Colloidal system:A Pharmaceutical Giant---Introduction

What are Colloids?

Colloidal System is defined as the heterogenous biphasic system in which the dispersed phase ranges within 1-10 angstrom to few micrometres in dimensions.This dispersed phase is nothing but the colloidal particles or colloids.
A colloidal system differs from other heterogenous and homogenous solutions in terms of the size of dispersed phase.
e.g- True solution has a size range of its dispersed phase less than 0.01 micrometre; While that of a heterogenous coarse dispersion varies between 10-1000 micrometres.

Examples of pharmaceutical colloids--silver colloid (germicidal),Iron-dextran injection (official and used in treatment of anaemia administered intramuscularly),Iron-sorbitol injection (consists of citrate,sorbitol and dextran complexed to ferric ions) etc.

Characteristics/Features of Colloids

1) Particle Size

The particle size as stated above varies between 1-10 angstrom to few micrometres.However, the major consideration of the particle size in this context is the colour it reflects is mainly due to its size. Larger the particle size, lesser wavelength of light it transmits.But this doesn't indicate that lesser particle size transmits larger wavelength of light.

e.g- Colloidal gold has a red colour while that of a gold sol having intermediate sizes of gold particles has a violet colour.

2) Particle Shape

The colloidal particles may possess different shapes like:

1)spherical (e.g-surfactants)
2)rod-shaped(e.g-Tobacco mosaic virus)
3)Flakes(e.g-Bentonite)
4)Ellipsoid(e.g-serum albumin)
5)Branched(e.g-cellulose)

The shape of the particles also reflect specific colour.

e.g-Gold colloid consisting of Spherical particles has red colour while those having disc shape have blue colour.

3) Surface Area

Since particle size is inversely propotional to surface area, colloids having smaller diameter have huge surface area.This is advantageous in the respect that it gears up some physico-chemical parameters associated with the system like it enhances solubility,absorption etc.

4) Surface Charge

The surface charge occurs on colloidal particles mainly due to 2 reasons:

1) Presence of dissociable or associable functional groups (-COOH,-OH etc.) which on dissociation in a suitable solvent gives a negative charge to the surface of colloids.

2) Adsorption of ions onto the surface from the solvent.

e.g- Adsorption of H+ ions in dilute HCl solution or H+/OH- in aqueous medium

The surface charge determines stability in the respect that presence of a huge amount of like charges creates repulsion among the colloidal particles thereby stabilizing the system.

e.g- Negatively charged colloids-sulphur,acacia,tragacanth
Positively charged colloids-Aluminium,gelatin

Pharmaceutical Applications

1) Therapy--- Colloidal system are used as therapeutic agents in different areas.

e.g- Silver colloid-germicidal
Copper colloid-anticancer
Mercury colloid-Antisyphilis

2) Stability--- As already stated, charges play an important role in determining the stability of a colloidal system.Colloids, especially lyophobic colloids having like charges on particle surface repel each other and prevent flocculation in suspensions.

e.g- Colloidal dispersion of gelatin is used in coating over tablets and granules which upon drying leaves a uniform dry film over them and protect them from adverse conditions of the atmosphere.

3) Absorption--- As colloidal dimensions are small enough,they have a huge surface area. Hence, the drug constituted colloidal form is released into the vicinity in large amount.

e.g- sulphur colloid gives a large quantity of sulphur and this often leads to sulphur toxicity.

4) Dissolution --- Due to huge surface area, the dissolution rate is very large as stated by Noyes-Whitney equation.

dC/dT=DA(Cs-Cb)/Ln

where, D=dissolution coefficient
dC/dT=Rate of diffusion
A= surface area of particle
Cs=concentration of stagnant layer (layer of solvent just adjacent to particle and is concentrated with the drug)
Cb=concentration of bulk solution (Often dilute and represents the rest of the solvent in the system)
L=length of stagnant layer
n=viscosity of the medium

5) Targeted Drug Delivery--- Liposomes are of colloidal dimensions and are preferentially taken up by the liver and spleen. Hence, principle of colloids is also used in targeted drug delivery system.