Week 8 disperse systems dosage forms Flashcards
Solutions
Dissolving a solute into a solvent;
-Molecular dispersion
-Contain small particles (ions or molecules)
-Transparent
-Do not separate
-Cannot be filtered
-Do not scatter light
Disperse systems
-One phase (a disperse phase) is distributed throughout a second
phase (a continuous phase/bulk phase or dispersant)
->Two immiscible phases in contact
Colloids
A colloid or colloidal dispersion = a two-phase system of matter; a
type of mixture intermediate between homogeneous mixtures and heterogeneous mixtures (size < 1 μm)
->In a phase colloid, small droplets or particles of one substance (the disperse phase) are dispersed in another (the continuous phase)
->In a molecular colloid, macromolecules are dispersed in a continuous phase
Properties of colloids
-Tyndall effect
-Brownian motion
-Adsorption
-Dialysis
-Electrophoresis
-Thixotropy
Tyndall effect
Colloids scatter light
-Light beam path through colloid is visible due to light scatter
As they are not homogeneously dispersed
Brownian motion
Under a microscope, the molecules of the liquid are in motion
->Because of the motion, the larger particles of the colloid are
being constantly bombarded by the smaller particles of the
liquid
->This motion and bombardment keeps the colloid particles suspended
Adsorption
A chemical is associated to the surface of a second agent
->Often reversible
->Can lead to conformational changes
->Manipulate physicochemical properties
Examples:
-Adsorption of gases on solids
-Adsorption of surfactants, polymers or proteins
Dialysis
->Separation of suspended colloidal particles from dissolved ions or molecules of small dimensions
->By means of their unequal rates of diffusion through the pores of semi-permeable membranes
Electrophoresis
->Migration of charged particles
->Surface dependent
Thixotropy
-Viscosity can change if you apply force
-Exhibit stable form at rest, Becomes fluid with shear /agitated
The particles can align and become thinner
Types of colloids
Based on the nature of the interaction between the dispersed phase and the continuous phase
Lyophobic – solvent hating (hydrophobic when the solvent is water)
Lyophilic – solvent liking (hydrophilic when the solvent is water)
Phase colloids
Lyophobic colloids
-Inorganic molecules or particles (insoluble)
-Little or no interaction between particles and dispersion medium
-Do not disperse spontaneously,
special procedures needed
-No effect on viscosity unsolvated
-Electrolytes: low concentrations may stabilise, higher
concentration causes instability
Lyophilic colloids
-Large organic molecules in colloidal size range
-Solvated by medium (H2O – this is hydrated)
-Disperse spontaneously
-Increase viscosity, forming gels at higher concentrations
-Electrolytes: generally stable but “salted out” by very high concentrations due to desolvation
Sol
Sol = a colloidal suspension of small solid particles in a continuous liquid medium
Gel
Gel = a colloidal suspension in which the liquid medium has become viscous enough to behave more or less as a solid
-Solid particles will arrange themselves into a 3D structure within the liquid solvent, giving the solution a rigid, definite shape as in a jelly
Types of gels
-Two-phase system (lyophobic)
-One-phase system (lyophilic)
Gels - two-phase system
If the gel contains small discrete particles
-Usually opaque, not as stable as single phase
-Colloidal particles aggregate to form a 3D sponge like network
-Flocculated small particles
-Network for elongated molecules
Gels - one-phase system
If the gel does not appear to hve small discrete particles
-Clear or translucent, with no definite boundaries between the dispersed macromolecules and the liquid
-Can be dispersed in water to provide a range of bases from mobile liquids to solids
-Matted fibres
-Crystalline and amorphous regions held by van de Walls forces
Gels formulation
Contains excipients;
GELLING AGENTS (increases viscosity to desired texture) ->
->Natural Gums -Tragacanth, carrageen, pectin, agar, alginic acid
->Semisynthetic Materials -Methylcellulose, Carboxymethylcellulose, hydroxyethylcellulose
->Synthetic - carbopol
-HUMECTANTS (to prevent drying out) - glycerol, sorbitol, propylene glycol
->PRESERVATIVES
->WATER
Foams
Foams = gases dispersed in a liquid/solid
->Formed by trapping many gas bubbles (a 3D network of surfactants in air)
-Used for the delivery of enemas and topical drugs
Also produced as an often unwanted by-product in manufacture ->troublesome, especially if the foam forms on shaking, why we need defoamers
Defoamers/antifoaming agents
Foam breakers and preventers
-Act as small droplets forming in the foam lamellae
-Displaces surfactants (which stabilise the thin films) at the air-water interface
Emulsions
A ‘pseudo‐stable’ (a bit stable) dispersion of at least two immiscible liquids, one of
which is dispersed throughout the other in the form of fine droplets (generally oil and water) stabilised by the presence of an emulsifying agent (known as emulsifier)
Suspensions (not colloid)
Course dispersion in which insoluble particles, generally > 1 μm, are dispersed within a liquid medium which is usually aqueous
-Settle out
-Shake to stay suspended
