Theory of disperse system

1. Disperse System

2. Comparison
Reference : Martin’s physical pharmacy and pharmaceutical sciences : physical chemical and biopharmaceutical principles in the pharmaceutical
sciences.—6th ed. / editor, Patrick J. Sinko ; assistant editor, Yashveer Singh.

4. Fundamental Properties
Particle properties Surface and
interfacial
phenomena
Instability
and
stabilization
Rheological
properties

5. Particle properties
Shape
Particle size and
size distribution
Shape: Important for understanding
behavior of suspensions during storage
Particle shape may have an impact on
packing of sediment, thereby on
product suspendability and stability.
Packing stability is weight to volume
ratio of sediment at equilibrium.
Particle shape affects viscosity of
colloidal dispersions.
Spherocolloids-low viscous, Linear –
more viscous
Changes in particle shape of colloids
affects its flow, sedimentation and
osmostic pressure

6. Particle size and size distribution:
Particle size affects absorption
Certain types of dosage forms require specific size range
Suspensions, aerosols delivering drugs into respiratory tract should contain particles in
the order of 1-5 μm and no particles larger than 10 μm
Size of a spherical particle expressed in terms of its diameter
Asymmetrical particles –equivalent spherical diameter is used to relate size of the
particle to the dia of perfect sphere having same surface area or same volume.
Particle size distribution: number distribution /weight distribution
A wide particle size distribution often results in high density suspension whereas
widely differing particle shape results in often produce low density slurries

7. Surface Properties & Interfacial phenomena
Two important factors : As particle size is reduced, specific surface is increased
:Presence of an electrical charge on the particle surface
Interface: Defined as a boundary between two phases
Surface free energy=Δ𝐺 = 𝑌𝑠𝑙 . ΔA; equilibrium will be reached when Δ𝐺=0
Particle Interactions: Both attractive and repulsive forces exist
These forces depend upon:
 the nature, size and orientation of species
 distance of separation between and among the particles of dispersed phase and
dispersion medium respectively.

8.  The balance between these forces determines the overall characteristics
of the system
 The particles in a disperse system with a liquid or gas being the
dispersion medium are thermally mobile and occasionally collide as a
result of Brownian motion .
 As the particles approach one another,both attractive and repulsive
forces are operative
 If attractive forces prevail, agglomerates results-indicates instability
 If repulsive forces dominate, a homogeneous dispersedly or stability
dispersion remains

9. Various types of attractive forces
a) Dipole-dipole or Keesome orientation forces
b) Dipole-induced dipole or Debye induction forces
c) Induced dipole –induced dipole or London dispersion forces
d) Electrostatic interactions between charged particles : Strongest
force-either attractive or repulsive and are effective over a
relatively long range and are dependent on the ionic charge and size
Forces that are weaker and effective over a shorter distance include
other types of electrostatic forces such as ion- dipole, ion-induced
dipole and (a-c).
(a-c) forces are referred to as Van der Waals forces.
short –range type of interaction, varying inversely with r6

10. 10
Electrical properties of interface
Particles dispersed in liquid media may become charged mainly in one of two
ways.
 The first involves the selective adsorption of a particular ionic species present
in solution. This may be an ion added to the solution or, in the case of pure
water, it may be the hydronium or hydroxyl ion.
 Second, charges on particles arise from ionization of groups (such as COOH)
that may be situated at the surface of the particle
 A third, less common origin for the charge on a particle surface is thought to
arise when there is a difference in dielectric constant between the particle
and its dispersion medium

11. • Some of the cations are adsorbed onto the surface, giving it a positive charge.
• Remaining in solution are the rest of the cations plus the total number of anions
added. These anions are attracted to the positively charged surface by electric
forces that also serve to repel the approach of any further cations once the initial
adsorption is complete.
• In addition to these electric forces, thermal motion tends to produce an equal
distribution of all the ions in solution.
• As a result, an equilibrium situation is set up in which some of the excess anions
approach the surface, whereas the remainder are distributed in decreasing
amounts as one proceeds away from the charged surface.
• At a particular distance from the surface, the concentrations of anions and cations
are equal, that is, conditions of electric neutrality prevail.
11

12.  aa′ is the surface of the solid. The
adsorbed ions that give the surface its
positive charge are referred to as the
potential-determining ions.
 Immediately adjacent to this surface
layer is a region of tightly bound solvent
molecules, together with some negative
ions, also tightly bound to the surface.
The limit of this region is given by the
line bb′
These ions, having a charge opposite to
that of the potential-determining ions,
are known as counterions or gegenions.
 The degree of attraction of the solvent
molecules and counterions is such that
if the surface is moved relative to the
liquid, the shear plane is bb′ rather than
aa′, the true surface.

13. • In the region bounded by the lines bb′ and cc′, there is an
excess of negative ions.
• The potential at bb′ is still positive because, there are
fewer anions in the tightly bound layer than cations
adsorbed onto the surface of the solid.
• Beyond cc′, the distribution of ions is uniform and electric
neutrality is obtained.
• Thus, the electric distribution at the interface is
equivalent to a double layer of charge, the first layer
(extending from aa′ to bb′) tightly bound and a second
layer (from bb′ to cc′) that is more diffuse. The so- called
diffuse double layer therefore extends from aa′ to cc′
13

14. • The potential at the solid surface aa′ due to
the potential-determining ion and is defined
as the difference in potential between the
actual surface and the electroneutral region
of the solution.
• The potential located at the shear plane bb′ is
known as the electrokinetic, or zeta,
potential, ζ.
• The zeta potential is defined as the difference
in potential between the surface of the tightly
bound layer (shear plane) and the
electroneutral region of the solution
electrothermodynamic (Nernst) potential, E.
14

15. DLVO theory
It relates the stability of a disperse system to the
electrolyte content in the continuous phase and
provides an insight into the factors responsible
for controlling the rate at which particles in
disperse systems come contact or aggregate.
DLVO theory applicable to colloids, suspensions,
and O/W emulsions.
VT = VA + VR

16. Attractive forces predominant at short distances &
the net interaction is attraction in the deep
potential energy minimum
At greater distances, the electrostatic repulsion
energy falls off more rapidly with increasing
separation distance than the van der Waals
attraction energy and the net interaction is
attraction in the shallow secondary minima
At intermediate distances, the electrostatic
repulsion predominates and net interaction is
repulsion with max potential Vmax
Stability of a disperse system is indicated by the
height of the maximum in the potential energy
curve Vmax
Value of Vmax necessary to prevent irreversible
contact of particles is around 50 mV

17. Instability and Stabilization concepts
Instability: due to reduced surface energy
Disperse system is stable as long as repulsive forces are sufficiently strong to
outweigh van der Waals or other attractive forces
Repulsive forces can either be: Electrostatic or Steric