Suspension

Question 1

what is a suspension

Answer 1

dispersion of finely divided solid particles in a liquid medium, which may be aqueous or non-aqueous

Question 2

size of coarse suspension

Answer 2

particle size >1 micron

Question 3

size of colloidal suspension

Answer 3

particle size <= 1 micron

Question 4

applications for suspension

Answer 4

1. oral
2. injection
3. external use

Question 5

oral applications of suspension

Answer 5

• provide means of administering insoluble drugs in liquid form
• provide means of supplying distasteful drugs in a form pleasant to taste
• provide an effective means of administering insoluble compounds for their absorptive or antacid properties in the treatment of gastric disorders

Question 6

injection use of suspension

Answer 6

provide a means of sustaining drug release as depot therapy

Question 7

external use of suspension

Answer 7

provide means of applying insoluble drugs to skin

Question 8

desirable properties of suspension

Answer 8

• chemically stable
• low rate of sedimentation of particles
• particles should be readily dispersed upon gentle shaking of container
• size of particles should remain fairly constant throughout storage
• should pour readily from container
• suspensions for injection should flow readily through syringe needle
• suspensions for external use should spread easily and yet not so mobile that they run off the surface when applied

Question 9

stokes law

Answer 9

v= (2r^2 (p1-p2)g)/9n

Question 10

flocculated suspension

Answer 10

• aggregates in dispersed phase
• rapid sedimentation rate
• porous sediment forms on prolonged standing
• relatively large volume
• easy to redisperse
• clear supernatant liquid
• rheological property of concentrated suspension- plastic flow with thixotropy

Question 11

deflocculated suspension

Answer 11

• individual particles in dispersed phase
• slow sedimentation rate
• compact sediment (cake) forms on prolonged standing
• relatively small volume
• difficult to redisperse
• cloudy supernatant liquid
• dilatant flow
• troublesome during production

Question 12

important factors for formulation of a suspension

Answer 12

1. dispersed phase
2. liquid vehicle
3. wetting agent
4. flocculating agent
5. thickening agent
6. pH buffer
7. preservative
8. colorant, flavour, fragrance

Question 13

dispersed phase

Answer 13

aka suspensoid, consisting of insoluble solid particles: lyophilic and lyophobic

Question 14

lyophilic solid

Answer 14

solvent-loving and do not require the use of wetting agent for dispersion in the liquid vehicle; may also increase the viscosity of suspension

Question 15

lyophobic solid

Answer 15

not solvent-loving and require the use of wetting agent for dispersion in the liquid vehicle, most drugs are hydrophobic

Question 16

liquid vehicle

Answer 16

dispersion medium / continuous phase; may be aq or non-aq, usually is water

Question 17

wetting agent

Answer 17

function to make the solid particles more penetrable by the dispersion medium, eg: replace air space in particle crevice
- usually pre-mixed with solid particles in the preparation of suspension

Question 18

moa of wetting agent

Answer 18

1. reduce solid-liquid interfacial tension

2. imparts hydrophilicity to particle surface

Question 19

examples of wetting agent for aq dispersion medium

Answer 19

1. surfactant: HLB 7-9 (eg. spans), used at concentrations below CMC
2. hydrophilic polymers (eg. acacia, tragacanth xanthan gum, polyvinylpyrrolidone)
3. polyhydric alcohols: miscible with water (eg. glycerin, propylene glycol, polyethylene glycols)

Question 20

flocculating agents

Answer 20

1. electrolytes
2. surface-active agents
3. hydrophilic polymers

Question 21

electrolytes

Answer 21

affect the zeta potential which governs interparticulate repulsion/attraction, interparticulate attraction occurs when zeta potential is reduced below critical value

Question 22

low concentration of electrolyte

Answer 22

0. 01-1%:
   - generally sufficient to reduce the zeta potential to just below the critical value for interparticulate attraction to occur
   - van da waals attractive forces cause the solid particles to aggregate to form flocs
   - on prolonged standing, the flocs will settle to form a porous sediment which is easy to redisperse

Question 23

high concentration of electrolyte

Answer 23

• reverse the sign of zeta potential and to a magnitude that will cause interparticulate repulsion
• on prolonged standing, the particles will settle and form a compact sediment which is difficult to redisperse

Question 24

valence of electrolyte

Answer 24

effectiveness of electrolyte in aggregating particles increases with valence of ions

• divalent ions are 10 times while trivalent ions are 100 times as effective as monovalent ion
• monovalent ions are sufficient to induce flocculation of weakly charged particles

Question 25

surface-active agents

Answer 25

1. ionic agents- behaving like electrolyte 2. non-ionic agents- used at concentration above cmc, micelles adsorbed at surface of the particles, where they serve as bridges between particles; flocculation brought about by bridging mechanism between particles

Question 26

thickening agent

Answer 26

function to assist suspension of solid particles, achieved through thickening of dispersion medium

Question 27

example of thickening agent

Answer 27

gums, gelatin, bentonite, carboxymethyl cellulose, polyvinylpyrrolidone

Question 28

small scle dispersion techique

Answer 28

1. mix solid drug with wetting agent in mortar 2. add a small portion of liquid vehicle containing the additives to produce a pourable mixture 3. transfer the mixture to measuring cylinder and make up to the required volume with remainder of the liquid vehicle

Question 29

large scale dispersion technique

Answer 29

1. add wetting agent to solid (drug) in mixer 2. add liquid vehicle containing additives 3. transfer the mixture to high shear equipment to produce a good dispersion (excessive shearing should be avoided)

Question 30

precipitation technique

Answer 30

1. organic solvent precipitation: water-insoluble drug is dissolved in water-miscible organic solvent and then added to distilled water under standard conditions 2. pH-induced precipitation: only applicable to drugs whos solubility affected by ph

Question 31

stability of suspension

Answer 31

index of stability (F) = Volume of sediment at time t / initial volume of total suspension = Ht/Ho more stable- less change in F over time

Question 32

redispersibility of suspension

Answer 32

- mechanical device is used to shake the suspension which has been put aside for dispersed phase to settle down - the number of shakes required to disperse the sediment is determined - ease of dispersibility is inversely proportional to the number of shakes

Question 33

hydrophilic polymer examples

Answer 33

Acacia, tragacanth, alginates

Question 34

low concentration hydrophilic polymers

Answer 34

<0.05% - generally sufficient to bring about flocculation - adsorbed at surface of particles, where they serve as bridges between particles

Question 35

high concentration hydrophilic polymers

Answer 35

1 protective colloid effect 0.05-0.1%: polymer forms a protective coat around each particle, separating them from one another 2. viscosity-enhancing effect >0.1%: polymer increases the viscosity of the medium and improve stability but reduces flowability of the suspension