Lecture 8: colloidal systems Flashcards
What are the classifications of dispersed systems?
- Molecular dispersion
- Colloidal dispersion
- Coarse dispersion
What category of dispersions do micelles fall in?
Colloidal dispersions
What are the pharmaceutical uses of colloids and nano drug delivery systems?
- To increase the dissolution of materials in pharmaceutical applications
- To enhance the stability of aqueous and oily pharmaceutical preparations
- To provide a controlled release and targeted delivery of therapeutics 4. To impart unique therapeutical properties to certain drugs
What are the types of colloidal systems?
- Lyophilic colloids
- Lyophobic colloids
- Association colloids
What kind of particles are present in a lyophilic colloidal suspension?
Particles that are solvated by the dispersion medium
“solvent loving”
What kind of particles are present in a lyophobic colloidal suspension?
Particles have little attraction for dispersion medium.
“solvent-hating”
What are association colloids?
Dispersions of micelles with aggregates of 50 or more surfactant molecules
Of the different types of colloidal systems, which are thermodynamically stable and which are thermodynamically unstable?
Thermodynamically unstable: lyophobic colloids
Thermodynamically stable: lyophilic and association colloids
What happens when a beam o light is passed through a colloidal system?
The light gets scattered and a cone shape is formed
What is the light scattering effect of colloids used for?
The determination of the size, shape and interactions
How does Brownian motion change for particles with different sizes?
Brownian movement is faster for smaller particles.
How does Brownian motion change for solutions with different viscosities?
Brownian motion decreases with increasing viscosity of the medium
How does diffusion rate change with temperature, particle size and viscosity?
Diffusion increases with temperature while it decreases with increasing particle size and viscosity
How does the velocity of sedimentation change with radius of the particle?
The velocity of sedimentation increases with increasing radius
How does the velocity of sedimentation change with viscosity of the medium?
The velocity of sedimentation decreases with greater viscosity.
What is viscosity?
The resistance to flow of a liquid
How does a Newtonian fluid behave with respect to its viscosity and shear stress?
Increasing shear stress has no effect of the viscosity, it remains constant
How does a plastic fluid behave with respect to its viscosity and shear stress?
Viscosity decreases with increasing shear rate until a minimum shear rate is reached, after which increasing shear stress has no effect of the viscosity, it remains constant
How does a pseudoplastic fluid behave with respect to its viscosity and shear stress?
Viscosity decreases with increasing shear rate
How does a dilatant fluid behave with respect to its viscosity and shear stress?
Viscosity increases with increasing shear rate
In a solid surface in contact with a solution of ions, what is the rigid layer composed of?
- The surface of the solid with the adsorbed potential determining ions
- The counter ions
In a solid surface in contact with a solution of ions, what is the diffuse layer composed of?
A combination of ions that have a charge opposite to the charge of the rigid layer
In a solid surface in contact with a solution of ions, what is the bulk solution composed of?
It’s a neutral ionic solution
In a solid surface in contact with a solution of ions, what is the electrical double layer?
The electrical double layer consists of the rigid and diffuse layer
but in high concentration the electric double layer doesn’t exist??
In a solid surface in contact with a solution of ions, what is Nernst potential?
It is the potential at the solid surface
So it’s basically the difference in potential between the actual surface and the electroneutral region of the solution.
In a solid surface in contact with a solution of ions, what is zeta potential?
- It is the electrical potential at the shear plane
- It is the difference in potential between the surface of the tightly bound layer and the electroneutral region of the solution
In a solid surface in contact with a solution of ions, what is the thickness of the electric double layer dependent on?
The type and the concentration of ions in the solution.
How does stability change with zeta potential?
Higher (absolute value of) zeta potential = higher stability
What happens if the (absolute value of) zeta potential is too low?
The colloidal system will coagulate
What happens to zeta potential at different pH?
At extreme pH, the absolute value of zeta potential is very high (which is good)
At the isoelectric point the zeta potential is zero (bad)
What are the ways by which flocculation of the colloidal suspension is prevented?
- Electrostatic stabilisation
- Steric stabilisation
- Protective sheath
How does electrostatic stabilisation work to reduce flocculation?
1.Provide an electric charge on the dispersed particles (EDL consideration).
2. Aggregation of particles are prevented by electrostatic forces of repulsion.
How does steric stabilisation work to reduce flocculation?
- Surround each particle with a protective sheath.
- Aggregation of particles prevented by steric hindrance of the adsorbed polymer
- Water shell bound to the hydrophilic surfaces gives rise to hydration repulsion.
How does protective sheath work to reduce flocculation?
- Hydrophobic colloids could be stabilized by the addition of large amounts of hydrophilic colloid, with the hydrophile being adsorbed on the hydrophobic particles.
- The hydrophile is the protective colloid
What is the gold number?
- When using a hydrophilic colloid as a protective sheath to stabilise a hydrophobic colloid, the protective property of the hydrophilic colloid is expressed in terms of the gold number
- It is the minimum weight (mg) of the protective colloid required to prevent a colour change from red to violet in 10 mL of gold solution on the addition of 1 mL of a 10% solution of sodium chloride.
