Disperse systems 1 Flashcards
Disperse systems definition
Consist of two or more components in a heterogenous mixture, one component is present as small particles or droplets (disperse phase), the other forms the continuous phase or dispersant
Disperse systems characteristics
Either component may be solid, liquid or gas
The size of the disperse phase particles may vary and they can act differently depending on their size
Colloidal and coarse dispersions
In colliodal dispersions, the particles are ~1nm to 1µm
In coarse dispersions, particles are > 1µm
The deciding factor will be the properties of the formulation- in a coarse dispersion, the relatively large particles are liable to sedimentation due to gravitational forces
Properties of an ideal disperse system medicine
Particles must not settle too rapidly
Sediment must be easily re-dispersed
Must easily flow out of the container
Small, uniform particle size
Advantages of disperse systems
Drugs susceptible to aqueous degradation may be formulated as a suspension
Improved bioavailability compared to solid e.g. increase surface area of kaolin»_space; increase adsorption of toxins
Particle size can be used to control absorption rates
Allows flexibility of formulation e.g. taste masking of bitter drugs
Disadvantages of disperse systems
Are as for all liquid formulations:
Stability problems in presence of liquid
Liquids (especially aqueous) are susceptible to microbial contamination
It is more difficult to mask the taste of a drug in a liquid than solid formulation (although it is easier in a suspension than a solution)
Administration of the correct dose can be less precise
Also, if not shaken accurate dosing cannot be assured
Colloids
Lyophobic/ lyophilic/ amphiphilic
Dispersions of particles sized between ~ 1nm and 1µm
Lyophobic- solvent hating
Lyophilic- solvent loving
e.g. an oil loving compound is lipophilic, a water loving compound is hydrophilic
A substance that froms a lyophilic colloid in one dispersant may not do so in a different dispersant
Lyophobic colloids
The particles have different characteristics to the dispersant, so display little, if any, attraction for the dispersant
The system is thermodynamically unstable
Will aggregate to lower their surface energy e.g. water insoluble drugs, clays, oils, inorganic particles dispersed in water
Lyophilic colloids
Particles have an affinity for the dispersant
Particles interact with dispersant molecules, leading to solvation (hydration)
This protective dispersant coat prevents coagulation so the dispersion is inherently stable e.g. proteins or gums in water
Amphilic colloids
Some molecules have regions with affinities for different dispersants
They orient at the surface to allow each part of the molecule to be in contact with its preferred phase
When the surface is full, the amphiphiles form spherical structures to allow them to remain in contact with their preferred phase
Stability of colloidal systems
When particles met they may rebound or become temporarily or permanently attached
The balance of forces between the particles dictates what happens
Attractive or repulsive forces- lyophobic and lyophilic colloids
Solvation forces- lyophilic colloids only
Salting out
Lyophobic colloid stability is more affected by the presence of electrolytes- the stability relies entirely on charges
Lyophilic colloid systems are unaffected by small amounts of electrolytes, as solvation effects keep the particles separate
But at high electrolyte concentrations some solvating molecules leave the colloidal particles to interact with electrolyte, allowing coagulation of the colloidal particles
Separating a mixture
Because different lyophilic colloids can have different degrees of solvation they require different concentrations of electrolyte to cause salting out and this can be used to separate a mixture of lyophilic colloids
