Surface chemistry: Elementary concepts of adsorption (excluding adsorption isotherms); Colloids: types, methods of preparation and general properties; Elementary ideas of emulsions, surfactants and micelles (only definitions and examples).
Elementary concepts of adsorption
The term adsorption implies the presence of excess concentration of any particular component in one of the three phases of matter (known as adsorbate) at the surface of liquid or solid phase (known as adsorbent) as compared to that present in the bulk of the material.
On the basis of the forces of attraction between adsorbent and adsorbate, two types of adsorption, namely, physisorption (i.e. physical adsorption) and chemisorption, may be identified.
The characteristics of physisorption are;
1. the forces of attraction are of van der waals type (weak forces).
2. Predominates at low temperature.
3. All gases show this adsorption at low temperatures.
4. Heat of adsorption is low, about 40 kJ/mol.
5. Reversible in nature.
6. Low activation energy (appx. 5 kJ)
7. Adsorption is multilayer.
The characteristics of chemisorption are:
1. the forces of attraction are of a chemical nature (strong forces)
2. Predominates at high temperature.
3. This is highly specific in nature.
4. Heat of adsorption is large 9appx. 80 to 420 kJ/mol)
5. Usually irreversible.
6. Large activation energy
7. Adsorption is monolayer.
The extent of adsorption of gases increases with increase in the pressure of the gases and it decreases with increase in temperature of the gas.
Colloids or sols are the substances whose sizes lie in between the solutes present in a true solution (e.g., salt, sugar) and the solutes present in suspension (e.g., sand). It can also be explained that colloids are intermediate between solutions and precipitates. The particles in colloids are larger than the molecules or ions that make up solutions. The diameters of colloidal particles may range from 1 to 100 nm. The particles in colloidal state do not settle down on standing, are not visible and they can pass through a filter paper. However, they do not pass through a perchment paper or animal membrane.
In the case colloids, we use the terms dispersed particles and dispersing medium. Dispersed particles are the colloidal particles. Milk is an example of a colloid; butterfat constitutes the dispersed particles and water is the dispersing medium.
Types of colloids
Colloids may consists of dispersed particles of various phases and dispersing mediums of various phases. These different types of colloids have different names.
Solid sol Coloured gems and glasses, some alloys, minerals
Sol Starch or proteins in water, paints, gold sol
Solid aerosol Smoke, dust, storm
Gel Jellies, cheese, butter, boot polish
Emulsion Emulsified oils, milk, cod liver oil, medicines
Liquid aerosol Mist, fog, cloud, insecticide sprays
Solid foam Styrene foam, rubber, occluded gases
Foam or froth Whipped cream, lemonade, froth, soap suds
Solid sol is a solid dispersed in a solid.
Sol is a solid dispersed in a liquid.
Solid aerosol is a solid dispersed in a gas.
Gel is a liquid dispersed in a solid
Emulsion is a liquid dispersed in a liquid.
Liquid aerosol is a liquid dispersed in a gas.
Solid foam is a gas dispersed in a solid.
A foam is a gas dispersed in a liquid.
The colloid (dispersing particles + medium) is a two phase system.
Properties of Colloids
Besides particle size, colloids have other identifying properties. Properties peculiar to colloids are (1) optical effects, (2) motion effect, and (3) electrical charge effect.
When a relatively narrow beam of light is passed through a colloid, such dust particles in the air, the light is scattered by the dust particles and they appear in the beam a bright, tiny specks of light. Many of us have seen this phenomenon.
The scattering of light in a colloid is due to the reflection of the light by the large colloidal particles producing a visible beam of light. This optical effect is named the Tyndall effect after John Tyndall, who investigated it in 1860.
If a colloid is viewed with a special microscope, dispersed colloidal particles appear to move in a zigzag, random motion. This erratic random motion is due to bombardment of the dispersed colloidal particles by the medium and this constant bombardment keeps the dispersed colloidal particles suspended indefinitely. They will not settle down. This movement of colloidal particles is called Brownian movement, and this could be observed because of the Tyndall effect.
Electrical charge effect
Very often a colloid will have ions from the dispersing medium adsorbed on its surface. This along with the Brownian motion of the colloid, prevents colloids from coagulating and precipitating.
To facilitate precipitation, an electrolyte of some type is added to the solution, which will neutralize the charge. For instance, in the case of qualitative analysis, if charge on colloids is interfering with the precipitation, an electrolyte of some type is added. This property is used in removing suspended particles from the effluent gases in industrial smokestacks., In the gas coming of out various industrial equipment is given an electrical charge. Before this exhaust gas goes into atmosphere, it is passed through charged plates. These charged plates attract the charged colloidal particles, which then are held on the plates till the current giving the plates their charge is shut off. Then the particles fall to the ground and can be collected. The response of colloid particles to electrical charge is electrical phoresis.
Emulsions are sols of liquid in liquid. Two types of emulsions may be distinguished, namely, oil-in-water and water-in-oil. To make emulsions stable, emulsifying agents such as soaps and detergents are added.
Any substance which can decrease the surface tension of water to a large extent is known as surfactant. Examples of soap and detergents. Such substances have larger concentrations at the surface of water as compared to the bulk of the solution.
Definition of micelle
Surfactants in solution are often association colloids, that is, they tend to form aggregates of colloidal dimensions, which exist in equilibrium with the molecules or ions from which they are formed. Such aggregates are termed micelles. (IUPAC MANUAL APPENDIX II (1972)).
What is colloidal micelle?
The heart of this new chemistry is the technology used to develop a "colloidal micelle." Sub-microscopic particles are created in a microscopic field similar to a magnetic field. It differs from traditional chemistry in that the molecular attraction is not the usual attraction between positive and negative poles. Rather, it is between like poles.
An analogy would be that negative attracts negative and positive attracts positive. The micelle has a hydrophilic (water seeking) pole and a hydrophobic (water repelling) pole. The hydrophobic poles attract each other, thus forming the interior of the micelle. The hydrophilic poles form a tough outer surface.
When a micelle comes in contact with a hydrocarbon molecule, the center of the micelle bonds to a similar hydrophobic hydrocarbon. It disrupts the attraction to other hydrocarbon molecules and/or to the surface. Detergent particles in water form particles with hydrophobic poles and hydrophilic poles. In the case of clothes, the hydrophobic pole of a detergent particle attracts the grease and dirt particles on the clothes and pull them apart. As such micelles come together, an outlayer or hydrophilic poles forms. These associative colloid particles are then washed away by water and leaving clothes clean.
The action of a single micelle is multiplied by billions of other micelles. The molecular level emulsification process penetrates highly viscous and sticky materials, lifting them from the surface to which they adhere.
Considering the damage inflicted on life and the planet by harsh cleaners and solvents, colloidal chemistry is an exceptionally advance towards environmental preservation.
Level I lecgtures are meant to explain the basic concepts of the topic. More advanced treatment of the topic will be attempted in level II and level III lectures.