Surface Chemistry

Surface Chemistry ADSORPTION 1. 2. 3. 4. Adsorption:- The phenomenon of increase in concentration at the surface due to molecular surface 5. force is known as adsorption. Adsorbate:- The substances, gases or liquids which are adsorbed on the surface of adsorbent are called 6. adsorbate. Adsorbent:- The solid substance on the surface of which adsorption takes place is called adsorbent. Absorption:- When a substance is uniformly distributed throughout the body of a solid or liquid, the phenomenon is called absorption. Sorption:- It may be defined as the process in which both adsorption and absorption take place simultaneously. Desorption :– Removal of the adsorbed substance from surface. Examples of adsorbents are activated charcoal, Pt, Pd, Ni, Silica gel, gelatin, Al2O3, starch. Adsorption 1. It involves higher concentration of the gas or liquid at the surface of a substance. 2. It is rapid in the beginning and slows Absorption 1. It involves uniform distribution of the molecular species throughout the bulk 2. It occurs at a uniform rate. down near the equilibrium. 3. It is a surface phenomenon. 3. It occurs throughout the body of the material. 4. It is a fast process. 4. It is a slow process. 61 Surface Chemistry but on the surface, particles are not surrounded by atoms or molecules on all sides and hence they possess unbalanced or residual attractive forces. MECHANISM OF ADSORPTION Adsorption is due to the fact that the surface particles of the adsorbent are in different state than the particles inside the bulk. Inside the adsorbent all the forces acting between the particles are mutually balanced Types of adsorption:- Depending upon the nature of forces between molecules of adsorbate and adsorbent, adsorption is of two types. Comparison of physi-sorption and chemi-sorption Physical Adsorption Chemical Adsorption (Activated ad.) 1. It is caused by intermolecular vander waal's forces. It is caused by chemical bond formation. 2. It is not specific. It is highly specific. 3. It is reversible. It is irreversible. 4. Heat of adsorption is low. – 20 to –40 kJ/mol Heat of adsorption is high. – 200 to –400 kJ/mol 5. No appreciable activation energy is involved. High activation energy is involved. 6. It forms multimolecular layers on adsorbent surface It forms unimolecular layer. under high pressure. Special point : Chemical Adsorption is endothermic  Ex. : Glass + H2 gas [Exception] FACTOR EFFECTING RATE OF ADSORPTION (2) Nature of adsorbate: (1) Nature of adsorbent : (A ) A dsor ption of Gas : Adsorption TC of gas Adsorption  Surface area of adsorbent or free valency of the solid.(So, powdered form of adsorbent is more effective) (B) Adsorption from solution: Adsorption  Concentration of solute in the solution. Volume of Gases at N. T. P. adsorbed by 1 g of Charcoal at 288 K Gas H2 N2 CO CH4 CO2 HCl NH3 SO2 Critical Temperature, Tc (K) 33 126 134 190 304 324 406 430 Volume adsorbed (mL) 4.7 8.0 9.3 16.2 48 72 181 380 Critical temperature increase Ease of liquification increase Extent of adsorption increase 62 Surface Chemistry (3) Effect of temperature :  Adsorbate + Adsorbent   Adsorption,  H = –ve, exothermic process. Normaly adsorption decrease with temp. because adsorption is exothermic process. Physical adsorption shows regular decrease with temp., but chemical adsorption first increase then decreases with temp. because it is specific, requires activation energy & in this process supplied heat may be used as its energy of activation. In solution (liquid + solid) Ex. : dyes solution + Charcoal 1 x  KC n m (4) Effect of Pressure:- Adsorption Isotherm. The extent of the adsorption of a gas on a solid is generally expressed as x m Where : x – Mass of adsorbate, m – Mass of adsorbent Theoretically x m P C = (Concn of liquid) (ii) Langmuir adsorption isotherm (Chemisorption, unilayer adsorption) : Langmuir 1916 postulated that : 1. The gases being adsorbed cannot form a layer more than a single molecule in depth. 2. There in no interaction between the adjacent adsorbed molecules. 3. A dynamic equilibrium exists between the adsorbed molecules and the free gas molecules. 4. The adsorption sites are equivalent in their ability to adsorb particels. The constants a and b are characteristic of a particular system at a particular temperature. x ac  m 1  bc For Solid + Liquid Applications of adsorption :(1) Production of high vaccum (2) Gas masks (3) Humidity control (4) Removal of colouring matter from solution 1 x n  Kp According to this .........(i) (5) Heterogeneous catalysis m K & n are constants. Value of K & n depends on (6) Seperation of inert gases (7) Softening of hard water nature of adsorbent & nature of adsorbate. (8) De ionisation of water x According to this increase with increase of P but m (9) In curing diseases x at high pressure becomes constant. (10) Cleansing agents m When taping logarithm of equation. (11) Froth floatation process (12) Adsorption indicators x 1 log = log K + log P m n (13) Chromatographic analysis Types of adsorption isotherm : (i) Freundlich adsorption isotherm (Physical adsorption) : Adsorbent – Solid , Adsorbate – Gas 63 Surface Chemistry (ii) N egat i ve cat al yst s: - The substance which CATALYSIS decreases the rate of chemical reaction is called The phenomenon in which the rate of a reaction is negative catalyst or inhibitor. It increase the activation altered (increased or decreased) by the presence of energy for the reaction. For example. a substance (Catalyst) is known as catalysis. 1 C H OH Catalytic reactions are divided into two types. Na2SO3 + O2  Na2SO4 2 (a) Homogeneous catalysis 1 Di Phenyla min e  C H COOH C6H5CHO + O2  6 5 (b) Heterogeneous catalysis 2 (a) Homogeneous catalysis:- When the reactants and (iii) Autocatalysts:- When one of the products of the the catalyst are in the same physical state, i.e. in the reaction begins to act as a catalyst, it is called autosame phase, it is called homogeneous catalysis. For catalyst. For example. In the initial stages the example:reaction is slow but as soon as the products come (i) Lead chamber process:- In this process for the into existence the reaction rate increases manufacture of sulphuric acid (H2SO4), NO (gas) is  CH 3 COOC 2 H 5 + H 2O   CH 3 COOH + used as a catalyst. C2H5OH (Auto- catalyst) No (g) 2SO2 (g) + O2 (g)    2SO3 (g)  2KMnO 4 + 5C 2O 4 H 2 + 3H 2 SO 4  (ii) Inversion of cane sugar:- In aqueous solution, it is 2MnSO4 + K2SO4 + 10CO2 + 8H2O (Autocatalysed by dilute acid (Hydrogen ions) catalyst) H SO (aq ) C12H22O11+H2O  C H O + C H O  6 12 6 6 12 6 (iv) Induced catalyst:- When a chemical reaction Sucrose Glucose Fructose enhances the rate of another chemical reaction it is (b) Surface catalyst or Heterogeneous catalysis:called induced catalysis. For example:When the catalyst and the reactants are not in the Soduim arsenite solution is not oxidised by air. If same physical state i.e. not in the same phase, it is however, air is passed through a mixture of called heterogeneous catalysis. For example. Na 3AsO 3 & Na2SO 3, both of them undergo (i) Decomposition of H2O2 (): simultaneous oxidation. The oxidation of sodium 2 2 4 Pt (S ) 2H2O2()   2H2O() + O2 (g) (ii) Habers process for NH3 Fe  Mo (s)   N2(g) + 3H2 (g)   (ii) Synthesis of CH3OH CO(g) + 2H2(g) 5 Cu   ZnO  Cr2 O 3 2NH3 (g) CH3OH() TYPES OF CATALYSIS Catalysts are divided into four types. (i) Positive catalysts (ii) Negative catalysts (iii) Autocatalysts (iv) Induced catalyst (i) Positive catalysts:- The substance which increase the rate of a reaction are known as positive catalyst. It decreases the energy of activation for the reaction. For example:MnO (s) 2KClO3 (s)   2KCl (s) + 3O2(g) Fe (s) N2 + 3H2  2NH (g)  3 sulphite, thus influences the oxidation of sodium arsenite. Na2SO3 + 1 2 O2  Na2SO4 Sod. sulphite air Na3AsO3 + 1 2 Sod.arsenite air Sod. sulphate. O2  Na3AsO4 Sod. aresenate Promoters:- Those substance which do not themselves act as catalysts but their presence increases the activity of a catalyst are called catalytic promoters or catalyst for a catalyst. Example:In the Haber process for the synthesis of ammonia, Fe is the catalyst while molybdenum (Mo) acts as a promoter. 2 Fe (Catalyst )    2NH (g) N2 (g) + 3H2 (g)   Mo (promoter ) 3 64 Catalytic Poison:- The substance whose presence decreases or destroy the activity of a catalyst is called catalytic poison. For example:- Carbon monoxide or H2S in hydrogen gas, acts as a poison for Fe catalyst in the Haber process for NH3, As2O3 acts as poison for Pt asbestos in contact process for H2SO4 Surface Chemistry Thus the reactants do not directly combine with each other, instead they react through the catalyst which provides an alternative pathway which involves lesser energy of activation. For example:- The function of nitric oxide [NO] as a catalyst in the formation of SO3 is explained as follows. Inhibitors:- Those substances which retard rate of a chemical reaction are known as inhibitor. For example: Ist step H3PO4, glycerol or acetamide decrease the rate of decomposition of hydrogen peroxide. NO 2 General characteristics of catalysts:(i) A catalyst remains unchanged in mass and chemical composition but can change their physical state. (ii) Only a very small amount of catalyst is sufficient to catalyse a reaction. 2 NO + O2  Catalyst reactant 2NO2 Intermediate IInd step fast + SO2  + NO  SO3 reaction intermediate reactant product catalyst regenerated Adsorption theory:- This theory explains the heterogeneous catalysis. The role of a solid catalyst in enhancing the reaction rate is explained on the basis of this theory in the following steps. (iii) A catalyst does not initiate a reaction. (i) The reactant molecules are adsorbed on the surface (iv) When a catalyst is a solid, it is usually more efficient of the catalyst at adjacent points. Adsorption leads when used in finely divided form. to higher concentration of the adsorbed reactant on the surface of a catalyst. (v) Generally catalyst does not change the nature of products. (ii) As adsorption is an exothermic process, the heat of adsorption provides the nescessary activation energy (vi) A catalysts does not change the equilibrium state of for the chemical reaction to proceed. a reversible reaction but helps to achieve the equilibrium state or position of equilibrium in lesser (iii) The adsorbed reactant molecules are tied on the solid time. surface of the catalyst. The bonds between the atoms of chemisorbed reactant molecules are weakened. (vii) The catalysts is generally specific in nature. The reactant molecules of sufficient energy combine (viii) Change rate constant of reaction. together and with the surface of the catalyst to form (ix) Does not change free energy of reaction. surface activated complex. (x) Participate in mechanism of reaction. This adsorbed activated complex is decomposed to form products at a definite faster rate. THEORIES OF CATALYSIS Intermediate compound formation theory:- (iv) The product molecules rapidly leave the catalyst surface to make room for the other reactant This theory explains homogeneous catalysis mainly. molecules to get adsorbed. Thus the chemical According to this theroy, the catalyst combines with combination between reactant molecules goes on at one of the reactants to give an intermediate the surface of the catalyst at a much faster rate. compound. This intermediate compound reacts with the other reactants and gives the product and regenerates the catalyst in its original form. 65 Surface Chemistry Catalysts in Industry:- Some of the important processes and their catalyst are given below. Industrial process Catalyst used 1. Perparation of O2 from KClO3(Homo) MnO2 2. Haber's process for ammonia (Hetero) Fe: Mo as promoter 3. Ostwald's process for HNO3 from NH3(Hetero) Pt Gauge 4. Chamber process for H2SO4 (Homo) Nitric oxide (NO) gas 5. Contact process for H2SO4 (Hetero) V 2O 5 6. Deacon's process for manufacture of Cl2(Hetero) CuCl2 7. Bosch process for H2 (Hetero) Fe2O3 + Cr2O3 (promoter) 8. Manufacture of vegetable ghee (Hetero) Ni, Cu as promoter 9. Hydrolysis of ester (Homo) aq H2SO4 10. Aldol condensation of aldehyde (Homo) — 11. Enolisation of ketone (Homo) H+ or —OH 12. Cracking of hydrocarbon (Hetero) Zeolites 13. Decomposition of O3 'Cl' atom 14. 2CO + O2  2CO2 'NO' ZEOLITE (Shape selective catalyst) : 1. OH COLLOIDAL SOLUTION The calatytic reaction that depends upon the pore structure of the catalyst & the size of the reactant is called shape selective catalyst. The pore size is generally in the range 260-740 pm. Thomas Graham (1861) studied the process of diffusion of dissolved substances through a parchment paper or an animal membrane and divided the substances into two classes :- Ex. Zeolite (honey comb – like structure) (i) Crystalloid 2. Zeolite is also called sodium alumino silicate, water softner & cationic exchanger (Exchange only cation from hard water). But this classification soon proved to be wrong since a crystalloid could behave as a colloid under different conditions and vice-versa. 3. Zeolite is 3d silicate. 4. Zeolite, before using as catalyst are heated in vacuum so that the water of hydration is lost and as a result Zeolite becomes porous. For example NaCl behaves as a crystalloid in aqueous medium and behaves as a colloid in benzene medium, whereas soap behaves as a typical colloid in water and behaves as a crystalloid in alcohol. * Application of Zeolite : Normally used in petro chemical industries for cracking of hydrocarbon & isomerisation. (ii) Colloid So new classification was given based on the size of solute particles. ZSM 5 ** R – OH    gasoline (Petrol) 66 Surface Chemistry Table : Comparison of True solutions, Colloids and Suspension S.No. Property True Solution Colloid Suspension < 10–7 cm or 10Å 10–7 to 10–5 cm > 10–5 cm or 103Å or 1 mor < 1 nm or 10Å to 103Å or 100 m or 1m to 100 m or > 100 nm or Crystalloidal (i) Particle size or 1 nm to 100 nm (ii) (iii) Visibility Not visible with any of Visible with Visible with naked the optical means ultramicroscope eye Not possible Not Possible Possible Not possible Possible Possible Seperation (a) with filter paper (b) with membranes (iv) Diffusion Diffuses rapidly Diffuse very slowly Does not diffuse (v) Settling Does not settle Does not settle but it Settles under may settle under gravity Centrifuge (vi) Nature Homogeneous Heterogeneous Heterogeneous (vii) Tyndall effect & does not show shows May or may not show Brownian moment TYPES OF COLLOIDAL SOLUTIONS A colloidal system is made of two phases. The substance distributed as the colloidal particles is called Dispersed phase or internal phase or the discontinuous phase. The continuous phase in which the colloidal particles are dispersed is called dispersion medium. For example, for a colloidal solution of copper in water, copper particles constitute the dispersed phase and water the dispersion medium. Depending on the physical states of dispersed phase or dispersion medium, colloidal solutions are of eight types 67 Surface Chemistry CLASSIFICATION OF COLLOIDAL SYSTEMS BASED ON DISPERSED PHASE AND DISPERSION MEDIUM Type of Colloidal Dispersed Dispersion Example Solution phase medium Foam gas Liquid Whipped Cream, Shaving Cream, Soda water Solid Foam gas solid Cork, pumice stone, Foam rubber Liquid Aerosol liquid gas Fog, mist, clouds, insecticide sprays. Emulsion liquid liquid milk, hair cream, *butter, cold cream. Gel liquid solid butter, cheese, curd, Jellies, boot polish Smoke (aerosol) solid gas dust, soot in air Sol (liquid) solid liquid ink, colloidal gold, paints, cell fluids. Solid sol solid solid Ruby glass (gold dispersed in glass), alloys gem stones, minerals. A colloidal dispersion of one gas in another is not possible since the two gases would give a homogenous molecular structure. CLASSIFICATION OF COLLOIDAL SYSTEMS : (1) Classification based on appearance :- If the (2) Classification based on interaction of phases :colloidal system has the appearance of a fluid, then it is called a Sol. On the basis of dispersion medium LYOPHILIC AND LYOPHOBIC SOLS Colloidal solutions in which the dispersed phase has different types of sol are possible. considerable affinity for the dispersion phase, are S.No. Dispersion medium Name of solution called lyophilic sols (solvent - liking). For example 1. Water Hydrosol or aquosol dispersion of gelatin, starch, gum and proteins in 2. Benzene Benzosol water. 3. Alcohol Alcosol Colloidal solutions in which the dispersed phase has 4. Air Aerosol. no affinity or attraction for the medium or for the The colloidal systems which have more rigid solvent are called Lyphobic colloidal (Solvent hating) structures are called Gel. Gelatin behaves both as a solutions. sol and a gel. 68 Surface Chemistry COMPARISION OF LYOPHOBIC AND LYOPHILIC SOLS S.No. Property 1. Preparation Lyophilic sol Lyophobic sol (Emulsoid) (suspensoid) Can be easily prepared by Can not be prepared easily shaking or warming the Special methods are required substance with solvent 2. Stability are more stable are less stable 3. Reversibility are reversible are irreversible 4. viscocity viscocity is much higher viscocity is nearly same than that of solvent as that of the solvent Surface tension is usually low Surface tension is almost 5. Surface tension same as that of solvent 6. 7. Hydration or These are highly solvated These are less solvated as solvation as the particles have great the particles have less affinity for solvent affinity for the solvent The particles have little The particles carry a charge or no charge at all characteristic charge either Charge positive or negative 8. Visibility Particles can not be seen Particles can be seen under under microscope microscope Coagulation or Precipitated by high Precipitated by low precipitation concentration of electrolytes concentration of electrolytes 10. Tyndall effect Less Scattering More Scattering 11. Migration in may or may not migrate migrate towards anode or electric field as they may or may not carry charge. cathode as these particles carrry charge. 9. 12. General Example Mostly of organic nature Mostly of Inorganic nature Ex. Gelatin, Starch, Gum, Ex. Transiton metal salt in water. Albumin & Cellulose Solution Gold, As etc. METHODS OF PREPARATIONS Lyophilic sols may be prepared by simply warming (a) Dispersion Methods : In this larger macrosized the solid with liquid dispersion medium eg. starch particles are broken down to colloidal size with water. On the other hand lyophobic sols have (b) Condensation methods : In this colloidal sized to be prepared by special methods. These methods particles are built up by aggregating single ions or fall into two categories molecules. This method is known as condensation method. 69 Dispersion method 1. Mechanical dispersion 2. Electro-dispersion 3. Ultrasonic dispersion 4. Peptization Aggregation or condensation method 1. 2. 3. 4. 3. Surface Chemistry DISPERSION BY ULTRASONIC VIBRATIONS By condensing vapours Exchange of solvents (Physical method) Change of physical state (Physical method) Chemical methods (a) Double decomposition (b) Oxidation (c) Reduction (d) Hydrolysis DISPERSION METHODS 1. MECHANICAL DISPERSION :The substance to be dispersed is grinded as finely as possible and then shaken with the dispersion medium to form a suspension. This suspension is passed through colloidal mill where the suspended particles are broken to produce particles of colloidal size. 2. ELECTRO-DISPERSION (BREDIG'S ARC 4. METHOD) Claus prepared mercury sol by subjecting Hg to sufficiently high frequency ultrasonic vibrations as shown in fig. The ultrasonic vibrations produced by the generator spread through the oil and strike the vessel containing Mercury under water. This produces clouds of Mercury which form the Mercury sol. PEPTIZATION The dispersion of a freshly precipitated material into colloidal solution by the action of an electrolyte in solution is termed peptization. The electrolyte used is called a Peptizing agent. A. few examples of sols obtained by peptization are : (i) Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride solution at once forms a dark reddish brown colloidal solution. Ferric chloride acts as a peptizing agent. (ii) Freshly precipitated silver chloride can be converted This method is suitable for the preparation of colloidal into a colloidal solution by adding a small amount of solutions of metals like gold, silver, platinum etc. An hydrochloric acid. arc is struck between the metal electrodes under the (iii) Cadmium sulphide can be peptized with the help of surface of water containing some stabilizing agent hydrogen sulphide. such as a trace of KOH. The water is cooled by The process of peptization thus involves the immersing the container in a ice bath. The intense adsorption of suitable ions (supplied by the heat of the arc vapourises some of the metal which electrolyte added - particularly a common ion) and condenses under cold water. electrically charged particles then split from the Ex. : Pt, Ag, Cu, Au, Sol are prepared by this precipitate as colloidal particles. method. 70 CONDENSATION METHODS (1) Physical Methods : Surface Chemistry (c) Reduction - Colloidal solutions of metals like gold, silver, platinum, lead etc. can be obtained when their salts solutions are acted upon by reducing agents. (a) By condensing vapours : Sols of Sulphur and 2AuCl3 + 3SnCl2 3SnCl4 + 2Au Sol. Mercury can be formed by passing the vapours of Sulphur and Mercury in cold water containing some (d) Hydrolysis :- Colloidal solution of many metal hydroxides are prepared by this method. Ferric NH4NO3. hydroxide sol is prepared by the hydrolysis of ferric (b) By exchange of solvent : when a true solution is chloride with boiling water. mixed with an excess of some other solvent in which FeCl3 + 3H2O Fe(OH)3 + 3HCl the solute is insolulble but solvent is soluble, a colloidal The hydroxide sol of Cr and Bi can also be prepared sol is formed. For eg. when a solution of sulphur in by this method. alcohol is poured in excess of water, a colloidal sol of sulphur is formed. * Purification of Colloidal Soln – (c) By excessive cooling : Molecules of certain substances condense together to form particles of colloidal size. For eg. colloidal sol of ice in an organic solvent like ether or chloroform may be prepared by freezing the mixture of water and the solvent. PROPERTIES OF COLLOIDAL (2) Chemical method : SOLUTIONS (1) Dialysis (2) Ultrafiltration (3) Ultracentrifugation The chemical method involve chemical reactions in 1. a medium in which the dispersed phase is sparingly soluble. A condition of super-saturation is produced but the actual precipitation is avoided. Some familiar 2. reactions used are : (a) Double decomposition (i) Arsenic sulphide sol: A 1% solution of arsenic oxide is prepared in hot water. The solution is cooled, filtered and is then gradually added to water saturated with hydrogen sulphide, whilst a stream of H2S is being passed through the solution. This is continued till an intense yellow coloured solution is obtained. As2 O3 + 3H2S  3H2O As2S3 (Yellow Sol) + Heterogenous - Colloidal particles in a solution differ in sizes and are not homogenously distributed through out the solution. Visibility – Colloidal particles can not be seen with naked eyes or with the help of microscope. Colloidal particles, however scatter light and become visible as bright spots in a dark background when seen through a ultramicroscope. In 1903 Zsigmondy devised a microscope which is based on the principle of scattering of light by sol particles. A strong beam of light is focussed by a series of lenses in a vessel containing the sol. The colloidal solution is observed at right angle to the beam of light. (b) Oxidation -A colloidal solution of sulphur is obtained by passing H2S into a solution of sulphur dioxide or oxidising agent Ex. : HNO3, Br2, Etc. 2H2S + SO2 2H2O + 3S (Sol) Sulphur sol can also be obtained when H2S is bubbled through an oxidising agent (bromine water or nitric acid) 71 Surface Chemistry 3. Filterability – Colloidal particles pass through an ordinary filter paper but do not pass through parchment and other fine membranes. 4. Surface tension and viscocity – For Lyophobic sols, surface tension and viscocity are not very different from those of the medium, as there is very slight interaction between the suspended particles 8. and the medium. On the other hand Lyophilic sols show a high degree of solvation of the particles and therefore, the properties of the medium are modified. Thus, the viscocity is much higher for the sol than for the medium. Furthermore the surface tension of the sol is lower than that of pure medium. 5. 6. 7. Colour – The colour of hydrophobic sol depends on the wavelength of the light scattered by the dispersed particles. The wavelength of the scattered light again depends on the size and the nature of particles. For example the colour of silver sol changes with the particle (suspended) diameter in solution. 9. Colligative properties - These properties depend on the number of solute particles in solution. In case of colloidal solutions, colloidal particles are the aggregates of ions or molecules and when compared to true solutions or normal solutions, the total no. of particles of solute in colloidal solutions are very less and hence these solutions exhibit colligative properties to lesser extent. Optical properties - Sols exhibit Tyndall effect When a beam of light is passed through a sol and viewed at right angles, the path of the light shows up as a hazy beam of cone (illuminated path of bluish light). This was first observed by Faraday and later by Tyndall and is known as Tyndall effect. The same phenomenon is observed when a beam of sunlight enters a dark room through a small slit. When light is focussed on a sol and observed perpendicular to the beam of light rays, a cone is seen which is called Tyndall cone. Kinetic properties - When a sol is examined with an ultramicroscope, the suspended particles are seen as shining specks of light. By following an individual particle, it is observed that the particle is in a state of continuous motion in zig-zag path. The continuous rapid zig-zag motion of a colloidal particle in the dispersion medium is called "Brownian movement or motion" (first observed by British botanist Robert Brown). The Brownian movement has been explained to be due to the unbalanced bombardments of the particles by the molecules of dispersion medium. Charge on colloidal particles - Colloidal particles always carry an electric charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol. The charge is due to the preferential adsorption of either positive or negative common ion present in excess. 72 Surface Chemistry Adsorption of charge : AgI + KI  AgI / I– (Negative Sol) (Excess) AgI + AgNO3  AgI / Ag+ (Positive Sol) (Excess) FeCl3 + H 2O  Fe(OH)3 / Fe+3 – (Positive Sol) FeCl3 + NaOH  Fe(OH)3 / OH (Negative Sol) SnO2 + 4H+  Sn+4 + 2H2O ; SnO2 / Sn+4 (In acidic medium )(Positive Sol) SnO2 + 2OH–  SnO 23  + H2O ; SnO2 / SnO 23  (In basic medium )(Negative Sol) A list of common sols with the type of charge on their particles. 1. Positively charged Negatively charged Metallic hydroxides Metallic sulphides like As2S3, Sb2S3, CdS e.g. Cr(OH)3, Al(OH)3 & Fe(OH)3 2. 3. Basic dye stuff example - Acid dye stuff example - Congored sols Methylene blue sol Metal like – Au, Ag, Cu, Pt etc. Proteins in acidic medium Proteins in basic medium Sols of starch, gum, gelatin, clay & charcoal ELECTRICAL DOUBLE LAYER The combination of the compact and diffused layer is referred as the Stern double layer. The diffused layer is only loosely attached to the particle surface and moves in the opposite direction under an applied electric field. The potential difference between the fixed layer and the diffused layer of opposite charge is called Electrokinetic Potential or Zeta Potential. The surface of a colloidal particle acquires a +ve/ve charge by selective adsorption of a layer of positive /negative ions around it. This layer attracts counter ions from the medium which form a second layer of –ve/+ve charges. The combination of the two layers of +ve and –ve charges around the sol particle was called Helmholtz double layer. According to modern view, the first layer of ions is 10. Electrophoresis - If electric potential is applied firmly held and is termed Fixed layer while the across two platinum electrodes dipping in a colloidal second layer is mobile which is termed as Diffused solution, the colloidal particles move towards one layer. or the other electrode, due to charge on them. The movement of sol particles under an applied electric potential is called "Electrophoresis". Depending upon the direction of movement of particles towards cathode or anode electrophoresis can be called "cataphoresis' or 'Anaphoresis'. Electrophoresis provides an experimental proof to show that the colloidal particles are charged particles. 73 11. Electr o osmosis - The medium will move in opposite direction to the dispersed phase under the influence of applied electric potential. The movement of dispersion medium under the influence of applied potential is known as 'Electro - osmosis'.(In presence of plasma membrane) 12. Coagulation or Precipitation We know that the stability of a lyophobic sol is due to the adsorption of positive or negative ions by the dispersed particles. The repulsion forces between the charged particles do not allow them to settle. If somehow, the charge is removed there is nothing to keep the particles apart from each other. In such (b) cases they aggregate or flocculate and settle down under the action of gravity. The flocculation and settling down of the discharged sol particles is called coagulation or precipitation. The precipitation can be brought about in five ways- (c) (a) By addition of electrolyte. (b) By electrophoresis. (c) By mixing two oppositely charged sols. (d) By boiling. (e) By repeated dialysis (a) By addition of electrolytes - (d) Surface Chemistry The higher the valency of the flocculating ion, the greater is its precipitating power. Thus for the precipitation of As2S3 sol (–ve) the precipitating power of Al3+, Ba2+, and Na+ ions is in the order Al3+ > Ba2+ > Na+ Similarly for precipitating Fe(OH)3 sol (positive) the precipitating power of [Fe(CN)6]–3, SO42– and Cl– ions is in the order [Fe(CN)6]3– > SO42– > Cl– The minimum concentration of an electrolyte in milli moles required to cause precipitation of 1 litre sol in 2 hours is called FLOCCULATION VALUE. The smaller the flocculating value, the higher will be the coagulating power of the ion. By Electrophoresis During electrophoresis the charged sol particles migrate towards the electrode of opposite sign. There they deposit their charge and then get coagulated (As the neutral particles can aggregate and change to suspension particles.) By mixing two oppositely charged sols The coagulation of two sols of opposite charge can be effected by mixing them.. For eg. Fe(OH)3 (positive sol) and Arsenic sulphide (negative sol) when mixed neutralize each other, join and coagulate. By boiling Sols such as sulphur and silver halides dispersed in water get coagulated when boiled due to increased collosions between sol particles and water molecules, which removes the adsorbed charged layer from the sol and therefore the sol particles settle down. By repeated dialysis - On continuous dialysis, besides the electrolytic impurities the adsorbed ions are also removed and so the sol gets precipitated. When an electrolyte is added in excess to a sol, then the electrolyte furnishes both the type of ions in solution. The oppositely charged ions get adsorbed on the surface of colloidal particles this causes neutralisation and there by the size and mass of (e) colloidal particle increases and it becomes a suspension particle. Due to greater volume and greater mass these suspension particles settle down COAGULATION OF LYOPHILLIC SOLS i.e they coagulate. The ion responsible for Lyophilic sols are stable due to charge and solvation neutralisation of charge on the particle is called the of the colloidal particles. When these two factors flocculating ion. are removed, a lyophillic sol can be coagulated. This is done (i) By adding electrolyte (ii) By adding Hardy Schulze Rule - This rule states that the suitable solvent. precipitating effect of an ion on dispersed phase of When solvents such as alcohol and acetone are added opposite charge increases with the valency of the to hydrophilic sol, the dehydration of dispersed phase ion. occur. Under this condition a small quantity of electrolyte can bring about coagulation. 74 13. Protection or Protective action. Lyophobic sols are readily precipitated by small amounts of electrolytes. However, these sols are often stablized by the addition of Lyophilic sols. The property of Lyophilic sols to prevent the precipitation or coagulation of a lyophobic sol is called protection. The Lyophilic sol used to protect a lyophobic sol from precipitation is referred to as a protective colloid. Lyophilic sols form a thin layer around lyophobic sol or around the ions furnished by electrolyte and therefore the coagulation can not take place (as the size does not increase much). Gelatin, Albumin, Gum Arabia, Potato Starch are some of the examples of Protective colloids. The Lyophilic colloids differ in their protective power. The protective power is measured in terms of "Gold number" introduced by Zsigmondy. The number of milligrams of a hydrophilic colloid that will just prevent the precipitation of 10 ml of standard gold sol on addition of 1 ml of 10% NaCl solution is known as Gold number of that protector (Lyophilic colloid). The precipitation of the gold sol is indicated by a colour change from red to blue when the particle size just increases. The smaller the gold number of a protective Lyophilic colloid, greater is its protection power. Gold Number of some hydrophilic colloids Lyophilic colloid Gold Number Gelatin 0.005 - 0.01 Egg albumen 0.08 - 0.1 Gum arabic 0.10 - 0.15 Potato - starch 25 Pr otection Capacity  1 Pr otection Number (Gold number) Surface Chemistry Gelatin and starch have the maximum and minimum protective powers. Congo rubin Number – Ostwald introduced congo rubin number to account for protective power of colloids. It is defined as the amount of protective colloids in milligrams which prevents colour change red to violet in 100 ml of 0.01% congorubin dye to which 0.16 g equivalent of KCl is added. EMULSIONS These are liquid-liquid colloidal systems There are two types of Emulsions (i) Oil dispersed in water (O/W types) (ii) Water dispersed in oil (W/O types) * In the first type water acts as dispersion medium. Examples of this type of emulsions are milk and vanishing cream. In milk, liquid fat is dispersed in water. * In the second system oil acts as dispersion medium Common examples of this type are butter and Cold cream. * Emulsions of oil and water are unstable and sometimes they seperate into two layers on standing. For stabilisation of an emulsion, a third component called emulsifying agent is usually added. The emulsifying agent form an interfacial film between suspended particles and the medium. The Principal agent for o/w emulsions are proteins, gums, soaps, etc. for w/o emulsion the principal emulsifying agents are heavy metal salts of fatty acids, long chain alcohols etc. Assoicated Colloids [Micelles] Substances whose molecules aggregate to form particles of colloidal dimensions are called associated colloids. The molecules of soaps and detergents are usually smaller than the colloidal particles. However, In concentrated solutions, these molecules associate and form aggregates of colloidal size. These aggregates of soaps or detergent molecules are called Micelles. Soaps and detergents are strong electrolytes and gives ions when dissolved in water C 17H35COONa  C17H35COO– + Na+ Soap 75 Surface Chemistry (a) Elastic gels - These are those which possesses the property of elasticity. They change their shape on applying force and return to original shape when the force is removed. Gelatin, starch and soaps are examples of substances which form elastic gels. (b) Non - elastic gels - These are the gels which are rigid eg. Silica gel. These are prepared by appropriate chemical action. Thus silica gel is produced by adding concentrated hydrochloric acid to sodium silicate solution of the correct concentration. The negative ions aggregate to form a micelle of colloidal size. The negative ion has a long hydrocarbon chain and a polar group (–COO–) at one end. In micelle formation, the long Properties of Gels : hydrocarbon chain (tail) which is insoluble in water 1. Hydration : A completely dehydrated elastic gel it directed towards the centre while the soluble polar can be regenereted by adding water to it. head is on the surface in contact with water. The charge on the micelle is responsible for the stability 2. Swelling : Partially dehydrated elastic gels imbibe water when immersed in the solvent. This results in of this system. increase in volume of the gel which is called swelling. GELS 3. Syneresis : Inorganic gels on standing, loose water A gel is a jelly like colloidal system in which a liquid is dispersed in a solid medium. 4. Gels may be classified into two types - and shrink. This process is called syneresis. Thixotropy : Some gels revert to liquid sols on agitation. This reversible sol-gel transformation is called Thixotropy. Types of colloids according to their size Multi Molecular Macro Molecular Associated colloids Formation by aggregation of Formation by aggregation These are the substances a large number of atoms or of big size molecules. which behave as normal elec- smaller molecules of substance. These are polymers with high trolytes at low concentration Ex. Gold, Sol, Sulphur sol molecular mass but get associated at higher Ex.  Starch, Cellulose, concentration and behave as Protein etc. colloidal solutions. These (Au) (S8) associated particles are also called micelles. Ex.  Soap & Detergent 76 APPLICATIONS OF COLLOIDS Colloids play an important role in our daily life and industry. Some of the important applications of colloids are listed below - 9. Surface Chemistry Adsorption indicators: Surface of certain precipitates such as silver halides have the property of adsorbing some dyes like eosin, fluorescein etc. In the case of precipitation titration of AgNO3 versus NaCl the indicator is adsorbed at the end point producing a characteristic colour on the precipitate. 1. Food. 2. Medicines. 3. Paints. 4. Electrical precipitation of smoke . 5. Purification of sewage water. 11. Colloidal solution of graphite in water is called "Aqua dag" while that in Oil is called Oil dag. 6. Formation of Delta 12. Gold solution in water is called Purple of cassius. 7. Purification of blood . 8. Tanning . Animal leather (skin) is soft due to the presence of very fine globules of oils and fats in the pores of skin. These globules are colloidal in nature. When this soft leather (skin) is placed in salt water, these globules of oils and fats coagulate and settle down in water. Now when the animal hides are dried they become hard. This process of making soft leather to hard leather by dipping it in salt water is called Tanning. Usually chromium salts are used for tanning. 13. Colloidion is cellulose nitrate & ethyl alcohol colloidal solution. 10. Industrial products. Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc are all colloidal solutions. Artificial rain : Can be caused by spraying oppositely charge dust or fine sand or precipitate like AgI (AgI has a crystal structure similar to ice) Critical Micelle concentration (CMC) : The concentration at which associated colloids or Micelles are formed is called CMC. Kraft Temperature (TK) : Temperature at which Micelles are formed. 77