Colloids, freezing organic

Milks of most species contain more water than any other constituent. Certainly this is true of the milks consumed by humans. The other constituents are dissolved, colloidally dispersed, and emulsified in water. The dissolved solutes in bovine milk aggregate about 0.3 M and depress the freezing point by about 0.54°C (see Chapter 8). The activity of water in milk, aw, which is the ratio of its vapor pressure to that of air saturated with water, is about 0.993. A small amount of the water of milk is bound , o tightly by proteins and by the fat globule membrane that it does not function as a solvent for small molecules and ions. Water content is usually determined as loss in weight upon drying under conditions that minimize decomposition of organic constituents, e.g., 3 hr at 98-100°C (Horwitz 1980). 

After washing several times with methanol, the microgel fraction is freeze-dried from benzene and residual solvents are. stripped off under reduced pressure (10-5 mbar). The microgels thus obtained are white, dust-like powders with a sedimentation volume of 20-50 ml/g. They can be redispersed to form colloidal organic solutions. 

The employed technique for this purpose was the so-called colloidal-probe AFM (Atomic Force Microscopy). A carbon microparticle with high degree of carbonization was attached to the top of the cantilever tip, forming the colloidal probe, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned here. 

An extensive literature review on organics characterisation lead to the conclusion that a combination of MF and RO followed by freeze drying is the state of the art technique to concentrate surface water to produce a stock of natural organic matter (NOM). This technique was applied to concentrate 5000 L of a local Australian surface water. Further, model compounds of the surface water systems to be examined were selected. These are well characterised and purified organics (IHSS FA FIA), and inorganic colloids (40 - 500 nm in diameter) in a carbonate buffer system containing mono- and multivalent cations. 

Latex is a colloidal dispersion of polymer in an aqueous solvent. This method is more suitable for those polymers that can be prepared via emulsion polymerization or those that have the ability to form emulsion. It consists of an aqueous dispersion/ stabilization of filler using a surfactant followed by the addition of the dispersed filler into the polymer latex. Nanocomposites can be obtained after freeze-drying the above mixture followed by melt processing. The latex method has several advantages including no requirement for organic solvent, reliability, ease of processing, and improved dispersion of the filler in the viscous polymer matrix [70]. 

The formation of micelles, or colloidal particles, by block copolymers in organic solvents has been described and reviewed by Price [2881. The molecular weight of polystyrene was estimated from specimens prepared by spraying and evaporation for TEM. Freeze etching a drop of solution rapidly frozen with liquid nitrogen [289] was described (Section 4.9.4) where the solvent was allowed to evaporate and a replica produced of the fracture surface. Another method [290]... 

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