ce in the efferent blood we assume that filtration equilibrium is established before the blood leaves the glomerular capillaries, i.e., that the glomerular hydrostatic pressure minus the efferent colloid osmotic pressure Posm equals the tubular pressure. The experimentally determined relation between the colloid osmotic pressure and the protein concentration C can be described as [16] ... 

The classical use of a micro filter funnel and folded paper or cotton wool always leads to loss of material and when a suction ball is used, the filtrate contains neither threads from the paper nor colloidal portions of the clarifying agent. If the solute crystallizes in the suction ball during or after filtration, then, when filtration is complete and the filter end has been cut off, the suction ball is heated in a bath with the open capillary upwards until the crystals have redissolved. 

For filtration of opalescent solutions containing colloidal impurities, the conical part of the filtration capillary is made longer (20-30 mm) and there are introduced into it, first, a well tamped cotton wool plug, then iron-free charcoal for polar solvents or alumina (chromatographic quality) for apolar solvents, and finally a second cotton wool plug. Since this filter is only slowly permeable, it is wetted only with the solvent, and the solvent necessary for the suction process is introduced from the other capillary after evaporation of the solvent through this free capillary end the latter is at once sealed. 

Colloidal Crystalline Arrays Colloidal spheres of silica and of polymers can be made relatively monodisperse, with standard deviations of 4% of the mean diameter for silica and 1% for polymer latexes. The spheres pack as shown in Figure 11.22a from fluid dispersions into fee (sometimes hep or bcc) colloidal crystals (CC) by gravity, by membrane filtration, or by capillary forces at the surface of an evaporating dispersion (80-82). The crystalline order of the materials is strictly at the length scale of the packed colloidal particles the packing of the atoms and molecules within the silica and polymer particles is totally amorphous. The CCs diffract... 

Colloid osmotic pressure A negative pressure that depends on protein concentration (mainly of albumin and globulins) and prevents excess filtration across the capillary wall. 

Hollow-fibre membrane modules are similar to the capillary type described above, but with fibres of outside diameters ranging from 80 to 500 pm. It is usual to pack a hollow-fibre module with many hundreds or thousands of these fibres, thus membrane area per unit volume is extremely hi. It should be apparent that filtration using hollow-fibre modules is only realistic with process fluids prefiltered to prevent fibre blockage fins limits the technology and it is applied mainly in UF. Also used in uhrafiltration is a spiral-wound membrane module which is often compared to a Swiss roD. The membrane and a spacer are wound round a former, with an appropriate permeate spacer flow is introduced and removed from the ends. This module design is not appropriate for solid-liquid separation, even when filtering colloids, because of the possibility of flow channel blockage and so it will not be discussed any finther. 

Particle size. Particles greater than 7 pm are larger than blood capillaries ( 6 pm) and become entrapped in the capillary beds of the lungs (which may have fatal effects). The majority of particles that pass the lung capillary bed accumulate in the elements of the RES (spleen, liver and bone marrow). The degree of splenic uptake increases with particle size. Removal of particles > 200 nm is due to a non-phagocytic process (physical filtration) in the spleen and phagocytosis (by Kupffer cells) by the liver. Particles < 200 nm decreases splenic uptake and the particles are cleared by the liver and bone marrow. Colloidal particles not cleared by the RES can potentially exit the blood circulation via the sinusoidal fenestration of the liver and bone marrow. 

To analyze the aqueous phase for any of these substances, it must first be separated from the polymer particles. Both flocculation and membrane filtration techniques can be used for this purpose and they are described in more detail below. The detection of the substances listed above can then be performed with the usual array of analytical methods used for characterizing aqueous media. For the determination of emulsifiers, electrolytes and water-soluble monomers, ion chromatography (IC) and high-performance liquid chromatography (HPLC) are particularly suitable. The techniques of choice for characterizing oligomers are gel permeation chromatography (GPC) and capillary electrophoresis (CE). As these analytical techniques are not specific to colloidal chemistry, they will not be described further here and the reader should consult the literature for more information. 

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