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