1. Colloids

2. Classes of solutions
True Solutions
(Suspensions)
Colloidal Solutions
Suspensions:
Heterogeneous mixtures
Relatively large particles
e.g. whole blood
many medicines (Shake well before using)

3. Colloids:
Heterogeneous mixtures (micro)
Dispersed particles: 1 to nm
Hydrophilic colloids (eucolloids)
Hydrophobic colloids (aggregation)

4. Colloids Colloids are mixtures of a solvent and suspended particles.
Particles are too small to see but are larger than molecules.
Due to their small size they do not settle out of solution.
There are several types of colloid:
aerosol (gas + liquid or solid, e.g. fog and smoke),
foam (liquid + gas, e.g. whipped cream),
emulsion (liquid + liquid, e.g. milk),
sol (liquid + solid, e.g. paint),
solid foam (solid + gas, e.g. marshmallow),
solid emulsion (solid + liquid, e.g. butter),
solid sol (solid + solid, e.g. pearl, opal).

5. Types of Colloids Dispersed Phase Continuous Name Gas Liquid Foam
Solid
Aerosol
Emulsion
Gel
Sol
Solid sol

6. Properties:
Brownian motion
Tyndall Effect (Reflection and light scattering)
Coagulation peptization
Dialysis

7. Tyndall Effect
Tyndall effect: ability of a Colloid to scatter light.
The beam of light can be seen through the colloid.

8. Removal of Colloidal Particles
Colloid particles are too small to be separated by physical means (e.g. filtration).
Colloid particles are coagulated (enlarged) until they can be removed by filtration.
Methods of coagulation:
heating (colloid particles move and are attracted to each other when they collide);
adding an electrolyte (neutralize the surface charges on the colloid particles).
Dialysis: using a semipermeable membranes separate ions from colloidal particles.

9. Hydrophilic & Hydrophobic Colloids
Focus on colloids in water.
“Water loving” colloids: hydrophilic.
“Water hating” colloids: hydrophobic.
Molecules arrange themselves so that hydrophobic portions are oriented towards each other.
If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water.

10. Hydrophilic and Hydrophobic Colloids
Most dirt stains on people and clothing are oil-based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water.
Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine.
Emulsifying agents help form an emulsion.

11. Hydrophilic and Hydrophobic Colloids
Sodium stearate has a long hydrophobic tail (CH3(CH2)16-) and a small hydrophobic head (-CO2-Na+).
The hydrophobic tail can be absorbed into the oil drop, leaving the hydrophilic head on the surface.
The hydrophilic heads then interact with the water and the oil drop is stabilized in water.

12. Particle Sizes Become Larger
Solutions Colloidal Dispersions Suspensions
All particles are on Particles of at least one Particles of at least
the order of atoms, component are large one component
ions, or small clusters of atoms, ions, may be individually
molecules (0.1-1 nm) or small molecules, or seen with a low-
are very large ions or power microscope
molecules ( nm) (over 1000 nm)
Most stable to gravity Less stable to gravity Unstable to gravity
Most homogeneous Also homogeneous, Homogeneous only
but borderline if well stirred

13. Solutions Colloidal Dispersions Suspensions
Transparent (but Often translucent or Often opaque but,
often colored) opaque, but may be may appear translucent transparent
No Tyndall effect Tyndall effect Not applicable
(suspensions cannot be transparent)
No Brownian Brownian movement Particles separate unless
movement system is stirred
Cannot be separated Cannot be separated Can be separated by
by filtration by filtration filtration
Homogeneous ———— to ———— Heterogeneous ——>

14. Colloids Tyndall effect
Light is scattered by particles suspended in water
Use the Tyndall effect to differentiate between a true solution and a suspension
True solution - a homogeneous mixture
ions and molecules are too small to scatter visible light
Suspension - particles are suspended in water
the particles are too large to be dissolved, and so are large enough to scatter light.

15. Colloids Colloid aka a colloidal dispersion
tiny particles are suspended in some medium
the particles are single large molecules or
the particles are groups of molecules or ions from 1 to 1000 nm.

16. Colloids
Type of colloid depends on the medium and the dispersed phase

17. Colloids

18. What stabilizes a colloid?
Electrostatic repulsion
A colloid is neutral
BUT, when a colloid is placed in an electric field, the particles all migrate to the same electrode.
The colloid attracts ions of the same charge which surround the colloid particle.
Ions of the opposite charge surround the first layer of ions.
Being surrounded by ion, the colloid particles repel other colloid particles, and so are unable to aggregate to precipitate out

19. To destroy a colloid (coagulation)
Colloids
To destroy a colloid (coagulation)
heat the colloid
heating increases velocities of particles
particles can collide with enough energy to knock off the ion barriers, thus allowing the colloid particles to aggregate and precipitate out.

20. To destroy a colloid (coagulation)
Colloids
To destroy a colloid (coagulation)
add an electrolyte
the electrolyte will neutralize the ion layers
Ex: deposition of clay where a river reaches the ocean. The high salt content of the seawater causes the suspension of clay particles to coagulate.