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Author Topic: How are colloids prepared?  (Read 4278 times)


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How are colloids prepared?
« on: 27/09/2011 11:01:04 »
LEIMAR asked the Naked Scientists:
What are the two general ways of preparing colloids?

What do you think?
« Last Edit: 27/09/2011 11:01:04 by _system »


Offline The Penguin

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How are colloids prepared?
« Reply #1 on: 18/10/2011 20:32:15 »
Colloids are simply mixtures of two phases within one material. Such as a foam where there liquid part is continuous with gas parts dispersed within. Both gas and liquid phases are the same material though. One way is through collection of gas phase into a solid/gas colloid such as smoke. Another way is by causing a liquid to turn into a gas by agitation such as whipping up a cream into a whipped topping.

Offline damocles

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How are colloids prepared?
« Reply #2 on: 19/10/2011 00:14:10 »
Throughout the last 34 years of my professional career I gave lectures on this sort of stuff. I will try to be brief and provide a simple and useful answer.

A colloid is a system which is a mixture of two or more phases where:
at least one phase has at least one dimension between about 5 nanometre and 1 micrometre.

What is the significance of such a definition? For most multi-phase mixtures the molecules at the phase interface are a minute fraction of the total. With colloids they become a significant fraction of the total. The result is that for colloidal systems, surface physical and surface chemical properties become a major factor in the system behaviour rather than just a minor perturbation.

One surface physical property is surface tension: all multi-phase systems have a strong tendency to minimize the area of phase interface. The individual phases like to gather into larger and separate "lumps". So you cannot prepare a colloid by grinding and sieving, for example. When the powder starts to reach the colloidal size range, you find that the grinding is doing as much welding the particles together as breaking them down.

Because colloidal systems are inherently unstable, indirect methods are necessary to prepare them. Here are not 2, but 3 general methods for preparing a hydrosol (colloidal system with tiny solid particles suspended in water). In each case the basic trick is to produce conditions where a large number of tiny seed crystals will form, but where there will be no opportunity for crystal growth.

(1) Non-polar, waxy solid (e.g. sulfur).

Dissolve a little of the solid material in an organic solvent that (1) is miscible with water and (2) boils at a much lower temperature than water. This usually means acetone. Pour this solution, slowly and dropwise, into water that is just below boiling, and well above the boiling point of acetone.

(2) very insoluble salt (e.g. silver iodide)

Prepare a very dilute (0.1 mM) solution of silver nitrate. Mix with an equal volume of potassium iodide which is about 5-10 times more concentrated (but still very dilute, meaning 0.5-1 mM). To generalize this to other insoluble salts, there needs to be an large imbalance between the cation and anion part of the insoluble salt in the solution, and it works best if the anion part has the larger concentration.

(3) metal (e.g. silver or gold)

Prepare a very dilute solution of a salt of the metal in very hot water and add a mild, water soluble reducing agent. For example a gold hydrosol can be prepared by dissolving a little gold in aqua regia, diluting, neutralizing with potassium carbonate, and reducing with formalin.


Offline imatfaal

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How are colloids prepared?
« Reply #3 on: 19/10/2011 10:31:13 »
Nice answer.  Takes me back to that distinctive smell of the chemistry class - thanks

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How are colloids prepared?
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