Colloids

Type of physical science: Chemistry

Field of study: Chemistry of solids

Colloids are particles that, in one of their dimensions, are not smaller than approximately 100 nanometers or larger than approximately 100,000 nanometers. The size of the particles controls much of the physical and chemical behavior of the colloid. In turn, these physical and chemical behaviors influence the industrial and scientific applications of colloidal chemistry.

Overview

Among the earliest artifacts of human creativity are shards of pottery prepared from natural deposits of clay. Prehistoric humans took advantage of the physical characteristics of colloids in the preparation of clay objects. The behavior of clay in the potter's hands or on the potter's wheel is controlled largely by the surface interactions of the clay particles and by interactions between the clay particles and the suspending fluid (water). The changes during firing, the changes in the hydration of the clay that lead to both drying and hardening, the chemical reactions of the slips (decorative coatings of clay applied as a liquid), and the glassing of the glazes, are practical applications of colloidal chemistry. Yet, millennia intervened between the discovery of these practical applications and the development of the science to explain these phenomena.

Size influences the chemical and physical behavior of particles. Atoms and most molecules are very small. Atoms and smaller molecules interact with their environment intimately over their whole surface. It does not matter how one approaches an unbonded atom; in terms of its chemistry, from every perspective it is essentially the same. This is less true of the atoms that make up a molecule; from each perspective, a molecule has a slightly different chemistry. Yet, the smaller is the molecule and the greater is its symmetry, the more likely it is that the molecule will behave uniformly from every perspective.

Colloidal particles are large enough that they have shape, contours, and the localization of charge and reactivity. Many biological molecules are colloidal in size and behavior. Proteins, starches, cellulose, and nucleic acids are colloidal molecules of biological origin. Other colloidal molecules are synthetic polymers constructed by covalently joining smaller molecules (monomers). These larger molecules have surfaces that, in different areas, have different chemical properties. The atoms within these macromolecules are greatly influenced in their reactions by their position within the macromolecule.

Atoms and smaller molecules are relatively accessible for chemical reactions, though there may be energetic barriers that prevent those reactions from occurring. Atoms and smaller molecules diffuse relatively rapidly in solutions and cannot be visualized with either the light or the electron microscope. In contrast, most particles that can be viewed with the naked eye or through a light microscope are relatively unreactive; they sediment rapidly and interfere with the normal passage of light. In classical physics, these particles may be the subject of study as they fall or are struck or otherwise move about.

Colloidal particles fall between these two ranges. They are larger than the atoms and molecules of classical chemistry and smaller than the macroscopic particles. It matters very much how one approaches a colloid particle. In a particle of clay (usually a minute plate or flake), the edges are often different from the faces. The interactions of the faces and the water that hydrates these particles influence the texture, consistency, and handling of the clay. The removal of the water during firing and the chemical reactions bonding the faces during the firing determine the nature of the product of the potter.

Colloids may be composed of almost any chemical elements and often form by the interactions of many smaller molecules. Such aggregation of molecules is involved in the formation of micelles of lipids in emulsions (such as fat in milk or latex in latex paints). Because of their large size, colloids do not behave as simple chemicals, but instead have parts of their chemical structure that interact with the environment and parts that are buried within the structure of the colloid itself. In a dispersion comparable to the solution of classical chemistry, colloids create scatter light. Such dispersions are said to be turbid. Under the microscope, larger colloidal particles can be seen to move as a result of collisions with the suspending medium (Brownian motion). Much of the early study of colloids was focused on the optical effects of colloidal metals, biological molecules, and lipid micelles, while details of the structure of colloids have been visualized using the electron microscope.

Because of their relatively small size, colloids do not behave like larger particles. They do not sediment rapidly without the application of forces much larger than the gravitational forces found on the earth's surface. Centrifuges were developed to sediment particles with this behavior. Larger colloids may sediment when exposed to forces only a few times larger than gravity. Ultracentrifuges, which have the potential to generate forces tens of thousands of times greater than the earth's gravity, can be used to separate and purify even relatively small colloidal species. The patterns of sedimentation under centrifugation are influenced by absolute particle weight, particle density, particle shape, and the state of hydration of the colloidal particles.

The chemist (or, more likely, the physical chemist) studying a colloid will examine some characteristics familiar to all chemists, including the elemental composition of the colloid, the proportion of the various elements, the types of bonding within the particle, the reactivity of the particle, and its solubility. The colloidal chemist will also look at uniquely colloidal features.

Colloids are often studied by examining them under the electron microscope, by observing their natural sedimentation and centrifugation, by electrophoresis (migration in an electric field), by measuring their effects on osmotic pressure, by determining particle weight, and by flocculation.

Colloid surfaces often bear charges that influence the behavior of the colloid particles among themselves and the interaction of these particles with their medium. The effects of these charges may be especially evident in electrophoretic studies and in flocculation experiments. The surface charge of colloids is often balanced by the adsorption on the surfaces of the colloid of equivalent amounts of counterions from the medium. Together the surface charge and the counterions make an electrical double layer. The counterions may play an important role in the repulsion of one colloidal particle by another. Counterions, especially those bearing more than one charge, may also play a role in the aggregation of colloidal particles and in flocculation.

In the absence of high-enough concentrations of counterions, water may bridge between the charges on the surfaces of colloidal particles. The oxygen in the water tends to interact with positive surface charges, while the hydrogen interacts with negative surface charges. Some of the water is relatively tightly bound to the colloidal surfaces. Water may also serve as a bridge between particles in the formation of more massive units. For example, this bound water may play a role in the formation of gels from colloidal materials such as proteins or polysaccharides.

Applications

Understanding the behavior of colloids is crucial to modern industrial chemistry.

Applications of colloid chemistry are found in computer technology, medicine, biochemistry, food chemistry, paint manufacturing, rubber processing, environmental chemistry, ion-exchange technology, dye production, photography, and ceramics manufacturing. In other industrial processes, colloidal properties affect the formation of precipitates, the ability of precipitates to be captured on a filter, the breaking of emulsions, the formation of foam, the recovery of catalysts from reaction mixtures, and the cleaning of surfaces produced by milling and manufacturing processes.

Many of the materials used in computers are colloidal (plastics and ceramics) or depend on colloidal materials in their manufacture (magnetic memory and the films for the manufacture of printed circuit boards). The development of electronic ceramics and plastics with high electrical resistivity, low dielectric constants, and thermal expansivity that matched the levels of other electrical components was a necessary step in the development of microelectronics. These developments have played an important role in bringing computer technology into the realm of everyday experience. The printed and miniaturized circuits that have lowered the cost of electronics would not be possible without these applications of colloidal chemistry.

Colloidal materials also play an important role in medicine. Absorbent materials used for packing and wrapping wounds, suturing materials, and the particles that form the basis for time-released medicines are colloidal. The understanding of human physiology and pathogenic biology is largely influenced by the processes and progress in biochemistry and molecular biology. Many of the procedures in these two areas are built around the colloidal characteristics of proteins, polysaccharides, nucleic acids, and lipids. The physical processes used in sorting biochemicals (centrifugation, electrophoresis, chromatography, and adsorption) are based on the colloidal chemistry of the biochemical. The nature of the biochemicals, their behavior in solution, and their reactivity are all functions of their colloidal chemistry. The pharmaceutical industry makes use of colloidal biochemistry and molecular biology in the synthesis and engineering of new drugs.

The food industry in developed countries depends on uniformity of product. The texture and consistency of many processed foods, including processed cheeses and meats, gravies, sauces, soups, and candies, is a result of the colloidal character of one or more of their ingredients. Whipped cream is an emulsion of air, milk fat, and soluble milk products. The stirring of ice cream as it freezes maintains ice crystals in the colloidal size range and assures the maintenance of colloidal-sized fat micelles. Bakery products are built around the chemistry of starch and protein and derive their individual textures from the interactions of these components in various combinations and from following various treatments.

Pigment particles are often colloidal. Paints are largely pigment particles suspended in a vehicle and glue. In latex paints, latex micelles form an emulsion in an aqueous vehicle, which allows latex paints to be water-washable when wet. The fusion of these latex particles as the paint dries assures that it will be resistant to water once it has set. The pigments of printing are also colloidal in size; the stability of the inks and paints is based on an understanding of the behavior of colloidal particles in dispersions. Glues as a base for these paints and inks may also be colloidal and, as they dry, they may become gels or have other surface reactions that solidify them and make them dry. Electrostatic printing processes depend on the attraction of colloidal pigment particles to temporarily charged regions of the paper or film that is being printed. The adsorption of dye materials to fibers in papers and cloth (whether the fibers are natural or synthetic) is a function of the surface chemistry of the fibers.

Rubber manufacturing is built around the latex emulsions in the saps of a number of plants and around the synthetic analogs of these latices. The manufacturing processes that produce larger rubber products depend on an understanding of the chemistry of the micelles of these complex organic molecules.

Colloidal particles play important roles in the environment. Fog and smoke are primarily colloidal liquid drops and solids, respectively, suspended in the atmosphere. The ability to "scrub" particles from smoke depends on an understanding of the surface and charge behavior of these colloids. Clay in the soil serves as a reservoir of minerals, which can be used by plants growing in that soil. The adsorption of minerals to the surface of the clay is a function of the chemistry of the clay particle. An understanding of this adsorption of ions to colloids can be used to improve agricultural systems and to limit the excessive use of fertilizers and other soil additives.

Ion exchange, a feature of colloidal materials, occurs when the cationic hydrogen from acid rain releases some of the minerals from soil clay particles. In some cases, the released minerals may support plant growth. In other cases, the released minerals may be toxic to plants or animals growing in the environment. The ion-exchange features of colloidal particles can be used in industrial applications to purify water or to create the unique ionic environments that are necessary for specific chemical reactions.

Many of the particles that compose the sediments in sewage are colloidal. The aggregation and flocculation of these colloids can account for much of the deposition of these sediments. By carefully manipulating the chemical environment in sewage treatment systems, engineers can separate the colloidal components of the wastes from the soluble components. Flocculation occurs when colloidal particles with surface charges are salted out by the addition of various ions to the dispersions. These ions interact at sites of surface charge to glue the colloidal particles in loose aggregates.

As an understanding of the behavior dispersions of colloidal particles improves, geologists can use this understanding to explain the environments that led to the formation of specific types of sedimentary rocks. Since fossils are frequently trapped within these rocks, an understanding of the behavior of colloidal particles will allow paleontologists to correlate ancient environments with the organisms that inhabited those environments.

The emulsions of photography include colloids in the matrix containing the pigments.

The particles of pigments that form during the developing process are also colloidal. Many of the advances in color photography, in high-resolution photography, and in high-speed photography have depended on the chemistry of the colloidally based emulsions.

Catalysis is the process that speeds the rate of chemical reactions. The catalyst serves as a site of reaction, while the reaction leaves the catalyst unchanged. Colloidal catalyst particles can be retained in one location, while the reaction solution (whether gas or liquid) may pass on, a result which is especially useful when the catalyst is expensive. Colloidal catalysts are used in the catalytic converters of automobiles and wood stoves, in contact lens-sterilizing systems, in industrial synthesis, and in smokestacks at industrial and power-generating sites.

Context

Although colloidal materials, including pigments, resins, gums, clays, horn, and bone, have played an important role in the artistic aspects of many cultures, colloids were not a subject of concerted scientific study until the middle of the nineteenth century. Much of the early work on colloids was meant to clarify the behavior of large molecules of biological origin, including proteins, starches, latices, oils, and fibrous polymers, including those found in wool, linen, and cotton. Since aqueous solutions were the basis of much of the study of inorganic chemistry, early attempts to study the colloidal molecules were also based on aqueous dispersions. The dispersions of macromolecules in water were recognized to have properties different from solutions of smaller molecules. The dispersions of larger biological molecules were categorized as colloids (from the Greek for glue or flour paste) to distinguish them from solids and simple solutions. Many of the characteristics of these macromolecular solutions were similar to those of dispersions of finely divided metals, another subject of the early studies of colloids.

In the middle of the nineteenth century, Thomas Graham studied the characteristics of molecules in solution. Among the molecules that he examined were some large biological molecules, including albumin (the proteins in egg white). Their behavior in solution was very similar to the behavior for silver chloride particles in suspension observed by Francesco Selmi a decade earlier. About the same time, Willibald G. Schmitt, and later Graham, observed that colloids do not pass through biological membranes in the same manner as smaller molecules.

The sorting by size through a thin, porous membrane was developed into the process of dialysis, which allowed partial purification of colloidal materials.

Further characterization and purification of colloidal materials was possible using the charge characteristics of the colloidal particles. Different colloidal species migrated in electric fields with rates that reflected their shape and surface charge density. These processes have been elaborated in the field of electrophoresis. By studying the migration of colloidal particles in well-defined electric fields, it is now possible to infer the molecular weights of the migrating particles, their charge densities and distributions, and the shape of their molecules by manipulating the medium in which the species is migrating. These manipulations include varying the specific pore sizes of the substrates (which may themselves be colloidal), adding chemicals to denature the migrating species, and altering the states of hydration of both the substrate and the migrating species.

Coincident with the developments in colloidal chemistry, which allowed the characterization of macromolecules, aggregates, and micelles, cell physiologists began to focus much of their study on the physical behavior of the cell body. Much of the physiology of cells results from the chemistry of proteins and polysaccharides, which are the major structural materials in the cytoplasm and the cell walls. It was also observed that much of the behavior of lipids in the cytoplasm and cell membrane is related to the formation of aggregates and micelles.

Advances in the colloid chemistry of biological systems have clarified the nature and function of macromolecules within the cell.

Physical chemists will continue their investigations into the chemical and physical behavior of large molecules. Applications of colloid chemistry in paint manufacturing, food processing, ceramics production, printing, geology, and photography will continue to be important, but applications of colloid chemistry in medicine, catalysis, plastics synthesis, information transfer, environmental chemistry, and computer technology are most likely to change the way in which humans live.

Principal terms

CHARGE: a property of an object that accounts for certain types of attractions and repulsions; charges are of two types, positive and negative, and opposite charges attract; within molecules, charges are derived from electrons (negatively charged) and protons (positively charged)

DISPERSION: the scattering of colloidal particles in a suspending medium; emulsions (liquid in a liquid), aerosols (liquid in a gas), and gels (solid in a liquid or gas) are examples of colloidal dispersions

FLOCCULATION: the formation of a precipitate through the aggregation of particles to form loose masses

HYDRATION: the formation of a unit including water and some other material; when bonded to the water, the material is said to be hydrated

MACROMOLECULE: a molecule of high molecular weight, especially those biological molecules synthesized by the joining together of smaller molecules

MICELLE: a colloidal particle formed by the aggregation of smaller dissolved molecules; through the formation of the micelle, the physical and chemical activities of the smaller molecule are modified

MOLECULE: an aggregation of atoms held together by forces strong enough that the aggregate has specific chemical characteristics that describe the unit

NANOMETER: one-billionth of a meter

Bibliography

Dickinson, Eric, ed. FOOD EMULSIONS AND FOAMS. London: Royal Society of Chemistry, 1987. Symposium proceedings with parts that are easily understood. Difficult language and formulas are presented in some sections. Describes the role of colloid chemistry in food processing. Illustrated, with line drawings, photographs, graphs, and tables.

Eicke, Hans-Friedrich, ed. MODERN TRENDS OF COLLOID SCIENCE IN CHEMISTRY AND BIOLOGY. Boston: Birkhauser-Verlag, 1985. A text by twenty-nine authors for college-level students contains difficult and technical language, including scientific formulas on occasion. Describes most features of colloid chemistry. The introductory chapter, "Birth, Life, and Death of Colloids," is an especially readable summary of the field. Illustrated, with graphs and tables.

Heimenz, Paul C. PRINCIPLES OF COLLOID AND SURFACE CHEMISTRY. New York: Marcel Dekker, 1977. This text is for advanced students with a background in physical chemistry, but it contains sections that are easy to understand. Meant to bridge the gap between courses in physical chemistry and the research literature in colloid chemistry. Illustrated, with photographs, line drawings, graphs, and tables. A particularly useful feature are the questions at the end of each section reviewing the reader's understanding.

Mittal, K. L., ed. SURFACE AND COLLOID SCIENCE IN COMPUTER TECHNOLOGY. New York: Plenum Press, 1987. Contains technical language and formulas, but some sections are nontechnical. This book describes ways in which colloid chemistry is used in the modern computer industry, including film preparation, microelectronics applications, and printing technology. Illustrated, with photographs, line drawings, graphs, and tables.

Svedberg, Thedor. COLLOID CHEMISTRY. New York: American Chemical Society Monograph Series, 1928. A text bridging between the nineteenth and twentieth century studies of colloids. Svedberg played an important role in the development of centrifugation techniques. Illustrated and contains some technical language.

Van Olphen, Hendrik. CLAY COLLOID CHEMISTRY. New York: Interscience, 1963. A text for advanced students with some sections that are more easily understood. Describes applications of colloid chemistry to clay technology, including manufacturing and drilling processes. Illustrated, with photographs, line drawings, graphs, and tables.

Van Olphen, Hendrik, and Karol J. Mysels, eds. PHYSICAL CHEMISTRY: ENRICHING TOPICS FROM COLLOID AND SURFACE CHEMISTRY. La Jolla, Calif.: Theorex, 1975. A text for advanced students and teachers of chemistry, but also includes nontechnical sections. Outlines ways in which many of the principles of chemistry can be illustrated using the unique feature of colloid chemistry. Illustrated with both graphs and tables.

Vold, Marjorie J., and Robert D. Vold. COLLOID CHEMISTRY: THE SCIENCE OF LARGE MOLECULES, SMALL PARTICLES, AND SURFACES. New York: Van Nostrand Reinhold, 1964. A text for beginning students of colloid chemistry which is easily understood. This volume is not very mathematical in its approach, but it does include some formulas. Chapter 6, "A Few Noteworthy Colloids," is especially interesting. Illustrated, with line drawings, photographs, graphs, and tables.

Colloids: categories, dispersion media, and disperse phases

The Formation of Aggregates

The Chemistry of Photography