Dewatering colloids

In order to investigate the phenomenon of dewatering of colloids, it is useful to briefly review the general definition of a colloid: colloids are a mixture of solid, fluid or gaseous substances, of which at least one is present in the form of tiny particles, drops or bubbles (down to macromolecule-size).  The so-called colloidal size of the particles (1 to 1000 nanometers, or from a millionth to a thousandth of a millimeter) leads to numerous properties and processes, which only occur in colloidal substances.

Let us look at that group of colloids, where solid material is divided or dispersed within a watery fluid.  If the quantity and density of the dispersed particles becomes large enough, and the particles begin to form netlike connections, a gel or jelly-like substance.  Further thickening and final dewatering of such gels show, among others, the following characteristics:

a) The dewatering occurs with certain gels not only through external factors such as pressure and evaporation, but also actively from within; that is, the water is continuously pressed out of the gel substance.  One can think of the process with pudding, which begins, after a certain time, to press a fluid out of itself.  This expulsion also occurs with small quantities where the pressure due to its own weight is negligable.  The same phenomenon is observable with colloidal milk products such as yogurt.  The colloidal particles, through chemo-physical interaction, combine into an increasingly dense and stable network, thereby forcing out a part of the water.  The scientific term for this is syneresis.

b) The dewatering process and subsequent drying-out occurs at a slow tempo.  This can be increased somewhat by pressure and temperature, but the gel nevertheless allows only a certain rate of movement to the expelled water, determined by the limited speed of diffusion of water through the colloidal substance, that is, through the network of colloidal particles.

The following photos show the restricted tempo of the pressing-out of water from a gel body, in intervals of half an hour.  (In addition to the active expulsion of water, the pressure from the body's own weight also contributed here to speeding up the dewatering.)

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c) Certain colloids, once they have become dewatered and de-colloidalized, cannot be brought back into the colloidal condition, or only under special circumstances.  The dewatering process is irreversible with these substances, as the colloidal particles, at first only loosely bound to each other, gradually form into dense aggregates.  In addition to the weaker chemophysical interactions, stronger chemical bonding is added, which ends the colloidal condition.  One terms such irreversible densification or flocculation as coagulation.

Such dried-out former colloids can often be moistened again, but the previous colloidal condition is not re-established.  The water is then in a more coarse capillary, outward condition, and no longer intimately interwoven with the colloidal particles.  The following photos show something of this irreversibility.  The gel body illustrated goes through a nine-day process of thickening and drying, which no longer allows a return to the earlier condition.  The last picture shows the swollen condition, resulting when the dried-out body is again immersed in water for three days.  It soaks up water like a sponge, but keeps the crumpled form it had when dry.  After this soaking it dries out relatively quickly, since the water no longer has to diffuse through the tiny pores of the colloidal condition.

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 One has a clear example of irreversible de-colloidalization and dewatering with particular glues of organic origin (the name colloid stems from Kolla = glue), which form nearly water-free networks during adhesion and drying.  These hardened glues are then more or less water-resistant.

With complex colloidal substances as, for example, the skins of animals, one can observe this irreversibility of dewatering and de-colloidalization.  Through the tanning process the skin is convrted to leather.  This leather can absorb water, but never neturns to the condition of a freshly-removed animal skin: an animal skin can be cut easily with a knife, which leather resists.  One can also wring water out of leather like a cloth, while an animal skin remains moist for long periods even with pressure and heat, since the water is contained in the fat-protein colloids of the skin cells.

d) With gels, a higher salt content slows down the drying process distinctly.  The diffusion of water then leads to transportation of the salt.

The following pictures show the de-watering process of a strongly salt-containing gelating body.  The third picture shows the condition after six days, the final picture after one month.  The de-watering thus proceeds very slowly, during which the salt is trasported to the exterior surface and deposited there.  Even after a month the body is still soft on the inside.

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With these observations as a background, we turn again to the origin of rocks.  In this world of rocks one can observe phenomena which are reminiscent of the properties described:

• Some of the cracks and clefts pictured in the first chapter can be pointed to as shrinkage cracks, in that the rock material expelled water and shrank in volume.  With the carbonate rocks (see the chapter on Limestone and Marble) this would have involved lime-related colloids formed during the deposition process.

• Certain minerals such as feldspar show, when weathered, the capacity to bind water and form colloids.  The silicious or aluminum-hydroxide gels are, in their chemistry, reminiscent of the rock world.  With them, the decolloidalization- and dewatering-process is irrevesible, in that the dehydrated silica or aluminum bonding does not return to the colloidal condition when wet again.

• During the dehydration and thickening of silica (polycondensation) molecular chains, rings, bands nets and layers are formed, whose multiplicity of structures corresponds remarkably to the forms of the silicate minerals occurring in nature.  In special cases, colloidal macromolecules or micelles ae created which form a silica gel, as forms the basis for the origin of chalcedony (agates, jaspers).

• There exists a geological phenomenon which in an impressive way illustrates the connection between, colloidality, dewatering and irreversibility:  The water agate consists of an agate sphere which contains gas and watery silicic acid in its hollow center.  The present dryness of the agate and the enclosed silicious water, hundreds of thousands of years old, shows that the nodule material has decolloidized and dewatered in an irreversible manner. Could it be that the water agate is a model for what occurred frequently worldwide in primeval times?