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C) Electrical properties
Most surfaces acquire a surface electric charge
when brought into contact with an aqueous
medium due to either ionization of functional
groups on the drug molecules and/or adsorption
of ions to the surface of the particles.
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Ionization
Surface charge of colloidal particle is controlled by the
ionization of surface groups
Examples:
Amino acids & proteins have carboxyl & amino groups
whose ionization depend on the pH as follow;
NH2-R-COO NH3-R-COO NH3-R-COOH
at high pH zwitter ion at low pH
alkaline medium iso-electric point acidic medium
Negatively charged zero charge positively charged
A protein is least soluble at its iso-electric point and is
readily precipitated
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• Iso electric point:
1. pH at which +ve charges = -ve charges,
2. i.e. net charge of the amino acid = zero.
3. It is a definite pH specific for each protein.
4. At this pH protein is least soluble&
precipitated.
Q; How can you precipitate insulin???
By adjusting the pH of the solution to the iso electric point
of insulin (pH 5.2).
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Ion adsorption
• Ions may be adsorbed on the surface of the colloidal
particle following immersion in an aqueous solution
containing electrolyte.
• In the absence of added electrolytes, preferential
absorption of hydroxyl ions onto the surface of particles
will occur.
• Hydronium ions are more hydrated than hydroxyl ions
and are therefore more likely to remain within the bulk
medium.
• Surfaces of sol in water are more often –ve charged than
+ve charged because cations are more hydrated than
anions so cations reside in the bulk while less hydrated
anions adsorbed on the surface.
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The electrical double layer
• The charged particles is surrounded by
an ionic cloud containing charges of
opposite sign to that of the particles (the
whole system is electrically neutral)
• Development of a net charge at the
particle surface affects the distribution of
ions in the surrounding interfacial region,
• As a result: concentration of counter ions
increase at the surface,
Thus, an electrical double layer exists
around each particle.
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Electro-kinetic phenomenon
• The movement of charged particles with respect to an
adjacent liquid phase is the base principle underlying
four electro-kinetic phenomena:
1. Electrophoresis
2. Electro-osmosis
3. Sedimentation potential
4. Streaming potential
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1- Electrophoresis:
The movement of colloidal particles under the influence
of electric field is called Electrophoresis.
In the electrophoresis cell, when an electric field is
applied across the colloidal solution, the colloidal
particles migrate to oppositely charged electrode.
• The velocity of the particles will
depend on their size and charge.
• Therefore electrophoresis can be
used to determine size and zeta
potential of colloidal particles.
• Used generally for separation of
protein using paper
electrophoresis
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1- Electrophoresis:
• The movement rate of protein is dependent on zeta potential,
molecular size and potential gradient where a mixture of
proteins can be resolved into separate zones if the mobilities of
the individual proteins are different
# The electrophoretic mobility of a protein at the iso-electric
point is zero
2. Electro-osmosis
▪ The opposite in principle to electrophoresis
▪ Is the movement of the liquid with respect to the particles
under the influence of potential gradient
▪ The dispersion medium itself begins to move in an electric
field
▪ Also used to determine zeta potential of colloidal particles.
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3. Sedimentation potential:
• The reverse of electrophoresis
• The creation of a potential when particles undergo
sedimentation.
• Also used to determine zeta potential of colloidal
particles
4. Streaming potential:
▪ The reverse of electro-osmosis
▪ Forcing a liquid to flow through a charged plug or bed of
particles creates the potential.
▪ Also used to determine zeta potential of colloidal particles
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Pharmaceutical applications of
colloids
1) Colloidal silver iodide, silver chloride & silver protein are
effective germicides & not cause irritation as ionic silver
salts.
2) Colloidal copper used in cancer.
3) Colloidal gold used as diagnostic agent.
4) Colloidal mercury used in syphilis.
5) Association colloids (SAA) are used to increase solubility &
stability of certain compounds in aqueous & oily
pharmaceutical preparations e.g. water soluble vitamins A, D
and K also essential oils and phenols of low water solubility.
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Pharmaceutical applications of
colloids
6) Efficiency of certain substances is increased when used in
colloidal form due to large surface area.
e.g. efficiency of kaolin in adsorbing toxins from GIT.
e.g. efficiency of aluminum hydroxide as antacid.
7) Blood plasma substitutes as dextran, PVP & gelatin are
hydrophilic colloids used to restore or maintain blood
volume.
8) Iron - dextran complex form non-ionic hydrophilic sols
used for treatment of anemia.
