Professor Dave here, I wanna tell you about colloids.
We’ve learned quite a bit about different kinds of mixtures.
For example, we’ve learned about homogeneous mixtures involving ionic or covalent solids
that disperse evenly in water or some other solvent to form a solution.
We have also seen certain heterogeneous mixtures where the components do not mix whatsoever,
like oil and water.
More often than not, a discussion of mixtures will end there.
But the reality is that there are other types of mixtures that do not fall strictly into
one of these extremes, but rather sit in between, so let’s find out what these could be.
One such type of mixture is called a suspension.
This is a heterogeneous mixture composed of large particles that are visible to the naked eye.
An example of this can be generated by mixing mud and water.
They will not remain completely separate, like oil and water do, nor will things disperse
perfectly evenly.
Instead, there will simply be a cloudy mix of water and suspended mud particles.
We can also create a mixture that is in between a suspension and a homogeneous solution.
This will usually form because the particle size is larger than ions or small molecules,
but smaller than the particles in a suspension.
Such a solution is called a colloid, or a colloidal dispersion.
To illustrate the difference between a solution, a colloid, and a suspension, let’s look
at saltwater, milk, and mud.
The difference in particle size is what determines the different characteristics of these mixtures.
The particles in a colloid are large enough to scatter light, which is a phenomenon called
the Tyndall effect.
This is why colloidal mixtures will appear cloudy or opaque, the way that particles in
the air are visible inside the beams of bright lights, which is what illuminates big event
spotlights.
Colloids like fog and smoke are what make these beams so bright, due to their scattering
ability.
Some colloidal particles consist of hundreds or thousands of molecules, but some are just
one large molecule, like a protein.
And when we discuss colloids, instead of solute and solvent, we will call the particulate
component present in small amounts the dispersed phase, and the substance the particulate is
dispersed in will be the dispersion medium.
The dispersed phase and the dispersion medium can each be of any phase.
Smoke is solid dispersed in gas.
Clouds, fogs, and mist involve liquid dispersed in gas.
Milk, mayonnaise, and butter involve liquid dispersed in liquid.
Jellies and gels are liquids dispersed in solids.
Foams and whipped cream involve gas dispersed in liquid.
So we can see that there are quite a wide variety of colloids.
Now that we know what colloids are, let’s discuss how they can be prepared.
In order to prepare a colloid, we must first produce particles of colloidal dimensions,
which means they must be above a size that would generate a solution but below a size
that would generate a suspension, and then we must distribute these particles throughout
a dispersion medium.
There are two ways we can prepare these particles.
The first is by breaking down larger particles.
This is how many paint pigments are produced.
The other way is by combining smaller molecules.
This is how clouds form.
Water molecules high up in the atmosphere where it gets very cold, are able to condense
enough to form tiny droplets that then aggregate.
If the water droplets become too large to remain in the cloud, we get precipitation,
which is what causes rain, sleet, and snow.
There are also some solid substances that spontaneously form colloidal systems when
mixed with water, like gelatin, or starch.
These substances contain particles that are already of colloidal size, the water simply
disperses them.
Another type of colloid is called an emulsion.
This occurs when two immiscible liquids are forcibly blended.
One liquid will break into colloidal droplets and disperse throughout the other liquid.
This is why oil spills in the ocean are hard to clean up, the action from waves causes
emulsions to form.
But sometimes, emulsions will coalesce and separate from the other liquid.
To combat this tendency, an emulsifying agent can be used.
This is a substance that inhibits the coalescence of the dispersed liquid.
Milk and mayonnaise are examples of emulsions.
Milk is an emulsion of butterfat in water, with a protein called casein acting as the
emulsifying agent, and mayonnaise is an emulsion of oil in vinegar with egg yolk components
acting as the emulsifying agents.
Sometimes dispersed colloidal particles will bear an electrical charge.
In any particular system, all the colloidal particles will have the same charge, which
helps keep the particles dispersed, since like charges repel one another.
We take advantage of the formal charges on colloidal particles to remove them from a
variety of mixtures.
We can place charged electrodes in a colloidal dispersion and the charged colloidal particles
will migrate to one of the electrodes.
Once this occurs, the colloidal particles lose their charge and consequently will coagulate
as a precipitate.
This is how we remove the carbon and dust particles in smoke.
If smoke passes through something called an electrostatic precipitator, the charged particles
in the colloid will be attracted to electrodes where they will be neutralized and then deposited
as dust.
This method is used in large-scale industrial processes, as well as simple air filters designed
for home use.
So now we know the difference between solutions, colloids, and suspensions, as well as quite
a few things about different types of colloids and their properties.