suspentions Flashcards
suspention
it is a heterogenous system in which solid particles (dispersed phase) is uniformly in liquid medium (dispersion medium)
suspention is biphasic (both phase visible) and heterogenous in nature
-dispesed phase= solid
dispertion medium= liquid
ex) chalk in water
ideal properties of suspention
-suspended moleules should not settle rapidly
-prevent from cake formation at bottom
-easy to pour
-stable
-easily flow out of syringe needles
types of suspention
-flocculated
-deflocculated
on the basis of sability, suspention may be a stabel or unstable
stable suspention
in which dispersed phase is dispersed well in dispertion medium
unstable suspentions
in which the dispersed phase is not well displaced in dispertion medium they settle down at bottom
flocculated suspentions
flocculated suspension are those suspension in which particles of dispersed phase aggregates together to form bunch of like structures called floccules
-rate of sedementation is high
-more stable
-it has large voids, so on settling down it form soft cake
-once shaken, it rediperses and comes back to stable condition
deflocculated suspension
there are those in which particles of dispersed phase remain aprt from each other and remain dispersed in the dispertion medium
-rate of sedimentation is low
-it have less voids so on setting down it form hard cake
-on shaken, its redispersion is very tough due to hard cake
-less stable
interfacial properties of suspended particles
in suspention, the solid particles is uniformly dispersed in liquid medium. the interface is formed in between the two phases, which influence the sability of suspension.
surface free energy
-during formulation of suspention, the solid reduced to fine po wder for better mixing in continous medium
-due to very fine partices, surface area of partiles are increase and also their surface free energy
-due to excess surface free energy, particles become highly energetics and tends to aggregate itself and resulting in formation of compact cake
-to reduce surface free energy and make suspention stable, we use wetting agents and surfacntats
-also system make flocculles to reduce surface free energy
ex)polysobate
formation of electrical double layer
-the ionic species present in solution get absorbed at the surface. due to ionization charges on the surface may arise. this produces the formation of electric double layer
-when surface charge is negative then theri will be layer of tightly bound solvent moleucles which is stern layer
-whereas the layer of positive charge, adjacent to the surface is called diffuse layer
- low elctrolyte concentration= deflocculated system
-more (increasing) electrolyte concentration= flocculated system
-high interaction forces- flocculated
-high repulsive forces- deflocculated
settling of suspentions
-also known as rate of sedimentation, because settling of suspention is depend on sedimentation rate
-the settiling down of suspended pticles of suspention at bottom is known as settling of suspention or also sedimentation
- rate of sedimentation increases = stability decreases
to make suspention stable
increase viscocity of dispertion medium by using some agents such as methyl cellulose, natural gums, acacia, tragacantin
-stokes law is valid for those suspention which does not contains more than 2% solid
types of formformulation is suspention
three various method which incolved in formulation of suspension and increase its stabiliy
- wetting agent
- controlled flocuolation
-structure vehicle
wetting agent
free energy that lead to formaltion of lymph, which is not a stable condition for suspention
so to tackle that we add wetting agent tothe suspension that reduce the surface free enegy and make suspension stable
ex) polysorbate
controlled flocculation
in this method we add electrolytes to the suspention
-when we add electrolyte + or - charge develop on the particles and they repel each other hence makes suspention stable
-if they some how make the floccules they will get easily redispersed
-we will take the electrolyte in such a way that it will be opposite charge for the solvent dispertion medium
