Colloidal Dispersed Fomrulations 4a 4b W2

Question 1

How do colloidal particles interact?

Answer 1

Non-covalent interactions between molecules and particles are governed by a
variety of specific and non-specific interactions.

We are focusing on 3 non-specific interactions

Question 2

3 non-specific interactions

Answer 2

Van der waals - exist between all bodies. These are attractive forces that occur due to the interactions between particles at a molecular level.

Electrostatic - exist only between charged molecules to surfaces

Thermal fluctuation - temperature dependant associated with the local concentration and configurational entropy of atoms or molecular groups that enhances teh steric repulsion

Question 3

What is the primary minimum

Answer 3

• the region of highly attractive
potential when particles are very close together (at the point of ‘hard’ contact).
• Particles reaching the primary minimum:
- experience overall strong attractive forces
- don’t have enough kinetic energy to escape
(KEp < VT)
- aggregate irreversibly

Not good for pharmaceutical suspensions

Question 4

What is the primary maximum

Answer 4

• The primary maximum is the region of highly repulsive potential when particles are close together.
• Particles approaching the primary maximum:
- experience overall repulsive forces
- remain dispersed as they cannot overcome the energy barrier
• Like an Arrhenius-like energy barrier, with a rate
constant and half-life

Good for pharmaceutical suspensions

Question 5

What is the secondary minimum

Answer 5

• The secondary minimum is the region of weakly
attractive potential when particles are (relatively)
far apart.
• Particles in the secondary minimum:
- experience overall (weaker) attractive forces
• Particles sit in an attractive energy ‘well’ but remain
separated from each other by the primary
maximum:
- particles loosely flocculate but are easily
redispersed by e.g. shaking

Good for pharmaceutical suspensions

Question 6

What is DLVO interaction

Answer 6

Is a model that describes the interaction between charged surfaces in a liquid, particularly in colloidal systems.

It explains how particles suspended in a liquid behave and what forces are at play when they approach each other.

The two key forces are:
- Van der Waals forces
- Electrostatic repulsion

Question 7

How does DLVO model the stability of colloids in aqueous medium

Answer 7

• calculates the overall energy of interaction
between particles and predicts their behaviour.
• Overall energy is the sum of attractive (van der Waals) and repulsive (double layer) interactions
• By convention, Vr is positive, Va is negative

Question 8

Attractive energy (Va) i dominated by….

Answer 8

Van de waals

for two sphericalparticles of radius a can be approximated as:

Vvdw = -(Aa/12D)

A = Hamaker constant for the particle material
and medium (~10-20 J)
a = particle radius (~10-9 -10-5 m)
D = interparticle distance (~10-9 -10-5 m)

Question 9

Electric Double Layer Force is created by

Answer 9

Repulsion between the diffuse layers of the 2 (liked charged) particles

Ves = Z x a x e^-kD

a constant (Z),
a geometric term (a)
and a decay rate (e-κD)
This is a form of the Poisson-Boltzmann equation

Question 10

Charges on a particle

Answer 10

Stern layer
- fixed layer of tightly bound counterions
- one hydrated ion radius deep

Shear surface
- boundary between the fixed and diffuse layers

Gouy-chapman
- diffuse layer o weakly bound counterions
- may extend up to 1-10nm

Electric double layer
- the stern and gouy layers together