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31 October 2003
Colorful Colloids
by Norm Stanley
Illustrated
by Brian Mansfield
"Dissolve finely beaten gold flakes in 'aqua regis', this is 'aquafort' [nitric acid] in which salmiac [ammonium chloride] has been dissolved. One obtains upon dessication a yellow residue which is water soluble. In acidic condition it is termed 'solution auri'. ...
"Take a large glass full of pure well water and add dropwise a few drops of the 'solution auri'. Then place a piece of cleanly scraped English tin into the glass. If it is left therein for some time it will look quite black, but after a few hours it begins to color the water red and finally the water reaches maximum brilliancy, The tin is then removed. ..."
Johan Christian Oreshall (1684)
This
ancient recipe describes the preparation of a gold hydrosol, or so-called
"liquid gold", by reduction of trivalent gold ("auric")
cation to metallic gold:
- Eq. 1:
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Figure 1. Click image to enlarge |
Other substances such as the vegetable gums exhibit colloidal behavior because their micelles are very large individual "macromolecules", held together by primary valence forces. Early in the last century the existence of macromolecules was a subject of sometimes heated controversy, the opposition holding that they were simply aggregates of smaller molecules, e.g., simple sugars or peptides. Evidence from electron microscopy and X-ray diffraction settled the issue in favor of macromolecules.
Macromolecules usually have highly extended linear or branched structures with molecular weights in the range of 10,000 to several million daltons. In solution they interact with each other and with the solvent to impede flow, causing the sol to be viscous1. They are termed "lyophilic colloids"because of this affinity for the solvent.. Soaps and other detergents are also lyophilic, or, more precisely, amphiphilic. Characteristically they have a hydrophilic end group, such as carboxyl (-COO-) or sulfate (-SO4-), attached to a long hydrocarbon chain.
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Figure 2. Click image to enlarge |
Although colloidal micelles are too small to be seen by light microscopy they do scatter light. This gives rise to the "Tyndall effect"2. If a beam of light is focused so as to pass through a cell containing the sol, the path of the light through the sol ("Tyndall cone") becomes visible (Fig. 1). The ultramicroscope (Fig. 2) uses this effect to visualize individual micelles. This consists of an optical train set up to pass a collimated light beam, perpendicular to the optical path of a microscope, through a cell (Fig. 3) mounted on the stage of the 'scope. With a colloidal sol in the cell magnification resolves the Tyndall cone into twinkling points of light reflected off individual micelles tumbling in Brownian motion. Construction of an ultramicroscope should provide a challenging project for an amateur who is into optics and microscopy.
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Figure 3. Click image to enlarge |
Blue gold hydrosol3
Prepare a 1% solution of chlorauric acid, HAuCl4, 4 by dissolving 0.1 g HAuCl4 in 9.9 mL distilled or deionized water. Carefully neutralize this solution with sodium carbonate solution, using litmus or HydrionTM paper as an indicator. Add 1 mL of this solution to 100 mL distilled or deionized water to give a neutral NaAuCl4 solution of about 0.01% concentration. Next prepare a very dilute solution of phenylhydrazine HCl, C6H5NH.NH2.HCl by dissolving a small crystal in 20 mL distilled or deionized water. Slowly add the phenylhydrazine solution to the gold solution while stirring gently; the solution will turn violet and then to blue as the reagent is added. The shade varies from violet to blue-black, depending on the amount of phenylhydrazine added. Van Klooster states that too little of the reagent gives a violet color, while a too strong solution produces a blue-black sol from which the gold precipitates after a while.
The blue gold hydrosol can be preserved in a clear glass bottle. Tall cylindrical bottles ("Oil sample bottles") make a nice exhibit. If not overdosed with phenylhydrazine, the sol should remain stable for many months. The sol appears blue and perfectly clear by transmitted light. By reflected light it appears somewhat turbid. Its optical heterogeneity can be shown by its Tyndall cone, which in my experience was quite pronounced and of a yellowish color.
The 1% NaAuCl4 stock solution used in this and the following demonstrations should be preserved in an amber glass bottle for protection from light.
Red gold hydrosol (by CO reduction)5
In this experiment Au+3 is reduced by passing a stream of carbon monoxide through NaAuCl4 solution:
- Eq. 2:
- Eq. 3:
Sulfuric acid promotes
the reaction by combining with the water formed in the reaction.
Set up a reaction train as shown in Figure 4. CO and CO2
are generated by heating oxalic acid with conc. sulfuric acid in a 250
mL Erlenmeyer flask. An absorption train of two 100 mL bottles,
each containing 30 mL sodium hydroxide solution (30 g NaOH in 100 mL water)
is used to remove CO2,
while allowing CO to pass into a 0.01% NaAuCl4
solution (prepared as in the previous experiment) contained in a 250 mL
boiling flask. Any unreacted CO is absorbed in a test tube containing
ammoniacal cuprous chloride6.
Place 10 g oxalic acid crystals in the Erlenmeyer flask and add 20 mL
conc. sulfuric acid to cover them. Heat very gently to obtain a
slow flow of gas through the absorption train and into the NaAuCl4
solution. (Caution: Although little or no gas should pass
through the CO absorption tube into the air, it is highly advisable to
carry out these operations in a hood or well-ventilated area. Carbon
monoxide is an insidious poison which can knock you out with little forewarning.)
The color changes occurring in the gold solution as reduction proceeds
are fascinating to watch.
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Figure 4. Click image to enlarge |
Initially no change
is seen in the nearly colorless solution, but on further passage of gas
it takes on a faint pink tinge which gradually deepens until the sol is
a beautiful ruby red. When no further color change is seen discontinue
heating and immediately open the pinchcock on the vent tube of the gas
generator (we don't want any caustic being sucked back into the hot acid
!)
Like the blue hydrosol, the red sol is perfectly clear by transmitted
light, but has a slightly turbid appearance by reflected light.
It shows a pronounced Tyndall effect of a yellowish-golden color.
Red gold hydrosol (by tannin reduction)7
Prepare 100 mL of 0.01% neutral chlorauric
acid solution (sodium chloraurate) as previously described. Also
prepare a 0.1% solution of tannin (tannic acid) by dissolving 0.1 g in
100 mL distilled or deionized water. Add a few drops of the
tannin solution to the NaAuCl4
solution contained in a 250 mL Erlenmeyer flask and heat over a Bunsen
flame or hot plate with constant shaking. A very clear sol of a
beautiful wine-red color, quite distinct from that of the sol produced
by carbon monoxide reduction, will be formed. The color may be deepened
somewhat by adding alternately small portions of gold and tannin solutions.
The outstanding peculiarity of this sol is its optical appearance.
Whereas both of the other sols were optically heterogeneous and showed
well-defined Tyndall cones as well as turbidity by reflected light, this
tannin-reduced sol showed only the faintest of Tyndall effects and had
no turbidity. This is suggestive of a higher degree of dispersity.
Also the sol is very stable, remaining unchanged after more than a year.
This suggests that the tannin also acts as a protective colloid to stabilize
the micelles.
Green gold hydrosol (by tannin reduction)8
Carrying out the reduction at elevated temperature
results in a green gold sol. I have not personally tried out this
preparation, but quote the recipe as referenced:
"Heat 200 cc of water and 4 cc. Of a 0.1 per cent gold chloride solution
to 60 C. Remove burner and add 5 cc. of a 20 per cent tannic acid
solution with stirring. A light green to olive green sol results.
"Repeating with 200 cc. of water, 10 cc. of a 0.1 per cent gold chloride
solution, and 10 cc. of the 20 per cent tannic acid solution results in
a dark green sol which is unstable.
"The first sol is protected and stable as long as the tannic acid
is protected from deterioration (add a few drops of phenol, chloroform,
or other disinfectant to prevent bacterial degradation of the tannic acid)."
Colloidal metals, notably silver, have germicidal properties. A century ago ArgyrolTM, a silver hydrosol stabilized with protein, was invented and commercialized by the American physician and eccentric art collector, Alfred C. Barnes (1872-1951) and his associate, Herman Hille. Wide acceptance of this product made Barnes a very wealthy man and enabled him to pursue his primary interest of art collecting. With the development of modern antibiotics in the middle of the last century Argyrol and other silver preparations fell into disuse. In recent years colloidal silver has again been promoted as a cure-all by alternative medicine devotees. Do-it-yourself kits for producing silver colloid electrochemically are being marketed9. Experimentation with these may be interesting, but I do NOT recommend imbibing the product. Accumulation of silver within the body can produce argyria, a condition which can leave the patient rather blue.
Colloidal silver was investigated by the 19th century American chemist, Matthew Carey Lea (1823-1897), an inventor of photographic processes. Here are some of the recipes for preparing "Carey Lea's silver":
Silver hydrosol (by reduction with ferrous citrate)10
Prepare the following solutions:
Silver nitrate (10%): Dissolve 1.1 g AgNO3 in 10 mL distilled or deionized water.
Ferrous sulfate (30%): Dissolve 4.3 g FeSO4.7H2O in 10 mL distilled or deionized water.
Sodium citrate (40%): Dissolve 6.6 g Na3C6H5O7.2H2O in 10 mL distilled or deionized water.
Combine the latter two solutions, neutralize to litmus with a few drops of conc. sodium hydroxide and quickly add it to the silver nitrate solution. Mendeleeff stated that a lilac-colored precipitate will be thrown down and subsequently turn blue. In my experience the mixture immediately turned dark blue, almost black, and part of the solution came down as a dark blue precipitate, with some remaining in colloidal suspension. On filtering the suspended silver passed through the filter while the precipitated silver was retained. Wash the precipitate on the filter with ammonium nitrate solution (7 g in 100 mL distilled or deionized water). Note that little or none of the precipitate dissolves to pass through the filter. Here the ammonium cation acts to neutralize the charge on the silver micelles thus inhibiting their dispersion. However the precipitated colloidal silver is "reversible", that is it can be dispersed by removing the neutralizing cations. Wash the precipitate on the filter with distilled or deionized water. The silver will now disperse in the water and pass through the filter. The resulting silver hydrosol may present a color ranging from bright red to a very deep
red-brown
This property of reversibility can be employed to purify the sol from the salts used in its preparation. Reprecipitate it by adding a saturated ammonium nitrate solution (12 g in 10 mL water. Decant off the supernatant liquid and wash the precipitate with two or three small quantities of 7% ammonium nitrate. Disperse the washed precipitate in 100 mL distilled or deionized water. The resulting silver hydrosol is bright red by transmitted light and chocolate brown by reflected light. This process can be repeated for further purification if desired.
As noted above, ammonium (and other monovalent cations) yield precipitates that are reversible. Divalent cations likewise precipitate hydrosols, but the effect of the double charge is not twice but many times that of the monovalent NH4+. Add a few drops of calcium chloride solution to silver hydrosol in a test tube. The resulting precipitate is irreversible and will not redisperse on washing with water.
Silver hydrosol (by reduction with dextrin)11:
Dextrin for this experiment can be prepared by careful heating of starch. The temperature for conversion of starch to dextrin is from 200 to 250 C. Spread finely powdered corn, wheat or potato starch in a thin layer in a pan and heat in an oven at ca. 225 C (500 F). A kitchen oven or the drying oven described in a previous Bulletin article12 can be used. Remove the pan from time to time and stir up the starch to promote uniform heat treatment and avoid burning. Continue heating until the powder is a uniform light brown. Bottle it as "dextrin".
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Click image to enlarge |
Dissolve 2 g dextrin and 2 g sodium hydroxide in100 mL distilled or deionized water. Gradually add 1.4 g silver nitrate dissolved in a small volume of water. Initially a brown suspension of silver oxide will be formed. This muddy brown liquid will slowly change color to a reddish chocolate as the dextrin slowly reduces the oxide to colloidal silver. Eventually it assumes a deep red color showing a fine bluish reflection, particularly after it is bottled. This is due to a thin film of metallic silver being deposited on the wall of the bottle. A few mL of this silver sol will color a large volume of water. When not too dilute, its color will be a beautiful red, perfectly transparent by transmitted light, but exhibiting a slight chocolate opacity by reflected light.
The hydrophobic nature of these silver sols becomes apparent when they are diluted by pouring into a larger volume of water. The sol shows little tendency to diffuse, sinking down as a colored cloud (Fig. 5).
References
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