Colloids and Formulations Flashcards
What is a solution?
A dispersion of molecules of material in a liquid medium and is clear
What is a suspension?
A dispersion of solid particles of material in a liquid medium and it’s generally cloudy
What are the size of particles in a colloidal suspension?
Between 1-100nm
What are the size of particles in a normal suspension?
More then 100nm
Why are suspensions good?
Many drugs aren’t water-soluble but can be suspended in an aqueous phase
What is a surface?
An interface between two phases, where one phase is solid
What do unsettling interactions mean for suspensions in terms of the DLVO theory?
Leads to instability, coalescence and settling
What do settling interactions mean for suspensions in terms of the DLVO theory?
Means the particles remain suspended
Give the equation and annotate for attractive interactions in the DLVO theory:
VA = - Aa/12H
* A = Hamaker constant for the
particle material and medium (~10-20J)
* a = particle radius (~10-9-10-7m)
* H = interparticle distance (~10-9m)
* R=2a+H
* VA= attractive energy
What forces dominate attractive interactions?
Van der Waals
Give the equation and annotate for repulsive interactions in the DLVO theory:
VR= 2 π ε, εo aΨδ x exp(-κH)/(1+H/2a)
- ε, εo = permittivities of the medium and vacuum
- a = particle radius (~10-9-10-7m)
- Ψδ = surface potential (would normally use ζ potential instead as more
easily measured) (~0-50mV) - κ = Debye-Huckel parameter ~109m-1, (1/κ = double layer thickness)
- H = interparticle distance (~10-9m)
Remove bottom part if k <10^5
What is the equation to calculate the total energy of interaction (VT)?
VT= VA+VR
What does a positive peak mean on a potential energy diagram?
Repulsion
Primary maximum
Repulsive barrier, will keep particles apart when close together
What does a negative dip mean on a potential energy diagram?
Attraction
Primary minimum
Attraction between two particles, if it goes far down, the particles are irreversibly aggragated
What is the secondary minimum on a potential energy diagram?
Where the repulsive and attractive lines on the graph come together horizontally in the centre
This means particles are attracted to each other but not strongly, so can easily be dispersed (forms flocks)
What will high temperatures do to particles in a suspension?
At high temperatures will promote primary minimum as more kinetic energy
Describe the electric double layer:
Any particle in an aqueous medium will have a charge on a surface
Stern layer, counter ions totally held to the charge in the middle
Diffuse layer, an imbalance of mobile co-ions
Sheer surface, the boundary between the fix layer and diffusive layer (electrical potential here called the zeta potential)
What is double layer thickness?
The distance between the stern layer and which charge is screened
1/ Debye huckel parameter (k)
Name two ways in how particles are stabilised:
- Steric stabilisation
- Electrostatic stabilisation
What is steric stabilisation?
Involving adsorption of an inert polymer on the surface of the particles
The surface prevents the particles to approach each other and Van der waals are too weak therefore easily separated
What is electrostatic stabilisation?
Involving a surface charge on the surface of the particles with the same charge will repel each other using Zeta potential
Zeta> +30mv = stable
Zeta< -30mv = unstable so can coagulate
What is the Zeta potential dependent on?
Stern potential and ionic strength of medium
Can govern sedimentation
What is a stern potential dependent on?
Presence of adsorbent material
What are two ways and how you can control particle behaviour?
Debye Huckel length parameter, k, dependant on ionic strength of medium so can be easily controlled, decrease ionic strength better
Stern potential, surface can be modified, large stern potential is better