Colloidal Dispersion: Protective colloids, Applications of Colloids in Pharmacy and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

STABILITY OF COLLOIDAL SYSTEMS: Colloidal particles undergo frequent collisions as a result of Brownian motion. Whether these collisions result in coagulation (permanent contact or close aggregation of particle), flocculation (temporary contact or large aggregates) or whether the particles rebound and remain uniformly dispersed, depends on the forces of interaction between the particles. The forces of interaction between the particles can be divided into two groups: attractive forces and repulsive forces.

Attractive forces (VA): van der Waals attractive forces between colloidal particles are considered to arise from dipole–dipole interactions and these forces varies as the inverse of the distance between particles.

Repulsive forces (VR): Electrical repulsive forces between colloidal particles arises because of the overlap of the diffuse parts of the electrical double layer and also because of osmotic pressure generated by the accumulation of ions between the particles. Repulsion between the particles decreases exponentially with increase in the distance between the particles.

Stability of lyophobic colloids—DLVO theory: The DLVO theory is based on a combination of attractive and repulsive forces and explains the aggregative instability of two particles at any given separation distance. According to the DLVO theory:

Total potential energy of interaction (VT) = VA + VR

Stability of lyophilic colloids: Lyophilic colloidal solutions or solutions of macromolecules are stabilized by electrical double- layer interaction and solvation. Lyophilic colloids are unaffected by the small amounts of added electrolyte because of the presence of the solvation layer of water surrounding the dispersed particles. At high electrolyte concentration, they loses their water of solvation and coagulates, i.e. a salting-out effect occurs. Addition of solvents such as alcohol and acetone could change a lyophilic colloid to lyophobic colloid.

Schulz–Hardy rule: Addition of electrolytes may cause precipitation or coagulation of colloidal dispersions due to reduction of zeta potential. The effectiveness of an electrolyte to cause precipitation or coagulation depends not only on its concentration but also on the valence of the ion causing precipitation. According to Schulz–Hardy rule, the higher the valence of the ion, the greater is the precipitation or coagulation power. For example, Al3+ is more effective than Mg2+ and Na+.

Hofmeister or Lyotropic series: The precipitation or coagulation power in lyophilic colloids is given by Hofmeister or lyotropic series. According to this series the precipitation or coagulation power is based on ability of electrolyte to dislodge solvent sheath from the colloidal particles. For example, anions in ascending order of precipitation power are iodide <bromide < chloride < acetate < sulphate < tartrate < citrate.

INTERACTION OF COLLOIDS:

Coacervation: It is the separation of a colloid-rich layer from a lyophilic solution on the addition of electrolyte or of a nonsolvent. Coacervation also occurs when two oppositely charged lyophilic colloids are mixed such as acacia and gelatin. Acacia at about pH 3 is negatively charged and gelatin at a pH below its isoelectric point is positively charged. A combination of solutions at about pH 4 results in coacervation.

Bridging: Bridging is a consequence of the adsorption of polymer molecule (lyophilic colloids) onto the surface of more than one colloidal particle. Bridging is seen when lyophilic colloids are added to lyophobic colloidal systems in small amounts. The adsorbed polymer molecules are long enough to bridge across the energy barrier between the colloidal particles thus resulting in formation of structured floc.

Protective colloid action: Mutually repulsive, nonionic polymeric materials such as methylcellulose, gums and nonionic surfactants adsorbs at the surface of colloidal particle and can stabilize a lyophobic colloidal solution to coagulation even in the absence of a significant zeta potential. At high concentration, lyophilic colloids saturate the surfaces of lyophobic colloidal particles and causes repulsion of the particles as a result of osmotic pressure effects. A polyelectrolyte may stabilize a lyophobic colloid by electrosteric stabilization.

Gold number: It is a measure of protective ability of hydrophilic colloid or polymer molecule. It is calculated as number of milligrams of hydrophilic colloid needed to prevent change in colour of gold solution (10 mL) from red to violet on the addition of 10% sodium chloride solution (1mL). The change in colour of gold solution is due to the change in particle size. The lower the gold number, the higher is the protective ability of the hydrophilic colloid. For example, gelatin with gold number 0.01 is more effective than acacia (gold number 0.2) and tragacanth (gold number 2.0).

Applications of colloids in pharmacy:

Table 1 – Applications of colloids in Pharmacy

 Multiple choice questions (MCQs)

1.The protective ability of colloids is measured as

a)Zeta potential

b)Streaming potential

c)Gold number

d)None of the above

2.If the gold number is less then the protective action will be

a)More

b)Less

c)Half

d)Zero

3.In sol, the dispersed phase and dispersion medium are

a)Solid and liquid

b)Liquid and liquid

c)Solid and solid

d)Liquid and gas

4.Homogenous phase (true solution) havng size range of

a)5 to 1.0µm

b)<0.1nm

c)>0.01 µm

d)<0.01 µm

5.Heterogenous phase (coarse dispersion) having size range of

a)5 to 1 µm

b)10 to 100 µm

c)10 to 1000 µm

d)1 to 10 µm

6.The following statement is appropriate for molecular dispersion

a)Diffusion rate is slow

b)Impermeable across semi permeable membrane

c)Does not scatter light

d)Particle size less than 1µm

7.The following statement is not appropriate for molecular dispersion

a)Shows tyndall effect

b)Does not scatter light

c)Diffusion rate is rapid

d)Does not settle on ultracentrifugation

8.The following statement is true for colloidal dispersion

a)Diffusion rate is slow

b)Impermeable across semipermeable membrane

c)Shows tyndall effect

d)All of the above

9.In _____ colloids, dispersed particles have a greater affinity to the dispersion medium

a)Lyophilic colloids

b)Lyophobic colloids

c)Association colloids

d)Hydrophobic colloids

10.Dispersion of rubber and polystyrene in benzene or ethyl methyl ketone is referred to as

a)Hydrophilic colloid

b)Lyophilic colloid

c)Lyophobic colloid

d)Hydrophobic colloid

11.Lyophobic colloids can be prepared by

a)Chemical method

b)Peptization method

c)Electric arc method

d)All of the above

12.Few colloids are translucent because of the

a)Tyndall effect

b)Splitting effect

c)Scattering

d)None of the above

13.The comparatively fast sedimentation rate of the dispersed phase produced by means of gravity other forces, this is characteristic of

a)Colloidal dispersion

b)Coarse dispersion

c)Both of the above

d)None of the above

14.The molecule in the molecular dispersion have size

a)Less than 10nm

b)More than 1nm

c)Less than 1nm

d)More than 10nm

15.The ____ colloids have strong attraction towards solvents.

a)Lyophilic

b)Lyophobic

c)Associated

d)None of the above

Solution:

1. c)Gold number
2. a)More
3. a)Solid and liquid
4. d)<0.01 µm
5. c)10 to 1000 µm
6. c)Does not scatter light
7. a)Shows tyndall effect
8. d)All of the above
9. a)Lyophilic colloids
10. b)Lyophilic colloid
11. d)All of the above
12. a)Tyndall effect
13. b)Coarse dispersion
14. c)Less than 1nm
15. a)Lyophilic

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 200-201.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 736-744.

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