Adsorption

Type of physical science: Condensed matter physics

Field of study: Surfaces

Except under conditions of ultrahigh vacuum, all interfaces between immiscible phases are covered with foreign matter. This material may be one or several molecular layers thick and may be strongly or weakly attached. Material held at interfacial surfaces such as this is said to be adsorbed at the surface.

Overview

On an atomic level, a surface of a liquid or solid phase may be viewed as being incomplete. Within the phase, the atoms are surrounded on all sides by other atoms of that material and are held to those other atoms by strong forces. At the surface, there is a lack of symmetry. The bonding potential of the surface atoms is fully satisfied only in the direction of the condensed phase. The excess bonding potential of the surface atoms results in the surface attracting matter from its surroundings and forming bonds to those materials. This is known as the process of adsorption.

The effect does not stop at this first layer of adsorbed material but can continue through several additional layers. In the case of adsorption at liquid interfaces, normal molecular motion keeps the layers somewhat indistinct. With solid surfaces, the lack of mobility of the solid particles gives greater structure to the adsorbed layers. This high degree of structure has made the investigation of these surfaces more amenable to study; more detail is known about adsorption on solid surfaces than at liquid surfaces.

The strength of the forces causing the attachment of adsorbed species, however, becomes lower with successive layers. It is the strength of this attaching force that is used to distinguish two types of adsorption. When the adsorbed material is strongly held to the surface, the force is comparable to the force of an ordinary chemical bond; this process is called "chemisorption." When the force is weaker, similar to that which is found holding molecules to one another in a liquid and which are overcome during evaporation, the process is known as "physical adsorption."

Chemical adsorption, chemisorption, is limited to the first layer of adsorbed material.

Once this monolayer coverage is attained, the additional layers become attached through physical processes. The laying down of these multilayers is likened to the condensation of a vapor into the liquid state. The formation of the first layer has received much study because the formation of this layer involves the greatest changes in the materials involved. Both the adsorbed species and the surface undergo changes during adsorption that modify their chemical behavior.

The adsorbed species may be either atoms or molecules and can be present either in the gas that is in contact with the condensed phase or in a liquid that contacts that phase. That is, adsorption can take place across a gas-solid interface, across a liquid-liquid interface, or across a liquid-solid interface. In each case, the amount of adsorption that occurs depends on the concentration of the adsorbing species present with more being adsorbed at higher concentrations.

Surfaces that appear to be smooth and clean to the eye are, at the molecular level, neither smooth nor clean. Most solids are collections of small crystals imperfectly joined to one another. These crystal boundaries are points of surface irregularities. Even when solids are grown as single crystals, these single crystals have imperfections where the exact orderly array of atoms and molecules has become disordered in the process of the crystal's growth. Either one of these cases results in a surface which, if viewed on a much expanded scale, presents a view of peaks, plateaus, plains, and valleys. These rough surfaces (at the atomic level) are also covered with atoms and molecules of any materials with which the solid has come into contact. A similar circumstance prevails at the boundary of two immiscible liquids or a liquid and a vapor. Some of the adsorbed material is strongly attached and some is only weakly held. The strength of the attachment depends on the chemical character of both the surface and the adsorbate.

For solids, clean surfaces can be prepared by heating the solid in an ultrahigh vacuum.

The heat provides energy to remove the adsorbate, and the low pressure decreases the opportunity for the removed material to become reattached. Pressures needed in the preparation of clean surfaces are on the order of 1 nanotorr, one one-thousandth of a billionth of the pressure of the atmosphere at sea level. Clean liquid surfaces are essentially not possible to produce.

Starting with a clean surface, the process of adsorption can be followed by focusing on the behavior of a passing molecule. Molecules are in constant, highly random motion so that sooner or later the molecule will collide with the surface. Depending on the energy present in the collision, the molecule may rebound from the surface or it may become somewhat attached.

These slightly attached molecules are able to migrate across the surface until they encounter a position of stability, at which point they become strongly attached to the surface. The more mobile molecules are physisorbed and the strongly attached molecules are chemisorbed. The chemisorbed molecules may continue to migrate across the surface, and the process continues until the surface is completely covered with one layer of adsorbate. Once the monolayer coverage is attained, other molecules from the fluid phase collide with this newly covered surface and may become physically adsorbed. Multiple layers are built up in this manner if the concentration of the adsorbate in the fluid phase is high enough. At low concentrations, the coverage often stops at a monolayer.

The chemisorbed molecule may not remain intact as it forms its attachment to the surface. It may be torn apart by the unsatisfied surface forces during the process of attaching to the surface. In this manner, what may be considered to be a surface compound is formed. This compound is a combination of atoms, including those of the surface, that are chemically bonded together. This change is important in one of the major applications of adsorption, catalysis.

The adsorption process is not a one-way process; the reverse, desorption, also occurs.

This reverse process was encountered in the discussion of making a clean surface. Desorption can be thought of as a reversal of the steps that occurred when the molecules were adsorbed.

Desorption happens when the adsorbate gains energy, possibly from heating, and breaks away from the surface attachment. The molecules that are physically adsorbed require relatively small amounts of energy to effect this removal. Chemically adsorbed molecules must have energy added that is in the range of energies needed to break chemical bonds within molecules. These energies can be measured and are one of the most important factors in understanding the difference between chemical and physical adsorption.

Applications

The uses of adsorption, excluding its use in providing information about surfaces, can be divided into three general categories: First, adsorption is used to remove unwanted molecules from a fluid phase, tie them up on the surface of a solid, and remove them by physically removing the solid from the fluid. Second, adsorption is used to attach molecules to a surface so that the molecules are held in place in order for other reactions involving them to take place. This is the phenomenon known as heterogeneous catalysis. Third, adsorption of molecules in a surface layer can modify both the physical and the chemical nature of the surface in ways that are useful.

In all adsorption processes, the importance of having a large amount of surface area available is clear. Finely powdered materials, small droplets, or bubbles have extremely large amounts of surface area compared with an equal mass of the material occurring in larger pieces.

This can be demonstrated by considering a cube of material 1 meter on each side. The amount of surface present is 6 square meters (six sides each 1 meter square). If that cube is cut into cubes that are each 1 centimeter on a side, there will be 1 million of them and each will have a surface area of 6 square centimeters. Totaling the area and converting from centimeters to meters yields 600 square meters. One can readily imagine the great increase in surface area that comes with even smaller division.

The first category of applications is familiar in many everyday situations. The purpose of placing open containers of baking soda or pieces of charcoal in freezers and refrigerators is odor control. The process occurs as the molecules responsible for the odors, as well as many other inoffensive molecules, become adsorbed on the surface of the powder and remain bound there until the powder is removed and disposed. Many products are moisture-sensitive, such as some pharmaceuticals and many optical instruments. In these cases, water vapor is the unwanted material. When these products are packaged, packets of solids are included, whose purpose is to adsorb water vapor from the surrounding air. The same, or similar materials, are used by hobbyists in the process of drying flowers for long-lasting floral arrangements. An industrially important application of this type is found in the processing of sugar cane. Sugar is dissolved from the plant material, yielding a highly concentrated sugar solution. This solution, however, contains impurities that impart color to solutions and that would color the crystalline sugar product. Charcoal is mixed with the solution and adsorbs the color producing impurities. The charcoal is then filtered out and clear, colorless sugar can be crystallized from the solution. The whole field of chromatographic separation of chemical components of mixtures, important to the research scientist and industrially, relies on adsorption.

Catalysis is an extremely important phenomenon, both industrially and biologically.

Many important chemical reactions are too slow to be of use at room temperature. These reactions can be sped up by applying heat, but often the chemicals are not stable enough to withstand the higher temperature and they decompose. An alternate mode of increasing the speed of chemical reactions is the use of catalysts. Catalysts have the ability to increase the rates of chemical reactions by factors of several thousand. Biological catalysts, called enzymes, are extremely effective and cause rate increases by factors of several billion. Enzymes are large biochemical molecules that adsorb smaller molecules at certain active sites on the enzyme surface. Catalysts that are in a different phase from the reacting chemicals are called heterogeneous catalysts. In each case of catalysis, the adsorbate molecule's bonds are altered through the adsorptive process that makes catalysts effective. The bonds that are changed by the surface forces make the adsorbed molecule more reactive than its precursor. Most people's closest contact with nonbiological, heterogeneous catalysis is the catalytic converter on automobiles. The purpose of these devices is to reduce the amount of unburned hydrocarbons that reach the atmosphere. The active part of these converters is a tiny amount of platinum metal dispersed on the surface of a honeycomb-like ceramic support. The surfaces of these platinum particles are the locations at which the reactions occur that convert partially burned fuel into carbon dioxide and water. The hydrocarbon fuel molecules are adsorbed on the platinum and are held there to react with oxygen from the air in the fuel mixture. This reaction itself produces heat, which further increases the reaction rate.

Oil, grease, and detergents may seem far apart and with nothing in common; however, the way that each of these accomplishes its purposes depends on adsorption. A component of each of these formulations is a substance in the class of materials known as surface active agents, or surfactants. This component is readily adsorbed at the surface and changes the character of that surface. One of the functions of oil and grease is to protect the integrity of surfaces that are in contact with each other by reducing friction. The mechanism through which this is accomplished is the adsorption of molecules present in the lubricant to form a monolayer on the surface of the metal. In this manner, a protective boundary layer is provided that prevents the metal surfaces themselves from coming into contact.

Detergents, in a similar fashion, form monolayers on particle surfaces and around oily droplets in a way that makes the combination hydrophylic, or compatible, with water. Materials that are compatible in this way are said to be "wet by water" as opposed to those hydrophobic materials that push water away. Materials that are wet by water can be kept in suspension in the water and floated away, which is the role of detergents in the washing process. An industrial application of the use of detergents is found in oil production, where detergents are pumped into the oil bearing rock strata of a well. The surfactants adsorb on the surface of oil droplets, increasing the mobility of the oil and thus allowing more oil to be produced from the well than would otherwise be possible. The process can greatly increase the percentage of oil that is recovered from the formation.

Context

Application of adsorption preceded, by a long time, the understanding of the process.

Understanding adsorption at a molecular level started to develop in the early 1900's and still commands research and study from points of view founded in chemistry, physics, biology, and several engineering disciplines. The role of adsorption in many phenomena is still being recognized and developed.

It is only with difficulty that a starting point for the development of an understanding of adsorption can be selected. Irving Langmuir was awarded the 1932 Nobel Prize in Chemistry for his work on surface chemistry. Langmuir worked in many fields of chemistry, but his largest single contribution was in the understanding of surface processes. He first evoked the concept of adsorption and used it in characterizing the catalytic effect of heterogeneous materials on reactions. Many of the experimental techniques developed by Langmuir are still in use but are somewhat modified to utilize new instrumentation.

Surfaces constitute only a very small fraction of the whole of materials, and it has required the development of sensitive measuring techniques to be able to distinguish their role from the role played by the bulk of the material. Therefore, the importance of surface effects on both chemical and physical processes was overlooked for many years. Once the door to understanding these effects was cracked open, however, advances occurred very rapidly.

Segments of industrial applications began to take on the aspect of science rather than art as more fundamental understanding of surface properties developed. This happened in industries manufacturing or relying on the use of catalysts, in industries concerned with lubrication, and in industries whose purposes called for blending together materials in phases that were normally immiscible. This later segment of manufacturing includes products ranging from coatings and cosmetics to foods and fire-fighting fluids.

With the continued refinement of experimental methods, knowledge of surface effects and the mechanism of their operation has expanded. Advances have been made that have carried scientists from the point of having only very inferential evidence about surfaces and their effects to the point of having more direct evidence of these factors. As these methods continue to be developed, the importance of adsorption to many processes will continue to expand.

Principal terms

HYDROPHILIC: attracted to and attracting water

HYDROPHOBIC: excluded from water or "water hating"

IMMISCIBLE: characterized by phases that differ significantly in chemical properties, which will not mix together when brought into contact but will separate into their own regions

MONOLAYER: an adsorbed layer of molecules covering the surface of a solid to a thickness of one molecule

PHASE: a region that has the same chemical and physical properties throughout; for example, an oil-water mixture separates into two phases

SURFACTANT: a molecule that has a greater affinity to the surface region in a solution than to the bulk of the solution

Bibliography

Farber, Eduard, ed. MILESTONES OF MODERN CHEMISTRY: ORIGINAL REPORTS OF DISCOVERIES. New York: Basic Books, 1966. Chapter 16 reprints and comments on Langmuir's 1915 paper, originally presented at the New York section of the American Chemical Society. This article, written in fairly nontechnical language, provides a look at the experimentation and the thinking that went into some of the very early understanding of adsorption and its importance to catalysis.

Gushee, David E., ed. CHEMISTRY AND PHYSICS OF INTERFACES. Washington, D.C.: Chemical Society, 1965. A collection of papers resulting from a symposium that were originally published in INDUSTRIAL AND ENGINEERING CHEMISTRY between September, 1964, and September, 1965.

Ruthven, Douglas. PRINCIPLES OF ADSORPTION AND ADSORPTION PROCESSES. New York: John Wiley & Sons, 1984. Although technical, the discussion of the range of applications of surface science will make it of interest. The general reader will be able to understand the role played by adsorption in the applications even without understanding the details.

Shaw, Duncan J. INTRODUCTION TO COLLOID AND SURFACE CHEMISTRY. Boston: Butterworths, 1980. This small book was written to fill a gap between the detailed treatment of surface science found in some texts and the overly general treatment found in others. While there is a fair amount of mathematics used, it is not essential to the understanding of the material presented. The liberal use of hand-drawn figures and the inclusion of some electron micrographs of surfaces is particularly helpful.

Vold, Robert D., and Marjorie J. Vold. COLLOID AND INTERFACE CHEMISTRY. Reading, Mass.: Addison-Wesley, 1983. A text for students needing a detailed knowledge of this branch of science. Nevertheless, the book contains much material that can be read and understood by an interested reader. In particular, chapter 5, "Some Applications of Surface Science;" should be of interest.

Absorption vs. adsorption

Liquefaction of Gases

The Atomic Structure of Liquids