Surface Chemistry

Properties of Colloids and Suspensions

  • Properties of Colloidal solutions:
    (a) Nature: Colloidal solutions are heterogeneous.
    (b) Colligative properties: Because of high molecular mass the measured value of colligative properties is negligible except for osmotic pressure.
    (c) Visibility: Particles are visible under Zsigmondy's ultra-microscope.
    (d) Colour: It depends upon the size of particles and the wavelength of the scattered light eg.
               (i) Colour of dilute milk appears red on seeing transmitted light while blue on seeing light scattered at 90°.
               (ii) Fine gold colloid is red in colour (purple of cassius) the colour changes to blue as the size grows.
    (e) Filter ability: Colloidal particles pass through ordinary filter paper but not through ultra filters, animals membrane, parchment / cellophane membrane.
    (f) Brownian movement (Robert Brown, 1827): It is a mechanical property. The zig-zag random motion of colloidal particle in colloidal solution due to impacts of the particles of dispersion medium was first studied by R.Brown and is termed as Brownian moment.
    (g) Tyndall effect: It is an optical property in which colloidal particles first absorb light and then scatter at right angle to the direction of propagation of the light. It was originally studied by Faraday and was further extended by Tyndall in 1869. It does not depend upon the charge of colloidal particles.
    (h) Electrophoresis: It is an electrical property in which charged colloidal particles move towards the electrode on application of electric field (of about 100 volts).
       * Movement of negatively charged colloidal particles like As2S3, S, Au, Ag, Pt acid dyes, etc towards anode is called anaphoresis.
       * Movement of positively charged colloidal particles like Fe(OH)3, Al(OH)3, Cr(OH)3 Bi, Pb, Fe basic dyes (eg. methylene blue) etc., towards cathode, is called cataphoresis.
       * The pH at which no electrophoresis takes place is called isoelectric point.
  • Applications:
    (i) Because different colloidal particles have different mobility separation from their mixture can be done by electrophoresis.
    (ii) Electrophoresis proves the presence of charge as colloidal particles.
    (iii) Charge on colloidal particles can be known qualitatively.
  • Stability of colloids:
    (a) Stability is directly proportional to the concentration.
    (b) Stability is inversely proportional to temperature.
    (c) Presence of some electrolyte to a certain limit stabilises the colloid.
    (d) Presence of lyophilic sol stabilises lyophobic sol.
  • Coagulation or Flocculation or Precipitation: Colloidal particles can be converted to suspended particles by
    (a) Addition of oppositely charged colloidal solution that causes the coagulation of both colloids.
    (b) Continuous electrophoresis
    (c) Prolonged dialysis
    (d) Addition of suitable electrolyte.
  • Coagulation or Flocculation value:
    (i) It is the minimum amount of an electrolyte in millimole L−1 of mixed solution, needed to coagulate a colloidal solution. Smaller the coagulation value greater is the coagulation power.
    (ii) Coagulation values of NaCl, BaCl2 and AlCl3 for As2S3 (negative) sol are respectively 51 millimoles per litre, 0.69 millimoles per litre and 0.093 millimoles per litre. Their coagulating power are \frac{1}{51} ie., 0.0196 (NaCl) > \frac{1}{0.69} i.e., 1.449(BaCl2) > \frac{1}{0.093} i.e., 10.75(AlCl3)
  • Protective power of lyophilic colloid (Zsigmondy):
    (i) Lyophilic sols are stable and stabilise lyopholic sols. Such sols are called protective colloids.
    (ii) Gold number: Gold number of protective colloid is the minimum number of milligram of the dry protective colloid needed to just prevent the change of colour of 10 mL standards red gold sol from red to blue (coagulation) by the addition of 1 cm3 of 10% NaCl solution.
    (iii) Protective power of a lyophilic sol is inversely proportional to the gold number.
    (iv) Gold number of starch is 20-25, gelatin is 0.005 - 0.01 etc.
    (v) Milk is stabilised by protein casein available in milk.
  • Hardy-Schulz rule:
    (a) Ions having charge opposite to the charge on colloidal particles, coagulate the sol.
    (b) Higher is charge of coagulating ion, higher is its coagulation power.
    (c) Coagulation power of an electrolyte is directly proportional to the rth power of the charge (valency) of the ion causing coagulation.
    (d) The coagulating power of an ion depend on both magnitude and sign of the charge of the ion.
    (i) For Fe(OH)3 positive sol, the order of coagulating power of ions is \tt [Fe(CN)_{6}]^{4-} > PO_4^{3-} > SO_4^{2-} > Cl^{-}
    (ii) For As2S3 negative sol, the order of coagulating power of ions is Sn4+ > Al3+ > Ca2+ > Na+
  • Applications of colloids:
    (a) Formation of delta: Colloidal negatively charged mud particles of river water coagulate by positively charged ions of seal water. The mud deposit formed is called delta.
    (b) Clarification of water: River water contains negatively charged colloidal mud particles. Addition of alum K2SO4 . Al2(SO4)3 . 24 H2O provides Al3+ ions for their coagulation.
    (c) Electrostatic precipitation of smoke: Smoke contains charged colloidal carbon particles. It is passed through two charged plates which attack these carbon particles and the smoke gets rid of charged carbon particles. This apparatus is called cottrell precipitator.
    (d) In medicine: Argyrol eye lotion contains silver sol, colloidal antimony is used to cure kala-azar, etc.
    (e) Blue colour of sky and lake water is due to scattering of light by colloidal particles. Milk appears blue if the light scattered at 90° to the direction of its passage into the milk is observed and emerging light gives red colour.
    (f) Oozing blood is coagulated by FeCl3 or potash alum. The action and the chemical being known as styptic.
    (g) In chrome tanning.
    (h) In rubber plating.
  • Tricks:
    1. The intensity of the scattered light in Tyndall effect, is directly proportional to the difference of refractive indices of the D.P and D.M. Lyophobic colloids show pronounced Tyndall effect because of high value of this difference.
    2. Lyophilic colloids are stable due to solvation of their colloidal particles.
    3. Lyophobic colloids are stable due to double layer of electric charge called as Helm Holtz electrical double layer.
    4. The fixed layer of charge and diffused layer of opposite charge create a potential difference called as "Electro kinetic potential (or) Zeta potential"
    5. During electrophoresis, the upper part of the colloidal solution becomes more concentrated w.r.t colloidal particles. Decantation of  this part rich in colloidal particles is called electro decantation.
    6. Colloidal graphite in an oil is called oil dag and in water as aqua dag. Both are lubricants for high friction machines.
    7. Milk is preserved by adding a small quantity of HCHO to it.
    8. Cloud burst: Tiny water droplets in clouds may have either negative or positive charge on them. When two clouds having opposite charges on water droplets mutually discharge {mutual coagulation} each it may rain or a cloud burst may take place.
    9. During coagulation of blood by the styptic potash alum, the negatively charged fibrinogen colloid coagulates to fibrin by Al3+

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1. Coagulation or flocculation value :
flocculating value ∝ \tt \frac{1}{coagulating\ power}