Colloids 4 (stability of colloids) Flashcards
Brownian motion of colloidal particles lead to?
> Permanent contact of particles: coagulation.
- Eventually large aggregates can be formed and sedimentation occurs.
> Temporary contact of particles: flocculation.
- Particles remain a small distance apart in an open structure. They may rebound and remain freely dispersed.
Define the following terms
A) Aggregation
B) Coagulation
C) Flocculation
A)
- Aggregation: collection of particles into groups.
B)
- Coagulation: particles are closely aggregated and impossible to redisperse.
C)
- Flocculation: particles are loosely grouped. Aggregates have an open structure in which the particles remain a small distance apart from each other.
What are some forces that influence the stability against coagulation or flocculation? (dependent on the interaction between particles)
- Van der Waals’ forces or electromagnetic forces of attraction
- Electrostatic forces of repulsion (zeta potential)
- Forces arising from solvation (ie particle-solvent affinity)
- Steric forces (geometry & conformation of molecules at particle surface).
How to stabilize lyophobic colloids?
Only stabilised by electrostatic repulsion
How does repulsion arise in colloidal systems?
- Osmotic effect produced by the increased number of charged species on the overlap of the diffuse regions of EDLs (electrical double layers) of two particles;
> The like charges on the surface of particles (charged particles repel each other)
How does attraction arise in colloidal systems?
- Van der Waals’ forces
Provide a diagram of a potential energy curve and describe its properties
Provide a diagram of a potential energy curve and describe its properties (continue)
What does the size of Vmax (primary maximum) depend on? What does reduction of Vmax lead to?
Depends on zeta potential which itself is dependent on”
- Ψ0 (Nernst potential)
- Electrolyte concentration (which affects length of EDL on magnitude of ζ)
Reduction of Vmax ultimately leads to instability and coagulation of hydrophobic colloid
What is done to achieve controlled flocculation?
Addition of electrolytes to compress the EDL and reduce the zeta potential: lowers the primary maximum (Vmax) and deepens the secondary minimum, resulting in an increased tendency for particles to flocculate in the secondary minimum (in suspensions more so)
What does the specific adsoprtion of ionic surfctants lead to?
Reduced zeta potential but does not compress the electric double layer
How are lyophobic colloids stabilised? What are they very sensitive to?
Lyophobic colloids are stabilized entirely by electric charges on their surfaces.
Very sensitive to added electrolytes
- Schulze-Hardy Rule: For ions having charge opposite to that of particles, the precipitation (coagulation) power increases rapidly with the valence or charge of the ions.
- Al+++> Ca++> Na+ (0.13 : 1.6 : 100)
How does concentration of electrolytes affect zeta potential
At a particular concentration, accumulation of counter-ions reduces zeta potential below its critical value (decreases Vmax), resulting in aggregation.
Mixing of oppositely charged colloidal particles –> mutual precipitation
How are lyophilic colloids stabilized? Is it affected by adding electrolytes?
Stabilized by the combination of electrical double layer interaction and solvation
- Unaffected by a small amount of added electrolytes; When conc. of electrolytes is high, salting out occurs.
- “Salting out” effect: Colloidal material loses its water of solvation to those electrolytes or ions which have strong hydration capacity/power, resulting in coacervation.
- Lyotropic series: order of increasing hydration eg Cs+ < Rb+ < NH4+ < Na+ < Li+
What is coacervation? What are some applications of coacervation in pharmaceutical formulation?
Coacervation: the separation of lyophilic colloid into two liquid layers (one is rich in colloidal aggregates) on the addition of another substance.
- The bottom layer is a colloid-rich layer - known as coacervate. The upper layer is colloid-poor with low viscosity
- Coacervation indicates the physical incompatibility of colloid with added substance.
- Coacervation can be induced by the addition of electrolytes, non-solvent or oppositely charged lyophilic colloids.
Application of coacervation in pharmaceutical formulation (deliberate):
- Microencapsulation
- Gelation
What is the effect of adding hydrophilic macromolecular material to lyophobic colloidal systems? Provide some examples and applications.
- moderate amounts of polyelectrolyte or polymer are added, a structured floc forms.
Application: removal of colloidal materials, water purification
- When large amounts of polymer (enough to cover the surface of particles) are added, lyophobic colloids will be stabilized by steric stabilization or protective colloid effect.
Particles with adsorbed polymers:
- steric interaction & osmotic effect –> repulsion.
- two particles do not approach each other closer than twice the thickness of the adsorbed layer –> no primary minimum (ie no strong attractive forces)
- Modified DLVO theory: VT= VA+VR +Vs (Vs: potential energy for steric stabilisation)
Describe how enthalpic stabilization is used to stabilize particles in polyoxyethylene chains (aquoeus dispersion)?
See attached image
