Colloid chemistry chapter 1

COLLOID CHEMISTRY Chapter - Introduction Dr Ngo Thanh An Concepts 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 Colloidal state: the state which consists of particles of the size nanometer to micrometer These "colloidal particles" are dispersed in a continuous medium Concepts Colloidal solution: intermediate of 'true solutions' and 'suspensions' True solutions are homogeneous system having the size of dispersed particles less than nm Suspensions are heterogeneous system and have particles which are more than 1000 nm in size Continuous phase and Dispersed phase to refer to the medium and to the particles, respectively, in the colloidal size range Interfacial phase: The properties of the molecules forming the interface are different from those in the bulk that these molecules are forming an interfacial phase Monodisperse – all particles are the same size Polydisperse – the particles are of many different sizes Concepts • 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 – liquid 2; solid – solid What is a dispersion? Concepts A dispersion is a collection of small particles in a matrix It has at least two phases: Dispersed phase Continuous medium What is a stable dispersion? A stable dispersion is one in which the dispersed particles remain single entities and not coagulate Coagulation leads to phase splitting and a complete change in the properties of the system In most situations we not want coagulation to occur but there in certain situations it might be beneficial For example, during purification of water, we want coagulation to occur so that all the impurities come together and can be disposed of easily Concepts Types of stabilities Thermodynamic stability (Of Theoretical Importance) 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 not coagulate for a very long period of time, it is, for all intents and purposes, a stable dispersion, and is called kinetically stable Concepts Place of colloid science Subject of colloid chemistry Classification of colloids 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 Lyophobic Colloids (solvent-fearing colloid): Lyophobic colloids are known by a variety of terms, depending on the nature of the phases involved Applications of colloids Applications of colloids Applications of colloids Applications of colloids 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 Applications of colloids 4) Cleansing action of soap: Soap solution is colloidal in nature It removes the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth 5) Formation of Delta: River water contains charged colloidal particles of clay, sand and many other materials Sea water is very big store house of a variety of electrolytes dissolved in it As soon as river water comes in contact with sea water coagulate the suspended colloidal particles which ultimately settle down at the point of contact Thus the level of the river bed rises As a result, water adopts a different course and delta is formed in due course of time Applications of colloids 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