Particle-Particle interactions

Question 1

Why do particles in a dispersed system tend to associate?

Answer 1

Because they have a large surface area, they associate in order to try to reduce their total surface energy.

Question 2

When particels meet, they may: (3)

Answer 2

1) Rebound and remain seperate
2) Become temporarily attached
3) Become permanently attached

Question 3

What dictates what happens when the particels do meet?

Answer 3

The balance of attractive and repulsive forces dictates what happens. Particle concentration also has a role.

Question 4

Attractive interactions (VA): What do these result from?

Answer 4

These result from Van der Waals forces between molecules in the surface layers of the interacting particles. They are short range forces, but there will be lots of them so will have a large effect.

Question 5

Equation: VA = -Aa/12H A= haymaker constant, a = radius, H=distance between particles
What effect will a change in H have on the attractive interactions?

Answer 5

H is multiplied by 12 so will have a big effect on the attractive interactions.

Question 6

What happens to the attraction as the distance (H) increases?

Answer 6

The attraction decreases as the distance increases.

Question 7

What charge are attractive forces all the time?

Answer 7

Negative charge.

Question 8

Repulsive interations (VR): What do these result from?

Answer 8

These result from electrical charges on the surfaces of particles due to

1) Adsorption of charged polymers or surfactants at the interface
2) Polarity difference between solid and liquid
3) Ionization of chemical groups at the surface of particles
4) Adsorption of small, inorganic ions onto the particle surfaces.

Question 9

Where do the repulsive interactions act over?

Answer 9

Approximately over the thinckness of the double layer

Question 10

What would happen to the VR if the distance between the particels increases?
Bonus point: in relation to VA

Answer 10

VR also decreases with distance between particles (more sharply than va)

Question 11

What do DVLO stand for?

Answer 11

Derjaguin and Landau, And Verwey and Overbeek theory

Question 12

What is the DVLO theory?

Answer 12

It describes the interaction between two particles in terms of Van Der Waals attractive forces (Va) and electrical repulsive forces (Vr):
Vt (total force) = Va + Vr

As particels approach each other they will come under attractive influence of Van Der Waals which will be opposed by repulsive forces of overlapping diffuse layers

Question 13

What charge is Vr on the graph?

Answer 13

Positive

Question 14

What charge is Va on the graph?

Answer 14

Negative.

Question 15

What force predominates at very low H (Interparticle distance)?

Answer 15

Va. Attraction predominates and the particles are very close together. This is the Primary Minimum

Question 16

What force predominates at intermediate distance?

Answer 16

Vr. REpulsion predominates in the middle of the electrical double layer and particels are more spread out. This is the Primary Maximum

Question 17

What is the force that predominates during large H (interparticulate distance)?

Answer 17

Va predominates. Even though Va decreases with increased H, Vr will decrease even more sharply becasue there are even less Van der waals forces therefore much less repulsion.
This is the secondary minimum.

Question 18

What does the height of the primary Maximum on the graph depend on?

Answer 18

The height will depend on the Vr and therefore the Zeta potential (zeta potential = indicates degree of repulsion)

Question 19

What is the force most favoured in pharmaceuticals?

Answer 19

Secondary Minimum

Question 20

Which axis on the graph tells us the total Forces?

Answer 20

Y axis.

Question 21

What is the force when the graph line crosses the X-axis?

Answer 21

No repulsive or attractive force at this point. therefore Vt = 0

Question 22

Why are particles at such high repulsion at the Primary Maximum?

Answer 22

Because the diffuse layer of the electrical double layer is overlapping, forcing eachother appart.
Repulsive forces will only act when the electrical double layer is overlapping

Question 23

WHat happens to the electrical double layer at the secondary minimum?

Answer 23

The electrical double layers are no longer overlapping therefore getting a low level of reversible attraction.

Question 24

What does the depth of the Secondary Minimum depend on?

Answer 24

Depends on particle size

Question 25

What is the binding of the two particels at Very low H?

Answer 25

Irreversible Van der Waal forces.

Question 26

What is the aggregation of molecules in the Primary minimum phase?

Answer 26

Coagulation - that is irreversible

Question 27

What is the aggregation of molecules in the Primary maximum phase?

Answer 27

Deflocculation

Question 28

What is the aggregation of molecules in the Secondary minimumphase?

Answer 28

flocculation that is reversible.

Question 29

Describe a formualtion that would be fully coagulated:

Answer 29

This is the primary minimum. The particels will settle very quickly becasue they are colsely aggregated and form big clumps. As the particles sediment, they trap the liquid vehicle with them, reducing the amount of free vehicle therefore formulation becomes more viscous. Impossible to redisperse.

Question 30

Describe a formualtion that would be Deflocculated:

Answer 30

This is the pripary maximum.
The particles remain seperate, and are small so will settle very slowly or not at all. Have a very low sedimentation value, so when they sediment, they don’t trap any liquid with them, and form very compact sediments in the bottom. Not easily redispersed on shaking but is possible

Question 31

Describe a formulation that would be Partially flocculated:

Answer 31

This is the Secondary minimum phase. There is formation of loose aggregates of particles which constantly break and reform. Aggregates are relatively large so sediment is rapid. As the particels sediment, they do trap liquid with them therefore have a high sedimentation value.
Shaking will re-disperse the sediment to a homogenous system. Most favoured in oharmaceuticals.

Question 32

WHat is the term used to describe the fully sedimentation of a partially flocculated system?

Answer 32

Clayed - the bottom is coloured, and the top transparrent.

Question 33

WHat is the term used to describe the fully sedimentation of a deflocculated system?

Answer 33

Caked - The whole system will be cloudy as it takes so long for the particels to sediment. But once they do, they form a very tight sediment at the bottom. Then the liquied will be transparent.

Question 34

The flocculated phase is the most common used in pharmacy, but what is the one characteristic that we would prefer to have that isn’t in Flocculated?

Answer 34

The SLOW SEDIMENTATION characteristic of a deflocculated system. But there is a risk of caking and irreversible sedimentation so we settle for flocculated.

Question 35

What is the best way to stabilise a suspension/colloid?

Answer 35

Altering the Zeta potential by wither altering the pH, or the electrolytes.
Altering the pH is the easiest and the most important.

Question 36

What are the other ways of stabilising a suspension/colloid?

Answer 36

• Bridging agents: Surface active agents, Polyvalent electrolytes, or hydrophilic polymers.
• Steric stabilisation: not a good option, involves the physical building of a barrier.

Question 37

What is the preferred Value of Zeta potential for a solution to make it most stable?

Answer 37

Over +30mV, or more negative than -30mV.

Question 38

What happens if the Seta potential is not higher than +30, or lower than -30mV?

Answer 38

The suspension is unstable and is coagulated.

Question 39

How does altering the pH change the Zeta potential?

Answer 39

More acidic, higher the Zeta potential, more basic, lower the Zeta potential.

Question 40

What are the pH values required to change the zeta potential to a level that means the suspension is stable?

Answer 40

The pH must be less than 4.5 (more acidic) >+30mV

Or pH must be more than 8 (more basic)

Question 41

What is the point where the graph crosses 0 on the Zeta potential graph called?

Answer 41

Isoelectric point.

Question 42

How do we alter the Zeta potential using electrolytes?

Answer 42

In this case, the electrolyte added is of opposite charge to the particles already in the suspension. This causes a decrease in the repulsive forces, and therefore a decrease in Zeta potential.

Question 43

What is the different amount of electrolytes that can be added to form the three diferent types of solutions?

Answer 43

Small amounts of electrolytes = flocculation (what we want)
Large amounts of electrolytes = Coagulation
Very large amounts of electrolytes = Charge reversal, deflocculation.

Question 44

What affects the ability of an electrolyte to flocculate particels?

Answer 44

The valency of the electrolyte: The Schultz-Hardy rule:
Trivalent electrolyte will have a bigger effect than a DIvalent electrolyte which will have more effect than a Monovalent.

Question 45

Which type of electrolyte is most often used and why?

Answer 45

Although Trivalent has most effect, it is not often used due to toxicity, most often used is Divalent.

Question 46

When would Bridging agents be used?

Answer 46

Only after trying changign the Zeta potetnial and if this was unsuccesful.

Question 47

What is the method of adding a surface active agent as a bridging agent?

Answer 47

This isusing non-ionic SAAs to adsorb onto more than one particle, to form a bridge, thereofre forming loosely flocculated structures.

Question 48

What is the method of Polyvalent Electrolytes as a bridging agent?

Answer 48

Adding electrolytes that have more than one charge along with a bridging agent i.e. can’t be Na+ but can be Ca2+ becasue these will bind to the bridging agent and the particle.

Question 49

What is the method of adding hydrophilic polymers of Bridging agents?

Answer 49

Addign long chain branched molecules that are hydrophilic will form a gel-like structure of the flocculating system, with part of teh chain adsorbing onto the particle, and the rest bridging to the other particles.

Question 50

Steric Stabilisation is:

Answer 50

Adding non-ionic polymers which will adsorb onto the particle surface, keeping the particles apart due to repulsive steric interaction.