Colloids review questions Flashcards
Define disperse systems and their two main components
disperse system = system in where one substance (the disperse phase) is dispersed or distributed as particles [more or less uniformly] throughout another phase (the dispersion medium or continuous phase).
Identify disperse systems based on the combination of their components’ states of matter and name each combination.
* cn be in solid, liq or gas, of the 9 possibilities 8 are colloidal dispersons
(gas in gas = molecular dispersion)
Classify and describe dispersion based on their particle size
Molecular dispersions (true dispersions; i.e. SOLUTIONS, size \< 1nm) Colloidal dispersions (particle size of 1 nm - 1μm) Fine and coarse dispersions (particle size \> 1 μm)
What is the size range for colloidal dispersions?
1-1000 nm
Define sol and gel as colloidal dispersions.
sol = dispersions of solids in liquid
Some colloidal dispersions under the proper conditions of concentration and temperature set to a solid or semi-solid ⇒ gels.
difference between true solutions and colloidal dispersions
True solutions contain molecules, typically <1 nm
Colloidal dispersions contain particles > 1 nm < 1000nm
*some cases such as with macromolecules (proteins, polymers, nucleic acids) while they are molecular solutions they are also considered colloids due to the size of the molecules that can range approximately 1-20 nm
three main types of colloidal systems?
Lyophobic (hydrophobic) Colloids
Lyophilic (hydrophilic) Colloids
Association Colloids
Compare lyophobic and lyophilic colloidal particles:
solvation, thermodynamic stability, redispersibility, preparation method/formation, sensitivity to electrolytes.
lyophobic:
- the degree of attraction is small or non-existent
- often unstable
- particles aggregating rather than remaining in contact with the dispersion medium
Lyophilic
- dom by repulsion
- disperse phase interacts appreciably with the dispersion medium
- affinity exists between the dispersed particles and the dispersion medium. This contributes to the stability of the system.
Gibbs free energy is defined as ∆G= ∆H-T∆S. How is this equation used to explain formation of lyophobic and lyophilic colloids.
Lyophobic
- little/no attraction, thermodynamically unstable so particles aggregate to lower surface free energy
- irreversible systems
- ∆G positive, ∆H positive
- Surface free energy NOT lowered by solvation; reverse process (agglomeration) spontaneous
Lyophillic
- thermodynamically stable and reversible
- ∆G = neg, ∆H = typically neg
- surface free enrgy lowered by solvation
- no floculation of coagulation
What are the two main methods for preparing lyophobic colloids? What is the main factor playing a role in stabilizing lyophobic particles?
Lyophobic colloids are formed by either dispersion or condensation
- Dispersion – mechanical disintegration or by addition of deflocculating agents (also called peptization)
- Condensation – physical and chemical
examples for lyophobic systems and formulations.
- water or aqueous solutions: water-insoluble drugs, clays (kaolin), colloidal sulfur, metal (colloidal gold, colloidal iron)
- Aurothioglucose colloidal dispersion, Iron Dextran Injection, Colloidal Sulfur topical preparation
. What types of lyophilic systems exist? What is the main factor playing a role in stabilizing lyophilic particles?
Two major classes:
- Water soluble polymers – size of individual molecules are colloidal in dimension
- Particulate dispersions – highly solvated
Stabilizing forces: hydration layer (main), electrical double layer (if applicable)
Give examples for lyophilic systems and formulations.
True Solutions
- water soluble polymers, acacia, povidone, protein solutions
Gelled Solutions
- high concentration of polymers
- e.g. gelatin or starch → set to gels on cooling
- methylcellulose → gels on heating
Particulate Dispersions
- contain solid particles, such as bentonite or microcrystalline cellulose
Define association colloids.
micelles - contain both large lyophobic moieties with strongly lyophilic groups within the same molecule and orient themselves accordingly in the solution.
What is ‘amphipatic’?
Lyophilic and lyophobic portions within the same molecule
Why is particle size an important consideration in dispersion stability?
Particle size is important in many ways, among these are
- Influence on sedimentation rate
- Increase in surface free energy and aggregation tendency
Explain how surface free energy is related to particle size and the stability of the system. Based on the equation ∆ F = γ ∆A
Essential characteristic common to all disperse systems; as particles become smaller, surface area to volume ratio becomes larger;
Explain the principle of Fick’s law of diffusion and how the different parameters influence the rate of diffusion.
Stokes law describes the sedimentation properties of particles. Explain the role of the different parameters in the context of formulation stability.
Name the parts of the electrical double layer (A-D), region E and potentials defined at different distances from the particle surface (1-3).
A – surface of particle
B – tightly bound layer
C – shear plane
D – diffuse part of double layer; layer with excess counter ions or also called Gouy- Chapman layer
E- electro-neutral region; bulk solution
1- surface potential
2- Stern potential
3- zeta potential
Define DLVO theory and the presence and balance of attractive and repulsive forces around colloidal particles.
a) name the axes (D,E,F) of the graph
b) explain curves VA, VR and VT
D – repulsive potential
E – attractive potential
F – distance between particles
VA – van der Waals attraction
VR – electrostatic repulsion
VT – total energy of interaction
explain the forces responsible for the potential minimum and maximum
A, B and C
what is the relationship between the balance of these forces and the physical stability of the particles?
A – primary minimum: attraction predominates - coagulation
B - secondary minimum – slight attraction predominates – flocculation
C – primary maximum – repulsive forces predominate due to the electrical double layer
The balance of these forces determine whether the particles will be individually dispersed or aggregate (either loosely or irreversibly).
