Colloids Flashcards
Classify molecular, colloidal, and fine and coarse dispersions based off particle size
Molecular: <1nm
Colloidal: 1nm-1um
Fine and coarse: >1um
What are lyophilic colloids?
Solvent loving colloids. Disperse phase interacts favourable with the dispersion medium such that the system is thermodynamically stable.
System forms spontaneously (negative Gibbs free energy)
Solvation and charge on particle controls stability
Not sensitive to electrolytes
What does a negative and a positive Gibbs free energy mean?
Negative: system forms spontaneously
Positive: system does not form spontaneously
What are lyophobic colloids?
Solvent hating systems
Disperse phase has little or no attraction to dispersion medium, causing particles to aggregrate– system is thermodynamically unstable
Do not form spontaneously (positive Gibbs free energy)
Charge on particles controls stability
Sensitive to electrolytes
What are association colloids?
Organic compounds that contain both large lyophobic entities and strongly lyophilic groups within the same molecule. Generally not in the colloidal size range.
How is surface free energy related to particle size and the stability of the system?
As particles become smaller, the surface area to volume ratio becomes larger, which leads to an increase in the surface free energy of the system; more stable system.
What is Brownian motion?
Results from random collisions of colloidal particles with the molecules of the dispersion medium
What is the Tyndall effect?
A beam of light directed at a colloidal sol will be scattered, making the sol appear turbid. Turbidity measures can be used to determine the concentration of particles, and light scattering measurements can be used to estimate particle size.
What type of microscope must be used to observe colloidal particles?
Must use an electron microscope
What is the electrical double layer?
The charge on the particle surface will attract oppositely charged ions (known as counter ions) to achieve electrical neutrality, causing the formation of an electrical double layer
Define shear plane and zeta potential
Shear plane signals the end of the tightly bound layer– the place where water is no longer bound to ions.
Zeta potential can be measured here, which is a good representation of the magnitude and sign of the electrical charge.
How does electrical potential change as we move away from the particle surface?
Electrical potential decreases as distance from the particle increases
What is the Debye length?
Debye length defines the thickness of the double layer.
2x the Debye length is the distance between particles.
Define deflocculated systems
Systems in which repulsive forces dominate, thus particles remain dispersed.
Have a high zeta potential
Colloidally/thermodynamically stable, but pharmaceutically unstable because caking is irreversible.
Define flocculated systems
Systems in which attractive forces dominate, and particles form flocs
Low zeta potential
Colloidally/thermodynamically unstable because of the aggregation, but pharmaceutically stable because can be easily redispersed.
