Colloid chemistry - Chapter 1: Introduction - Ngo Thanh An

Nội dung text: Colloid chemistry - Chapter 1: Introduction - Ngo Thanh An

1. The term colloid was developed by Graham in 1861, from the Greek meaning glue-like. Graham defined a colloid in terms of its inability to pass through a fine membrane. Traditionally, the colloidal domain is defined as extending over a range of dimensions from a few nanometers to a few tens of micrometres.
2. • Surface is the term used when considering the dividing phase between: gas – liquid; gas – solid. • Interface is the term used when considering the dividing phase between: solid – liquid; liquid 1 – liquid 2; solid 1 – solid 2.
3. Types of stabilities 1. Thermodynamic stability (Of Theoretical Importance) 2. Kinetic stability (Of Practical Importance) A system is called thermodynamically unstable when there exists a state where the system will have lower energy than it currently has. A dispersion can be thermodynamically unstable, but can still be kinetically stable. If in a dispersion (even if it is thermodynamically unstable) the particles do not coagulate for a very long period of time, it is, for all intents and purposes, a stable dispersion, and is called kinetically stable.
4. 1. Lyophilic Colloids (solvent-loving colloid): The classical use of the term lyophilic colloids refers to soluble macromolecular materials in which the individual particles (macromolecules such as synthetic polymer chains or proteins) are of colloidal dimensions. However, there are macromolecules of colloidal dimensions containing both lyophobic and lyophilic components. 2. Lyophobic Colloids (solvent-fearing colloid): Lyophobic colloids are known by a variety of terms, depending on the nature of the phases involved.
5. Colloids play a very significant role in nature and in our daily life. Some of the important applications of colloids are discussed below: 1) Medicines: Medicines in colloidal form are easily adsorbed by the body tissues and hence are more effective. 2) Sewage disposal: Colloidal particles of the dirt, mud etc. carry electric charge, hence when sewage water is passed through the plates kept at a high potential, the colloidal particles are coagulated due to electrophoresis and the suspended matter gets removed. 3) Purification of water: The precipitation of colloidal impurities present in water can be done by adding certain electrolytes like alum etc. the negatively charged colloidal particles of impurities get neutralized by the Al3+ ions and settle down and pure water can be decanted off.
6. 6) Smoke precipitation: Smoke particles are actually electrically charged colloidal particles of carbon in the air. Precipitation of smoke particles is carried out by Cottrell precipitator which is based on the principle of electrophoresis. Smoke is allowed to pass through a chamber having a number of metal plates attached to a metal wire connected to a source of high potential. Charged particles of smoke get attracted by oppositely charge electrode get precipitated after losing their charge and the hot air passes out through the chimney. The dust particles are also removed in this process. Thus the nuisance of smoke in big industrial cities can be avoided. 7) Photography: A colloidal solution of silver bromide in gelatin is applied on glass plates or celluloid films or paper to from sensitive plates in photography.
7. 11) Other applications: a) Blue color of the sky is due to the scattering of light by colloidal dust particles in air (Tyndall effect). Similarly, sea water looks blue due to scattering of light by the colloidal impurities present in sea water. b) Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by the comet in its path. c) Blood is a colloidal solution and the stoppage of bleeding on applying ferric chloride solution is due to coagulation of blood forming a clot.
8. Average diameter
9. Sedimentation
10. Electrophoresis Electrophoresis: motion of particles under influence of electric field