Disperse Systems (Suspensions)

8/2/2019 Disperse Systems (Suspensions)

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DISPERSE SYSTEMS

SUSPENSION


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DISPERSE SYSTEMS

The term "Disperse System" refers to asystem in which one substance (TheDispersed Phase) is distributed, in discrete

units, throughout a second substance (thecontinuous Phase or vehicle).

Each phase can exist in solid, liquid, or

gaseous state .


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SUSPENSION

mixture of two substances, one of which isfinely divided and dispersed in the other.

Suspensions:

S-S,

L-S (OR S-L),

G-S

Colloidal suspension 1 nm to 0.5 m

Coarse suspension 1 to 100 m

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A suspension of liquid droplets or fine solidparticles in a gas is called an aerosol.

Blood is an example of suspensions

Suspensions are useful for administeringinsoluble or poorly soluble drugs or insituations when the presence of a finelydivided form of the material in the GI tract

is required.

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The Difference Between Solution &Suspensions

When the 2 substances totally mix it iscalled a solution.

E.g. Solute + Solvent = Solution

(sugar) + (water) = Solution

We then say sugar is soluble in water, it hasdissolved.


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The Difference Between Solution &Suspensions

Suspensions

Sometimes when we mix substances they stayin clusters. We therefore say it is insoluble in

water. E.g. Chalk + Water = Suspension

Eventually the particles sink to the

bottom to form sediment.


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Uses of Suspensions

To administer an insoluble compound as aliquid. To lessen the unpleasant taste of an insoluble

compound by: formulating a vehicle in which the drug is not

soluble using an insoluble form of the drug (ie. salt

form or prodrug) adsorbing the drug onto an insoluble carrier

To modify the release rate of the drug. To improve the stability by reducing the fraction

of drug in solution

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Properties of an Ideal Suspensionare:

Uniform dispersion

Palatable

Pleasing odor and color

No grittiness

Easy to pour yet not watery

No cap-lock

Temperature insensitive

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particles should settle slowly

formulation should allow the easy

redispersion of sedimented particles a flocculated suspension is desirable than

a deflloculated suspension

a suspension should not be too viscous toreduce the sedimentation rate

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Properties of an Ideal Suspensionare:


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Routes of Administration

Oral

Ocular

Rectal

Parenteral

Topical

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Examples of PharmaceuticalSuspensions:

1. Antacid oral suspensions

2. Antibacterial oral suspension

3. Dry powders for oral suspension(antibiotic)

4. Analgesic oral suspension

5. Anthelmentic oral suspension

6. Anticonvulsant oral suspension

7. Antifungal oral suspension

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Preparation

Large scale: colloid mill

: mortar and pestle small scale

witting agents: alcohol, glyrecin(hygroscopic liquids)

METHODS OF PREPARATION

(1) use of controlled flocculation .

(2) use of structured vehicle.

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Packaging and Storage ofSuspensions:

1) Should be packaged in wide mouth containershaving adequate air space above the liquid.

2) Should be stored in tight containers protected

from: freezing.

excessive heat & light.

3) Label: "Shake Before Use" to ensure uniformdistribution of solid particles and thereby uniformand proper dosage.

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FACTORS TO BE CONSIDERED

Sedimentation.

Particle size.

Nernst and zeta potential. Deflocculation and flocculation.

Electrokinetic Properties.

Density of the vehicle. Viscosity of the vehicle.

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F=0.5 F=1.0 F=1.5

Sediment VolumeF={volume of sediment Vu}/{original volume Vo}

Vo

Vu

Vo

Vu

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Sedimentation volume-

F = Vsed/Vtot

The value of F normally lies between 0 to1 for any pharmaceutical suspension.

The value of F provides a qualitive

knowledge about the physical stability ofthe suspension.

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External Forces Acting on ParticlesGravity Brownian Movement

Sedimentationequilibrium: Gravityis neutralized byBrownian movement.


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SETTLING & AGGREGATION

The suspension shall form loosenetworks of flocks that settlerapidly, do not form cakes and are

easy to resuspend.

Settling and aggregation mayresult in formation of cakes

(suspension) that is difficult toresuspend or phase separation(emulsion).

flock

cake

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Nernst potential

The difference in electric potentialbetween the actual surface of the particleand the electroneutral region is referred to

as Nernst potential.

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Zeta potential

Potential difference between the ions inthe tightly bound layer and theelectroneutral region, referred to as zeta

potential.


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Deflocculation

Deflocculation of particles is obtainedwhen the zeta potential is higher thanthe critical value and the repulsiveforces supersede the attractive forces.

These deflocculated particles when

sediment form a close packedarrangement with the smaller particlesfilling the voids between the larger ones.-SOLID HARDCAKE


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Flocculation When this zeta potential goes below the critical

value, the attractive forces supersede therepulsive forces and flocculation occurs.

These loosely packed particles or floccs settlefaster than the defflocculated particles becauseof their larger sizes.

But unlike deffloculated particles this sedimentof floccs does not form solid cake.

This sediment of floccs is easy to redisperse by

minute agitation.


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Degree of flocculation-

= Ffloc/Fdefloc

(Vsed/Vtot)floc=-------------------

(Vsed/Vtot)defloc


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When the total volume of both the

flocculated and the deflocculatedsuspensions are same, the degree offlocculation,

= (Vsed)floc/(Vsed)defloc


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THIXOTROPIC SUSPENSION

A thixotropic suspension is the onewhich is viscous during storage butloses consistency and become fluid

upon shaking. A well-formulated thixotropic

suspension would remain fluid long

enough for the easy dispense of adose but would slowly regain itsoriginal viscosity within a short

time. SUSPENSION


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Disperse Systems (Suspensions)

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