SURFACE CHEMISTRYIntroduction : Surface chemistry is that branch of chemistry which deals with study of the phenomena occuring at the surface or interface, i.e. at the boundary separating two bulk phases. In this chapter our main emphasis will be on three important topics related to surface chemistry, viz., adsorption, colloids and emulsions. Adsorption : The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid resulting into a higher concentration of the molecules on the surface is called adsorption. As a result of adsorption, there is a decrease of surface energy. The process of removal of an adsorbed substance from the surface on which it is adsorbed is called desorption. It is the reverse of adsorption and can be brought about by heating or by reducing the pressure. Adsorbent and adsorbate : The substance on the surface of which adsorption occurs is known as adsorbent. The substances that get adsorbed on the solid surface due to intermolecular attractions are called adsorbate. Charcoal, silica, gel, alumina gel are good adsorbents because they have highly porous structures and have large surface area. Colloids on account of their extremely small dimensions possess enoromous surface area per unit mass and are, therefore, also good adsorbents. Sorption : When both adsorption and absorption take place simultaneously. Eg : Dyes get adsorbed as well as absorbed in the cotton fibre i.e. sorption takes place. Thermodynamics of adsorption : Adsorption is an exothermic process. Therefore H of adsorption is always negative. When a gas is adsorbed the entropy of the gas decreases i.e. S is negative. Adsorption is thus accompanied by decrease in enthalpy as well as entropy of the system, for a process to be spontaneous requirement is that G must be negative. On the basis of equation, G = H - TS, G can be negative if H has sufficiently high negative value as - TS is positive. Thus, in an adsorption process, which is spontaneous, S is negative, and H is also sufficiently negative and as a combination of these two factors, G is negative. H becomes less and less negative as adsorption proceeds further and further. Ultimately H becomes equal to TS and G becomes zero. This is the state at which equilibrium is attained. Enthalpy of adsorption Hadsorption : It is the amount of the heat released when 1 mole of an adsorbate gets adsorbed on a particular adsorbent at adsorption equilibrium. It depends upon the nature of both the adsorbate as well as adsorbent. RESONANCE Difference between physical adsorption and chemical adsorption Physical Adsorption Chemical Adsorption The forces between the adsorbate molecules and the adsorbent are weak van der Waal’s forces. Low heat of adsorption of The forces between the adsorbate molecules and the adsorbent are strong chemical forces. High heat of adsorption of the order of 20-40 kJ mol–1. the order 200-400 kJ mol–1 Usually occurs at low It occurs at high temperature and temperature decreases with increasing temperature. It is reversible. The extent of adsorption depends upon the ease of liquefication of the gas. It is irreversible. There is no correlation between extent of adsorption and the ease of liquefication of gas. It is less specific in It is highly specific in nature, all gases are nature and occurs only adsorbed on the surface when there is bond of a solid to some extent. formation between adsorbent and adsorbate molecules. It forms multimolecular It forms mono-molecular layers. layer. extent of adsorption extent of adsorption (x/m) increases with (x/m) increases with pressure pressure Adsorption of gases on solids : The extent of adsorption of a gas on a solid surface is affected by the following factors: The nature of the gas (i.e. nature of the adsorbate). The easily liquefiable gases such as HCl, NH3, Cl2 etc. are adsorbed more than the permanent gases such as H2, N2 and O2. The ease with which a gas can be liquefied is primarily determined by its critical temperature. Higher the critical temperature (Tc) of a gas, the more easily it will be liquefied and, therefore, more readily it will be adsorbed on the solid. Gas SO2 CH4 H2 TC 330K 190 K 33 K Nature of adsorbent. The extent of adsorption of a gas depends upon the nature of adsorbent. Activated charcoal (i.e. activated carbon), metal oxides (silica gel and aluminium oxide) and clay can adsorb gases which are easily liquified. Gases such as H2, N2 and O2 are generally adsorbed on finely divided transition metals Ni and Co. Activation of adsorbent. (a) Metallic adsorbents are activated by mechanical rubbing or by subjecting it to some chemical reactions. Surface Chemistry - 1 then according to Le-Chaterlier’s principle.... the surface area is increased and therefore. x m x m Temperature Temperature The initial increase in chemisorption with increase in temperature is because of activation energy required. x/m will depend upon the power of pressure which lies between 0 to 1. (x/m) = kp1/n we get log (x/m) = logk + (1/n) log p. in the intermediate range of pressure.2 . He also assumed that the layer of the adsorbed gas was unimolecular. If the above equilibrium is subjected to increase in temperature.. (d) x ap a ap A plot of m/x against 1/p gives a straight line with slope and intercept equal to 1/a and b/a respectively.. At very high pressure x/m =a/b . This may be expressed as : (x/m) = constant or (x/m) p0 0 (since p = 1) or (x/m) = kp0.. The constant k and n can be determined as explained below : Taking logarithms on both sides of Eq. It is represented by the relation : x ap . shows a maximum in the curve and then it decreases continuously. (c) x/m increases linearly with p... m 1 bp b 1 ... This may be expressed as (x/m) p1/n or (x/m) = kp1/n Slope=(1/n) log(x/m) intercept = log k log p One of the drawbacks of Freundlich isotherm is that it fails at high pressure of the gas.. At low pressure the graph is almost straight line which indicates that x/m is directly proportional to the pressure.. This is why the chemical adsorption is also known as “Activated adsorption”. The above relationship is also called Freundlich’s adsorption isotherm. (b) x/m becomes constant i. they are sub-divided into smaller pieces. fractional power of pressure. (c) For determination of the parameters ‘a’ and ‘b’.. it gives O2. Freundlich Adsorption isotherm : The variation of extent of adsorption (x/m) with pressure (P) was given mathematically by Freundlich. He considered adsorption to consist of the following two opposing processes : (i) Adsorption of the gas molecules on the surface of the solid (ii) Desorption of the adsorbed molecules from the surface of the solid He believed eventually a dynamic equilibrium is established between the above two opposing processes. RESONANCE first time for some substance which alter rate of chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction and the phenomenon is known as catalysis.. As a results.. A graph between degree of adsorption (x/m) and temperature ‘t’ at a constant pressure of adsorbate gas is known as adsorption isobar.... This equation applicable only when adsorbate substance form unimolecular layer on adsorbent surface. This may be expressed as : (x/m) p or (x/m) = kp where k is constant...e.. the adsorbing power increases. Eg : Potassium chlorate when heated at 653K to 873K.. When MnO2 is used in this reaction the O2 is quickly at the low temperature hence MnO2 is a catalyst 2KCIO3 2KCI + 3O2 Surface Chemistry . At low pressure according to Eq.(b) At very low pressure : x/m = ap . Catalysts : Berzillus in 1835 used the word catalyst Where n can take any whole number value which depends upon the nature of adsorbate and adsorbent.... This isotherm works particularly well for chemisorption.e. Physical adsorption decreases continuously with increase in temperature whereas chemisorption increases initially.. Thus. It is based on the assumption (a) that every adsorption site is equivalent and (b) that the ability of a particle to bind there is independent of whether or not nearby sites are occupied.. the desorption will be favoured. At high pressure according to Eq.... (a) may be written in its inverse form. Langmuir Adsorption Isotherm : Langmuir derived it on theoretical considerations based on kinetic theory of gases.. Effect of temperature. Mostly the process of adsorption is exothermic and the reverse process or desorption is endothermic... with increase in temperature. Eq. (a) m 1 bp where ‘a’ and ‘b’ are Langmuir parameter..(b) To increase the adsorbing power of adsorbents. the surface is fully covered and change in pressure has no effect and no further adsorption takes place which is clear from the Figure... i. At high pressure the graph becomes almost constant which means that x/m becomes independent of pressure. chemical adsorption... Type of colloid 2.P. HCl( ) (ii) CH3 COOCH3 () CH3 COOH(aq) Table : Type of Colloidal Systems H2SO 4 ( ) (iii) C12H22O11(aq. On the basis of physical state of D. : There are two types : (i) Lyophilic colloids / liquid loving sols / intrinsic colloid.) H2O( ) DP DM Solid Solid Solid Solid Liquid Liquid Liquid Liquid Gas Solid Liquid Gas Sol Aerosol Gel Emulsion Liquid Aerosol Gas Solid Solid Sol Gas Liquid Foam C6H12O6 (aq.P. (ii) Lyophobic colloids / solvent hating colloid / extrinsic colloid. Fe(OH)3. the solution of metals like Ag and Au. and gem stones Paints. hair cream Fog. dust Cheese. proteins. starch. the catalytic activity is localised on the surface on the catalyst. + D. 1 2 Property Lyophilic colloids Lyophobic colloids Ease of preparation These are formed only by special methods There are easily formed by direct mixing. The common examples of lyophilic colloids are glue.M. For example.3 .No. * Colloidal solution are often termed as sol.M.P. Reversible or These are reversible in irreversible nature nature 3 Particles nature 4 Stability The particles of colloids are true molecules but are big in size These are very stable These are irreversible in nature. Homogeneous Catalysis : When catalysts and reactants are in same phase then the process is said to be homogeneous catalysis and NO( g ) Eg : (i) 2SO 2 (g) O 2 (g) 2SO3 (g) Classification of colloids : 1. Colloid solution : It is a heterogeneous system consisting of 2 phase : (1) Disperse Phase (D.P) : The phase which is dispersed through the medium is called dispersed phase or discontinuous phase or internal phase. Ex. RESONANCE Solid Sol Examples Some coloured glasses. It is also known as continuous phase or outer phase or external phase. Name of colloidal system Water Hydro sol or aqua sol Alcohol Alco sols Benzene Benzo sols Air Aero sols * Aquadag & oildag are colloidal solution of graphite in water & oil respectively. egg albumin. intermediate compound formation theory and the old adsorption theory. gelatin. In Gold sol.M. (2) Dispersion Medium (D. colloidal solution may be divided into eight system. On the bases of physical state of D. for D. Ni( s ) (iv) Vegetable oils () H2 (g) Vegetable ghee (s). Gold is D.P. hydroxides like Al (OH)3. soap lather. etc. The colloidal solution in which the particles of the dispersed phase have a great affinity (or love) for the dispersion medium. Colloid Solution : Colloid State : A substance is said to be in colloidal state when the size of the particle of disperse phase is greater than particle of true solution and less than that of suspension solution particle. Examples : Let us consider addition of H2 gas to ethelene in presence of Ni catalyst. butter. On the Basis of interaction of D.M. insecticide sprays Pumice stone.M. foam rubber Froath.P and water is D. * Solution of gas in gas is not a colloidal system because it form homogeneous mixture. cloud. On the basis of D. then process a know as heterogenous catalysis and catalyst is called heterogeneous catalyst Pt ( s ) Eg : (i) 2SO 3 (g) O 2 (g) 2SO3 (g) Fe( s ) (ii) N2 (g) 3H2 (g) 2NH3 (g) Pt ( s ) (iii) 4NH3 (g) 5O 2 (g) 4NO(g) 6H2O(g) Adsorption Theory of Heterogeneous Catalyst : This theory explain the mechanism of heterogeneous catalyst. The particles are aggregates of many molecules These are unstable and require traces of stabilizers Surface Chemistry . are called lyophilic colloids. : Colloidal solution are classified as D. rubber.) C6H12O6 (aq. and D. This theory is combination of two theory.) Glucose Fructose Heterogenous Catalysis : When catalysts and reactants are in different phases. mist. their range of diameters between 1 and 1000 nm (10–9 to 10–6 m).M. 3. metal sulphides like As2S3 Distinction between lyophilic and lyophobic colloids S. The colloidal solutions in which there is no affinity between particles of the dispersed phase and the dispersion medium are called lyophobic colloids. cell fluids Smoke. Colloidal solution = D. the reaction take places as follows.M. and D.M) : A medium in which colloidal particles are dispersed is called dispersion medium. jellies Milk. whipped cream. CH3(CH2)11SO4– Na+. 9 Tyndall effect They do no show tyndall effect They show tyndall effect. (ii) Hydroxy Sol. cellulose. Similarly. 8 Viscosity The viscosity and surface tension of the sols are less than that of the dispersion medium The viscosity and surface tension are nearly the same as that of the dispersion medium. Macromolecular colloids : In this types.Ag.P. but that start flattening with increase in concentration and ultimately form sheet or film like structures which have a thickness of two molecules. Its value depends upon the nature of D. eosin (c) Sulphide Sol. particles carry similar type of charge. In these colloids. Fe(OH)3. Cr(OH)3. TiO2. the mechanism of micelle formation is same as that of soaps. in case of detergents. TiO2. Fe(OH)3. These colloids are quite stable and resemble RESONANCE Aggregation of RCOO– ions to form an ionic micelle. etc. the particles of the dispersed phase are sufficiently big in size (macro) to be of colloidal dimensions. Micelles may contain as many as 100 molecules or more. dust particle in water. etc. (ii) Non Metal sols : S. Multimolecular. Critical micell concentration [CMC] : The minimum concentration required for micell formation is called critical micell concentration. the particles are held together by van der Waal’s forces. I2. The formation of micelles takes place only above a particular temperature called Kraft Temperature (Tk) and above a particular concentration called Critical Micelle Concentration (CMC). Associated colloids (Micelles) : There are some substances which at low concentrations behave as normal strong electrolytes but at higher concentrations exhibit colloidal behaviour due to the formation of aggregated particles. gelatin. Ex. Organic Colloids (i) Homopolar sol .5 Action of electrolytes No effect The addition of small amount of electrolytes causes precipitation (called coagulation) of colloidal solution. Fe2O3. the micelles are spherical in shape. These associated particles are also called micelles. Surface active agent (surfactants. charcoal. These macromolecules forming the dispersed phase are generally polymers having very high molecular masses. Fe2O3. The particles move in a specific direction either towards anode or cathode depending upon their charge in an electric field 7 Hydration The particles of colloids are heavily hydrated due to the attraction for the solvent. Soap. Surface Chemistry .In this type of colloid. On the basis of charge of on particles (i) positive Sol (a) Metal Oxide & Hydroxide . HgS. sodium lauryl sulphate viz.SnO2.g. AI(OH)3. Surface active agents such as soaps and synthetic detergents belong to this class. these colloids revert back to individual ions. * Also CMC increase with decreasing polarity of the D. Cu (b) Acidic dye . Cr(OH)3 (iv) Salt Sol .congo red. Hence.CdS. The particles of colloids are not appreciably hydrated. sols of gold atoms and sulphur (S8) molecules. These are called lamelar micelles or McBain Micelles. AI(OH)3. which decrease the surface tension) like soaps and detergents form micelle beyond CMC (~10–3 mol/litre for soaps). and D. AgI. Sols. Au. macromolecular and associated colloids : Multimolecular colloids : In this type. Sb2S3. eg. (ii) negative Sol (a) Metal sol . Sol of rubber in benzene which contain . Pt.. As2S3. For eg. (d) Natural sol . * Usually longer the hydrophobic chain.AgBr. Pt. * The micelles ‘formation takes place only above a particular temperature called as Kraft Temperature (Tk ). Naturally occuring macromolecules are starch. Mechanism of micelle formation: Let us take the example of soap solutions. Graphite (iii) Sol of oxide and hydroxide : SnO2. latex rubber. the particles consist of an aggregate of atoms or small molecules size less than 1 nm. On dilution. starch carbon particle in smoke.M. clay.Blood. These are also known as associated colloids. For example.M. the polar group is – SO4– along with the long hydrocarbon chain. Associated colloids (Micelles): These are the substances which behave as normal strong electrolytes at low concentration but behave as colloidal particles at higher concentration. smaller is its CMC. 6 Charge on particles The particles do not carry any charge. Au. true solutions in many respects.Starch sol 5. The aggregated particles thus formed are called micelles. As2 S3. 4.ve charge colloidal particle of latex. On the basis of chemical composition: Inorganic Colloids (i) Metal sols : Cu. These colloids have both lyophobic and lyophilic parts. For soaps. (b) Basic Dyes Methylene blue.4 . e. proteins. vismark brown. gum. *At CMC. enzymes. Ag. the CMC is ~ 10–4 to 10–3 mol L–1. ) + S (sol). silver solution are obtained by following method. *It is a suitable technique for oils also. These are stabilized by adding some suitable stabilizer. RESONANCE (Fig. The intense heat produced vaporises the metal. 2 AuCl3 + 3 HCHO + 3H2O 2Au(sol) + 3HCOOH + 6HCl. Phenolphthalein. (i) By Exchange of solvent : When a true solution is mixed with an excess of the other solvent in which the solute is insoluble but solvent is miscible. silver. oxidation.5 .. Some of the methods employed are given below : (a) Mechanical dispersion (By colloidal mill) : Here the substance is first finely powdered. AlCl3 or CrCl3 . or by bubbling O2 (g) through a solution of H2S : 2H2S (aq.Preparation of lyophobic colloidal sols : [A] Condensation methods : In these methods particles of atomic or molecular size are induced to combine to form aggregates having colloidal dimensions. a colloidal sol is obtained.M. sulphur sol can be prepared 2 by this methods. It is shaken with the D. *Sol of gold is also known as purple of cassius. (b) Electrical disintegration or Bredig's Arc method : This process involves dispersion as well as condensation. which then condenses to form particles of colloidal size by surrounding cooling mixture (ice). AlCl3 + 3H2O Al(OH)3 (sol) + 3HCl The colloidal sol of sillicic acid is also obtained by hydrolysis of dilute solution of sodium silicate with hydrochloric acid. As2O3(in hot water) + 3H2S (saturated solution in H2O) Double decomposit ion As2S3(sol) + 3H2O CHOHCOOK | + 3H2S CHOHCOOSbO CHOHCOOK | + Sb2S3 (orange sol) + 2H2O CHOHCOOH (ii) Oxidation : A colloidal sol of sulphur is obtained by passing H2S into a solution of sulphur dioxide. Na4SiO4 + 4HCl Si(OH)4 (sol) + 4NaCl. Oxidation SO2 + 2H2S(saturated solution in H2O) 3S(sol) + 2H2O Sulphur sol can also be obtained when H2S is bubbled through Br2 water or nitric acid (oxidizing agent). copper.) + Br2 (aq. This suspension is passed through a colloidal mill. platinum. *A slight trace of KOH is added in water to stabilized colloidal solutions. These molecules then aggregate leading to formation of sols. to form a suspension.) + O2 (g) 2H2O (l) + 2S (sol). Colloidal sols of less reactive metals such as gold. when a solution of phenolphthalein in alcohol is poured in excess of water a white sol of phenolphthalein is found. *This is the latest method for preparation of metal oxides and metal sulphide sols from their coarse suspension.) 2HBr (aq. (iii) By condensing vapours of a substance into solvent [B] Dispersion Methods : In these methods large particles of the substance are broken into particles of colloidal dimensions in the presence of dispersion medium. FeCl3 + 3H2O Fe(OH)3 (sol) + 3HCl . (ii) Excessive cooling (i) Double decomposition : When a hot aqueous dilute solution of arsenous oxide (As2O3) is mixed with a saturated solution of H2S in water. reduction or hydrolysis. mercury etc. (d) Peptization: The term has originated from the digestion of proteins by the enzyme pepsin. lead etc. I . electric arc is struck between electrodes of the metal immersed in the dispersion medium as shown in fig. can be prepared by this method. In this method. For example. a colloidal sol of arsenous sulphide (As2S3) is obtained. : Bredig's Arc method) (c) Ultrasonic dispersion : Ultrasonic vibration (having frequency larger than audible range) can bring about the transformation of coarse suspension or liquids like oil. when a solution of sulphur in alcohol is poured in excess of water. Colloidal solutions can be prepared by chemical reactions leading to formation of molecules by double decomposition. (iii) Reduction : Colloidal sol of metals like gold. (a) Chemical methods. into colloidal range. 2H2S (aq. (b) Physical methods : The following physical methods are used to prepare the colloidal solutions. For this purpose chemical as well as physical methods can be applied. NH2NH2 can also be used as reducing agent. a colloidal sol of sulphur is obtained. (iv) Hydrolysis : A colloidal sol of metal hydroxides like Al(OH)3 or Cr(OH)3 is obtained by boiling a dilute solution of FeCl3 . This method also comprises both dispersion and condensation. (purple of cassius) Re duction 2 AuCl3 3 SnCl2 3 SnCl4 2Au(sol) Re duction AgNO3 + tannic acid silver sol. Peptization may be defined as (the process of converting a precipitate into colloidal sol by shaking Surface Chemistry . due to high average molecular masses of colloidal particles. Thus in sols of silicic acid. Sn4+ . Purification of Colloidal Sols The colloidal sols obtained by various methods are impure and contain impurities of electrolytes and other soluble substances. they have to be removed. The particles were seen to be in constant zig-zag motion as shown in fig. Example of Tyndall Effect Blue colour of sky and sea water. Light thrown from a projector in cinema hall.. Visibility of tail of comets. Application of Tyndall Effect : (i) In making ultramicroscopes. etc. observed that if a strong beam of light is passed through a colloidal sol placed in a dark place. Brownian movement arises because of the impact of the molecules of the dispersion medium with the colloidal particles. the values of the colligative properties observed experimentally are very small. blood serum. the path of the beam gets illuminated. Hence. However.P and D. SnO2 + 4HCl Sn4+ + 2H2O + 4Cl– . this phenomenon was observed in case of colloidal particles when they were seen under an ultramicroscope. (ii) In finding heterogenity of solution. SnO2 + Sn4+ SnO2 / Sn4+ . Mechanical Properties : Brownian movement: Robert Brown. AgCl : Cl– . FeCl3 Fe3+ + 3Cl– .6 . (a) dispersed phase and (b) dispersion medium. this difference is appreciable and therefore the tyndal effect is quite well defined but in lyophilic sols the difference is very small and the tyndal effect is very weak. Eye +3 +3 microscope +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3 Charge Colloidal particals of Fe (OH)3 (ii) Freshly prepared stannic oxide on treatment with a small amount of dilute hydrochloric acid forms a stable colloidal sol of stannic oxide.it with dispersion medium in the presence of a small amount of electrolyte). However. However. Filterability : Colloidal particles pass through an ordinary filter paper. in 1869. Fe(OH)3 + Fe3+ Fe(OH)3 | Fe3+ +3 +3 +3 +3 + 3+ Fe Peptizing agent ppt of Fe (OH)3 +3 +3 +3 +3 +3 Optical Properties-Tyndall effect : Tyndall. Important properties of colloidal sols : Tyndall cone Light source Scattered light True solution Colloidal solution *The intensity of scattered light depends on the difference between the refractive indice of the D. This phenomenon is due to scattering of light from the surface of colloidal particles. The illuminated path of beam is called Tyndall cone. Visibility : Due to scattering caused by the colloidal particles. (iii) Freshly precipitated silver chloride can be converted into a colloidal sol by adding a small amount of hydrochloric acid. Only osmotic pressure measurements are used in determining the molecular mass of polymers. which is due to the scattering of light by the colloidal particles. elevation in boiling point. as the size of the particle increases. Colligative Properties : Colloidal sols show the colligative properties viz. RESONANCE pollen grains placed in water do not remain at rest but move about continuously and randomly. discovered in 1827 that Heterogeneous character : Colloidal sols are heterogeneous in character as they consist of two phases. Appearance of dust particle in a semi darked room. These impurities may destabilise the sol. CdS : S2– . For example: (i) When freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3 (peptizing agent) it adsorbs Fe3+ ions and thereby breaks up into small-sized particles. mole fraction of the dispersed phase is very low. the particle do not pass through other fine membranes. there is little or no tyndal effect. The electrolyte used for this purpose is called peptizing agent. It has been postulated that the impact of the molecules of the dispersion medium on the colloidal particle are unequal leading to zig-zag motion. albumin. it will appear as a bright spot moving randomly. A very important method of removal of soluble impurities from sols by a semipermeable membrane is known as dialysis. Surface Chemistry . Later on. This zigzag motion is called Brownian movement. In lyophobic colloids. a botanist. SnO2 .M. This phenomenon is called Tyndall effect. In a true solution there are no particles of sufficiently large diameter to scatter light & hence the beam is invisible. (iv) Cadmium sulphide can be peptised with the help of hydrogen sulphide. depression in freezing point and osmotic pressure. Hence. relative lowering of vapour pressure. Factors Affecting Brownian Movement : (i) If particles is large then brownian movement becomes less. form a second layer. : A set up for electrophoresis.M. D = dielectric constant of medium. Charge on colloidal particles : Colloidal particles are either positively charged or negatively charged. Due to similar nature of the charge carried by the particles. Streaming potential : A potential difference is developed across a porous partition when the dispersion medium of a charged colloid is forced through it. AgI in contact with AgNO3 forms positively charged colloidal sol due to adsorption of Ag+ ion. Sedimentation potential or Dorn potential : When the charged colloidal particles are made to settle down under centrifugal field. Fe(OH)3 sol prepared by the hydrolysis of FeCl3 solution adsorbs Fe3+ and this is positively charged. (Fig. The existence of the electric charge is shown by the phenomenon of electrophoresis. As2S3 + H2S As2S3 S2– : 2H+. no Brownian movement is observed. This charge is due to preferential adsorption of either positive or negative ions on their surface. RESONANCE Diffused part. under the influence of the electric field'. This process is reverse of electrophoresis. This phenomenon is termed as electromosis. This effect is called Dorn effect and the potential difference thus developed is called Dorn potential or sedimentation potential. silver and platinum particles in their respective colloidal sols are negatively charged while particles of ferric hydroxide. It involves the 'movement of colloidal particles either towards the cathode or anode. This is called Streaming potential. they repel each other and do not combine to form bigger particles. The combination of two layers of opposite charge around the colloidal particle is called Helmholtz electric double layer. The dispersion medium has an equal and opposite charge making the system neutral as a whole. – Fe(OH)3 + FeCl3 Fe(OH)3 Fe3+ : 3Cl Fixed part As2S3 colloidal sol is obtained when As2O3 is saturated with H2S : As2O3 + 3H2S As2S3 + 3H2O.M. The charge of opposite ions of fixed and diffused layer double layer results in a difference in potential between two opposite charge layer is called the electrokinetic potential or zetapotential which can be given by : Z= 4 u D where = viscosity coefficient. FeCl3 + 3H2O Fe(OH3) + 3HCl . when the dispersed particle becomes big enough to acquire the dimensions of suspension. Electric Double Layer Theory or Helm-holtz Electric double layer : The surface of colloid particles acquire a positive or negative charge by the selective (preferential) adsorptions of common ions carrying positive or negative charge respectively to form first layer. Isoelectric point : The H+ concentration at which the colloidal particles have no charge is known as the isoelectric point. Positive charge on colloidal sol is due to adsorption of Fe3+ ion (common ion between Fe(OH)3 and FeCl3). AgI + AgNO3 [AgI]Ag+ : NO3– .) Electrosmosis : When movement of colloidal particles is prevented by some suitable means (porous diaphragm or semi permeable members). (iii) The brownian movement does not change with time & remains same for months or even for a year. This process is reverse of electro-osmosis. (ii) Brownian movement increases with increasing temperature. begins to move in an electric field. SnO2 + 4H+ Sn4+ + 2H2O SnO2 + Sn4+ SnO2 Sn4+ SnO2 in alkaline medium forms negatively charged colloidal sol due to adsorption of SnO32– formed. a sol is stable and particles do not settle down.the effect of the impacts average out and the Brownian movement becomes slow. aluminium hydroxide are positively charged.7 . That is why. As2S3 adsorbs S2– ions (common between H2S and As2S3 and thus is negatively charged). There is adsorption of common ion present in excess. SnO2 + 2OH– SnO32– + H2O SnO2 + SnO32– SnO2 SnO32– Electrical Properties (Electrophoresis) : The particles of the colloids are electrically charged and carry positive or negative charge. At this point stability of colloidal particles becomes very less & do not move under influence of electric field. gold. u = velocity of colloidal particles when an electric field is applied. Ultimately. Surface Chemistry . The apparatus used for electrophoresis as shown in fig. SnO2 in acidic medium forms positively charged colloidal sol due to adsorption of Sn4+ formed. it is observed that the D. Arsenious sulphide. there occurs a charge separation and a potential difference is developed. AgI in contact with KI forms negatively charged colloidal sol due to adsorption of I– ion AgI + KI AgI I– : K+. Importance : (i) In confirmation of kinetic energy. This layer attract counter ions from D. The first layer of ions is firmely held and is termed as fixed layer while the second layer is mobile which is termed as diffused layer. (iii) Stability of colloidal solution : Brownian movement does not allow the colloidal particles to settle down to gravity & thus is responsible for their stability. (ii) Determination of Avogadro numbers. 03 . and oil is D. greater its power to cause precipitation. the precipitation of AgI adsorb iodide ions from the D. the coagulating power of cation is in the order of Al3+ > Ba2+ > Na+.M with the formation of fixed layer and negatively charged colloidal solution form.005 . is a silver sol protected by organic material used as eye drop. Ex.2 Gum arabic Dextrin Starch 0. when an electrolyte is added in larger concentration.8 . the particles will come near to each other to form aggregates and settled down under the force of gravity. and liquid water is D.P. Inversion of phase : The conversion of emulsion of oil in water (o/w) into water in oil (w/o) or vice versa is called the inversion of phase. a third component is added called emulsifying-Agent form an interfacial film between D. Fixed layer + K K K + + I– I – I – I K+ I– – + K I AgI – I– I– I– I– K+ Zetapotential K+ Protecting power + K K+ Diffused layer Coagulation/Flocculation : The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids.2 20 .25 6 – 6. however when KI solution is added to AgNO3 solution positive charge sol result due to the adsorbs of Ag+ ions from D. It may be noted that smaller of the gold number. This process of aggregation of colloidal particles into an insoluble precipitate by the addition of some suitable electrolyte is known as coagulation. and D.0. :. This is known as Hardy-Schulze. AgI/I– AgI/Ag+ Negative charged Positively charged.M. coagulation value coagulating power of electrolyte A coagulating power of electrolyte B RESONANCE Gold Number = 1 gold number weight of lyophilic sol in mg 10 volume of gold sol in mL The gold numbers of a few protective colloids are as follows : Gelatin Haemoglobin Egg albumin 0. coagulatio n value B Types of emulsions : Depending on the nature of the dispersed phase. the charge is removed. the emulsions are classified as (a) Oil in water emulsions (b) Water in oil emulsions (a) Oil in water emulsions (o/w) : This type of emulsions is formed when oil D.0. and water D. Gold Number : Zpsigmondy (1901) introduce a term called gold number it is defined as ‘’the minimum amount of the protective colloid in milligrams which when added to 10 ml of a standard gold sol is just sufficient to prevent a colour change from red to blue on the addition of 1 ml of 10% sodium chloride solution. : . Example : Milk is an emulsion in which liquid fat is D. Emulsion are unstable and some time they are separated into two layers on keeping still.M.P. = coagulation value A Surface Chemistry . somehow. Emulsion droplets are bigger than sol particles and can be seen under an ordinary microscope and sometimes even with a magnifying glass. Ex.01 0.1 . If. for the stabilising of an emulsion. The neutral particles then start aggregating giving particles of larger size which are then precipitated. The stability of the lyophobic colloids is due to presence of charge on colloidal particles.water type emulsions. This fixed layer attracts counter ions from the medium forming a second layer.07 0. Emulsions : Pair of immiscible liquid is called emulsion.P.15 .M. the particles of the sol take up the ions which are oppositely charged and thus get neutralised.0.25 Uses of protective action : (i) Gelatin is added in the preparation of ice cream to protect the particle of ice. (b) Water in oil emulsions (w/o) : This type of emulsions is formed when water is D.P. and casein is emulsifying agent. the coagulating power of anion is in the order of [Fe(CN)6]4–>PO43–>SO42–>Cl– In case of negative charged sol. greater will be protecting power of the protective colloid.0. Coagulation value = Hardy-Schulze Rule : According to this rule greater is the valency of coagulating ion. Coagulated sol Coagulation value or Flocculation value : The minimum concentration of electrolyte in millimoles required to cause coagulation of one litre of colloidal solution is called coagulation value. It is express in terms of millimoles/litre.in . (ii) Protargol and Argyrol. At lower concentration of electrolytes. Coagulating power 1 . coagulation takes place and the same cannot be reversed simply by shaking.Cold cream and cod liver oil. However.Example : When silver nitrate solution is added to KI solution. + + + ++ + + ++ + + + + + + + + ++ + + + + ++ + + + ++ + + ++ + + + + + + + + + ++ + + + + + + + ++ + + + + + ++ +++ + + + + ++ + + + + + + ++ + + + + + + + + + ++ + + ++ + + + ++ + + ++ ++ + ++ + ++ + + + + + ++ + ++ + + + + + + ++ + + + +++ + +++ + + + + + + + + ++ + + + + + + + + ++ + + + + + + + + + + ++ + + + + + + + ++ + + ++ + + + + + + + + + + ++ + + + + + + + ++ + + ++ Neutralised sol particles millim oles of electrolyte volume of sol in litre Comparision of relative coagulating power of two electrolyte for the same colloidal solution: The coagulation value decrease with increase in charge of the coagulating ion.M. the aggregation of particles is called flocculation that can be reversed on shaking while at higher concentration of electrolyte. In case of positive charged sol.M.Milk and vanishing cream are oil .