Colloid

Principle

Colloids are all types of, generally, liquid like mixtures exciting of a solvent and a (semi-)macro-molecular substance. More precisely, a colloidal solution or colloidal dispersion is a type of mixture intermediate between a homogeneous mixture (also called a solution) and a heterogeneous mixture with properties also intermediate between the two.

Typical membranes restrict the passage of dispersed colloidal particles more than for ions or dissolved molecules. Many familiar substances, including butter, milk, cream, aerosols (fog, smog, and smoke), asphalt, inks, paints, glues, and sea foam are colloids. The size of dispersed phase particles in a colloid range from 1 nm to 1μm. Dispersions where the particle size is in this range are referred to as colloidal aerosols, colloidal emulsions, colloidal foams, or colloidal suspensions or dispersions. Colloids may be colored or translucent because of the Tyndall effect (see Light: scattering), which is the scattering of light by particles in the colloid.

When the substance is a liquid or solid, the mixture is a Suspension. When both phases are liquid, than the mixture is an Emulsion.

Since there are 3 basic aggregation states one would expect 9 kinds of solids but since gas is always soluble in another gas, 8 types remain. Table 1 gives these combinations.

Table 1 Types of colloids

		Dispersed Medium
		Gas	Liquid	Solid
Continuous Medium	Gas		Liquid Aerosol Examples: fog, mist	Solid Aerosol Examples: smoke, dust
	Liquid	Foam Examples: whipped cream, nose sprea	Emulsion Examples: milk, hand cream, salve	Sol Examples: paint, pigmented ink, blood
	Solid	Solid Foam Examples: aerogel, styrofoam, pumice	Gel Examples: gelatin, jelly, cheese, opal	Solid Sol Examples: cranberry glass, ruby glass

Applications

They are numerous, as Table 1 indicates, also in medicine.

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Interaction between colloid particles

The following forces play an important role in the interaction of colloid particles:

Excluded Volume Repulsion: this refers to the impossibility of any overlap between hard particles.

Electrostatic interaction: Colloidal particles often carry an electrical charge and therefore attract or repel each other. The charge and the mobility of both the continuous and the dispersed phase are factors affecting this interaction.

Van der Waals forces: This is due to interaction between two dipoles (permanent or induced). The van der Waals force and is always present, is short range and is attractive.

Entropic forces: According to the second law of thermodynamics, a system progresses to a state in which Entropy (irreversible increase of disorder at microscopic scale) is maximized. This can result in effective forces even between hard spheres.

Steric forces between polymer-covered surfaces or in solutions containing non-adsorbing polymer can modulate interparticle forces, producing an additional repulsive steric stabilization force or attractive depletion force between them.

Stabilization of colloid suspensions

Stabilization serves to prevent colloids from aggregating. Steric and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges leading to very large charge double-layers of the continuous phase around the particles. In this way the specific density differences are so small that buoyancy or gravity forces are too little to overcome the electrostatic repulsion between charged layers of the dispersing phase.

The charge on the dispersed particles can be observed by applying an electric field: all particles migrate to the same electrode and therefore must all have the same sign charge.

Destabilizing a colloidal suspension

Unstable colloidal suspensions form flocks as the particles aggregate due to interparticle attractions. This can be accomplished by a number of different methods:

· Removal of the electrostatic barrier that prevents aggregation of the particles (by addition of salt to a suspension or changing the pH). This removes the repulsive forces that keep colloidal particles separate and allows for coagulation due to van der Waals forces.

· Addition of a charged polymer flocculant (bridging of individual colloidal particles by attractive electrostatic interactions).

· Addition of non-adsorbed polymers called depletants that cause aggregation due to entropic effects.

· Unstable colloidal suspensions of low volume fraction form clustered liquid suspensions wherein individual clusters of particles fall to the bottom or float to the top, since Brownian forces become too small to keep the particles in suspension. Colloidal suspensions of higher volume fraction can form colloidal gels with viscoelastic properties. These gels (e.g. toothpaste) flow like liquids under shear but maintain their shape when shear is removed. It is for this reason that toothpaste can be stays on the toothbrush after squeezing out.

Colloidal particles are large enough to be observed by Confocal microscopy.

Just as a solution, a colloid has an osmotic effect (see Osmosis).