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This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.

 

A colloid, also called a colloidal dispersion, is a two-phase heterogeneous mixture made up of a dispersed phase of tiny particles that are distributed evenly within a continuous phase. For example, homogenized milk is a colloid made up of tiny particles of liquid butterfat (the dispersed phase) suspended in water (the continuous phase). In comparison to true solutions, the continuous phase can be thought of as the solvent-like substance and the dispersed phase as the solute-like substance.

Each type of colloid has a name. A solid sol is one solid dispersed in another solid, such as colloidal gold particles dispersed in glass to form ruby glass. A solid emulsion is a liquid dispersed in a solid, such as butter. A solid foam is a gas dispersed in a solid, such as Styrofoam or pumice. A sol is a solid dispersed in a liquid, such as asphalt, blood, pigmented inks, and some paints and glues. An emulsion, sometimes called a liquid emulsion, is a liquid dispersed in another liquid, such as mayonnaise or cold cream. A foam is a gas dispersed in a liquid, such as whipped cream or sea foam. A solid aerosol is a solid dispersed in a gas, such as smoke and airborne particulates. An aerosol, sometimes called a liquid aerosol, is a liquid dispersed in a gas, such as fog, which is tiny water droplets suspended in air. All gases are inherently miscible (completely soluble in each other), so by definition there is no such thing as a gas-gas colloid. Some colloidal substances are a mixture of colloid types. For example, smog is a combination of liquid and solid particles dispersed in a gas (air), and latex paint is a combination of liquid latex particles and solid pigment particles dispersed in another liquid. Table 18-1 summarizes the types of colloids and their names.

What About Gels?

Many reference sources incorrectly list gel as a type of colloid, describing a gel as a liquid dispersed phase in a solid continuous phase, which is properly called a solid emulsion. In fact, a gel is a type of sol in an intermediate physical phase. The density of a gel is similar to the density of the dispersing liquid phase, but a gel is physically closer to solid form than liquid form. Prepared gelatin is a good example of a typical gel. Mary Chervenak adds, “I think toothpastes are defined as colloidal gels with viscoelastic properties.”

Table 18-1. Types of colloids

Phase of colloid Continuous phase Dispersed phase Colloid type
solid solid solid solid sol
solid solid liquid solid emulsion
solid solid gas solid foam
liquid liquid solid sol
liquid liquid liquid emulsion
liquid liquid gas foam
gas gas solid solid aerosol
gas gas liquid aerosol
gas gas gas n/a

What differentiates a colloid from a solution or a suspension is the size of the dispersed particles. In a solution, the dispersed particles are individual molecules, if the solute is molecular, or ions, if the solute is ionic. Particles in solution are no larger than one nanometer (nm), and usually much smaller. In a colloid, the dispersed particles are much larger, with at least one dimension on the close order of 1 nm to 200 nm (=0.2 micrometer, μm). In some colloids, the dispersed particles are individual molecules of extremely large size, such as some proteins, or tightly-bound aggregates of smaller molecules. In a suspension, the dispersed particles are larger than 100 nm.

These differing particle sizes affect the physical characteristics of solutions, colloids, and suspensions, as follows:

  • Solutions, and (usually) colloids, do not separate under the influence of gravity, while suspensions eventually settle out. In a colloid, the interactions among the tiny particles of the dispersed phase with each other and/or with the continuous phase are sufficient to overcome the force exerted by gravity on the tiny particles of the dispersed phase. In a suspension, the force of gravity on the more massive particles of the dispersed phase is sufficient to cause them to settle out eventually, although it may take a long time for that to occur. (If the particles of the dispersed phase are less dense than those of the continuous phase, as for example in a mixture of oil dispersed in water, the dispersed phase “settles” out on top of the continuous phase, but the concept is the same.)
  • Solutions do not separate when centrifuged, nor do colloids except those that contain the largest (and most massive) dispersed particles, which may sometimes be separated in an ultracentrifuge.
  • The particles in solutions and colloids cannot be separated with filter paper, but suspensions can be separated by filtering.
  • Solutions pass unchanged through semipermeable membranes–which are, in effect, filters with extremely tiny pores–while suspensions and all colloids except those with the very smallest particle sizes can be separated by membrane filtration.
  • Flocculants are chemicals that encourage particulate aggregation by physical means. Adding a flocculant to a solution has no effect on the dispersed particles (unless the flocculent reacts chemically with the solute) but adding a flocculant to a colloid or suspension causes precipitation by encouraging the dispersed particles to aggregate into larger groups and precipitate out.
  • The particles in a solution affect the colligative properties of the solution, while the particles in a colloid or suspension have no effect on colligative properties.
  • Solutions do not exhibit the Tyndall Effect, while colloids and suspensions do. The Tyndall Effect describes the scattering effect of dispersed particles on a beam of light. Particles in solution are too small relative to the wavelength of the light to cause scattering, but the particles in colloids and suspensions are large enough to cause the light beam to scatter, making it visible as it passes through the colloid or suspension.

Mary Chervenak comments

Synthetic latexes, some of which have small enough particles to be considered aqueous colloidal suspensions, appear blue for this reason.

Figure 18-1 shows the Tyndall Effect in a beaker of water to which a few drops of milk had been added. I used a green laser pointer for this image because the much dimmer red laser pointer I used when I actually did the lab session proved impossible to photograph well, even thought it was clearly visible to the eye. The bright green line that crosses the beaker is the actual laser beam, reflected by the colloidal dispersion. The green laser pointer is bright enough that the scattered light illuminates the rest of the contents of the beaker as well.

Figure 18-1

Figure 18-1. The Tyndall Effect

Table 18-2 summarizes the physical characteristics of solutions, colloids, and suspensions. It’s important to understand that there are no hard-and-fast boundaries between solutions, colloids, and suspensions. Whether a particular mixture is a colloid or a suspension, for example, depends not just on the particle size, but the nature of the continuous phase and the dispersed phase. For example, note that the particle size of colloids may range from about 1 nm to about 200 nm, while the particle size of suspensions may be anything greater than 100 nm. Furthermore, particle sizes are seldom uniform, and may cover a wide range in any particular mixture.

So, is a particular mixture with a mean particle size of 100 nm a colloid or a suspension? It depends on the nature of the particles and the continuous phase. Solutions, colloids, and suspensions are each separated by a large gray area. Near the boundaries between types, it’s reasonable to argue that a substance is both a solution and a colloid, or both a colloid and a suspension. As George S. Kaufman said, “One man’s Mede is another man’s Persian.”

Mary Chervenak comments

“Quod cibus est aliis, aliis est venenum.” (What to some is food, to others is poison.)

Table 18-2. Physical characteristics of solutions, colloids and suspensions.

Characteristic Solution Colloid Suspension
Type of particle individual molecules or ions very large individual molecules or
aggregates of tens to thousands of smaller molecules
very large aggregates of molecules
Particle size < 1 nm ~ 1 nm to ~ 200 nm > 100 nm
Separation by gravity? no no (usually; otherwise, very slowly) yes
Separation by centrifugation? no yes, for more massive dispersed particles yes
Captured by filter paper? no no yes
Captured by membrane? no yes (usually) yes
Precipitatable by flocculation? no yes yes
Exhibits Tyndall Effect? no yes yes
Affects colligative properties? yes no no

In this chapter, we’ll prepare various colloids and suspensions and examine their properties.

Everyday Colloids and Suspensions

  • The protoplasm that makes up our cells is a complex colloid that comprises a dispersed phase of proteins, fats, and other complex molecules in a continuous aqueous phase.
  • Detergents are surfactants (surface-active agents) that produce a colloid or suspension of tiny dirt particles in an aqueous continuous phase.
  • Photographic film consists of an emulsion of gelatin that serves as a substrate for a suspension of microscopic grains of silver bromide and other light-sensitive silver halide salts.
  • Many common foods, including nearly all dairy products, are colloids or suspensions.
  • Toothpaste, shaving gel, cosmetic creams and lotions, and similar personal-care products are colloids.
  • Water treatment plants use flocculants (chemicals that cause finely suspended or colloidal dirt to clump into larger aggregates and settle out) as the first step in treating drinking water.

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