Colloids By Sitwat Rafi


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Colloids By Sitwat Rafi

  1. 1. COLLOIDS BY : Sitwat Rafi
  2. 2. INTRODUCTION A colloid is a substance microscopically dispersed throughout another substance. The word colloid comes from a Greek word kolla, which means glue thus colloidal particles are glue like substances. These particles pass through a filter paper but not through a semipermeable membrane. Colloids can be made settle by the process of centrifugation.
  3. 3. The colloidal system consist of two phases: A dispersed phase ( A discontinuous phase ) A dispersion medium ( A continuous phase ) The dispersed-phase particles have a diameter of between approximately 1nm – 0.5um . Such particles are normally invisible in an optical , though their presence can be confirmed with the use of an ultramicroscope or an electron microscope.
  4. 4. Homogenous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. If the dispersed phase is solid and the dispersion medium is liquid then it is called colloidal suspension but if the dispersed phase and dispersion medium both are in liquid state then it is known as colloidal emulsion.
  5. 5. The dispersed-phase particles or droplets are affected largely by the surface chemistry present in the colloid. Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color. Colloidal solutions (also called colloidal suspensions) are the subject of interface and colloid science. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.
  6. 6. . Examples of colloids are silver solutions, milk, synthetic polymers and blood etc. Blood Milk
  7. 7. INTERACTION BETWEEN PARTICLES : The following forces play an important role in the interaction of colloid particles: 1) EXCLUDED VOLUME REPULSION: IN LIQUID THEORY STATE : In liquid state theory, the 'excluded volume' of a molecule is the volume that is inaccessible to other molecules in the system as a result of the presence of the first molecule. The excluded volume of a hard sphere is eight times its volume—however, for a two-molecule system, this volume is distributed among the two particles, giving the conventional result of four times the volume.
  8. 8. 2) ELECTROSTATIC INTERACTION : Colloidal particles often carry an electrical charge and therefore attract or repel each other. The charges of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting this interaction. 3) VAN DER WAALS FORCE : van der Waals force (or van der Waals interaction), is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds, the hydrogen bonds, or the electrostatic interaction of ions with one another or with neutral molecules or charged molecules
  9. 9. 5) STERIC FORCES : Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.
  10. 10. TYPES OF COLLOIDS: Classification of Colloids based on Physical State of Dispersed Phase and Dispersion Medium:
  11. 11. Classification of Colloids based on Nature of Interaction Between Dispersed Phase and Dispersion Medium: Lyophilic Colloids Lyophobic Colloids Association colloids
  12. 12. Lyophilic Colloids • Lyophilic colloids are liquid loving colloids (Lyo means solvent and philic means loving).  Ease of Preparation: • As these colloids are liquid loving, their solutions are easy to prepare and can be prepared directly by mixing colloid with liquid.  Stability: • Lyophilic sols are relatively stable as strong forces of interaction exist between colloidal particles and liquid.
  13. 13. Reversibilty: If large quantity of liquid is added to precipitations or the colloidal solution is stirred properly lyophilic sols can regain their original state. This shows that lyophilic sols are also reversible in nature.
  14. 14. Lyophobic Colloids Lyophobic colloids are liquid hating colloids (Lyo means solvent and phobic means hating). Ease of Preparation: As these colloids are water hating, their solutions are easy to prepare and cannot be prepared directly by mixing colloid with liquid. Special methods are employed to prepare lyophobic solutions. Stability: Lyophobic sols are less stable as weak forces of interaction exist between colloidal particles and liquid.
  15. 15. Reversibilty: Lyophobic colloids do not regain their original state as coagulated mass cannot be dispersed into colloidal form. This shows that lyophobic solutions are also irreversible in nature.
  16. 16. Association colloids • These are the colloids which behave as normal strong electrolytes at low concentrations but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. The associated colloids are usually formed by surfactants (surface active agents) like soaps and synthetic detergents. These agents form micelles when present in solution at a concentration greater than critical micellar concentration (CMC).
  17. 17. PREPARATION OF COLLOIDS Lyophilic and lyophobic colloidal solutions (or sols) are generally prepared by different types of methods. Some of the common methods are as follows. Dispersion methods : In these methods, larger particles of a substance (suspensions) are broken into smaller particles. The following methods are employed.
  18. 18. A) Mechanical dispersion: In this method, the substance is first ground to coarse particles. • It is then mixed with the dispersion medium to get a suspension. • The suspension is then grinded in colloidal mill. • It consists of two metallic discs nearly touching each other and rotating in opposite directions at a very high speed about 7000 revolution per minute. • The space between the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal size. • Colloidal solutions of black ink, paints, varnishes, dyes etc. are obtained by this method.
  19. 19. In this method, the coarse particles along with dispersion medium is brought into colloidal state by grinding it in colloidal mill, ball mill or ultrasonic disintegrator. The solid particles along the dispersion medium are fed into the colloidal mill. The mill consists of two steel plates nearly touching each other and rotating in opposite directions with high speed (7000 rev/min).
  20. 20. The solid particles are torn off to colloidal size and are then dispersed in liquid to give the sol colloidal graphite (lubricant) and printing inks are made by this method. Recently, a mercury sol has been prepared by disintegrating a layer of mercury into sol particles in water, by means of ultrasonic vibrator.
  21. 21. (B) By electrical dispersion or Bredig’s arc method: oThis method is used to prepare sols of platinum, silver, copper or gold. o The metal whose sol is to be prepared is made as two electrodes which immerge in dispersion medium such as water etc. oThe dispersion medium is kept cooled by ice. oAn electric arc is struck between the electrodes. oThe tremendous heat generate by this method and give colloidal solution. oThe colloidal solution prepared is stabilised by adding a small amount of KOH to it.
  22. 22. This process involves dispersion as well as aggregation. Colloidal solutions of metals such as gold, silver, platinum etc. can be prepared by this method. In this method electric arc is struck between electrodes of metal immersed in the dispersion medium.
  23. 23. (C) By peptisation: The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte is called peptization. The electrolyte is used for this purpose is called peptizing agent or stabilizing agent. Cause of peptisation is the adsorption of the ions of the electrolyte by the particles of the precipitate. Important peptizing agents are sugar, gum, gelatin and electrolytes. Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing Fe3+ or OH– ions, viz. FeCl3 or NH4OH respectively. Fe(OH)3 + FeCl → *Fe(OH)3Fe]3+ + 3Cl– Precipitate electrolyte Colloidal sol
  24. 24. Preparation of colloids by condensation method: In condensation method, the smaller particles of the dispersed phase are aggregated to form larger particlesof colloidal dimensions. Some important condensation methods are described below:1) By change of physical state: Solutions of substances like mercury and sulphur are prepared by passing their vapours through a cold water containing a suitable stabilizer such as ammonium salt or citrate.
  25. 25. 2) By excessive cooling: A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution,separately combine to form particles of colloidal size. 3) By exchange of solvent: Colloidal solution of certain substances such as sulphur,phosphorus which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of water. For example alcoholic solution of sulphur on pouring into water gives milkey colloidal solution of sulphur.
  26. 26. 4) Chemical methods: Some chemical reactions used to aggregate smaller particles of atomic or ionic size to form large particles of colloidal dimensions. Colloids can be prepared by following chemicals methods. a) Oxidation: Addition of oxygen and removal of hydrogen is called oxidation. For example: Colloidal solution of sulphur can be prepared by oxidizing an aqueous solution of H2S with a suitable oxidizing agent such as bromine water. H2S + Br2 → 2HBr + S 2H2S + SO2 → 2H2O + 3S
  27. 27. b)Reduction: Addition of hydregen and removal of oxygen is called reduction. For example: Gold sol can be obtained by reducing a dilute aqueous solution of gold with stannous chloride. 2AuCl3 + 3SnCl2 → 3SnCl4 + 2Au c) Hydrolysis: It is the break down of water. Sols of ferric hydroxide and aluminium hydroxide can be prepared by boiling the aqueous solution of the corresponding chlorides. for example. FeCl3 + 3H2S → Fe(OH)3 + 3HCl
  28. 28. d) Double decomposition: The sols of inorganic insoluble salts such as arsenous sulphide, silver halide etc may be prepared by using double decomposition reaction. For example: Arsenous sulphide sol can be prepared by passing H2S gas through a dilute aqueous solution of arsenous oxide. As2O3 + 3H2S → As2S3(OH)3 + 3H2O
  29. 29. Purification of colloids There are three common methods used for purification of colloids: Dialysis Electrodialysis Ultra filteration
  30. 30. Dialysis The removal of ions or molecules from colloidal dispersion by process of diffusion across the membrane.
  31. 31. Electro dialysis The process of dialysis is very slow. The process is speeded up by application of electrical potential. This is called electro dialysis.
  32. 32. Application of electro dialysis: Artifical kidney machine make use of electro dialysis.
  33. 33. Ultra filtration Ultra filtration is a process of high pressure filtration through a semi permeable membrane in which colloidal particles are retained while the small sized solutes and the solvent are forced to move across the membrane by hydrostatic pressure forces. The process makes use of the filter paper made up of nitrocellulose or colloidons.
  34. 34. Application of ultra filtration: Ultra filtration is a vital process that takes place in the kidneys.
  36. 36. Related to the motion of particles with respect to the dispersion medium: Brownian movement: Zigzag movement of colloidal particles continuously and randomly. Diffusion: Movement of particles from a region of higher concentration to one of lower concentration.
  37. 37. Sedimentation : Settling down of particles under the influence of gravity at a very slow rate.
  39. 39. Depends on the presence of a charge on the surface of a particle ELECTROPHEROSIS: Motion of charged particles related to the fluid under the influence of an applied electric field. ELECTRO-OSMOSIS : Motion of liquid induced by an applied potential. COAGULATION: Process of precipitating a colloidal solution.
  41. 41. Properties of a substance relative with light Faraday Tyndall Effect: The phenomenon in which light is scattered by particles of matter in its path. It enables a beam of light to become visible by illuminating dust particles, etc. Light scattering: Used to give information about particle size and shape and for determination of molecular weight of colloids.
  42. 42. ADVANTAGES OF COLLOIDS Colloids allow the dispersion of normally insoluble materials, such as metallic gold or fats. These can then be used more easily, or absorbed more easily. Colloidal gold, for example, can be used in medicine to carry drugs and antibiotics, because it is highly nonreactive and non-toxic, unlike silver. Pharmaceutical industry makes use of colloidal solution preparation in many medicines. A wide variety of medicines are emulsions. An example is Cod Liver Oil. Paint industry also uses colloids in the preparation of paints. Asphalt emulsified in water is used for building roads.
  43. 43. In milk, the colloidal suspension of the fats prevents the milk from being thick, and allows for easy absorption of the nutrients. Sewage water contains particles of dirt, mud etc. which are colloidal in nature and carry some electrical charge. These particles may be removed by using the phenomenon of electrophoresis. The sky is the empty space around earth and as such has no colour. It appears blue due to the scattering of light by the colloidal dust particles present in air (Tyndall effect).
  44. 44. The sugar present in milk produces lactic acid on fermentation. Ions produced by acid, destroy the charge on the colloidal particles present in milk, which then coagulate and separate as curd. Soap solution is colloidal in nature. It removes the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth.
  45. 45. DISADVANTAGES OF COLLOIDS Colloids are frequently hard to extract or purify, which can lead to large losses in analysis or extraction having a large impact on the economics of the process.
  46. 46. PHARMACEUTICAL APPLICATION OF COLLOIDS: • Some important application of colloids are discussed below:
  47. 47. 1. THERAPY: • Colloidal material are used for a variety of pharmaceutical application including therapeutic & diagnostic agents, drug delivery system. • Example: copper colloids ( anti cancer) & mercury colloids ( anti syphilis).
  48. 48. 2.DRUG PREPARATION: • Drug substances may also be prepared as colloidal sized particles to improve bioavailability or therapeutic activity .Example: colloidal sulphur etc.
  49. 49. 3.TABLETS COATING: • Colloidal dispersion of gelatin is used in coating over tablets & granules which upon drying leaves uniformly dry film over them & protect them from adverse condition of the atmosphere.
  50. 50. 4.MEDICINES: • Most of the medicines are colloidal such as Calcium & Gold are administered by injections to raise the vitality of human system.
  51. 51. 5.FOOD ITEMS: • Large numbers of food particles which we use in our daily life are colloidal in nature. • Example: Milk, butter, & ice cream etc
  52. 52. 6.NUCLEAR MEDICINES: Colloidal dispersion containing radioactive isotopes are being used as diagnostic & therapeutic agents in nuclear medicines. Example: Colloidal gold is made by reducing a solution of AgCl either by treatment with ascorbic acid.
  53. 53. THANK YOU!