6. Colloidal properties of any colloidal substance

colloidal properties

1. COLLOIDAL STATE

2. Introduction Solution is an intimate mixture of two or more chemical substances. In solution , the dissolving agent is the solvent ( dispersion medium ) and the substance which dissolves is the solute ( dispersed phase ). State of matter of a solution may be solid, liquid or gas which are in the form of atoms , ions, or molecules.

3. Scottish chemist Thomas Graham (1861) studied the ability of dissolved substances to diffuse into water across a permeable membrane . Crystalline substances (sugar , urea, sodium chloride, etc.) passed through the membrane Others like glue, gelatin and gum arabic did not passed through the membrane The former called crystalloids and the latter colloids In, Greek, kolla = glue ; eidos = like He thought that the difference in the behavior of ‘crystalloids’ and ‘colloids’ was due to the particle size . Thomas Graham ( 1805-69) Any substance could be converted into a colloid by subdividing it into particles of colloidal size.

4. Colloidal systems consist of finely divided particles of any substance dispersed in any medium. The dispersed entities are molecules or aggregates of molecules. These are smaller than coarse, filterable particles but larger than atoms and small molecules. Examples of colloidal systems from daily life

5. A colloidal system is defined as a two phase heterogeneous system. Colloidal System = Dispersed phase + Dispersion medium Definition of Colloids

6. A mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. The substances are present as larger particles than those found in solution, but are too small to be seen with a microscope. There are no strict boundaries on the size of colloidal particles, but they tend to vary between 10 -9  m to 10 -6  m in size . The mixture is also called a colloidal solution, colloidal system, or colloidal dispersion. The three forms in which all matter exists are solid, liquid or gas. Colloidal systems can be any combination of these states. A colloidal system is not a true solution but it is not a suspension either because it does not settle out like a suspension will over time. Colloids are larger than most inorganic molecules and remain suspended indefinitely. They are large molecules, such as proteins, or groups of molecules . They have many properties, depending on their large specific surface .

7. True solution The solute particles are dispersed in the solvent as single molecules or ions. Thus the diameter of the dispersed particles ranges from 1Å to 10 Å . Ex. sugar or salt in water. Suspension The dispersed particles are aggregates of millions of molecules . The diameter of these particles is of the order 2,000 Å or more . Ex. sand stirred into water. Colloidal solutions/colloidal dispersions are intermediate between true solutions and suspensions The diameter of the dispersed particles in a colloidal dispersion is more than that of the solute particles in a true solution and smaller than that of a suspension. When the diameter of the particles of a substance dispersed in a solvent ranges from about 10 Å to 2,000 Å , the system is termed a colloidal solution, colloidal dispersion, or simply a colloid. The material with particle size in the colloidal range is said to be in the colloidal state.

8. Characteristics of True Solutions, Colloidal Solutions and Suspensions Sr. No. True solutions (Molecular solutions) Colloidal solutions (Colloidal dispersion) Suspension (Coarse dispersion) 1 The particle size are in the order of 1 nm or 10Å The particle sizes lie in the range of 1 nm – 200 nm i.e. 10Å–2000Å The particle sizes are greater than 200 nm or 2000Å 2 Particles are invisible under the electron microscope Particles are invisible even under the most powerful microscope but its scattering effect can be observed in ultra-microscope. Particles are visible to the naked eye or at least through a microscope. 3 Particles diffuse readily through parchment membrane Colloidal particles do not diffuse through parchment membrane Particles do not diffuse 4 Particles readily pass through ordinary filter paper as well as through parchment membrane Particles pass through filter paper Particles do not diffuse through filter paper or parchment membrane 5 No scattering occurs Particles scatter light. This scattering of light is called Tyndall effect They do not show Tyndall effect

9. Various types of colloid dispersions with examples Dispersed phase Dispersion medium Colloidal system Examples Gas Gas Homogeneous mixture Mixture of gases Liquid Aerosols of liquids Fog, mist, clouds, insecticide sprays Solid Aerosols of solids Smoke, dust storm Gas Liquid Foam, Froth Soda water, whipped cream, froth, soap lather , etc . Liquid Emulsion Milk, medicines, shampoo, creams, emulsified oils Solid Sols Milk, starch in water, proteins in water, gelatin in water, cell fluids. Gas Solid Solid foam Pumice stone, foam rubber Liquid Gels Dahi, Yoghurt, Jellies, cheese, jams Solid Solids sols Minerals, gem stones, glass, alloys. Paints, gold

10. Lyophilic sols (solvent-loving) Those in which the dispersed phase exhibits a definite affinity for the medium or the solvent. In case water acts as the dispersed phase, the lyophilic colloid is called hydrophilic colloid . These are generally stable due to the strong attractive forces operating between the two phases. These are reversible in nature. On evaporating the sol, the dispersed phase obtained can be easily reconverted into the solution by simply agitating it with the dispersion medium. Lyophobic sols (solvent-hating) Those in which the dispersed phase has no attraction for the medium or the solvent. Such solutions are relatively less stable and are not easily prepared. These can be easily precipitated by heating the sol or on adding small amount of electrolyte to it. These are irreversible . The solid obtained by precipitation cannot be reconverted into colloidal solution by simply shaking it with the dispersion medium. Colloidal solutions of gold, silver, Fe(OH) 3 , As 2 S 3 etc. are lyophobic. Lyophobic sols need stabling agents for their preservation. H. Freundlich (1925) classified the colloid systems into two categories Classification of colloids

11. Properties of Sols Heterogeneity A colloidal solution is heterogeneous in nature and consists of two phase i.e ., the colloidal particles (dispersed phase) and the dispersion medium. Since the colloidal particles are small in size, a sol appears to be homogeneous to the naked eye. However , their heterogeneous nature can be confirmed by seeing under electron microscope . Difusibility Colloidal particles do not diffuse through parchment or other fine membrane. They can be separated from solution by dialysis. Filterability Ordinary filter papers have large pore size. Therefore, colloidal particles can pass through them. However , they cannot pass through animal and vegetable membranes and ultra filter papers. This idea tells the basis of separation of colloidal particles from those of crystalloids.

12. Stable nature Colloidal solutions are quite stable and the colloidal particles do not settle at the bottom. This is because of the constant motion of colloidal particles . Colligative properties As the colloidal particles are the aggregates of simple molecules, the number of particles per liter of the sol is relatively much smaller than in a true solution. Values of the colligative properties such as elevation in boiling point, depression in freezing point and lowering of vapour pressure are much smaller as compared to true solutions. However , the osmotic pressure of colloidal solutions, though smaller than true solutions is measurable and gives information regarding the number of particles present in one kg of the dispersion medium .

13. OPTICAL PROPERTIES OF SOLS Sols exhibit Tyndall effect When a strong beam of light is passed through a sol and viewed at right angles, the path of light shows up as a hazy beam or cone. This is due to the fact that sol particles absorb light energy and then emit it in all directions in space. This ‘scattering of light’, as it is called, illuminates the path of the beam in the colloidal dispersion. The phenomenon of the scattering of light by the sol particles is called Tyndall effect . The Tyndall effect was first discovered by (and is named after) the Irish physicist John Tyndall (1869). The diameters of the particles that cause the Tyndall effect can range from 40 to 900 nanometers. The illuminated beam or cone formed by the scattering of light by the sol particles is often referred as Tyndall beam or Tyndall cone. The hazy illumination of the light beam from the film projector in a smoke-filled theatre. The light beams from the headlights of car on a dusty road, are familiar examples of the Tyndall effect. True solutions do not show Tyndall effect. Since ions or solute molecules are too small to scatter light , the beam of light passing through a true solution is not visible when viewed from the side .

14. Tyndall effect (Illustration)

15. Ultramicroscope Sol particles cannot be seen with a microscope Zsigmondy (1903) used the Tyndall phenomenon to set up an apparatus named as the ultramicroscope. An intense beam of light is focused on a sol contained in a glass vessel. The focus of light is then observed with a microscope at right angles to the beam. Individual sol particles appear as bright specks of light against a dark background ( dispersion medium ). It may be noted that under the ultramicroscope, the actual particles are not visible. It is the larger halo (Circle of light) of scattered light around the particles that are visible. Thus an ultramicroscope does not give any information regarding the shape and size of the sol particles. Principle of the Ultramicroscope

16. Milkotester or Electronic Milk Tester (EMT ) Milkotester is used to determine Fat % in milk. The photometric method for determining fat in milk is based on the measurement of light scattering by a diluted sample. A calcium chelating agent, ethylenediaminetetraacetate (EDTA) eliminates the turbidity caused by the casein micelle, so the light scattering is due solely to the fat in the milk. The amount of light scattered is dependent on number and size of fat globules. If the size distribution is uniform the amount of light reaching the photocell will be proportional to the fat content. The results are read from a galvanometer scale graduated in percent fat. The Milkotester are calibrated at the factory against the Rose-Gottlieb method which is the accepted international reference method.

17. KINETIC PROPERTIES OF SOLS Brownian Movement An illustration of Brownian movement 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 undergoing a constant rapid motion. It moves in a series of short straight-line paths in the medium, changing directions abruptly. The continuous rapid zig-zag movement executed by a colloidal particle in the dispersion medium is called Brownian movement or motion . This phenomenon is so named after Sir Robert Brown who discovered it in 1827 . Suspension and true solutions do not exhibit Brownian movement.

18. Origin of charge on sol particles All the dispersed particles of a particular sol carry a positive or a negative charge. They acquire this charge by Adsorption of ions from the aqueous medium Ionization of surface groups ELECTRICAL PROPERTIES OF SOLS The sol particles carry an electric charge

19. The most important property of colloidal dispersions is that all the suspended particles posses either a positive or a negative 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 sol particles acquire positive or negative charge by preferential adsorption of positive or negative ions from the dispersion medium. For example, a ferric hydroxide sol particles are positively charged because these adsorb Fe 3+ ions from ferric chloride (FeCl 3 ) used in the preparation of the sol. Since the sol as a whole is neutral, the charge on the particle is counterbalanced by oppositely charged ions termed counter ions (in this case Cl – ) furnished by the electrolyte in medium. Adsorption of ions from dispersion medium gives charge to Sol particles which do not settle on account of mutual repulsions.

20. By Adsorption of ions The charge on the sol particles originates by the selective adsorption of ions common to the particles from the dispersion medium. Examples . Ferric hydroxide sol particles are positive because they adsorb the common ion Fe 3+ from the aqueous medium. Arsenic sulphide sol particles acquire a negative charge since they adsorb the common ion S 2 – from the medium. Fe(OH) , sol particles adsorb Fe 3+ ions and become positive As 2 S 3 sol particles adsorb S 2- ions and acquire negative charge

21. It is not necessary that a particular sol particles always adsorb the same kind of ions. In fact, the particles may adsorb the anions or cations whichever are in excess and acquire the corresponding charge . For example, AgCl sol produced by the addition of AgNO 3 solution to sodium chloride solution , bears a positive charge if Ag + ions are in excess. On the other hand, if Cl - ions are in excess , the AgCl sol particles acquire a negative charge. When Ag+ ions are in excess, AgCl sol particle adsorbs these ions and becomes positive when Cl - ions are in excess, the AgCl particle adsorbs these and acquires a negative charge

22. Ionization of Surface groups (a) Charge on Soaps and Detergent sols Soaps and detergent sol particles are aggregates of many molecules. The hydrocarbons tails of the molecules are directed to the center, while the groups – COO - Na + (or – OS O 3 - Na + ) constitute the surface in contact with water. As a result of ionization of the surface groups, the particle surface is now made of the anionic heads, COO - (or OS O 3 - ). This makes the sol particle negative.

23. (b ) Charge on Protein sols Protein sol particles possess both acidic and basic functional groups . In aqueous solution at low pH, the –NH 2 group (basic) acquires a proton to give – NH 3 + while at high pH the —COOH group (acidic) transfers a proton to OH – to give —COO – . The protein sol particle has positive charge at low pH and negative charge at high pH . At an intermediate pH ( isoelectric point), the particles will be electrically neutral. The changes in the charge of the protein sol are shown by the direction of movement of the dispersed phase in electrophoresis. Charge on protein sol changes with pH

24. The Theory of Electrical Double layer The surface of colloidal particle acquires a positive charge by selective adsorption of a layer of positive ions around it. This layer attracts counter ions from the medium which form a second layer of negative charges. The combination of the two layer of + ve and – ve charges around the sol particle was called Helmholtz Double layer. Helmholtz thought that positive charges next to the particle surface were fixed, while the layer of negative charges along with the medium were mobile . This theory was later modified by Guoy, Stern and others. Helmholtz double layer The electrical double layer (Stern)

25. When a solid is present in contact with a liquid, double layers of ions appear at the surface of separation. One part of double layer is fixed on the surface of the solid - called fixed part of double layer - either consists of positive ions or negative ions. The second part of the double layer consists of mobile or diffuse layer of ions which extend into the liquid phase - contains positive and negative ions The net charge is equal and opposite to that on the fixed part of the double layer. In Figure (a), the fixed part carries positive charge while in Figure (b), the fixed part carries negative charge. In each case the net charge on mobile layer is equal and opposite to that on the fixed layer. This theory can be safely applied to colloidal systems. The ions, which get preferentially adsorbed on the colloidal particles, are held in the fixed part of double layer. These ions give the characteristic charge to the colloidal particles. On the other hand, the ions with opposite charge are present in the mobile portion of double layer and give opposite charge to this layer.

26. The presence of oppositely charged ions on the fixed and mobile portion of the double layer leads to the appearance of a difference of potential between the two layers. Zeta potential ( ζ ) Zeta potential is a term used for electrokinetic potential in colloidal dispersions. It is usually denoted by Greek letter zeta ( ζ ), hence called ζ -potential. The potential difference across an electric double layer usually between a solid surface and a liquid dispersion medium.

27. Zeta potential ( ζ ) values ranges from + 100 mV to – 100 mV. A dividing line between stable and unstable aqueous dispersions is generally taken at either +30 or -30 mV. Colloidal particles with zeta potentials more positive than + 30mV are normally considered stable. Colloidal particles with zeta potentials more negative than -30mV are normally considered stable . The casein micelles have a surface (zeta) potential of about −20 mV at pH 6.7.

28. Coagulation or Precipitation of a sol Colloidal particles in a sol carry the same charge. Being similarly charged, they repel each other. The repulsive forces between the similarly charged particles do not allow them to come closer. Hence , they remain scattered or suspended throughout the dispersion medium. However , if the charges are removed by some means, the colloidal particles come together to form bigger aggregates and settle down under the influence of gravity . This process by which the colloidal particles come closer and result in the precipitation of the colloidal solution is called coagulation. Stability of colloidal particles Electrical charge Water of hydration

29. Addition of electrolytes Addition of solvents By mixing two oppositely charged sols Boiling By electrophoresis By persistent dialysis Different ways to coagulate or precipitate sols

30. Addition of electrolytes When electrolyte is added to a sol, the charged colloidal particles adsorb the oppositely charged ions produced by the dissociation of electrolyte. The charge on the colloidal particles is neutralized and the resulting neutral particles come together to form bigger aggregates and thus get precipitated. When NaCl is added to negatively charged sol of As 2 S 3 , arsenous sulphide settles down as precipitate . The Na+ ions neutralize the negative charge on the colloidal particles, after losing the charge, the particles come together to form bigger aggregates which finally settle down as yellow precipitate .

31. Points must be kept in mind Addition of electrolyte brings about coagulation of colloidal solution. Bleeding is stopped by the application of alum (electrolyte). B lood is coagulated by alum and thus blood vessel gets sealed. Ions of opposite charge to that of the colloidal particles is effective in causing coagulation of a solution. In the precipitation of positively charged Fe(OH) 3 solution, anion is of importance. It does not matter greatly what the cation is? Coagulating capacity of various electrolytes is different and depends on the following: Valency of the ion. The precipitating effect increases with increase in the valency of ion. It also depends on the nature of the colloidal sol. Lyophobic colloidal sols are easily coagulated while lyophilic sols like gelatine, starch, etc. are coagulated with difficulty and with lare excess of electrolyte.

32. Coagulation or flocculation Various chemicals (e.g. polymers) are used to induce flocculation or coagulation. These are known as flocculants or coagulants . The intermolecular and surface forces play important roles in the stabilization and coagulation of colloids. The terms coagulation and flocculation are widely used in colloid chemistry to mean aggregation of the particles. Coagulation implies the formation of compact aggregates leading to the macroscopic separation of a coagulum Flocculation implies the formation of a loose or open network which may or may not separate macroscopically. The minimum amount of an electrolyte required to cause precipitation of 1L of a colloidal solution is called flocculation value or coagulation value of electrolyte for the sol. Generally expressed in millimols/L. The reciprocal value is called flocculating or coagulation power .

33. Addition of solvent Addition of solvent causes dehydration of colloidal particles. i.e. acetone, ethanol, etc. have the ability to bind the water. Water of hydration is one of the factor for stability of colloidal particle. Based on this principle, alcohol test in milk is performed to judge the heat stability of milk. Alcohol coagulation test is simple and rapid. It is based on poor stability of milk proteins in presence of alcohol. 68% ethyl alcohol by weight or 75% by volume is used (Density 0.8675 g/ml at 27°C) . The test aids in detection of abnormal milk such as colostrum, milk from animals in late lactation, milk from animals suffering from mastitis and milk in which mineral balance has been disturbed. Alcohol test

34. When two colloidal sols of opposite charges are mixed, the charge of one is neutralised by the charge of the other and aggregation of the particles and precipitation of colloids takes place. The process is called mutual coagulation. In order to bring about mutual coagulation , the two colloids must be mixed in proper proportions. For example, when Fe(OH) 3 and As 2 S 3 are mixed in proper proportion, these get precipitated. Mutual Coagulation

35. Lyophobic sols can also be coagulated by boiling. This is because at higher temperatures , the colloidal particles collide together more frequently and vigorously so that they coalesce to finally form precipitate. For example, egg albumin, a hydrophobic colloid is coagulated by hot water or hot oil in the poaching or frying of eggs. By boiling

36. By electrophoresis Electrophoresis  is a general term that describes the migration and separation of charged particles (ions) under the influence of an electric field. It consists of two electrodes of opposite charge (anode, cathode), connected by a conducting medium called an electrolyte . Under the influence of electric field, the colloidal particles move towards the oppositely charged electrodes. If the process is carried out for a long time, the colloidal particles come in contact with the electrode and lose their charge. Consequently, they get precipitated .

37. The stability of colloidal sols is due to the presence of a small amount of electrolyte. If the electrolyte is completely removed by persistent dialysis, the particles left will get coagulated. By persistent dialysis The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane is called dialysis . It’s principle is based upon the fact that colloidal particles can not pass through a parchment or cellophane membrane while the ions of the electrolyte can pass through it.

38. Haemodialysis is the purification of blood by using a semipermeable membrane. In hemodialysis, blood is purified by removal of toxic products that generally are uraemic in nature. Haemodialysis treatment is for patients suffering from severe renal diseases where, the transfer of waste molecules from blood occurs by diffusion or ultrafiltration (UF). The transport of molecules from high concentration to lower concentration is referred to as diffusion

39. Protective action of sols Lyophobic sols are readily precipitated by small amounts of electrolytes. However these sols are often stabilized by the addition of lyophilic sols. The property of lyophilic sols to prevent the precipitation 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. Example: -casein protects  s1  s2 and -caseins in milk.

40. Analysis of colloids For analysis of colloids – Purification or isolation of colloids is necessary. By precipitation or chemical treatment – colloids can be isolated – but will not remain in natural state. Centrifugation is the way of purification and isolation of colloidal particles. The rate at which particles settle down depends on difference in densities of dispersion medium and dispersed phase. For isolation of colloidal particles, ultracentrifuge is used – process called as ultracentrifugation . The rpm used is 70,000 to 5,00,000. Each colloidal particle have its own sedimentation coefficient at which it will be sediment down.

41. ELECTROPHORESIS Electrophoresis is a general term that describes the migration and separation of charged particles (ions) under the influence of an electric field. An electrophoretic system consists of two electrodes of opposite charge (anode, cathode), connected by a conducting medium called an electrolyte. The separation effect on the ionic particles results from differences in their velocity ( v ) , which is the product of the particle's mobility (m) and the field strength (E ) : The mobility ( m ) of an ionic particle is determined by particle size, shape, and charge, and the temperature during the separation, and is constant under defined electrophoretic conditions. Electrophoretic conditions are characterized by the electrical parameters (current, voltage, power), and factors such as ionic strength, pH value, viscosity , pore size, etc., which describe the medium in which the particles are moving .  

42. Sol – A colloidal system – Solid in Liquid Dispersed phase is Solid and Dispersion medium is Liquid It may be hydrophilic or hydrophobic in nature – depends on whether it has attraction for dispersion medium. Ex. Casein in milk . Gel – A colloidal system – Liquid in Solid Dispersed phase is liquid and Dispersion medium is Solid It may be characterized by semi-solid, elastic structure. Ex. Curd, cheese, Jellies. Emulsion – A colloidal system – Liquid in Liquid Dispersed phase is liquid and Dispersion medium is Liquid Emulsion are of two types: Oil-in water (ex. Fat in milk) Water-in-oil (ex. Water in butter, cod liver oil.)

43. Milk as a colloidal system Milk is a complex biological fluid - Major constituents of milk are – Fat, lactose, proteins, minerals and water. They exist in different physical forms. Water is present as a dispersion medium. Caseins (protein) exists as a sol or as colloids Fat as an emulsion Lactose and whey proteins ( -Lactalbumin, -Lactoglobulin, serum albumins and Igs ) as a true solution. Part of minerals as a true solution and part in colloidal system. Casein as a colloidal particles Milk is a biological fluid – casein is a bio colloid. Casein micelles are present in milk as spherical particles- diameter range from 30 to 300 nm . Average size in cow milk – 70-110 nm Average size in buffalo milk – 110-160 nm They are highly hydrated, and contain, on a dry matter basis , ̴ 94% casein and ̴ 6% inorganic materials . These inorganic materials are collectively referred to as micellar calcium phosphate (MCP) and consist primarily of calcium and phosphate, with lower levels of magnesium and citrate .

44. The Colloidal Behavior of Casein Micelles The dominant structural feature of skim milk ( i.e. fat-free milk) is the casein micelle. They are a class of four phosphoproteins αS 1 -, αS 2 -, β- and κ-caseins Represent the majority of proteins present in milk from most mammalian species Casein proteins are ‘disordered’, i.e. flexible polymers. When dissolved into water, their primary structure prevents tight folding of the peptide chain, partly due to the many proline residues. Moreover, the primary structure of e.g. β-casein protein looks like a polymeric surfactant molecule. It has an amphiphilic molecular structure, i.e. a hydrophilic ‘head’ and a long and flexible hydrophobic ‘tail’. Such a molecular structure tend to spontaneously associate into structures denoted as ‘association colloids’. Amphiphilic character of the casein proteins significantly influences the formation of the casein micelles. Most casein molecules are part of this unique protein aggregate.

45. Schematic illustration of the relative size of the different colloidal entities present in raw milk: the fat globule, the casein micelle, whey protein and lactose

46. Freeze fracture electron microscopy image of a casein micelle Sketch of the casein micellar structure (dual binding model) inside the micelles are the α and β caseins at the micellar surface κ-casein in located The points stand for the colloidal calcium phosphate clusters

47. EMULSIONS These are liquid-liquid colloidal systems. An emulsion may be defined as a dispersion of finely divided liquid droplets in another liquid. Generally one of the two liquids is water and the other, which is immiscible with water, is designated as oil. Either liquid can constitute the dispersed phase . Types of Emulsions There are two types of emulsions. Oil-in-Water type (O/W type ) Water-in-Oil type (W/O type) Two types of Emulsions

48. Examples of Emulsions Milk is an emulsion of O/W type. Tiny droplets of liquid fat are dispersed in water. Stiff greases are emulsions of W/O type, water being dispersed in lubricating oil. Preparation of Emulsions The dispersal of a liquid in the form of an emulsion is called emulsification. This can be done by agitating a small proportion of one liquid with the bulk of the other. It is better accomplished by passing a mixture of the two liquid through a colloid mill known as homogenizer . The emulsions obtained simply by shaking the two liquids are unstable. The droplets of the dispersed phase coalesce and form a separate layer. To have a stable emulsion, small amount of a third substance called the Emulsifier or Emulsifying agent is added during the preparation. This is usually a soap , synthetic detergent, or a hydrophilic colloid.

49. Role of Emulsifier The emulsifier concentrates at the interface and reduces surface tension on the side of one liquid which rolls into droplets. Soap , for example, is made of a long hydrocarbon tail (oil soluble ) with a polar head —COO - Na + (water soluble). In O/W type emulsion the tail is pegged into the oil droplet , while the head extends into water. Thus the soap acts as go-between and the emulsified droplets are not allowed to coalesce.

50. WHAT ARE GELS ? A gel is a jelly-like colloidal system in which a liquid is dispersed in a solid medium. For example, when a warm sol of gelatin is cooled , it sets to a semisolid mass which is a gel. The process of a gel formation is known as Gelation. Explanation Gelation may be thought of as partial coagulation of a sol. The coagulating sol particles first unite to form long thread-like chains . These chains are then interlocked to form a solid framework. The liquid dispersion medium gets trapped in the cavities of this framework . The resulting semisolid porous mass has a gel structure .

51. Two types of Gels Gels may be classified into two types: Elastic gels T hose which posses 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 . Elastic gels are obtained by cooling fairly concentrated lyophilic sols. The linkages between the molecules (particles) are due to electrical attraction and are not rigid. Non-elastic gels Those which are rigid e.g., 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 resulting molecules of silicic acid polymerise to form silica gel. It has a network linked by covalent bonds which give a strong and rigid structure.

52. Properties of Gels Hydration : A completely dehydrated elastic gel can be regenerated by addition of water. But once a nonelastic gel is freed from moisture, addition of water will not bring about gelation. Swelling: Partially dehydrated elastic gels imbibe water when immersed in the solvent. This causes increase in the volume of the gel and process is called Swelling. Syneresis: Many inorganic gels on standing undergo shrinkage which is accompanied by exudation of solvent. This process is termed Syneresis. Thixotropy: Some gels are semisolid when at rest but revert to liquid sol on agitation. This reversible sol-gel transformation is referred to as Thixotropy. Iron oxide and silver oxide gels exhibit this property. The modern thixotropic paints are also an example.