disperse systems Flashcards
what is a colloid
a disperse system where one phase is in the form of tiny particles/droplets
what type of colloid is an emulsion
liquid in liquid
what type of colloid is a suspension
solid in liquid
name two types of colloids
emulsion- liquid in liquid
suspension- solid in liquid
why use disperse systems
single phases may not be able to provide all the formulation requirements
what is an emulsion
liquid medicine where one insoluble liquid is suspended as microscopic droplets in another liquid, usually oil and water=immiscible so 2 separate phases formed, molecular film of emulsifier at interface
what occurs at the interface of oil and water in emulsions
molecular film of emulsifier
types of emulsions
oil in water
water in oil
water oil water
oil water oil
why do emulsions need emulsifiers
stability
how are emulsions produced
homogenisation
what are suspensions
liquid medicine where insoluble solid (drug) is dispersed as fine particles in liquid
what is a vehicle
liquid which particles are suspended in
difference between emulsion ad suspension
emulsion is only 2 immiscible liquids, suspension can be any phase
describe what an emulsion and suspension look like under microscopes
emulsion- circles, molecular film at interface
suspension- irregular particles
physical signs of instability
separation and caking
what is separation and describe what it looks like
particles fall due to difference in density, redisperse by shaking
looks like a gradient
what is caking and describe what it looks like
high density sediment of particles, difficult to redisperse
solid layer at bottom
ways to keep particles suspended/prevent sedimentation
reduce particle size, density match with vehicle, increase viscosity of vehicle
what can be used to increase viscosity
thickening agents
entangling soluble polymers- long chains entangle
charge dispersal
pvp, gelatin, cellulose ethers (carboxymethyl), natural polysaccharides (xanthax gum, starch)
what is charge dispersal
small plate like charged particles dispersed in liquid which thicken the viscosity, particles repel due to charge so liquid resists movement
name some substances that can be used to do charge dispersal
carbomer, bentonite, colloidal silicon dioxide
what are flocs
loose weakly bonded particle aggregates
how to keep suspension particles suspended
flocculation
add surfactant to adsorbs surface charge or add soluble polymer to adsorb to polymer, shaking bottle will break up the flocs and redisperse to make a uniform suspension
how does caking form
particles repel each other then fall into a dense cake, weight of particle presses down on particles below them then theyre forced to aggregate
why can flocculated structures be redispersed
flocculated structure is very porous so it doesnt cake (when it settles theres still lots of gaps)
what is ostwald ripening
smaller particles have greater solubility than large particles, dissolved molecules crystallise on surface of larger particles so the lattice and size increases
particle size/distribution changes due to temperature fluctuations
what can increase the rate of ostwald ripening and why
surfactants because they increase solubility
how to prevent caking
flocculation
