Section 7.3.4 Solids Flocculating

Sections 7.3.1 to 7.3.4 consider solids suspension, solids dispersion, solids dissolving, and solids flocculating respectively. General issues related to mixing using a fluidized bed are given in Section 7.3.5. 

In a 500 ml. wide-mouthed reagent bottle place a cold solution of 25 g. of sodium hydroxide in 250 ml. of water and 200 ml. of alcohol (1) equip the bottle with a mechanical stirrer and surround it with a bath of water. Maintain the temperature of the solution at 20-25°, stir vigorously and add one-half of a previously prepared mixture of 26-5 g. (25 -5 ml.) of purebenzaldehyde (Section IV,115) and 7 -3 g. (9-3 ml.) of A.R. acetone. A flocculent precipitate forms in 2-3 minutes. After 15 minutes add the remainder of the benzaldehyde - acetone mixture. Continue the stirring for a further 30 minutes. Filter at the pump and wash with cold water to eliminate the alkali as completely as possible. Dry the solid at room temperature upon filter paper to constant weight 27 g. of crude dibenzalacetone, m.p. 105-107°, are obtained. Recrystallise from hot ethyl acetate (2-5 ml. per gram) or from hot rectified spirit. The recovery of pure dibenzalacetone, m.p. 112°, is about 80 per cent. 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

Problems which arise with certain precipitates include the coagulation or flocculation of a colloidal dispersion of a finely divided solid to permit its filtration and to prevent its re-peptisation upon washing the precipitate. It is therefore desirable to understand the basic principles of the colloid chemistry of precipitates, for which an appropriate textbook should be consulted (see the Bibliography, Section 11.80). However, some aspects of the colloidal state relevant to quantitative analysis are indicated below. 

The process of dispersing a gel or a flocculated solid to form a sol is called peptisation, and is briefly dealth with on page 421 and in Section 11.8. 

Suspensions are coarse dispersions of finely divided solids in a liquid. The solid particles have a mean particle size greater than 0.1 pm in diameter. Pharmaceutical suspensions are administered orally, topically, and parenterally and should avoid the following problems sedimentation, caking, flocculation, and particle growth. Physicochemical principles in the solid/liquid interface will be discussed in this section as they pertain to the preparation of good pharmaceutical suspensions. 

In some cases, the formation of a colloid is not desirable, as in the precipitation of a solid from solution (see Section 11.2). Especially with metal snlfides, the solid precipitate may appear as a colloidal suspension with particles small enongh to pass through ordinary filter paper (Fig. 11.22). If this happens, precipitated solid can be separated out only by flocculation, centrifugation, or forcing the suspension through a membrane, such as cellophane, that permits passage of only the small solvent molecules. 

Interesting effects are observed when a dispersion contains both larger and smaller particles the latter are usually polymer coils, spherical or cylindrical surfactant micelles, or microemulsion droplets. The presence of the smaller particles may induce clustering of the larger particles due to the depletion attraction (see Section 5.4.S.3.3, above) such effects are described in the works on surfactant-flocculated and polymer-flocculated emulsions. Other effects can be observed in dispersions representing mixtures of liquid and solid particles. Yuhua et al. ° have established that if the size of the solid particles is larger than three times the size of the emulsion drops, the emulsion can be treated as a continuous medium (of its own average viscosity), in which the solid particles are dispersed such treattnent is not possible when the solid particles are smaller. 

Particle diameter <1 pm solids concentration <0.1%. Related to flocculants for thickening (Section 16.11.5.9). 

Note A stable dispersion of small solid or liquid particles may also show a kind of phase separation when conditions in the liquid are changed in such a way that attractive forces between the particles become dominant. A separation into a condensed phase (high volume fraction of particles) and a very dilute dispersion would then result. The interfacial tension between these phases is very small, e.g., a few pN m 1. Conditions for this to occur are (a) that the particles are about monodiperse and of identical shape and (b) that the attractive forces do not become large (because that would lead to fractal aggregation see Section 13.2.3). Since these conditions are rarely met in food systems, we will not further discuss the phenomenon. Nevertheless, phenomena like depletion flocculation (Section 12.3.3) show some resemblance to a phase separation. 

Agglomeration in Liquid Suspensions (Section 7.4.6) Solid particles that are suspended in liquids may agglomerate as a result of two basically different phenomena Flocculation is the aggregation of solid particles into relatively loose conglomerates (floes) after collision and coalescense have occurred. Adhesion may be enhanced by the addition of polymers (so called flocculants). 

---