Definitions

dispersion medium

Colloid

[kol-oid]

A colloid is a type of mechanical mixture where one substance is dispersed evenly throughout another. Because of this dispersal, some colloids have the appearance of solutions. A colloidal system consists of two separate phases: a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). A colloidal system may be solid, liquid, or gaseous. Many familiar substances are colloids, as shown in the below chart.

The dispersed-phase particles have a diameter of between approximately 5 and 200 nanometers. Such particles are normally invisible to an optical microscope, though their presence can be confirmed with the use of an ultramicroscope or an electron microscope. Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. The dispersed-phase particles or droplets are largely affected by the surface chemistry present in the colloid.

Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.

Colloidal systems (also called colloidal solutions or colloidal suspensions) are the subject of interface and colloid science. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.

Classification of colloids

Because the size of the dispersed phase may be difficult to measure, and because colloids have the appearance of solutions, colloids are sometimes identified and characterized by their properties. For example, if a colloid consists of a solid phase dispersed in a liquid, the solid particles will not diffuse through a membrane, whereas with a solution the dissolved ions or molecules will diffuse through a membrane.

Colloids can be classified as follows:

  Dispersed Phase
Gas
Liquid
Solid
Continuous Medium Gas
NONE
(All gases are mutually miscible)
Liquid Aerosol
Examples: fog, mist, clouds
Solid Aerosol
Examples: smoke, air particulates
Liquid
Foam
Example: whipped cream
Emulsion
Examples: milk, mayonnaise, hand cream
Sol
Examples: paint, pigmented ink
Solid
Solid Foam
Examples: aerogel, styrofoam, pumice
Gel
Examples: gelatin, jelly, cheese, opal
Solid Sol
Example: cranberry glass

In some cases, a colloid can be considered as a homogeneous, (not heterogeneous [meaning not the same]) mixture. This is because the distinction between "dissolved" and "particulate" matter can be sometimes a matter of approach.

Hydrocolloids

A hydrocolloid is defined as a colloid system wherein the colloid particles are dispersed in water. A hydrocolloid has colloid particles spread throughout water and depending on the quantity of water available can take on different states, e.g., gel or sol (liquid). Hydrocolloids can be either irreversible (single-state) or reversible. For example, agar, a reversible hydrocolloid of seaweed extract, can exist in a gel and sol state, and alternate between states with the addition or elimination of heat.

Many hydrocolloids are derived from natural sources. For example, carrageenan is extracted from seaweed, gelatin has bovine (cow) and fish origins, and pectin is extracted from citrus peel and apple pomace.

Gelatin desserts like jelly or Jell-O are made from gelatin powder, another effective hydrocolloid. Hydrocolloids are employed in food mainly to influence texture or viscosity (e.g., a sauce).

Hydrocolloid-based medical dressings such as Duoderm are used for wound treatment and for acne.

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 of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting this interaction.
  • van der Waals forces: This is due to interaction between two dipoles that are either permanent or induced. Even if the particles do not have a permanent dipole, fluctuations of the electron density gives rise to a temporary dipole in a particle. This temporary dipole induces a dipole in particles nearby. The temporary dipole and the induced dipoles are then attracted to each other. This is known as 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 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 steric repulsive force (which is predominantly entropic in origin) or an attractive depletion force between them.

Stabilization of a colloidal dispersion

Stabilization serves to prevent colloids from aggregating. Steric stabilization and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. Different phases generally have different charge affinities, so that a charge double-layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is 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 dispersion

Unstable colloidal dispersions form flocs as the particles aggregate due to interparticle attractions. In this way photonic glasses can be grown. This can be accomplished by a number of different methods:

  • Removal of the electrostatic barrier that prevents aggregation of the particles. This can be accomplished by the addition of salt to a suspension or changing the pH of a suspension to effectively neutralize or "screen" the surface charge of the particles in suspension. 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. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively-charged colloidal silica particles can be flocculated by the addition of a positively-charged polymer.
  • Addition of nonadsorbed polymers called depletants that cause aggregation due to entropic effects.
  • Physical deformation of the particle (e.g., stretching) may increase the van der Waals forces more than stabilization forces (such as electrostatic), resulting coagulation of colloids at certain orientations.

Unstable colloidal suspensions of low-volume fraction form clustered liquid suspensions, wherein individual clusters of particles fall to the bottom of the suspension (or float to the top if the particles are less dense than the suspending medium) once the clusters are of sufficient size for the Brownian forces that work to keep the particles in suspension to be overcome by gravitational forces. However, colloidal suspensions of higher-volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels, such as toothpaste, flow like liquids under shear, but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.

Colloids as a model system for atoms

In physics, colloids are an interesting model system for atoms. Micrometre-scale colloidal particles are large enough to be observed by optical techniques such as confocal microscopy. Many of the forces that govern the structure and behavior of matter, such as excluded volume interactions or electrostatic forces, govern the structure and behavior of colloidal suspensions. For example, the same techniques that can be used to model ideal gases can be used to model the behavior of a hard sphere colloidal suspension. In addition, phase transitions in colloidal suspensions can be studied in real time using optical techniques, and are analogous to phase transitions in liquids.

Colloids in biology

In the early 20th century, before enzymology was well understood, colloids were thought to be the key to the operation of enzymes; i.e., the addition of small quantities of an enzyme to a quantity of water would, in some fashion yet to be specified, subtly alter the properties of the water so that it would break down the enzyme's specific substrate, such as a solution of ATPase breaking down ATP. Furthermore, life itself was explainable in terms of the aggregate properties of all the colloidal substances that make up an organism. As more detailed knowledge of biology and biochemistry developed, the colloidal theory was replaced by the macromolecular theory, which explains an enzyme as a collection of identical huge molecules that act as very tiny machines, freely moving about between the water molecules of the solution and individually operating on the substrate, no more mysterious than a factory full of machinery. The properties of the water in the solution are not altered, other than the simple osmotic changes that would be caused by the presence of any solute.

References

  • Lyklema, J. Fundamentals of Interface and Colloid Science, vol.2, page.3.208, 1995
  • Hunter, R.J. Foundations of Colloid Science, Oxford University Press, 1989
  • Dukhin, S.S. & Derjaguin, B.V. Electrokinetic Phenomena, J.Willey and Sons, 1974
  • Russel, W.B., Saville, D.A. and Schowalter, W.R. Colloidal Dispersions, Cambridge, 1989 University Press
  • Kruyt, H.R. Colloid Science, Volume 1, Irreversible systems, Elsevier, 1959
  • Dukhin, A.S. and Goetz, P.J. Ultrasound for characterizing colloids, Elsevier, 2002
  • Rodil, Ma. Lourdes C., Chemistry The Central Science, 7th Ed. ISBN 013533480

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