suspension PHT 312 l..

heehawultraMechanics

Feb 22, 2014 (3 years and 6 months ago)

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Suspension

Definition


A
Pharmaceutical suspension is a
coarse dispersion
in which
internal phase is
dispersed uniformly
throughout the
external
phase.


The
internal phase consisting of insoluble
solid particles
which is
maintained uniformly
throughout the suspending
vehicle with
aid of single
or combination
of suspending
agents.


The
external phase (suspending medium)
is generally
aqueous in
some instance, may be
an organic
or oily liquid for non oral
use.

2
-
Degree of flocculation (
β)


Degree of flocculation: is the ratio of the sedimentation volume of the flocculated
suspension F, to the sedimentation volume of the deflocculated suspension, F¥

ß = F / F¥

(Vu/Vo) flocculated

ß =
--------------------

(Vu/Vo) deflocculated

When the total volume of both the flocculated and the deflocculated suspensions
are same;

ß = (Vu)
floc

/ (Vu)
defloc

-
The minimum value of ß is
1
; this is the case when the sedimentation volume of
the flocculated suspension is equal to the sedimentation volume of deflocculated

suspension.

-

ß is more fundamental parameter than F since it relates the volume of flocculated
sediment to that in a deflocculated system

Electrokinetic

Properties

Zeta Potential


The zeta potential is defined as the difference in potential between
the surface of the tightly bound layer (shear plane) and electro
-
neutral region of the solution.

The ions that gave the particle its charge, are called potential
-

determining ions.


Immediately adjacent to the surface of the particle is a layer of tightly
bound solvent molecules, together with some ions oppositely charged
to the potential determining ions.


These two layers of ions at the interface constitute a double layer of
electric charge (shear plane).

Zeta potential governs the degree of repulsion between the adjacent,
similarly charged, dispersed particles.


If the zeta potential is reduced below a certain value, the attractive
forces exceed the repulsive forces, and the particles come together.
This phenomenon is known as flocculation.


Particles carry charge may acquire it from
adjuvants

as well as
during process like

crystallization, grinding processing, ionization of functional group
of the particle, adsorption of ions from solution e.g. ionic
surfactants
.

VI.
Deflocculation

and flocculation

Flocculated Suspensions


In flocculated suspension, formed
flocs

(loose aggregates) will cause increase

in sedimentation rate due to increase in size of
sedimenting

particles.



Hence, flocculated suspensions sediment more rapidly. Here, the sedimentation
depends not only on the size of the
flocs

but also on the porosity of
flocs
.

In

flocculated suspension the loose structure of the rapidly
sedimenting

flocs

tends to preserve in the sediment, which contains an appreciable amount of
entrapped liquid.


The volume of final sediment is thus relatively large and is easily
redispersed

by
agitation.


Even the smallest particles are involved in

flocs
, so the supernatant appears clear
.

Deflocculated suspensions


In deflocculated suspension, individual particles are

settling.


rate of sedimentation is slow. which prevents entrapping of liquid medium which

makes it difficult to re
-
disperse by agitation.


This phenomenon called ‘
cacking
’ or ‘claying’. In deflocculated suspension,

larger particles settle fast and smaller remain in supernatant liquid

so supernatant appears cloudy