Somewhere between the sizes of an atom and a grain of sand lies the realm of small particles called colloids. As will become evident, they are everywhere. The simplest colloidal materials, also generally known as suspensions or dispersions, consist of two mixed phases. The continuous or dispersing phase may be gas, liquid, or solid (or even plasma, the fourth phase of matter). Air, water, and plastics are common examples. The colloid particles make up the dispersed or suspended phase when uniformly distributed in the second, continuous phase. The dispersed matter may also be gas, liquid, or solid, and any combination in more complex suspensions. Colloidal dispersions are considered homogeneous mixtures even though they can be heterogeneous at or below the microscale.
Colloids are typically defined as having at least one linear dimension approximately between 1 nanometer (3.94 × 10 −8 inches) and 1 micrometer, or micron (3.94 × 10 −5 inches). A length-scale restriction for colloids is straightforward but arbitrary and misleading; there are no absolute bounds. It is best to think of a colloid in terms of how the material behaves with respect to inertial or body forces, such as gravity and fluid flow. For example, particles that do not quickly settle out or float to the top of their dispersed phase are considered colloidal. Wood fibers used in paper-making behave as colloids in water even though they can be tens of microns in diameter and many millimeters in length; individual fibers would not be considered colloidal in air where they would settle in seconds.
Classification of Materials
Most kinds of colloidal suspensions maintain microscopic phase separation between two or more materials while having a uniform macroscopic distribution of each. Aerosols are fine liquid droplets or solid particulates that form in a gas phase. The inverse of an aerosol is a foam—gas bubbles or pockets trapped in a liquid or solid medium. Liquid foams often have neither a truly continuous phase nor a dispersed colloid in the usual sense. The bubbles are frequently very large with liquid films for separating them that are thin enough to exhibit colloidal characteristics. Soapsuds in a bathtub are a prime example of this phenomenon.
The scientific study of colloids began during the late 1800s with the continuing development of high-resolution microscopy and light-scattering theory (by Lord Rayleigh in 1871). Optical microscopy can view objects as small as approximately one-half the wavelength of light used, or roughly three-tenths of a micron for white light. Much smaller objects may still be located under the microscope by light scattering, but particle size becomes indistinguishable. Particle sizes can be measured to thousandths of a micron by carefully measuring the intensity of light scattered at different relative angles. Electron microscopy is capable of reproducing images at this scale, sometimes with atomic resolution.
Aerosols of dust, smoke, mist, and fog are the most convenient colloids for observing light-scattering phenomena outside of the laboratory. This is demonstrated by the orange or red color of sunsets that results from dust and smoke blocking shorter wavelengths of light more than longer ones: violet, indigo, blue, and green over yellow, orange, and red. When the sun is higher, the light path through the atmosphere is much shorter with fewer dust particles to scatter it. Sunrise is not usually as red as sunset because larger particles entrained in hotter, daytime air currents settle out in the calmer and cooler night atmosphere. Light scattering also explains why yellow or amber fog lights actually improve visibility in fog and higher-intensity headlights do not. Increasing intensity yields increased scattering, whereas eliminating shorter wavelengths reduces the effect.
Liquid drops dispersed in a second liquid medium are called emulsions and may be the most important of all colloids, including blood and milk. Solid colloids suspended in liquid are called sols, or hydrosols, when waterborne. Solid suspensions include ice cream, sedimentary rock formations, colored plastics and ceramics filled with pigment particles (as opposed to dyes), and other synthetic composites. Composites are made from two solids to form a hybrid material with the desirable properties of each constituent: for example, a plastic continuous phase for its light weight with a metal or ceramic filler for strength.
Colloidal suspensions of polymers and surfactants present two distinct groups compared to other dispersed or dissolved materials and will be discussed in more detail below.
Interfaces and Interparticle Forces
Surface area is more important than size for defining colloids. They behave differently than classic objects, or macroscopic bodies, because of their high surface-to-volume ratio, or more accurately, surface-to-mass. The force due to gravity acts on the mass while intermolecular forces effectively act at particle surfaces. If these interparticle or surface forces are large enough to alter the effects of gravity on an object, then it has the characteristics of a colloid. For example, gold is more than nineteen times as dense as water and should sink in it. But a nanometer-sized gold sphere (a few hundred atoms) has so little mass that the forces of water molecules hitting it from all directions are enough to make it effectively buoyant. This effect of thermal energy in a dispersing fluid is called Brownian motion . The colloidal gold may remain stable for many years as long as it never collides with another interface, that is, the boundary between phases. The concept behind the adage "like dissolves like" is as important for colloids as it is for solutions.
A nanometer (nm) is one-billionth of a meter (10 −9 m, or 3.94 × 10 −8 inches), approximately six water molecules end-to-end. A micron (μm) is 1,000 nanometers or one-thousandth of a millimeter (10 −6 m, or 3.94 × 10 −5 inches); the thickness of one strand of hair is typically 50–100 microns (1.97–3.93 × 10 −3 inches) in diameter. Fog and clouds contain water droplets of smaller diameters but still above 1 micron. Red blood cells are about 7 microns (2.76 × 10 −4 inches), smoke particles under a micron to 10 nanometers (3.94 × 10 −5 inches to 3.94 × 10 −7 inches). Very large molecules called polymers (e.g., DNA , cellulose, nylon) are also colloids and may be as large as tens of nanometers, comparable to viruses.
The Scottish botanist Robert Brown (1773–1858), who also discovered the cell nucleus, first noticed small pollen particles wiggling and moving randomly under a microscope. Higher temperatures result in larger kinetic energies in the dispersant molecules, keeping even larger particles from sedimenting or floating. Thus, temperature is also a determining parameter in the definition of a colloid.
Atomic and molecular interactions determine both chemical solubility and colloidal stability. Similar molecules attract each other more than dissimilar ones. This universal attraction between everything, called London dispersion, is the result of electron motion. Dispersion forces are partly responsible
for keeping gold from dissolving in water since the two are not very similar. With this logic, a second gold particle in water would be attracted to the first and stick to it. This is called aggregation and cohesion; the aggregation of two unlike particles or interfaces is called adhesion.
More than London dispersion forces are often present. Even though a single molecule or atom has no net charge , it often has stable partial charges of equal positive and negative magnitudes, together called a dipole. Dipole interactions are attractive and often grouped with London dispersion under the collective label van der Waals interaction. Water is a highly polar molecule and gold is easily polarized, arguing for the even greater probability of aggregation. But as a metal, the gold interface with water easily builds up surface charge that repels other gold interfaces through the electrostatic or coulombic interaction. Nearly all materials acquire surface charge in water, more often negative than positive. Several other interparticle forces exist, but van der Waals and electrostatic forces usually determine colloidal behavior.
Surfactants, Adsorption, and Micelles
Molecules and surfaces are often qualitatively categorized as either hydrophilic (water-loving) or hydrophobic (water-fearing). Surface active agents or surfactants have both characteristics. One end of a simple surfactant is hydrophilic, the polar or ionic head group. The rest of the molecule is the hydrophobic tail, such as an oily hydrocarbon. Having very dissimilar parts in solution attracts the molecule to surfaces, as in detergency.
Surfactants concentrate at interfaces by adsorption to remove lyophobic (solvent-fearing) parts from the solvent. This behavior lowers the liquid's surface tension; that is to say, it lessens the imbalance of intermolecular forces between the solvent and its surroundings. The surface tension of pure water causes it to bead up on hydrophobic surfaces, such as a water-proofed jacket or a rain-treated windshield. Liquids with lower surface tensions, such as oils and alcohols, bead up to lesser extents. Water can be made to wet hydrophobic material by adding surfactant; this is sometimes called "breaking" the surface tension.
At low concentrations, surfactants form traditional solutions. At higher concentrations, surfactant molecules may self-assemble into colloidal-scale structures in bulk solution referred to as micelles. They can be spheres, cylinders, vesicles , or bilayer sheets with nanometer to micron dimensions. Above the critical micelle concentration (CMC), micelles begin to grow by molecular association or aggregation. Hydrophobic tails escape water by sticking together in a core shielded by a shell of head groups. Surfactant solubility may sometimes be too low at room temperature for micellization; not enough molecules can dissolve to start associating.
Emulsions and Foams
Oil in vinegar (mostly water) for salad dressing is a common emulsion . However, this is not a stable colloidal system. Usually, salad dressings must be shaken vigorously to redisperse the oil before pouring. The kinetic energy of shaking breaks up the oil into small droplets. But the oil quickly separates again, aggregating into a large, hydrophobic phase. This is called coalescence for droplets since they not only attach but also merge into a single, larger drop. The interfacial tension between oil and water is very high; in other words, they do not mix easily. The acetic acid in vinegar can act as a surfactant, but a much better one is needed to reduce the oil—water interfacial tension enough for stable emulsion formation.
Mayonnaise, on the other hand, is a relatively stable emulsion due mostly to high viscosity (more precisely, viscoelasticity), though surfactants are also present. The oil and water in mayonnaise cannot separate into phases because the emulsion droplets do not have enough energy for much movement. In less viscous emulsions, surfactants are responsible for stability. They reduce interfacial tension for the formation of small particles that either repel or very weakly attract each other. Brownian motion must be able to counter the effects of interparticle attraction, sedimentation, or creaming, which is floatation. Micellar suspensions could also be considered microemulsions, although this is debatable.
Stable foams may be formed by surfactant solutions. Thin liquid films separate gas bubbles, which can be colloidal but are usually much larger. Once formed, gravity eventually drains the liquid until the films break. Viscous additives can slow drainage and increase bubble lifetime significantly. Solid emulsions and foams are less common, the dispersing phase being solid while a liquid or gas phase is dispersed.
One surfactant crucial to life occurs naturally in the lung, without which babies could never take their first breaths in air. Pulmonary surfactant decreases the surface tension of liquid (mostly water) in the lungs to almost zero so that tiny air sacs called alveoli can expand to get oxygen into the blood. Water has very high surface tension due to strong molecular attractions, including polar and hydrogen bonding . Molten salts, mercury, and other liquid metals have much higher surface tensions, but nearly all liquids at room temperature have lower surface tensions than water.
Solid Dispersions, Gels, and Polymer Solutions
"Colloid" comes from the Greek word meaning "glue," which was traditionally a sol. Nearly all paints are sols. Particles of pigment, binder, and filler create the color, strength, and substance of the solid coating after drying. Most paints must be shaken to disperse the particles evenly before application. Many polishing compounds are highly concentrated sols, perhaps even a paste. Rather than separating, these may set over time; the solid particles aggregate. If they pack loosely, the solid network can trap solvent to form a sol-gel. Stirring may redisperse the particles into a sol state again. Silica is known to form gels and gel layers in water.
Polymers, or macromolecules, in solution are lyophilic colloids, implying that they dissolve. However, polymer can still be separated from solvent by physical means, unlike traditional solutions. This is a further implication of its colloidal status. Larger molecules may be filtered or centrifuged from small ones, although some solvent inevitably remains. Even here the physical separation criterion for solutions is not absolute, since zeolites can filter dissolved molecules by size exclusion to tenths of nanometers and atomic isotopes may be separated by centrifugation.
Polymers can also form gels, such as gelatin, as a solid suspension. Entangled polymer chains trap solvent. Some polymer mixtures form solid-in-solid solutions, such as poly(ethylene) and poly(propylene) blends in clothing fabrics. Polymers can also be surfactants, such as starch with hydrophilic and hydrophobic segments.
Colloids are extremely important to both commerce and life. The Information Age of the late twentieth century nurtured many advancements allowing more detailed investigation of colloidal materials. Lasers and computers, of course, have greatly affected all areas of science. In return, colloids play a major role in the semiconductor industry. Silica–alumina sols polish silicon wafers that go into diode lasers, memory chips, and micro-processors. Everyone takes advantage of colloidal suspensions, especially since the human body contains so many with its cells, proteins, and DNA. As society pushes to make so many things smaller, more functional, and more efficient, colloid science will become increasingly pertinent to technological developments.
The silica-based mineral opal may be considered a solid emulsion when enough water is trapped to have microscopic domains larger than the usual hydration layer. A solid foam coffee cup, thermos, or packing filler is made from polymer expanded with microscopic air pockets. Porous polymer and ceramic membranes could also be viewed as solid foams. Ice cream is a more complicated colloid and may be considered both solid foam and solid suspension. It has at least three phases: ice crystals, air bubbles, and frozen fatty cream. Higher-quality ice cream has more finely dispersed phases for a smoother taste.
Nanotechnology is the interdisciplinary field that has evolved from the study of colloids and the techniques of integrated circuit fabrication. Biotechnology and the biological sciences are replete with numerous colloidal systems, including cells, cell membranes, viruses, bacteria, proteins, and DNA. Most of the hype in nanotechnology has centered on the future creation of microelectromechanical (MEM) devices and even nanobots. All endeavors in nanotechnology must deal with fundamental issues common to colloids, interparticle forces most directly. All areas of science are involved in nanotechnology, and it even began to have an impact on disciplines like business, economics, and law by the end of the twentieth century.
SEE ALSO Solution Chemistry .
D. Eric Aston
Evans, Fennell, and Wennerström, Håkan (1999). The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet , 2nd edition. New York: Wiley–VCH.
Hiemenz, Paul C., and Rajagopalan, Raj (1997). Principles of Colloid and Surface Chemistry , 3rd edition, revised and expanded. New York: Marcel Dekker.
Liger-Belair, Gérard (2003). "The Science of Bubbles." Scientific American 288(1): 80–85.
Morrison, Ian D., and Ross, Sydney (2002). Colloidal Dispersions: Suspensions, Emulsions, and Foams. New York: Wiley Interscience.
Shaw, Duncan J. (1992). Introduction to Colloid and Surface Chemistry , 4th edition. Boston: Butterworth-Heinemann.
Special Nanotechnology Issue (2001). Scientific American 285(3): 32–91.
"Colloids." Scientific American. Available from http://www.sciam.com .
"Physical Sciences Research." Bell Laboratories. Lucent Technologies. Available from http://www.bell-labs.com/org/physicalsciences/index.html .