The document discusses the stability of colloidal systems. It explains that lyophilic sols are stabilized by electrostatic repulsion between charged colloidal particles, while lyophobic sols are thermodynamically unstable but can be stabilized by imparting a small charge to particles. The DLVO theory describes the electrostatic repulsive and van der Waals attractive forces acting on particles. Addition of electrolytes can reduce repulsion and cause coagulation. Protective colloids can stabilize hydrophobic colloids through adsorption. The gold number test measures the minimum amount of protective colloid needed to prevent coagulation of a gold sol.

1. Stability of Colloids
Dr. A. SUMATHI
Professor & Head
Department of Pharmaceutics

2. Stability of Colloids
 The presence & magnitude or absence of a charge on a colloidal
particle is an important factor in the stability of colloidal system
 Lyophilic sols
 These are stabilized by the presence of charge on the colloidal
particles & surrounding of each particle with a solvent sheath
that prevents mutual adherence
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Dr. A. SUMATHI COLLOIDS

3. Stability of Colloids
 Lyophobic sols
 These are thermodynamically unstable
 The like charges produce a repulsion that prevents
coagulation of the particles
 Addition of a small amount of electrolyte that stabilizes by
imparting a charge on the particles
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Dr. A. SUMATHI COLLOIDS

4. Stability of Colloids
DLVO Theory
 Derjaguin Landau Vervey Overbeek Theory
 The forces on colloidal particles in a dispersion are due to
electrostatic repulsion & London type Vander Waals forces of
attraction
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Dr. A. SUMATHI COLLOIDS

5. DLVO (Derjaguin Landau Vervey Overbeek) Theory
 These forces result in potential energies of Repulsion (VR) &
Attraction (VA) between the particles
 There is a attraction near the origin & a high potential barrier of
repulsion at moderate distances
 A shallow secondary attraction is sometimes observed at longer
distances of separation
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Dr. A. SUMATHI COLLOIDS

6. DLVO (Derjaguin Landau Vervey Overbeek) Theory
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Dr. A. SUMATHI COLLOIDS

7. Stability of Colloids
 Addition of electrolyte can bring about coagulation of colloidal
particles
 The mixing of oppositively charged colloids will result in mutual
agglomeration
 If salt is sufficiently added to Lyophilic colloids, agglomeration &
sedimentation of the particles will result. This phenomenon is
called Salting out
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Dr. A. SUMATHI COLLOIDS

8. Sensitisation & Protective Colloidal Action
 The addition of small amount of Hydrophilic or Hydrophobic
colloid to a Hydrophobic colloid of opposite charge will sensitize or
coagulate the particles
 This is due to reduction of zeta potential below the critical value
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Dr. A. SUMATHI COLLOIDS

9. Sensitisation & Protective Colloidal Action
 The addition of large amount of Hydrophilic substances stabilizes
the system
 Hydrophiles being adsorbed on the Hydrophobic particles
 This phenomenon is known as Protection and the added
Hydrophilic sols are called as Protective Colloids
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Dr. A. SUMATHI COLLOIDS

10. GOLD NUMBER
 It is the minimum weight in milligrams of the protective colloid
required to prevent a colour change from red to violet in 10 ml of a
Gold sol on the addition of 1 ml of a 10 % solution of Sodium
chloride
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Dr. A. SUMATHI COLLOIDS

11. Thank You
 Dr. A. SUMATHI