Colloids Flashcards
Define disperse systems and their 2 main components
a system in which one substance is dispersed or distributed as particles throughout another phase
- Disperse phase
- Dispersion medium continuous phase)
Which dispersion is not considered a colloidal dispersion
gas in gas dispersion (molecular dispersion)
What are the particle sizes of each type of dispersion
Molecular dispersion: less than 1nm (true dispersion)
Colloidal dispersion: 1nm - 1microm (1000nm)
Fine and coarse dispersion: greater than 1microm
What is the difference between true solutions and colloidal dispersions
Colloidal dispersions have larger particle size
True solutions don’t scatter light and appear clear while colloidal dispersions scatter light and appear opaque
What are 3 main types of colloidal systems
Lyophobic: “solvent hating” the degree of attraction is small or non-existent
- unstable, particles aggregating
Lyophilic: “solvent loving” the disperse interacts nice with the dispersion medium
- stable
Association colloids:
Contain both lyophobic and strongly lyophilic groups
Characteristics of lyophobic and lyophilic colloids
Lyophobic:
- Irreversible
- Do not form spontaneously
- Delta H, Delta G is +
- Delta S is negative??
- Surface free energy NOT lowered by solvation
- Makes the reverse spontaneous
- High sedimentation
Lyophilic:
- Reversible
- Form spontaneously
- Delta H, Delta G is -
- Delta S is positive
- Surface free energy lowered by solvation
- Low sedimentation
What are 2 methods to prepare lyophobic colloids
Dispersion: grinding in a mortar, colloid mills, ultrasonic generators
Condensation: physical and chemical
What are types of lyophilic systems
True solutions
- Water soluble polymers, acacia, povidone, protein solutions
Gelled Solutions
- High concentration of polymers
- gelatin or starch –> set to gels on cooling
- methylcellulose –> gels on heating
Particulate dispersions
- Contain solid particles, such as bentonite or microcrystalline cellulose
Why is particle size important in dispersion stability
-Influence on sedimentation rate
- Increase in surface free energy and aggregation tendancy
What are 2 ways to decrease surface free nergy to improve physical stability of the dispersion?
What is the equation
Surface free energy (delta F) = interfacial tension x surface area
By decreasing both values.
To keep the particles small with large surface area the main choice is decreasing the interfacial tension
What are the 2 kinetic properties of colloidal particles. Define them
Brownian motion:
results from random collisions of colloidal particles with molecules of the dispersion medium
Diffusion:
colloid particles spontaneously diffuse from a high to low concentration
What 2 values can decrease sedimentation when increased
Density of dispersion medium
Viscosity of dispersion medium
Describe the Tyndall effect
A laser directed at a colloidal sol will be scattered or absorbed, appearing turbid.
The particle size + concentration is directly proportional to the degree of light scattering
What type of microscopy can you see colloidal particles in
Electron microscopy
What causes the electrical double layer and its role.
- The charge on the particle surface will attract oppositely charged ions to achieve electrical neutrality
It determines the distance between particles in the dispersion (which affects stability)
Define shear plane and zeta potential
Shear plane is the boundary after the tightly bound layer that marks the mobile portion of the double layer
- potential measure at shear plane is called zeta potential
How does electrical potential change as we move away from the partcile
Zeta potential decreases from the surface potential to the stern potential and decays slowly
How does the Debye length influence the interaction between lyophobic particles
Debye length is the length of the double layer and 2 of them indicate the distance between particle
*****Define Deflocculated and Flocculated particles and the zeta potential in each systems
Flocculated (colloidly unstable, pharm. stable)
- Particles remain dispersed loose aggregations (easy to redisperse)
- attractive forces
- low zeta potential
- faster sedimentation
- clumps
- no caking
Deflocculated (colloidly stable)
- Particles are dispersed (difficult redispersion)
- repulsive forces
- High zeta potential
- slower sedimentation
- no clumps
- Caking
How can you lower the zeta potential
By adding counter ions
Which can be redispersed and which cannot
Flocculation, Aggregation, Coagulant
Flocculation: a loose aggregation
Aggregation: particles associate, but can be overcome by vigorous shaking
Coagulant cannot be redispersed, irreversible
What are examples of flocculating agents and their mechanism of action
Electrolytes: decrease zeta potential
Lyophilic macromolecules: protective colloid effect
What are protective colloids?
When lyophilic macromolecules are added to lyophobic colloids
- they absorb onto other particle surfaces preventing close contact of particle and therefore preventing coagulation (stable)
What provides the stability for lyophobic systems? what about lyophilic?
Lyophobic:
- stabilized by the electrical double layer
- Affected by electrolytes
Lyophilic
- Combination of electrical double layer and solvation
- Unaffected by small amounts of electrolytes (can become lyophobic with large amounts of electrolytes)
(if hydration or polymer increases, flocculation or coagulation occurs)
