  When a gas or liquid is brought into contact with a solid, and they are taken into the solid to form a solution, it's called absorption. Molecules can also become attached the outside of the solid. This is called adsorption.
Adsorption is a process with which you are already familiar. When you use an activated charcoal filter on your water faucet to make your drinking water taste better, you are using adsorption. Adsorption uses a solid to remove particles from a liquid or gas that passes across it. The particles stick to its surface. Adsorption has been in use for thousands of years ... its first application was probably the use of bone ashes to remove colour from syrups or alcohol. Today, adsorbant materials are widely used; for example, drinking water at water treatment plants is passed through activated charcoal to purify it. This page will explain how adsorption works, and provide some more examples of how it is used. All compounds are adsorbant to some extent. In order for a solid to be a good adsorbant, it must have a surface composed of molecules which provide a good attractive force. When molecules in the gas or liquid pass near the surface of the solid, they stick. This can happen in one of two ways: 
1: Physisorption (van der Waals and electrostatic forces)This is the most common form of adsorption. The molecules are attracted by van der Waals forces, and attach themselves to the surface of the solid. The molecules remain intact, and can be freed easily (forces are small, and short-range). 2: Chemisorption (by chemical bonding) The molecules undergo a chemical bonding with the molecules of the solid, and this attraction may be stronger than the forces holding the solid together. If the molecules are removed, they may form different compounds. Another feature of a good adsorbant is a large surface area. The bigger its surface area, the more molecules it can trap on its surface. Generally this means that a good adsorbant will either be composed of very tiny particles, or be very porous ... full of many tiny little holes on its surface that effectively increase its surface area by many many times times. Adsorbents can also be both. Activated charcoal, for example, can have a total surface area of up to 2000 square metres per gram!  On the left is an electron microscope image of a piece of activated charcoal, which is the most commonly used adsorbant. You can see that it is full of holes, which makes its total surface area very large. There is lots of surface to hold molecules, which remain in place by van der Waals forces (physisorption).Charcoal is made by heating wood without oxygen. This removes the impurities, but leaves all the carbon in the wood mostly unburnt. About 25% of the original volume remains, and it's mostly carbon. Charcoal can then be burned itself, and very little smoke will result, because mostly it's just carbon combining with oxygen to produce carbon dioxide and carbon monoxide (invisible gases). Activated charcoal is charcoal made from wood, coconut shell, or bone, which has been treated with oxygen to remove all the impurities sticking to its surface inside the pores. It's ready to adsorb more! While activated charcoal is the most common adsorbant, there are many others. Alumina (aluminum oxide and hydroxide) and silicagel (a compound containing silicon dioxide) are commonly used to remove water vapour from air. Silicagel is what's in those little packets stuffed inside a new piece of electronic equipment. Chemicals like these will readily adsorb (by chemisorption) water molecules; see our experiment with diaper chemicals.  A complex molecule called zeolite, a crystalline solid containing silicon, aluminium and oxygen atoms, (see picture at right) is also commonly used by industries, such as in petrochemical cracking, ion-exchange (water softening and purification), and in the separation and removal of gases and solvents. The zeolite molecule contains weakly attached smaller molecules, which it will exchange (by chemisorption) for others in the fluid or gas that passes through it, such as the calcium and magnesium ions in 'hard' water. Many washing detergents also contain zeolites, which are used to trap impurities in your dirty clothing. Commercial waste water containing heavy metals, and nuclear effluents containing radioactive isotopes, can also be cleaned up using such zeolites. Metals such as copper and aluminum could also be considered chemisorptive adsorbants, since they collect molecules like sulphur and oxygen from the air, and bind them to their surfaces by chemical reactions. (This results in the greenish compound that collects on the surface of copper roofs, like on the Canadian Parliament buildings, and the whitish surface of aluminum, which is aluminum oxide.)  Just about anything suspended or dissolved in a fluid can be removed by adsorption, including molecules that give the liquid colour or a bad taste. Cooking oils, for example, are passed through an adsorbant, to remove their strong colour. Activated charcoal is commonly used to remove odour molecules from air and water, and bad taste molecules from water. When used to purify water, a filter is then needed to remove traces of charcoal from the water. Activated charcoal is also used in gas masks. Adsorptive materials can also be used to control what reactants are produced in a chemical reaction. In this case, the adsorbant is a catalyst, since it is unaffected by the gasses passing over it. This is what a catalytic converter on an automobile does ... it binds some of the pollutant chemicals in the exhaust gases to its surface. Adsorbant molecules can be designed and built to trap certain chemicals. Polar substances (whose molecules have a north and south pole), such as water or alcohols, are most easily trapped by polar adsorbents such as zeolites, alumina, or silica gel. On the other hand, nonpolar adsorbents, like charcoal and some polymers, have more affinity with organic compounds, oil or hydrocarbons. |
