Colloids PDF

COLLOIDS  COLLOIDS □ A colloid is a substance microscopically dispersed throughout another substance. □ The word colloid comes from a Greek word 'kolla', which means glue thus colloidal particles are glue like substances. □ These particles pass through a filter paper but not through a...

COLLOIDS  COLLOIDS □ A colloid is a substance microscopically dispersed throughout another substance. □ The word colloid comes from a Greek word 'kolla', which means glue thus colloidal particles are glue like substances. □ These particles pass through a filter paper but not through a semipermeable membrane. □ Colloids can be made settle by the process of centrifugation. The Colloidal System Consist of two Phase System: A dispersed phase( A discontinuous phase ) A dispersion medium ( A continuous phase) The dispersed-phase particles have a diameter of between approximately 1nm – 100nm. □ Such particles are normally invisible in an optical, though their presence can be confirmed with the use of an ultramicroscope or an electron microscope. SOLUTIONS ❑ Made up of particles or solutes and a solvent ❑ The solvent part of the solution is usually a liquid, but can be a gas. ❑ The particles are atoms, ions, or molecules that are very small in diameter. COLLOIDAL MIXTURE ❑ Has particles that are not as small as a solution and not as large as a suspension. ❑ The particles are intermediate in size. SUSPENSIONS ❑ Made up of particles and a solvent its particles are larger than those found in a solution. ❑ The particles in a suspension can be distributed throughout the suspension evenly by shaking the  Comparison of the Properties of Solutions, Colloids, And Suspensions Property True Solution Colloid Suspension Particle Size Less than 1 nm 1 to 100 nm More than 100 nm Appearance Clear Cloudy Cloudy Homogeneity Homogeneous Homogeneous or Heterogeneous Heterogeneous Transparency Transparent but Often translucent Often opaque but often coloured and opaque but can can be translucent be transparent Separation Does not separate Can be seperated Separates or settles Filterability Passes through Passes through Particles do not filter paper filter paper pass through filter paper Examples of colloids are milk, synthetic polymers, fog,blood, jam, shoe polish, smoke, etc.  CLASSIFICATION OF COLLOIDS ❑ Based of physical state of dispersed phase an dispersion medium. ❑ Based of nature of interaction between dispersed phase and dispersion medium. ❑ Based on molecular size in the dispersed phase. ❑ Based on appearance of colloids. ❑ Based on electric charge on dispersion phase. Based on physical state of dispersed phase and dispersion medium BASED ON NATURE OF INTERACTION BETWEEN DISPERSED PHASE AND DISPERSION MEDIUM LYOPHILIC COLLOIDS □ Colloidal solution in which the dispersed phase has a great affinity for the dispersion medium. □ They are also termed as intrinsic colloids. □ Such substances have tendency to pass into colloidal solution when brought in contact with dispersion medium. □ If the dispersion medium is water, they are called hydrophilic or emulsoids. □ The lyophilic colloids are generally self- stabilized. □ Reversible in nature and are heavily hydrated. □ Example of lyophilic colloids are starch, gelatin, rubber, protein etc.  LYOPHOBIC COLLOIDS □ Colloidal solutions in which the dispersed phase has no affinity to the dispersion medium. □ These are also referred as extrinsic colloids. □ Such substances have no tendency to pass into colloidal solution when brought in contact with dispersion medium. □ The lyophobic colloids are relatively unstable. □ They are irreversible by nature and are stabilized by adding small amount of electrolyte. □ They are poorly hydrated. □ If the dispersion medium is water, the lyophobic colloids are called hyrophobic or suspenoids. □ Examples: sols of metals like Au, Ag, sols of metal hyroxides and sols of metal sulphides.  Based on molecular size in the dispersed phase. □ MULTIMOLECULAR COLLOIDS Individual particles of the dispersed phase consists of aggregates of atoms or small molecules having diameter less than 10-7cm. The particles are held by weak vander waal’s forces. Example; gold sol, sulphur sol □ MACROMOLECULAR COLLOIDS The particles of dispersed phase are sufficiently large in size enough to be of colloidal solution. These are called Natural ASSOCIATED COLLOIDS These colloids behave as normal electrolytes at low concentrations but behave as colloids at higher concentrations. These associated colloids are also referred to as micelles. Sodium stearate (C18H35NaO2)behave as electrolyte in dilute solution but colloid in higher concentrations. Examples: Soaps , higher alkyl sulphonates , polythene oxide.  HOW DOES SOAP WORK? When greasy dirt is mixed with soapy water, the soap molecules arrange themselves into tiny clusters called micelles. The water-loving (hydrophilic) part of the soap molecules sticks to the water and points outwards, forming the outer surface of the micelle. The oil-loving (hydrophobic) parts stick to the oil and trap oil in the center where it can't come into contact with the water. With the oil tucked safely in the center.  Based on appearance of colloids SOLS ▪ When a colloidal solution appears as fluid. ▪ The sols are generally named as dispersion medium. ▪ When the dispersion medium is water, the sol is known as hydrosol or aquosol. ▪ When the dispersion medium is alcohol or benzene it is called alcosol and benzosol respectively. GELS ▪ When a colloidal solution appear as solid. ▪ The rigidity of gel varies from ▪ substance to substance.Examples : jelly, butter, cheese, curd.  BASED ON ELECTRICAL CHARGE ON DISPERSION PHASE □ POSITIVE COLLOIDS When dispersed phase in a colloidal solution carries a positive charge. Examples : Metal hyroxides like Fe(OH)3, Al(OH)2, methylene blue sol etc. □ NEGATIVE COLLOIDS When dispersed phase in a colloidal solution carries a negative charge. Examples : Ag sol, Cu sol  METHOD OF COLLOIDS Mechanical Dispersion Electrical dispersion or Bredig’s Arc Method Peptisation Ultrasonic Irradiation Condensation Method A) Mechanical Dispersion: The substance is first ground to coarse particles. It is then mixed with the dispersion medium to get a suspension.The suspension is then grinded in colloidal mill. □It consists of two metallic discs nearly touching each other and rotating in opposite directions at a very high speed about 7000 revolution per minute. □ The space between the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal size. □Colloidal solutions of black ink, paints, varnishes, (B) By Electrical Dispersion Or Bredig’s Arc Method: □This method is used to prepare sols of platinum, silver, copper or gold. □The metal whose sol is to be prepared is made as two electrodes which immerge in dispersion medium such as water etc. □ The dispersion medium is kept cooled by ice. □An electric arc is struck between the electrodes. □The tremendous heat generated by this method give colloidal solution. The colloidal solution prepared is stabilized by (C) By Peptisation: □ The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte is called peptisation. □ Cause of peptisation is the adsorption of the ions of the electrolyte by the particles of the precipitate. □ The electrolyte used for this purpose is called peptizing agent or stabilizing agent. □ Important peptizing agents are sugar, gum, gelatin and electrolytes.  D) Ultrasonic Irradiation The passage of ultrasonic waves through a dispersion medium produces ulternating regions of cavitation and compression in the medium. The cavities collapse with great force and cause the breakdown of coarse particles dispersed in the liquid.  Condensation Method ▪ In condensation method, the smaller particles of the dispersed phase are aggregated to form larger particles of colloidal dimensions. ▪ Some important condensation methods are described below: a) Solutions Of Substances like mercury and sulphur are prepared by passing their vapours through a coldwater containing a suitable stabilizer such as ammonium salt or citrate. B) By Excessive Cooling: A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in solvent. The molecules of water which can no longer be held in solution, separately combine to form particles of colloidal size. c) By Exchange Of Solvent: Colloidal solution of certain substances such as sulphur, phosphorus which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of water. For example alcoholic solution of sulphur on pouring into water gives milky colloidal solution of sulphur. D) Chemical Methods: Colloids can be prepared by following chemicals methods.. 1)Oxidation: Addition of oxygen and removal of hydrogen is called oxidation. For example: Colloidal solution of sulphur can be prepared by oxidizing an aqueous solution of H2S with a suitable oxidizing agent such as bromine water. H2S+ Br2 → 2HBr + S 2H2S + SO2 → 2H2O + 3S 2)Reduction: Addition of hydrogen and removal of oxygen is called reduction. For example: Gold sol can be obtained by reducing a dilute aqueous solution of gold with stannous chloride. 2AuCl3 + 3SnCl2 → 3SnCl4 + 2Au 3) Hydrolysis: It is the break down of water.Sols of ferric hydroxide and aluminium hydroxide can be prepared by boiling the aqueous solution of the corresponding chlorides. For example. FeCl3 + 3H2S → Fe(OH)3 + 3HCl 4) Double Decompostion The sols of inorganic insoluble salts such as arsenous sulphide, silver halide etc may be prepared by using double decomposition reaction. For example: Arsenous sulphide sol can be prepared by passing H2S gas through a dilute aqueous solution of arsenous oxide. As2O3 + 3H2S → As2S3(OH)3 +3H2O  Purification of Colloids There are three common methods used for purification of colloids: ▪ Dialysis ▪ Electrodialysis ▪ Ultrafiltration  Dialysis □The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane. □It’s principle is that colloidal particles can not passthrough a cellophane membrane while the ions of the electrolyte can pass through it. □The impurities slowly diffused out of the bag leaving behind pure colloidal solution. □ The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag.  Electro Dialysis The process of dialysis is very slow. The process is speeded up by application of electrical potential. □ This is called electro dialysis.  Application of electro dialysis Artificial kidney machine make use of electro dialysis. Ultra filtration ▪Ultra filtration is a process of high pressure filtration through a semi permeable membrane in which colloidal particles are retained while the small sized solutes and the solvent are forced to move across the membrane byhydrostatic pressure forces.  Application of ultra filtration: Ultra filtration is a vital process that takes place in the kidneys. PROPERTIES OF COLLOIDS  PHYSICAL PROPERTIES OF COLLOIDS Heterogeneity: Colloidal solutions consist of two phases-dispersed phase and dispersion medium. Visibility of dispersed particles:The dispersed particles present in them are not visible to the naked eye and they appear homogenous. Filterability: The colloidal particles pass through an ordinary filter paper. However, they can be retained by animal membranes, cellophane membrane and ultrafilters. Stability: Lyophilic sols in general and lyophobic sols in the absence of substantial concentrations of electrolytes are quite stable. Colour: The colour of a colloidal solution depends upon the size of colloidal particles present in it. Larger particles absorb the light of longer wavelength and therefore transmit light of shorter wavelength.  OPTICAL PROPERTIES TYNDALL EFFECT When an intense converging beam of light is passed through a colloidal solution kept in dark, the path of the beam gets illuminated with a bluish light. This phenomenon is called Tyndall effect and the illuminated path is known as Tyndall cone. The Tyndall effect is due to the scattering of light by colloidal particles. Tyndall effect is not exhibited by true solutions. This is because the particles present in a true solution are too small to scatter light. Tyndall effect can be used to distinguish a colloidal solution from a true solution. The phenomenon has also been used to devise an instrument known as ultra microscope. The instrument is used for the detection of the particles of colloidal dimensions.  MECHANICAL PROPERTIES BROWNIAN MOVEMENT The continuous zigzag movement of the colloidal particles in the dispersion medium in a colloidal solution is called Brownian movement. Brownian movement is due to the unequal bombardments of the moving molecules of dispersion medium on colloidal particles. The Brownian movement decreases with an increase in the size of colloidal particle. This is why suspensions do not exhibit this type of movement.  ELECTRICAL PROPERTIES OF COLLOIDS ELECTROPHORESIS The movement of colloidal particles towards a particular electrode under the influence of an electric field. If the colloidal particles carry positive charge, they move towards cathode when subjected to an electric field and vice versa.  ELECTROSMOSIS The movement of dispersion mediumunder the influence of an electric field in the situation when the movement of colloidal particles is prevented with the help of a suitable membrane. During electrosmosis, colloidal particles are checked and it is the dispersion medium that moves towards the oppositely charged electrode.  Flocculation value: The coagulating power of an electrolyte is usually expressed in terms of its flocculation value which may be defined as the minimum concentration (in millimoles per litre) of an electrolyte required to cause the coagulation of a sol. A smaller flocculation value indicates the greater coagulating power of the electrolyte. Thus, Coagulating power α 1 Flocculation value The coagulation of colloidal solution can also be achieved by any of the following methods. By electrophoresis By mixing two oppositely sols Bypersistent dialysis  Biological Significance Of Colloidal Systems Protoplasm is a Colloidal system: Protoplasm is colloidal in nature. Amoeboid movements: Amoeboidmovements in amoeba occur  Fruits: Turns store a large amount of proteins and starch. They also exhibit colloidal properties. Blood: High plasma protein forms a colloidal system in blood. This system maintains the pH and osmotic concentration of blood.  ADVANTAGES OF COLLOIDS □Colloids allow the dispersion of normally insoluble materials, such as metallic gold or fats. These can then be used more easily, or absorbed more easily. □Colloidal gold, for example, can be used in medicine to carry drugs and antibiotics, because it is highly non-reactive and non-toxic. □Pharmaceutical industry makes use of colloidal solution preparation in many medicines. A wide variety of medicines are emulsions. An example is Cod Liver Oil. □Paint industry also uses colloids in the preparation of paints. □In milk, the colloidal suspension of the fats prevents the milk from being thick, and allows for easy absorption of the nutrients. □Sewage water contains particles of dirt, mud etc. which are colloidal in nature and carry some electrical charge. These particles may be removed by using the phenomenon of electrophoresis. □The sky is the empty space around earth and as such has no colour. It appears blue due to the scattering of light by the colloidal dust particles present in air (Tyndall effect). □Asphalt emulsified in water and is used for building roads. □The sugar present in milk produces lactic acid on fermentation. Ions produced by acid, destroy the charge on the colloidal particles present in milk, which then coagulate and separate as curd. □Soap solution is colloidal in nature. It removes the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth. □ Large numbers of particles which we use food in our daily life are colloidal in nature. Example: Milk, butter, & ice cream etc