Colloidal State C455 2024-2025 PDF

Damanhour University Faculty of Science Chemistry Department Colloidal state 2024-2025 Colloidal state 24-25 Course description Course Name Colloidal state Course Code Learning Outcomes Describes the colloidal system...

Damanhour University Faculty of Science Chemistry Department Colloidal state 2024-2025 Colloidal state 24-25 Course description Course Name Colloidal state Course Code Learning Outcomes Describes the colloidal system Classifies colloidal systems Expresses the preparation and purification techniques of colloidal solution Expresses the different properties of colloids Describes micelle and emulsion Classifies emulsions Describes the gel and their properties Evaluates the electrical properties of colloids Describes electric double layer and zeta potential Expresses colloidal stability Assessment Quantity Percentage (%) Oral test 1 10 Midterm Exam 1 10 Homework 1 10 Final Exam 1 70 Total (%) 100 1 Colloidal state 24-25 Course plan Week 1 Types of solutions, The colloidal state, Phases of colloids. Week 2 Classification of colloids, Mechanism of Micelle Formation Week 3 Coagulation of Colloidal Solutions, Week 4 Properties of Colloidal Solution Week 5 Electrical properties of colloidal solution Week 6 Preparation of colloidal solutions Week 7 Protection of Colloids Week 8 Purification of colloidal solutions Week 9 Emulsions Week 10 Gels Week 11 -12 Application of Colloids 2 Colloidal state 24-25 Examples of Colloids 3 Colloidal state 24-25 THE COLLOIDAL STATE 1.1 Definitions Thomas Graham (1861) soluble substance divided into classes, "crystalloids" and "colloids”, according to their powers of diffusion across vegetable or animal membranes. Substances such as salt, sugar and urea, which diffuse rapidly, were termed crystalloids on account of the fact that they are rapidly obtained in the crystalline form. The other class includes many amorphous substances like gelatin, starch and gum which exhibit little or no tendency to diffuse through the membrane and were, because of their gluey nature, called colloids (Greek: kola = glue and edios = like). Since the time of Graham our conception of colloids has undergone a radical change. The difference in the rate of diffusion of crystalloids and colloids through a membrane can only be explained by assuming that colloids in the dissolved state yield bigger particles which cannot easily penetrate through the minute pores of the membrane whereas, crystalloids in solution are broken down to tiny molecules or even fragments of molecules (ions) which can diffuse through quickly. It is true that colloids readily yield a particle size in solution, which hinders their diffusion through a membrane, but certainly it does not mean that other substances are debarred from being brought down to that particle size. Gold, copper and other metals, which are ordinarily insoluble in water, have all been obtained in colloidal state. Sodium chloride, according to Graham, is a crystalloid but it has been obtained in the colloidal state in benzene. Soap behaves as a colloid in water and a crystalloid in alcohol. In face of such facts Graham's distinction, which implies that colloids and crystalloids are particular kind of substances, is no more tenable 4 Colloidal state 24-25 for all substances by employing suitable methods can be obtained in the colloidal condition. We should thus speak of the colloidal state of matter just as we speak of the liquid, solid and gaseous state of matter, rather than to call a particular material as colloid or crystalloid. Although the term colloid has lost its original significance, it is still used to describe such organic substance as gelatin, gum, albumin, etc., which practically always readily yield colloidal solutions. Table 1: Types of solutions according to states 5 Colloidal state 24-25 1.2 Colloidal State Certain solutes such as starch, glue, gelatin etc. could not pass through the parchment membrane while the ordinary solutes such as sodium chloride, urea, sugar etc. can easily do so. Thus, colloid is not a substance, but it is a state of a substance which depends upon the molecular size- comment? The three main states of matter that exists in real life are Solid, Liquid and Gas. We can say that a colloidal substance does not represent a separate class of substances. When the size of the solute particle lies between 1 nm and 1000 nm, it behaves as a colloid. Hence, we can say that colloid is not a substance but a state of the substance which is dependent on the size of the particle. 1.3 Types of Solutions On the basis of particle size of the substance, the solutions may be divided into three types. These are: 6 Colloidal state 24-25 1) True solutions 2) Suspensions 3) Colloidal solutions. 1) True solution is a homogeneous solution which contains small solute particles (molecules or ions) dispersed throughout a solvent. For example : The solution of sodium chloride in water. The particle size is less than 1 nm. The particles of a solute in a true solution are invisible even under microscope and as particles can pass through ordinary filter paper as well as through animal membrane. 2) Suspension is a heterogeneous mixture which contains small insoluble particles. The particle size is more that 1000 nm. For example: Dirt particles in water. The particles of a suspension may not be visible to the naked eye but are visible under a microscope. The particles of a suspension can neither pass through an ordinary filter paper nor through animal membrane. 3) Colloidal solution is a heterogeneous solution which contains particles of intermediate size. For example: Milk. a) The particles of a colloidal solution have diameters between 1 to 1000 nm. b) Such particles cannot be normally seen with naked eye. c) light reflected by them can be seen under an ultramicroscope. d) The particles of a colloidal solution can pass through ordinary filter paper but not through animal membrane. 7 Colloidal state 24-25 e) In a colloid, the dispersed phase may consist of particles of a single macro molecule (such as synthetic polymer or protein) or an aggregate of many atoms, molecules or ions. f) Colloidal particles have an enormous surface area per unit mass. Table 2: Types of solutions and their properties Properties True solution Colloidal solution Suspension Particle size Smaller than 10 Å 10 Å – 1000 Å More than 1000 Å Appearance Clear Generally clear Opaque Nature Homogeneous Heterogeneous Heterogeneous Separation by Not possible Not possible Possible filtration Separation by Not possible Possible Possible cellophane paper Visibility Not visible under Visible under Visible to naked eye microscope ultra- microscope Brownian motion Not observable Occurs May occur The size of dispersed particles in colloidal solutions is more than that of solute particles in a true solution and smaller than that a suspension. 8 Colloidal state 24-25 1.4 Phases of colloids and their Classification Colloidal solution is of heterogeneous nature and consists of two phases i.e. a dispersed phase and a dispersion medium. 1) Dispersed phase It is the component present in small proportion and is just like a solute in a solution. For example: In the colloidal solution of silver in water, the silver acts as a dispersed phase. 2) Dispersion medium It is generally component present in excess and is just like a solvent in a solution. Classification of Colloids The colloids are classified on the basis of the following criteria : ✓ Physical state of dispersed phase and dispersion medium. ✓ Natural of interactions between dispersed phase and dispersion medium. ✓ Type of particles of the dispersed phase. 1) Classification based on the Physical state of the Dispersed phase and Dispersion medium A common method of classifying colloids is based on the phase of the dispersed substance and what phase it is dispersed in. The types of colloids include sol, emulsion, foam, and aerosol, Table 3. 9 Colloidal state 24-25 TABLE 3: Types of colloidal systems Dispersion Dispersed Type of Colloid Example Medium Phase Solid Solid sol Ruby glass, colored glasses with dispersed metals Solid Liquid gel Pearl, cheese, Jellies, minerals with liquids Gas Solid foam Lava, pumice Solid Sol Paints, cell fluids - AgCl, Au, AsO3, or S in water Liquid Liquid Emulsion Milk, oil in water, water in oil Gas Foam Soap suds, whipped cream Solid Aerosol Smoke Gas Liquid Aerosol Fog, mists, clouds 10 Colloidal state 24-25 A: Dispersed phase B: Dispersion phase Depending upon the nature of the dispersion medium, colloidal solutions are sometimes given specific names. For example: Dispersion medium Name of colloidal solution Water Hydrosols or aquasol Alcohol Alcosols Benzene Benzosols Air Aerosols Gels A gel is a colloidal system in which a liquid is dispersed in a solid. The lyophilic sols may be coagulated to give a semi solid jelly like mass which encloses all the liquid present in the sol. 11 Colloidal state 24-25 The process of gel formation is called gelation and the colloidal system formed is called gel. The Common examples of gel are : gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide, etc. Classification of gels Gels may be classified into two types : (1) Elastic gels (2) Non-elastic gels (i) Elastic Gels These are the gels which possess the property of elasticity.They readily change their shape on applying force and return to original shape when the applied force is removed. They change to solid mass on dehydration which can again be converted into gel by addition of water followed by heating and cooling. When these gels are placed in contact with water, they absorb water and swell. This property is called Imbibition. Examples are gelatin, agar, starch etc. (ii) Non-elastic Gels These are the gels which are rigid and do not have the property of elasticity. They change into solid mass on dehydration which becomes rigid and cannot be converted into original form by heating with water. They do not show the phenomenon of imbibition. For example: silica gel. Emulsions 12 Colloidal state 24-25 A dispersion of tiny droplets of one liquid in another liquid is known as an emulsion. Any two immiscible liquids can yield an emulsion. Either liquids can be dispersed in the other. Thus we can have an emulsion of nitrobenzene in water by shaking a little of nitrobenzene with a relatively large excess of water, while an emulsion of water in nitrobenzene would result on agitating a little on agitating a little water with a large excess of nitrobenzene. a) Types of Emulsions In general there are two types of emulsion recognized : (1) Oil in water emulsions, and (2) Water in oil emulsions H2O Oil Oil H2O H2O Oil Oil H2O Oil in Water Water in Oil Two types of emulsions. Water is usually one of the components and the other is an oil or a liquid insoluble in water, which takes the place of oil. Emulsions composed entirely of water and oil is not stable. The dispersed droplets at once come together (coalesce) and form a separate layer. To stabilize an emulsion the addition of a third substance, known as an emulsifying agent or emulsifier, is essential. 13 Colloidal state 24-25 Emulsifier Oil Water Role of emulsifier. The emulsifier concentrates at the oil-water interface and forms a film sufficiently tough to prevent the coalescence of the droplets. Soaps, gelatin, and gum are useful emulsifying agents. According to Bancroft, an emulsifier at the water-oil interface lowers the surfaces tension on the side of one liquid more than it does on the other and thus the interface will tend to curve around the second liquid, which is dispersed as droplets. b) Preparation of Emulsions. (1) Agent is water method: The emulsifying agent is dissolved in water and the oil is added to it bit by bit with considerable agitation. This method results in emulsions of water type. (2) Agent in oil method: The emulsifier in oil and water is added to it resulting in an emulsion of the water in oil type. (3) Nascent soap method: This method is useful for emulsions stabilized by soap. This fatty acid part of the emulsifier is dissolved in oil and the alkaline 14 Colloidal state 24-25 part in water and the two are mixed. The formation of soap at the interface results in the formation of a stable emulsion. c) Examples and uses Obviously, an emulsified substance is more effective than if it were in massive from. It offers much greater surface for action. Thus many pharmaceutical preparations are emulsions e.g., emulsions of Cod-liver oil and Hulibut-liver oil. The digestive juices in the stomach and elsewhere more readily act upon the emulsified oil. The disinfectants, phenyl and Lysol, yield an emulsion of the oil-in-water type when poured into water. 2) Classification based on Nature of Interaction between Dispersed Phase and Dispersion Medium Depending upon the nature of interactions between dispersed phase and the dispersion medium, the colloidal solutions can be classified into two types as: (i) lyophilic and (ii) lyophobic sols. (i) Lyophilic colloids The colloidal solutions in which the particles of the dispersed phase have a great affinity (or love) for the dispersion medium, are called lyophilic colloids. a) These solutions are easily formed and the lyophilic colloids are reversible in nature. The reversible; If the dispersion medium is separated from the dispersed phase the sol can be again formed by simply remixing it with the dispersion medium. b) These sols are quite stable and cannot be easily coagulated. 15 Colloidal state 24-25 Examples of lyophilic colloids are gum, gelatin, starch, proteins, rubber,… (ii) Lyophobic colloids The colloidal solutions in which there is no affinity between particles of the dispersed phase and the dispersion medium are called lyophobic colloids. Such solutions are formed with difficulty. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking. Therefore, these are not stable. Further, once precipitated, they do not form the colloidal sol by simple addition of dispersion medium. Hence, these are irreversible in nature. These sols need some stabilising agents for their preservation. In case, the dispersion medium is water, the lyophobic sol. Is called hydrophobic colloid. Difference between Lyophilic and Lyophobic Colloids Property Lyophilic colloids Lyophobic collloids Preparation easily formed by direct mixing. formed only by special methods. Particles The particles are true molecules and particles are aggregrates of many nature are big size. molecules. The particles are not easily visible The particles are easily detected Visibility even under microscope. under ultramicroscope. Stability They are very stable. Unstable, require stabilizer Action of not easily precipitated by small They are easily precipitated by electrolytes amount of electrolytes. Very large small amount of electrolytes. 16 Colloidal state 24-25 quantities required for coagulation Nature are reversible in nature. are irreversible in nature. The particles do not carry any The particles move in a specific Charge on charge. may migrate in any direction anode or cathode particles direction. depending upon their charge. The particles of colloids are not The particles are heavily hydrated Hydration appreciably hydrated due to the due to the attraction for the solvent. hatred for the solvent. The viscosity is much higher than The viscosity is nearly the same as Viscosity that of the dispersion medium. dispersion medium. Surface surface tension is usually lower than surface tension is almost the same Tension dispersion medium as dispersion medium. Tyndall weak Tyndall effect. show Tyndall effect. effect 3) Classification Based on type of particles of dispersed phase. Depending upon the type of the particles of the dispersed phase, the colloids are classified as: 1) Multimolecular colloids 2) Macromolecular colloids 3) Associated colloids. 1) Multimolecular colloids When on dissolution, atoms or smaller molecules of substances (having diameter less than 1 nm) aggregate together to form particles of colloidal dimensions, the particles thus formed are called multimolecular colloids. Therefore, in these sols the dispersed phase consists of aggregates of atoms or molecules with molecular size less than 1 nm. 17 Colloidal state 24-25 For example: sols of gold atoms and sulphur (S8) molecules. In these colloids, the particles are held together by van der Waals forces. 2) Macromolecular colloids These are the substances having big size molecules (called macro molecules) which on dissolution form solution in which the dispersed phase particles have size in the colloidal range. Naturally occurring macro-molecules are starch, cellulose, proteins, enzymes, gelatin etc. Artificial macro-molecules are synthetic polymers such as nylon, polythene, plastics, polystyrene etc. Since these macromolecules have large sizes comparable to those of colloidal particles, the solutions of such molecules are called macromolecular colloidal solutions. Thus, the common examples of macromolecular colloids are starch, cellulose, proteins, plastics, etc. 3) Associated colloids These are the substances which when dissolved in a medium behave as normal electrolytes at low concentration but behave as colloidal particles at higher concentration due to the formation of aggregated particles. The aggregate particles thus formed are called micelles. For example: In aqueous solution soap (sodium stearate) ionises as: 18 Colloidal state 24-25 C17H35COONa ⇔ C17H35COO¯ + Na+ In concentrated solution, these ions get associated to form an aggregate of colloidal size. The colloidal behaviours of such substances is due to the formation of aggregates or clusters in solutions. Such aggregated particles are called micelles. Thus, micelles are the cluster or aggregated particles formed by association of colloids in solution. The common examples of micelles are soaps and detergents. 1.5 Mechanism of Micelle Formation Micelles are generally formed by the aggregation of several ions or molecules with lyophobic as well as lyophilic parts. The micelle may contain as many as 100 molecules or more. When sodium stearate is dissolved in water, it gives Na+ and C17H35COO‾ ions. 19 Colloidal state 24-25 C17H35COONa ⇔ C17H35COO¯ + Na+ The stearate ions associate to form ionic micelles of colloidal size. The stearate ion, C17H35COO¯ consists of two parts: 1) a non-polar part which consists of long chain hydrocarbon part. It is called non-polar tail. This part is insoluble in water but soluble in oil or grease. It is also called water repelling or hyhdrophobic part. 2) a polar group which consists of carboxylate ion, COO‾. It is called polar- ionic head. It is soluble in water and insoluble in oil or grease. It is water attracting or hydrophilic part. a) The stearate ions are therefore, present on the surface with their COO¯ groups in water and the hydrocarbon tail staying away from it and remains at the surface. b) Inside water, these molecules have a unique orientation which keeps the hydrocarbon portion out of water. c) At critical micelle concentration, the anions are pulled into the bulk of the solution and form a clusters of molecules in which the hydrocarbon tails are in the interior of the cluster and ionic ends are at the surface of the cluster. This formation is called micelle formation and the aggregate thus formed is known as ionic micelle. Examples of micelles are: (i) Sodium palmitate [C15H31COONa] (ii) Cetyl trimethyl ammonium bromide CH3(CH2)15(CH3)3 N+Br¯ (iii) Sodium lauryl sulphate [CH3(CH2)11SO3O¯Na+] 20 Colloidal state 24-25 In case of detergents e.g., sodium lauryl sulphate, CH3(CH2)11OSO3¯Na+ , the polar group is SO42- along with the long hydrocarbon chain. Therefore, the mechanism of micelle formation is same as that of soaps. Cleaning Action of Soap 1) The cleansing action of soap is due to its tendency to act as micelle and form emulsions. 2) A soap is composed of long chain of alkyl group called tail and a polar part COO¯ ion called head. 3) The dirt in the cloth is due to the presence of dust particles in fat or grease which stick to the cloth. 4) When the cloth is dipped in aqueous soap solution, the soap and the dirt come in contact with each other. 21 Colloidal state 24-25 5) The soap molecules form micelle around the oil droplet in such a way that the hydrophobic part of stearate ions is in the oil or grease droplet while the hydrophilic part projects out of the grease droplet like the bristles. 6) Each oil droplet is surrounded by a number of negatively charged carboxylate ions. 7) Since the polar groups can interact with water, the oil droplets surrounded by stearate ions is now pulled in water and hence removed from dirty surface. 8) Since similar charges repel each other, the oil droplets break up and form small droplets or globules. 9) The negatively charged sheath around the globules prevents them from coming together and form aggregates. These small droplets get dispersed in water forming emulsion. 10) The hand rubbing or the agitation due to the washing machine causes dispersion of the oil or grease throughout the soapy water. These are washed away with water along with dust particles. In this way grease or dirt are removed from the surface of the cloth. Macromolecular Multimolecular Colloids Associated Colloids Colloids aggregation of a large consist of aggregates of number of ions which atoms or molecules They consist of large behave as colloidal size which generally have size molecules. particles at higher diameter less than 1 nm. concentration. They behave as normal The atoms or molecules The molecules are electrolyte at low are held by weak van der flexible and can take concentration and waal forces. any shape. colloidal only at high concentration. Their molecular masses They have high Their molecular masses are not very high. molecular masses. are generally high. 22 Colloidal state 24-25 lyophobic character. lyophilic lyophilic and lyophobic 2. Coagulation of Colloidal Solutions 1) A small amount of an electrolyte is necessary for the stability of the colloidal sol. The ions of the electrolytes are adsorbed on the sol. particles and impart them some charge; positive or negative. 2) The charged colloidal particles repel one another and are prevented from coming close together to unite into bigger particles. 3) If somehow the charge is removed the particles will come nearer to each other to form aggregate (or coagulate) and settle down under the force of gravity. 23 Colloidal state 24-25 For example: In the presence of a large excess of the electrolyte, the charge on the particles of the dispersed phase is neutralized and as a result, they come closer, grow in size and ultimately form precipitates. Thus, the phenomenon of precipitation of a colloidal solution by the addition of excess of an electrolyte is called coagulation or flocculation. 2.1 Mechanism of Coagulation When an electrolyte is added to the sol., the colloidal particles take up ions carrying opposite charge from the electrolyte. As a result, their charge gets neutralized, and this causes the uncharged particles to come closer and to get coagulated or precipitated. For example: If BaCl2 solution is added to As2S3 sol, the Ba2+ ions are attracted by the negatively charged sol as particles and their charge gets neutralized. This leads to coagulation. Hardy Schulze Rule The coagulation capacity of different electrolytes is different. It depends upon the valency of the active ion or called flocculating ion, which is the ion carrying charge opposite to the charge on the colloidal particles. According to Hardy Schulze rule, greater the valency of the active ion or flocculating ion, greater will be its coagulating power. 24 Colloidal state 24-25 According to Hardy Schulze law, the quantity of electrolyte required to coagulate a definite amount of colloidal solution depends upon the valency of the coagulating ion. The coagulating ions are the ions of electrolyte which carry the opposite charge of the colloidal particles. The greater the valency of the coagulating ion, the greater the coagulation power. The coagulation of a negatively charged sol (As2S3) is done by adding a positively charged colloid. The coagulating values of different electrolytes are given below: Electrolyte Cation Coagulating value NaCl Na+ 52 MgCl2 Mg2+ 0.72 BaCl2 Ba2+ 0.69 AlCl3 Al3+ 0.093 Coagulation ∝ 1/ Coagulating value (Coagulation is inversely proportional to coagulating value) So the order of coagulating power of cations for negatively charged sol As2S3. Al3+ > Ba2+ > Mg2+ > Na+ For coagulation of a positively charged sol (Fe(OH)3) the coagulating ion is an anion, the power of coagulating value are given below : 25 Colloidal state 24-25 Electrolyte Anion Coagulating value KBr Br – 138 K2SO4 SO42- 0.210 K2C2O4 C2O42- 0.238 K3[Fe(CN)6] [Fe(CN)6]3- 0.096 So the order of coagulating power of anion for positively charged sol Fe(OH)3. Br– < SO42- < [Fe(CN)6]3- The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a sol is known as Coagulation value or flocculation value. The coagulating power is inversely proportional to coagulation value or flocculation value. 2.2 Methods of Coagulation Apart from the addition of the electrolyte, the coagulation of a colloidal sol can be affected by the following methods: 1) adding of electrolyte When the electrolytic solution is added to a solution then the colloid’s particles attract the opposite charged electrolyte ions and neutralized. After neutralization colloid particles attract each other and precipitated. Example: When NaCl electrolyte is added As2S3 negative solution then yellow precipitate is formed. Na+ neutralize the negative colloid particles in this case. 26 Colloidal state 24-25 2) By Mutual Precipitation When two- oppositely charged sols are mixed in equimolar proportions, they mutually neutralize their charge, and both get coagulated. For example: if positively charged Fe(OH)3 sol and negatively charged As2S3 sol are mixed, both the sols get coagulated. 3) By Electrophoresis In the electrophoresis the particles of the dispersed phase move towards the oppositely charged electrodes. If the process is carried for a long time the particles will touch the electrode, lose their charge, and get coagulated. 27 Colloidal state 24-25 Electrophoresis 4) By Persistent Dialysis The stability of colloidal sols is due to the presence of a small amount of electrolyte. If the electrolyte is completely removed by repeated dialysis, the particles left will get coagulated. 4) By heating or cooling On heating thermal energy of colloidal particles increases so much ha overcomes the repulsive forces between them, and particles unite o form larger particles. The charge on colloidal particles is due to preferential adsorption. The adsorption is inversely proportional to temperature. Due to heating, the desorption of adsorbed particles takes place. 28 Colloidal state 24-25 The sols get coagulated on heating, for example: coagulation of egg In some cases, cooling the sol also results into its coagulation. For example: coagulation of milk i.e., on cooling milk fats start floating on the surface. 2.3 Coagulation of Lyophilic sols Lyophilic sols are more stable than lyophobic sols. The stability of lyophilic sols is due to two factors: 1) Charged an 2) Solvation of the colloidal particles. When these two factors are removed, a lyophilic sol can be coagulated. This can be done (i) by adding electrolyte and (ii) by adding suitable solvent. For example: when solvents such as alcohol or acetone are added to hydrophilic sols, it results into dehydration of dispersed phase. Under this condition a small quantity of electrolyte can cause coagulation. 29 Colloidal state 24-25 2.4 Coagulation of lyophobic solutions: Lyophobic sols are less stable than lyophilic colloid. Hence they are more easily coagulated. The stability of lyophobic sol is only due to the charge on the colloidal particles. This factor can be removed by adding only electrolytes. 3. Protection of Colloids Lyophobic sols such as those of metals like gold, silver etc. can be easily precipitated by the addition of a small amount of electrolytes. They can be prevented from coagulation by the previous addition of some stable lyophilic colloids like gelatin, albumin, etc. This is because when a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around the lyophobic particles, and this protect them from electrolytes. If a small amount of gelatin is added to gold sol, it is not readily precipitated by the addition of sodium chloride. This process of protecting the lyophobic colloidal solutions from precipitation by the electrolytes due to the previous addition of some lyophilic colloid is called protection. The colloid which is added to prevent coagulation of the colloidal sol is called protecting colloid. According to a recent view the sol. acquires the stability of the protective colloid merely on account of the mutual absorption of their particles and it is immaterial whether the sol. particles adsorb the protective colloid or vice versa. In fact it is reasonable to assume that if the protected particles are smaller than the protecting particles the former will be adsorbed by the latter (Fig. a) and if the protected particles are bigger than the protecting particles the latter will be adsorbed by the former (Fig. b). 30 Colloidal state 24-25 Protected particles Protecting particles a: When protected particles are b: When protected particles are smaller than protecting particles bigger than protecting particles Gold number: The different protecting colloids differ in their protecting powers. It is known as the number of milligrams of the protective colloid required to prevent the coagulation of a 10 mL of a given gold sol when 1 mL of a 10% solution of sodium chloride is added to it. The coagulation of gold sol is indicated by change in colour from red to blue. Smaller the value of the gold number, greater will be protecting power of the protective colloid. Lyophilic colloid Gold no. Gelatin 0.005-0.01 Haemoglobin 0.03-0.07 Egg albumin 0.1- 0.2 Gum Arabic 0.15- 0.25 31 Colloidal state 24-25 Starch 20-25 Therefore, reciprocal of gold number is a measure of the protective power of a colloid. Gelatin is the best protective colloid. 4. Properties of Colloidal Solution 4.1 Physical Properties (a) Heterogeneous Character The colloidal solutions are heterogeneous in nature consisting of two phases (1) dispersed phase and (2) dispersion medium. Because of the small particle size, the colloidal solutions generally appear to be homogeneous to the naked eye but their heterogeneity can be confirmed by seeing under electron microscope. (b) Stable Nature The colloidal solutions are quite stable. Their particles in a state of motion and do not settle down at the bottom of the container. Particles of certain colloidal sols, which have comparatively large size may settle down but very slowly. (c) Filterability The colloidal particles can pass through ordinary filter papers because the size of the colloidal particles is lesser than the size of the pores of filter paper. 32 Colloidal state 24-25 However, they cannot pass through animal and vegetable membranes and ultrafilter papers. This forms the basis of separation of colloidal particles from those of crystalloids. 4.2 Colligative Properties All molecular colloids in solution exhibit colligative properties. Since most of these colloids have high molecular masses the depression of freezing point, elevation of boiling point or lowering of vapor pressure is so small that these methods fail. In chemistry, colligative properties are properties of solutions that depend on the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the nature of the chemical species present. They can at best be used up to the molecular mass of a few thousand. The osmotic pressure, however, is quite marked for even high molecular mass polymers and this method finds wide use for the determination of molecular masses of a variety of polymers like proteins, cellulose and its derivatives, rubber, and a host of man-made polymers. Modified osmotic pressure equation is used and the molecular mass is calculated from the intercept of a linear plot of π/C vs C. The extrapolation of the straight line is carried to zero concentration of the solution [π is the measured osmotic pressure and C is the concentration expressed in g.L-1 solution]. 33 Colloidal state 24-25 Colligative properties are mostly studied for dilute solutions, whose behavior may often be approximated as that of an ideal solution. For osmotic pressure measurements, the colloid must be completely free from any electrolyte or any low molecular mass non-polymeric material to obtain any reliable value. It depends only on the number of dissolved particles in solution and not on their identity. Non-colligative properties depend on the identity of the dissolved species and the solvent. 34 Colloidal state 24-25 4.3 Mechanical Properties (a) Brownian Movement When colloidal solutions are viewed through a powerful ultramicroscope, the colloidal particles are seen to be in a state of continuous zig-zag motion. This motion was first observed by the British botanist, Robert Brown in 1827. The pollen grains suspended in water do not remain at rest but move about continuously and randomly in all directions. The colloidal particles also are moving at random in a zig-zag motion. This type of motion is called Brownian movement after the name of its discoverer, Robert Brown. 35 Colloidal state 24-25 The Brownian motion is independent of the nature of the colloid but depends on the size of the particles and viscosity of solution. Smaller the size and lesser the viscosity, faster is the motion. Cause of Brownian Movement The Brownian movement has been explained to be due to the unbalanced bombardment of the particles by the molecules of the dispersion medium. The molecules of the dispersion medium are constantly colliding with the particles of the dispersed phase. The impacts of the dispersion medium particles are unequal, thus, causing a zig-zag motion of the dispersed phase particles. When a molecule of dispersion medium collides with a colloidal particle, it is then displaced in one direction. Then another molecule strikes it, displacing it to another direction and so on. This process gives rise to a zig-zag motion. Brownian movement has a stirring effect which does not allow the particles to settle down and hence is responsible for the stability of the sols. However, if the size of the dispersed phase particles increases, then the chances of unequal bombardment decrease. 36 Colloidal state 24-25 (1) Brownian movement provides a direct demonstration of the ceaseless motion of molecules as postulated by kinetic theory. (ii) The Brownian movement explains the force of gravity acting on colloidal particles. This helps in providing stability to colloidal sols by not allowing them to settle down. (iii) It has also helped in the determination of Avogadro’s number. (b) Diffusion The sol particles diffuse from higher concentration to lower concentration region. However, due to bigger size, they diffuse at a lesser speed. (c) Sedimentation The colloidal particles settle down under the influence of gravity at a very slow rate. This phenomenon is called sedimentation and is used to determine the molecular mass of macromolecules. 4.4 Optical Properties : Tyndall Effect When a strong beam of light is passed through a true solution placed in a beaker, in a dark room, the path of the light does not become visible. However, if the light is passed through a sol, placed in the same room, the path of the light becomes visible when viewed from a direction at right angle to that of the incident beam. This phenomenon was first observed by Faraday and later studied in detail by Tyndall and therefore, it is called Tyndall effect. 37 Colloidal state 24-25 The cause of Tyndall effect is the scattering of light by the colloidal particles i.e., these particles scatter light in all directions in space. The scattering of light illuminates the path of beam in the colloidal dispersion. The particles in true solution are too small in size to cause any scattering i.e.. The Tyndall effect is not observed in true solutions. Thus, the phenomenon of scattering of light by colloidal particles as a result of which the path of the beam becomes visible is called Tyndall effect. The illuminated path of the beam is called Tyndall cone. The Tyndall effect can be observed due to scattering of dust particles, when a beam of sunlight enters a dark room through a slit. Tyndall effect is observed only when the following two conditions are satisfied : (i) The diameter of the dispersed phase particles is not much smaller than the wavelength of light used. (ii) The refractive indices of the dispersed phase and the dispersion medium are largely in magnitude. 38 Colloidal state 24-25 Tyndall effect (a: true and b) colloidal solution) Importance of Tyndall Effect The Tyndall effect has also been used to devise an instrument called utramicroscope. In this instrument, an intense beam of light is focused on the colloidal solution contained in a glass vessel. The focus of light is then viewed with a microscope at right angles to the beam. Individual colloidal particles appear as spots of bright light against a dark background. Ultra-microscope does not make the actual colloidal particles visible but only the light scattered by the colloidal particles can be seen through a microscope. The colour of colloidal solutions depends on the wavelength of the light scattered by the dispersed particles. The wavelength further depends on the size and nature of the particles. It has been observed that the colour of colloidal solutions also changes with the manner in which the observer receives the light. For example: a mixture of milk and water appears blue when viewed by the reflected light but if transmitted light is viewed it is red. Similarly, gold sol is red in colour when the particles are fine but as the size of the particles increases, its colour changes to purple, then blue and finally golden. 4.5 Electrical Properties 39 Colloidal state 24-25 The particles of the colloidal solutions possess electrical charge, positive or negative. The presence of charge is responsible for the stability of these solutions. The sol particles carry some charge while the dispersion medium has no charge. For example: the colloidal solutions of gold, arsenious sulphide (As2S3) are negatively charged while those of Fe(OH)3 and Al(OH)3 have positive charge. In the case of silver chloride sol, the particles may either be positively or negatively charged. Various views have been put forward regarding the origin of charge on the colloidal particles. (a) Due to Frictional Electrification The frictional electrification due to the rubbing of the dispersed phase particles with that of dispersion medium results in some charge on the colloidal particles. But the dispersion medium must also get some charge because of the friction. (b) Due to Dissociation of the Surface Molecules In aqueous solution of soap (sod. palmitate) which dissociates into ions as: C15H31COONa ↔ C15H31COO¯ + Na+ 40 Colloidal state 24-25 The cations (Na+) pass into the solution while the anions (C15H31COO¯) have a tendency to form aggregates due to weak attractive forces present in the hydrocarbon chains. (c) Due to Selective Adsorption of ions The particles constituting the dispersed phase adsorb only those ions preferentially which are common with their own lattice ions. For example: if silver nitrate solution is added to an aqueous solution of potassium iodide, the precipitated silver iodide will adsorb negative I¯ ions (common ions) from the dispersion medium to form a negatively charged sol. AgI + I¯ ——> AgI : I¯ However, if silver iodide is formed by adding potassium iodide to silver nitrate solution, the sol will be positively charged due to the adsorption of Ag + ions (common ions) present in the dispersion medium. AgI + Ag+ ———> AgI | Ag+ Thus, the ion which is common with their own lattice ions is preferentially adsorbed. 41 Colloidal state 24-25 If ferric chloride is added to excess of hot water, a positively charged sol of hydrated ferric hydroxide is formed. This is because of adsorption of Fe 3+ ions. However, when ferric chloride is added to sodium hydroxide (NaOH) solution, a negatively charged sol is obtained due to the adsorption of OH¯ ions. FeCl3 + H2O (excess) → Fe2O3.xH2O | Fe3+ FeCl3 + NaOH (aq)→ Fe2O3.xH2O | OH¯ 42 Colloidal state 24-25 Based on the nature of the charge, the colloidal solutions have been classified into positively charged and negatively charged colloids. a) Electrophoresis b) Electroosmosis (a) Electrophoresis The presence of the charge on the sol particles and its nature whether positive or negative can be determined with the help of a phenomenon known as electrophoresis. The colloidal particles move towards positive or negative electrodes depending upon their charge under the influence of electrical field. The phenomenon of movement of colloidal particles under an applied electric field is called electrophoresis. The particles accumulate near the negative electrode, the charge on the particles is positive, On the other hand, if the sol particles accumulate near the positive electrode, the charge on the particles is negative. a) The apparatus consists of U-tube with two platinum electrodes in each limb. b) Take a sol of As2S3 in the U-tube. c) The intensity of the colour of the sol in both the arms is same. d) Now pass the current through the sol. 43 Colloidal state 24-25 e) After some time, it is observed that the colour of the sol near the positive electrode became intense than the initial colour. This indicates that the As2S3 particles have accumulated near the positive electrode. f) This indicates that the particles of As2S3 are negatively charged and they move towards oppositely charged (positive) electrode and accumulate there. When an electric current is passed through positively charged Fe3(OH) sol, it is observed that they move towards negatively charged electrode and get accumulated there. Thus, by observing the direction of movement of the colloidal particles, the sign of the charge carried by the particles can be determined. (b) Electroosmosis When the movement of the colloidal particles is prevented by some suitable means and the molecules of the dispersion medium are allowed to move under the influence of applied potential, the phenomena is called electroosmosis. Electroosmosis is the phenomenon of the movement of the molecules of the dispersion medium under the influence of electric field whereas colloidal particles are not allowed to move. 4.6 Electrokinetic phenomena Electrophoresis is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic 44 Colloidal state 24-25 phenomenon was observed for the first time in 1807 by Ferdinand Frederic Reuss (Moscow State University), who noticed that the application of a constant electric field caused clay particles dispersed in water to migrate. It is ultimately caused by the presence of a charged interface between the particle surface and the surrounding fluid. It is the basis for a few analytical techniques used in biochemistry for separating molecules by size, charge, or binding affinity. Electrophoresis of positively charged particles (cations) is called cataphoresis, while electrophoresis of negatively charged particles (anions) is called anaphoresis. Electrophoresis is a technique used in laboratories to separate macromolecules based on size. The technique applies a negative charge, so proteins move towards a positive charge. This is used for both DNA and RNA analysis. Polyacrylamide gel electrophoresis has a clearer resolution than agarose and is more suitable for quantitative analysis. In this technique DNA foot-printing can identify how proteins bind to DNA. It can be used to separate proteins by size, density, and purity. It can also be used for plasmid analysis, which develops our understanding of bacteria becoming resistant to antibiotics. Suspended particles have an electric surface charge, strongly affected by surface adsorbed species, on which an external electric field exerts an electrostatic Coulomb force. According to the double layer theory, all surface charges in fluids are screened by a diffuse layer of ions, which has the same absolute charge but opposite sign with respect to that of the surface charge. The electric field also exerts a force on the ions in the diffuse layer which has direction opposite to that acting on the surface charge. This latter force is not actually applied to the particle, but to the ions in the diffuse layer located at some distance from the particle surface, and part of it is 45 Colloidal state 24-25 transferred all the way to the particle surface through viscous stress. This part of the force is also called electrophoretic retardation force. When the electric field is applied and the charged particle to be analyzed is at steady movement through the diffuse layer, the total resulting force is zero: Considering the drag on the moving particles due to the viscosity of the dispersant, in the case of low Reynolds number and moderate electric field strength E, the drift velocity of a dispersed particle v is simply proportional to the applied field, which leaves the electrophoretic mobility μe defined as: The most well-known and widely used theory of electrophoresis was developed in 1903 by Smoluchowski where is the dielectric constant of the dispersion medium, is the permittivity of free space (C2 N-1 m-2), η is dynamic viscosity of the dispersion medium (Pa s), and is zeta potential (i.e., the electrokinetic potential of the slipping plane in the double layer). The Smoluchowski theory is very powerful because it works for dispersed particles of any shape at any concentration. Unfortunately, it has limitations on its validity. It follows, for instance, from the fact that it does not include Debye length κ. However, Debye length must be important for electrophoresis, as follows immediately from the Figure on the right. Increasing thickness of the double layer (DL) leads to removing point of retardation force further from the particle surface. The thicker DL, the smaller retardation force must be. 46 Colloidal state 24-25 Detailed theoretical analysis proved that the Smoluchowski theory is valid only for sufficiently thin DL, when particle radius a is much greater than the Debye length: This model of "thin Double Layer" offers tremendous simplifications not only for electrophoresis theory but for many other electrokinetic theories. This model is valid for mostaqueous systems, where the Debye length is usually only a few nanometers. It only breaks for nano-colloids in solution with ionic strength close to water. The Smoluchowski theory also neglects the contributions from surface conductivity. This is expressed in modern theory as condition of small Dukhin number: In the effort of expanding the range of validity of electrophoretic theories, the opposite asymptotic case was considered, when Debye length is larger than particle radius: Under this condition of a "thick Double Layer", Hückel predicted the following relation for electrophoretic mobility: This model can be useful for some nanoparticles and non-polar fluids, where Debye length is much larger than in the usual cases. There are several analytical theories that incorporate surface conductivity and eliminate the restriction of a small Dukhin number, pioneered by Overbeek and Booth. Modern, rigorous theories valid for any Zeta potential 47 Colloidal state 24-25 and often any aκ stem mostly from Dukhin-Semenikhin theory. In the thin Double Layer limit, these theories confirm the numerical solution to the problem provided by O'Brien and White. Various combinations of the driving force and moving phase determine various electrokinetic effects. According to J.Lyklema, the complete family of electrokinetic phenomena includes: Electrophoresis, as motion of particles under influence of electric field. Electro-osmosis, as motion of liquid in porous body under influence of electric field. Diffusiophoresis, as motion of particles under influence of a chemical potential gradient. Capillary osmosis, as motion of liquid in porous body under influence of the chemical potential gradient. Sedimentation potential, as electric field generated by sedimenting colloid particles. Streaming potential/current, as either electric potential or current generated by fluid moving through porous body, or relative to flat surface. Colloid vibration current, as electric current generated by particles moving in fluid under influence of ultrasound; electric sonic amplitude, as ultrasound generated by colloidal particles in oscillating electric field. 4.7 Electrical double layer: The model which gave rise to the term 'electrical double layer' was first put forward in the 1850's by Helmholtz. In this model he assumed that no electron transfer reactions occur at the electrode and the solution is composed only of electrolyte. The interactions between the ions in 48 Colloidal state 24-25 solution and the electrode surface were assumed to be electrostatic in nature and resulted from the fact that the electrode holds a charge density (qm) which arises from either an excess or deficiency of electrons at the electrode surface. For the interface to remain neutral the charge held on the electrode is balanced by the redistribution of ions close to the electrode surface. Helmholtz's view of this region is shown in the figure below Electrical double layer formation on negative sol with the potential drop occurring in a linear manner between the two plates. It is perhaps no surprise that when impedance analysis is performed on electrochemical systems the response due to the electrolyte redistribution is modelled in terms of capacitive elements. 49 Colloidal state 24-25 5. Preparation of colloidal solutions Preparation of Lyophilic Colloids The lyophilic colloids have strong affinity between particles of dispersed phase and dispersion medium. Therefore, these colloidal solutions are readily formed by simply mixing the dispersed phase and dispersion medium under ordinary conditions. For example: The substances like gelatin, gum, starch, egg albumin etc. pass readily into water to give colloidal solution. They are reversible in nature because these can be precipitated and directly converted into colloidal state. Lyophobic sols can be prepared by mainly two types of methods: 1) Condensation methods 2) Dispersion methods. 1) Condensation Methods In these methods, smaller particles of dispersed phase are condensed suitably to be of colloidal size. This is done by the following methods : a) By chemical Reactions The chemical reactions given ahead may be used to prepare lyophobic colloidal solutions. (i) Oxidation: A colloidal sol of sulphur is obtained by bubbling H2S gas through the solution of bromine water, sulphur dioxide, etc. H2S + Br2 —-> 2HBr + S 2 H2S + SO2 ——> 2H2O + 3S 50 Colloidal state 24-25 (ii) Reduction: The colloidal solutions of metals are obtained by reduction of their compounds. For example: a solution of AuCl3 is reduced with SnCl2. 2 AuCl3 + 3 SnCl2 ———> 3 SnCl4 + 2 Au The reaction can also be carried out with formaldehyde. 2 AuCl3 + 3HCHO + 3 H2O ——> 2 Au + 3 HCOOH + 6 HCl The gold sol, thus prepared, has a purple colour and is called purple of cassius. (iii) Hydrolysis: A colloidal solution of ferric hydroxide is prepared when a concentrated solution of ferric chloride is added drop wise to hot water. FeCl3 +3H2O ——> Fe(OH)3 +3HCl (iv) Double decomposition: As2S3 sol is obtained by passing H2S through dilute solution of arsenious oxide in water. As2O3 + 3H2S —–> As2S3 +3H20 (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 the solvent. The molecules of water which can no longer be held in solution, separately combine to form particles of colloidal size. 51 Colloidal state 24-25 (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. (d) By change of Physical State Sols of substances like mercury and sulphur are prepared by passing their vapours through a cold water containing a suitable stabilizer such as ammonium salt or citrate. 2) Dispersion Methods In these methods, larger particles of a substance (suspension) are broken into smaller particles. (a) Mechanical dispersion In this methods, 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 a colloidal mill. It consists of two metallic discs nearly touching each other and rotating in opposite directions at a very high speed. 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, dyes, etc. are obtained by this method. 52 Colloidal state 24-25 Mechanical dispersion (b) By Electrical Dispersion or Bredig’s arc Method This method is used to prepare sols of metals such as platinum, silver, copper or gold. The metal whose sol is to be prepared is made as two electrodes immersed in dispersion medium such as water. The dispersion medium is kept cooled by surrounding it with a freezing mixture. An electric arc is struck between the electrodes. The tremendous heat generated by the arc vaporises the metals which are condensed immediately in the liquid to give colloidal solution. The colloidal solution prepared is stabilised by adding a small amount of KOH to it. Preparation of colloidal solution by Bredig´s Arc method. 53 Colloidal state 24-25 (C) Ultrasonic dispersion Ultrasonic vibrations (having frequency more than the frequency of audible sound) could bring about the transformation of coarse suspension to colloidal dimensions. Claus obtained mercury sol by subjecting mercury to sufficiently high frequency ultrasonic vibration. Ultrasonic dispersion of Hg in water (d) By Peptization The process of converting a freshly prepared precipitate into colloidal form by the addition of a suitable electrolyte is called peptization. The electrolytes used for the purpose are called peptising agents. Cause of Peptization When an electrolyte is added to a freshly prepared precipitate, the suitable ions from the added electrolyte are adsorbed by the particles of the precipitate. The charged particles repel one another and form colloidal solution. For example: On treating a precipitate of iron (III) oxide with a small amount of FeCl3 solution, gives a reddish brown coloured colloidal solution. 54 Colloidal state 24-25 Fe(OH)3 + Fe3+ ——–> Fe(OH)3Fe3+ A precipitate of silver chloride can be peptised by shaking with a dilute solution of silver nitrate to give a colloidal solution of silver chloride. AgCl + Ag+ ——-> AgCl.Ag+ 6. Purification of colloidal solutions Impurities especially electrolytes which can destabilize the sols must be eliminated to make the colloidal solutions stable. The following methods are commonly used for the purification of colloidal solutions. 6.1 Dialysis The process of separating the particles of colloids from those of crystalloids by means of diffusion through a suitable membrane is called dialysis. Principle: Colloidal particles cannot pass through a parchment of cellophane membrane while the ions of the electrolyte can pass through it. a) The colloidal solution is taken in a bag made of cellophane or parchment. 55 Colloidal state 24-25 b) The bag is suspended in fresh water. The impurities slowly diffuse out of the bag leaving behind pure colloidal solution. c) The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag. d) Dialysis can be used for removing HCl from the ferric hydroxide sol. e) To increase the process of purification, the dialysis is carried out by applying electric field. This process is called electrodialysis. Electrodialysis The dialysis process is slow and to speed up its rate, it is carried out in the presence of an electrical field. When the electric field is applied through the electrodes, the ions of the electrolyte present as impurity diffuse towards oppositely charged electrodes at a fast rate. The dialysis carried out in the presence of electric field is known as electro-dialysis. Electro-dialysis 56 Colloidal state 24-25 Ultra-filtration It is the process of removing the impurities from the colloidal solution by passing it through graded filter papers called ultra-filter papers. a) These filter papers are permeable to all substances except colloidal particles. Colloidal particles can pass through ordinary filter paper because the size of the pores is too large. b) However, the size of the pores of filter paper can be reduced by impregnating them with colloid or gelatin solution to stop the flow of colloidal particles. c) The colloidal solution generally used is 4% solution of nitrocellulose in a mixture of alcohol and ether. d) An ultra-filter paper may be prepared by soaking the filter paper in a colloidal solution, hardening by dipping in formaldehyde solution and then finally drying it. e) With these ultra-filter papers impurities of different sizes can be effectively removed. In this method, sol is poured over the ultrafilters which allows solution of impurities to pass through but retains the colloidal particles. f) The colloidal particles left on the ultra-filter paper are then stirred with fresh dispersion medium (solvent) to get a pure colloidal solution. This is a slow process and to speed up the process, pressure or suction is applied. 57 Colloidal state 24-25 Ultra-Centrifugation In this method, the colloidal sol is taken in a tube which is placed in an ultra centrifuge. On rotation of the tube at high speeds, the colloidal particles settle down at the bottom of the tube and the impurities remain down in the solution called centrifugate. The settled colloidal particles are mixed with an appropriate dispersing medium to regenerate the sol. 58 Colloidal state 24-25 Application of Colloids Most of the substances we come across in our everyday life are colloids and we mainly colloids ourselves. The food we eat, the clothes and shoes we wear, the wooden furniture we use, the houses we live in, the very books and newspapers we read, are all largely composed of colloids. Let’s us examine our dinner table from this angle. Article Type of Colloidal system 1. Halwa Gel structure 2. Steaming hot food Droplets of water dispersed in air 3. Biscuits Air dispersed in fat 4. Milk Fat globules dispersed in water 5. Butter Water dispersed in fat 6. Cheese Fat dispersed in solid casein 7. smoke from cigarettes Carbon dispersed in air 8. Ice cream Ice particles dispersed in cream 9. Fruits Juice dispersed in the solid tissue of fruit 10. Lassi Suspension of casein in water We will now discuss a few important applications of colloid chemistry. Colloidal Medicines Colloidal medicines are more effective on account of their easy assimilation and adsorption (i) Argyrol and protargol are protected colloidal solution of silver. They are used as a cure for granulations. 59 Colloidal state 24-25 (ii) Colloidal gold, manganese, calcium, etc., are used for intra-muscular injections to raise the vitality of human system in diseases like tuberculosis and rickets. (iii) Colloidal sulphur is used as a germ killer, especially in plants. (iv) Colloidal antimony has been shown to be effective in curing kalazar. Smoke Precipitation Every day we see so much smoke coming out of the chimneys. Production in big cities cannot be avoided due to the existing large number of factories, which are an essential feature of modern civilization. Now, smoke is a colloidal system consisting of carbon particles dispersed in air. The carbon particles remain in colloidal suspension because they carry an electrical change, which prevents them from setting. If somehow their charge could be removed, the smoke particles would precipitate. To achieve this, Cotterell devised an electrical precipitator in which the carbon particles in smoke are made to settle down by bringing them in direct contact with oppositely charged metallic plates. A modern form of this precipitator is shown below. 60 Colloidal state 24-25 High voltage wire Chimney Smoke + ASH Smoke precipitation The smoke is led through a chamber provided with a metallic knob charge to a very high potential. The charged particles are attracted to the knob, lose their charge, and fall down while the gases free from smoke pass up the chimney. Sewage Disposal Dirt particles dispersed in water carry a charge on them and may be separated by Cataphoresis. A system of two tanks fitted with metallic electrodes is used to effect the separation. 61 Colloidal state 24-25 Coagulated Dirt Dirt Sol. + Electrode Electrode Sewage disposal. The solid dirt particles dispersed in water are coagulated on the oppositely charged electrodes. The deposit thus obtained may be utilized as a manure. Purification of Water Impure water contains clay particles and bacteria, etc., suspended in it. When alum is dissolved in such water, the coagulation of colloidal impurities takes place. Alum furnishes aluminium ions Al3+ on solution, which discharge the negatively charged sol. The clear water can then be decanted off. Formation of Delta The river water carries with it sand particles and many other substances in the suspended state. Seawater, on the other hand, contains a number of electrolytes dissolved in it. When the river water meets seawater the colloidal sand and other suspended materials present in the former are precipitated by the electrolytes of the latter and delta is formed. 62 Colloidal state 24-25 Cl- Mg2+ Na+ Cl- Na+ Cl- Na+ Cl- Na+ Cl- Ocean Cl- Coagulated sand particles (Delta) River Negatively charged sand particles Formation of Delta (illustration). Rubber Industry Rubber is obtained from the sap of certain trees as an emulsion of negatively charged rubber particles in water (Latex). On boiling, the protective layer of the protein, naturally covering the particles, is broken and the rubber particles are exposed to the coagulating effect of salts present in the dispersion medium. The coagulated mass is vulcanized (treated with sulphur) and sold as rubber. Metal and wooden articles can be "rubber plated" by cataphoresis when the negatively charged rubber particles are deposited on the article, which is made the anode in such a process (Anode process). Tanning Both hides and leather are gel structures containing proteins in the colloidal state. When hides are soaked in tannin which itself is a sol., the 63 Colloidal state 24-25 mutual coagulation of the positively charged particles of the hide and the negatively charged particles of tannin take place. This process called tanning imparts hardness to leather, which now has a less tendency to undergo putrefaction. Among the material-tanning agents, chromium salts are used with advantage for the precipitation of the hide material and the process is called chrome tanning. Cleanning Action of Soap Action of soap is two-fold : (a) It forms a colloidal solution in water and removes dirt by simple adsorption at the surface of the sol. particles; and (b) It emulsifies the greasy or oily materials, which are there by detached from the body or cloth. Other substances, which are usually sticking on account of the grease, are also automatically released when the latter is emulsified. Soap solution Water Oil layers Fabric Oil Drops Water Soap solution 64 Colloidal state 24-25 Chemical Warfare smokes or mist screens are formed by the dispersion of irritant or otherwise harmful substances by bombs, portable sprays, etc. Gas masks are essential to filter out irritant or toxic smokes, which are adsorbed by a suitable adsorbent like colloidal animal charcoal. Photographic Plates These are thin glass plates coated with gelatin containing a fine suspension of silver bromide. A solution of potassium bromide and gelatin is mixed with a solution of silver nitrate. Insoluble silver bromide appears in the form of a fine suspension protected by gelatin. This is then painted on the glass plates. Colloids in Nature (i) Rain: When air saturated with water vapour reaches a cool region, colloidal particles of water in air are formed due to condensation. Further condensation results in the formation of bigger drops of water, which fall due to gravity. Sometimes oppositely charged clouds meet each other and mutual coagulation results in rainfall. Bancroft succeeded in causing artificial rain by throwing electrified sand from an aeroplane and thus coagulating the mist hanging in the air. Mist is, of course, colloidal system with water dispersed in air. (ii) Blue Colour of the Sky: Under the ultra-microscope the colloidal particles appear to be bluish stars. Only blue light is scattered and the rest of it is absorbed. There are numerous dust and water particles floating about in 65 Colloidal state 24-25 the sky. They scatter blue light and make the sky look bluish. Without this scattering, sky would have been all-dark. (iii) Tail of comets: When a comet flies with tremendous velocity, some solid particles are left behind floating in air and produce Tyndall Cone which forms its tail. (iv) Blood: Blood is a colloidal solution of an albuminoidal substance and is coagulated to a clot by trivalent aluminium ions of alum. or ferric ions of ferric chloride. This explains the stoppage of bleeding by the application of alum or ferric chloride. The greater part of the oxygen taken up by blood in the lunges is adsorbed on the surface of colloidal red corpuscles and not dissolved in ordinary sense. Carbon monoxide gas is a deadly poison to human system because once the corpuscles adsorbed this gas they cease to function as "oxygen Carriers". (v) Milk: Milk is an emulsion of fat in water, the emulsifying agents being albumin and casein. It is very easily digestible because emulsified fat offers large surface to the action of digestive juices secreted in the mouth, stomach and elsewhere inside the body. (vi) Soils: Good soils should be colloidal in nature. They can then retain more moisture for emergency and moreover the nourishing materials are supplied to the plant by the process of adsorption. The decayed animal refuse humus serves as a protective colloid for the soil. 66 Colloidal state 24-25 QUASTIONS 1- Distinguish between lyophobic and lyopilic colloids. Explain Schulze-Hardy law. 2- Discuss the precipitation of colloids by electrolytes. What is the influence of the ions carrying the same charge as the colloid on its stability. 3- Describe one important for the preparation of colloids. How is the sign of the charge on a colloidal particle determined? 4- What is meant by: (a) Tyndall effect, and (b) Brownian movement? 5- Explain the term colloidal solution. Describe the different methods used for the preparation of these solutions. Illustrate your answer with suitable examples. 6- Write a short note on the optical properties of colloids. 7- Explain why (a) a colloidal solution is not precipitated in the presence of gelatin. (b) A colloidal solution contains electrically charged particles. (c) Alum is used in town water supply. (d) A beam of light passes through a colloidal solution of god. 8- Write notes on the following: (a) Brownian movement, (b) Peptization, and (c) Dialysis. 9- Describe briefly the properties of colloidal solutions of the following: (a) Gold, (b) soap. 10- (a) Name any two general methods for producing colloidal suspensions and illustrate one of them briefly. (b) Explain the cleansing action of soaps. 11- What do you understand by "hydrophobic" and "hydrophilic" colloids? How are an arsenic sulphide solution obtained and its charge determined? 67 Colloidal state 24-25 12- How would you prepare colloidal solution of ferric hydroxide and gold? Give an account of optical and electrical properties of colloids. 13- What do you understand by (a) colloidal solution, (b) Emulsion and (c) gel? Give one example of each. 14- Write a general account of the properties have lyophilic and loyphobic sols. 15- Describe briefly the general methods of preparation of colloidal solutions. Explain dialysis, peptization and gold number. 16- Describe the properties of colloidal solutions. How are they prepared? 17- Write a brief note on the preparation, properties and applications of colloids? 18- What are "colloidal solutions"? Give important methods of preparing lyophobic sols. Explain the terms (a) Coagulation of colloids by electrolytes, (b) Brownian movement, (c) Cataphoresis and (d) Tyndall effect. 19- Indicate the general methods of preparing colloidal solution. Write notes on (a) gold number and (b) Cataphoresis. 20- Compare and contrast the properties of hydrophobic colloids and those of hydrophilic colloids. Give suitable examples to illustrate your answer. 21- Discuss some of the important method for preparation of colloids. 22- What is meant by (a) Tyndall effect, (b) Brownian movement, (c) Cataphoressis, (d) Dialysis? 23- Discuss some important methods of preparation and purification of colloids. 68 Colloidal state 24-25 Choose the correct answer 1. This method is commonly used for destruction of colloid (a) Addition of electrolytes (b) Condensation (c) Dialysis (d) Filtration by animal membrane 2. Smoke is one of the………….. (a) Sol (b) Aerosol (c) Emulsion (d) none of the above 3. The ability of an ion to coagulate a given colloid is dependent on the (a) only charge (b) magnitude (c) both magnitude and charge (d) sign of the charge 4. The position occupied by non-polar hydrophobic and polar hydrophilic group in a micelle is (a) non-polar towards the outside and polar towards inside (b) polar towards the outside and non-polar towards inside (c) both present only at the surface (d) both distributed throughout 5. Depending upon the type of the particles of the dispersed phase, the colloids are classified as: (a) micromolecular, macromolecular and multimolecular (b) micromolecular, macromolecular and associated (c) Associated, macromolecular and multimolecular (d) none of the above 6. …………… consist of large size molecules.? (a) Macromolecular Colloids (b) Associated Colloids (c) Multimolecular Colloids (d) None of the above 7. A colloidal solution consists of (a) dispersed phase (b) dispersion medium (c) dispersed phase in dispersion medium (d) dispersion medium in dispersed phase 8. Which of the following acts as the best coagulating agent for ferric hydroxide sol? (a) Potassium ferrocyanide (b) Potassium chloride (c) Potassium oxalate (d) Aluminium chloride 9. Tyndall effect confirms the (a) gravity effect on the sol. Particles (b) light scattering by the sol. particles (c) heterogeneous nature of sols. (d) Brownian motion of the sol. particles 69 Colloidal state 24-25 10. the term anaphoresis refers to: (a) electrophoresis of positive charges (b) electrophoresis of negative charges (c) both a and b (d) None of the above 11. The scattering of light by the dispersed phase called: (a) Brownian movement (b) Tyndall effect (c) Electrophoresis (d) None of the above 12. The lyophilic sols are: (a) reversible in nature (b) may be reversible and nonreversible (c) irreversible in nature (d) None of the above 13. The movement of the dispersion medium under the influence of applied potential is known as: (a) osmosis (b) diffusion (c) electro-osmosis (d) Electrophoresis 14. The …………the gold number of the hydrophilic colloid, the greeterthe protective power. (a) greater (b) constant (c) lower (d) None of the above 15. The charge on the colloidal particles is developed due to: (a) frictional electrification (b) self-dissociation (c) selective adsorption (d) All of the above Put T (true) or F (false) for the following sentences: 1. Lyophobic sols are less stable than lyophilic colloid. Hence, they are more hardly coagulated. 2. The protected particles can be adsorbed on the surface when their particles smaller than protecting ones. 3. The stability of lyophobic sol is only due to the charge on the colloidal particles. 4. Colligative properties depend only on the number of dissolved particles in solution and their identity. 5. Tyndall effect is observed when the diameter of the dispersed phase particles is much smaller than the wavelength of light used. 70 Colloidal state 24-25 6. The colour of gold changed from red to golden when the size of the particles increases. 7. For silver iodide sol, the particles may either be positively or negatively charged according to the amount of iodide. 8. when ferric chloride is added to sodium hydroxide solution, a positively charged sol is obtained due to the adsorption of Fe3+ ions. 9. When a strong beam of light is passed through a true solution, the path of the light does not become visible. 10.The sol particles diffuse from higher to lower concentration at a lesser speed, due to bigger size of particles. 11.the number of milligrams of the colloid required to prevent the coagulation of a one liter of a given gold sol called gold number. 12.Lyophobic sols are less stable due to their charged and the solvation of the colloidal particles. 13.If positively charged Fe(OH)3 sol and negatively charged As2S3 sol are mixed, both the sols get protected. 14.The coagulating power is directly proportional to coagulation value. 15.Multimolecular colloids consist of aggregates of atoms or molecules which generally have diameter less than 1 nm. 71