Colloids

The solutes in solutions that we have been considering up to this point are ions or small molecules

• They form homogeneous solutions with the solvent
• They do not slowly settle out of solution, or sink to the bottom of the solution, over a period of time
• Gravitational forces are small compared to the kinetic energy of the molecules in solution

As solutes get larger, at some point they my start to settle out, or sink to the bottom of the solvent

• Gravitational forces are greater in comparison to the kinetic energy of the solution
• In this case, we no longer have a homogeneous solution, but rather, a heterogeneous mixture

Another property of "large" solute molecules, or high-molecular weight components in a mixture, is the interaction of such molecules with visible light.

• Particles can scatter light when their physical dimensions are similar to the wavelength of the light
• Light scattering of this type will be manifest as the sample appearing "cloudy"
• Visible light has wavelengths of from ~400nm to ~750nm
• Particles with physical dimensions (i.e. diameter) of at least 400nm to 750nm can scatter visible light and will therefore appear cloudy
• A Carbon-Carbon bond has dimensions of approximately 0.15nm, therefore, a carbon-containing molecule would need to have on the order of several thousand carbons in a chain to be of a size to scatter visible light

Between the tiny solutes we have been considering up to this point, and solutes that are so large that they settle out of solution, are homogenous mixtures involving "big" solutes

• These solutions are termed "colloidal dispersions", or just "colloids"
• Colloids are somewhere between a homogenous solution and a heterogenous mixture
• They are small enough to where random collisions keep them dispersed throughout the solution, and the won't settle out due to the effects of gravity, but they are not really dissolved by the solvent
• They can be detected by their light scattering properties (i.e. their large molecular size). The scattering of light indicates that dispersed throughout the solvent is a "solute" that is actually comprised of large chunky bits. Although too small to settle out, their presence is considered to represent a heterogenous mixture, rather than a homogenous solution
• Colloidal dispersions can be gasses, liquids or solids. Here are some examples:

• Hydrophilic means "water loving"
• Hydrophilic colloids have solutes with structural groups exposed on their surface that are able to hydrogen bond with water (electronegative groups with or without hydrogen covalently attached)
• Since "like dissolves like" hydrophilic colloids form aqueous colloidal dispersions

• Hydrophobic means "water fearing"
• Hydrophobic colloids have solutes with surface groups that cannot hydrogen bond, and typically involving groups that can only interact via dispersion forces
• Hydrophobic colloids cannot be prepared in water - unless some type of chemical alteration is done to the solute.
• If the solute can adsorb ions onto its surface, then it may be able to interact strong enough with water. (Note: adsorb means to stick to the surface).
• Another strategy is to combine the solute with another molecule that has two distinct ends to it. One end is hydrophobic in nature, the other hydrophilic. The hydrophobic end binds to the hydrophobic solute, and the hydrophilic end can interact with water (and solubilize the hydrophobic solute). Examples of this are soap molecules, and the digestive juices called bile (helps to dissolve fats in the diet in the aqueous environment of our bodies).

This can be a little tough to do because you cannot centrifuge them out, and often cannot filter them out. Some times heat or addition of electrolytes can cause colloid particle to clump together - or coagulate

• Heat increases the kinetic energy and rate of intermolecular collisions between colloidal particles. Sometimes they have a natural tendency to stick together. Thus, larger aggregates are built up that eventually settle out of solution or can be filtered out.
• If some colloidal solutes are solubilized in aqueous solution by surface ions, then the addition of counter ions can cause such particles to stick together (by eliminating the charge repulsion of like-charges)
• As seen with the section on osmosis, semi-permeable membranes can also separate colloids. This is the basis of kidney dialysis.