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 - when adjacent atoms come close enough that their outer electron clouds just barely touch

Electrostatic - the attractive/repulsive force between objects with opposite/identical electric charges

Thermal fluctuation

Question 3

What is the primary minimum

Answer 3

• the region of highly attractive potential when particles are very close together

• Particles reaching the primary minimum experience overall strong attractive forces and 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 and remain dispersed as they cannot overcome the energy barrier

Good for pharmaceutical suspensions

Question 5

What is the secondary minimum

Answer 5

the region of weakly attractive potential when particles are (relatively) far apart.

• Particles 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

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,

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