Electrical properties of Dispersed systems

Question 1

How do dispersed particles act to try to reduce surface area and surface energy?

Answer 1

Dispersed particles have large surface area so when meet each other they tend to stick to particles that are like themselves rather than be in contact with the continuous phase. (like for like)

Question 2

What will happen if the particels become permanently attached?

Answer 2

They will become larger particles/clumps that will likely sediment and may become impossible to re-disperse.

Question 3

What decides if the particles bind or bounce off each other?

Answer 3

The electrical properties of the particles - the attractive and repulsive forces. The charge on the particle is a very important factor for the stability of these systems

Question 4

What are the three possible ways a surface can aqquire an electrical charge when in contact with water?

Answer 4

Ion dissolution
Ionisation
Ion adsorption

Question 5

What is ion dissolution?

Answer 5

If the product is made up of more than one ion e.g. SILVER IODIDE, the different ions will have different solubilities. Silver ions are more soluble than iodide ions so when placed in water, the silver will readily dissolve into the bulk of water, leaving the iodide ions behind. There is now an imbalance in charges, with an overall surface charge that is negative.

Question 6

What is ionisation?

Answer 6

If the molecules that are on the surface of the particle can be ionized the particle may acquire a surface charge.

Question 7

What influences the Ionisation?

Answer 7

This is influenced by the pKa of the ionisable groups and the pH of the dispersant.

Question 8

What is the charge at the isoelectric point?

Answer 8

pka = pH

There will not be an overall net charge.

Question 9

What will be the overall charge if the pH is below the pKa?

Answer 9

Cations are gained, anions are lost, net POSITIVE CHARGE

Question 10

What will be the overall charge if the pH is above the pKa?

Answer 10

cations are lost and anions are gained resulting in a NEGATIVE CHARGE

Question 11

What is Ion Adsorption?

Answer 11

This is the opposite of Ion dissolution.
A surface charge can be acquired by the unequal adsorption of ions onto the surface.
When put into water, the overall charge will often become negative, because the cations ar generally held more tightly in the water (are more hydrated), leaving the anions free to adsorb to the surface of the uncharged molecule

Question 12

Describe the Ion distribution in disperse systems

Answer 12

A disperse system is an overall electrically neutral system, but the surface charge of the particles influence the distribution of ions in the rest of the liquid. If the particles have a negative surface charge, they will repell other -ively charged molecules, and attract +ively charged molecules.

Question 13

What is the Electrical Double Layer?

Answer 13

The layer around each particle with a different composition from the rest of the system. (the system is overall neutral) It can be split to the:
Inner region
Diffuse region
Electrically neutral region.

Question 14

What is the inner region of the Electrical Double Layer?

Answer 14

The surface charge of the particle itsel and the tightly adsorbed ions on the surface

Question 15

What is the Diffuse region of the Electrical Double Layer?

Answer 15

Area just beyond inner region that isn’t as tightly bound with charges. There is a mixture of charges here (but more of one than the other)

Question 16

What is the Electrically neural region of the Electrical Double Layer?

Answer 16

Region outside the double layer. Equal amount of cations and anions present.

Question 17

Describe what would happens in the Electrical Double Layer?

Answer 17

Imagine putting a partcle into contact with a solution containing ions. The surface of the particle may become charged. Counter (opposite) ions are then attracted to the charged surface (inner region). Further away from the surface, the effect is less pronounced, but there is still an excess in the counterions, but some co-ions are present (Diffuse region).
Further still the distribution of ions is uniform and the area is electrically neutral (equal amount of each ion)

Question 18

What is the True surface in Ion distribution in disperse systems:

Answer 18

The edge of the particle itself

Question 19

What is the stern plane of the Ion distribution in disperse systems:

Answer 19

The plane going through the middle of the adsorbed particles

Question 20

What is the Shear plane Ion distribution in disperse systems:

Answer 20

The plane going around the adsorbed layer

Question 21

What is the edge of the EDL (electrical double layer)?

Answer 21

The layer before the electrical neutral region.

Question 22

What does the EDL theory tell us?

Answer 22

That there is an uneven distribution of ions in a disperse system. There will therefore be some areas in the formulation that are more charged than other areas, and the difference in charges between the areas is the POTENTIAL.

Question 23

Electrical Potential: What are we interested in?

Answer 23

We are interested in measuring the potentials against the ENR (electrical neutral region) i.e. the difference between the ENR and other parts of the EDL.

Question 24

What happens to the potential as we increase the distance from the surface of the particle?

Answer 24

The potential decreases with distance from the surface i.e. the differences in the charges decreases.

Question 25

What represents the highest potential?

Answer 25

The difference between the ENR and the particle surface

Question 26

How would a graph representing the electrical double layer look?

Answer 26

The graph drops rapidly and linearly to the stern plant, then has a more gradual exponential decrease to zero where difference is so far from particle that there is no real difference in charge so have no potential.

Question 27

Where (and why) is the graph when at the ENR?

Answer 27

The graph is on 0 because there is no potential (no difference in charge). This is because the adsoebed ions act as a screen that reduces the attraction between the true surface and the counterions further away.

Question 28

What is the DeBye Huckel length?

Answer 28

This is the distance from the Stern plane to the ENR and it represents the thicknes of the double layer. (1/k)

Question 29

What does the DeBye Huckel length help us understand?

Answer 29

It helps us understand the affect the particle is going to have on the greater formulation.

Question 30

What part of the graph represents the DeBye Huckel length?

Answer 30

The exponential region of the graph (after the stern plane)

Question 31

What is K in the DeBye Huckel length formulation?

Answer 31

K is a value which varies within formulation. It is dependent on the electrolyte concentration of the liquid phase so as a formulator we can influence this with the conc of electrolytes

Question 32

How can we reduce the DeBye Huckel length (1/k)?

Answer 32

We can reduce it by adding electrolytes, which will increase K, and therefore decrease the DeBye Huckel length (reduce the exponential phase on the graph)

Question 33

What contains lots of electrolytes that could affect formulations by affecting the DeBye Huckel lenth?

Answer 33

Flavourings and Preservatives.

Question 34

The potential between the actual surface of the particle and the Electroneutral region of the solution is the :

Answer 34

Nernst Potential (E)

Question 35

The potential between the Stern Plane and the Electroneutral region is the:

Answer 35

Stern Potential

Question 36

The potential between the Shear plane and the Electroneutral region is the:

Answer 36

Zeta Potential

Question 37

Why is the Zeta potential so important?

Answer 37

It is the only value that we can influence as formulators

Question 38

Describe the Zeta Potential:

Answer 38

It is The potential between the Shear plane and the Electroneutral region. It varies between formulations and can be used as a measure of the forces acting in a system.

Question 39

What does the Zeta potential indicate?

Answer 39

It indicates the degree of repulsion between adjenct (neighbouring), similarly charged, dispersed particles

Question 40

The higher the Zeta potential...

Answer 40

The more stable the system is likely to be

Question 41

What should the Zeta potential be for a dispersion to remain stable?

Answer 41

The zeta potential should be either greater than +30mV or less than -30mV

Question 42

What would happen in a dispersion with a Zeta potential of 0mV?

Answer 42

There is no difference in charge between the shear plane and the electroneutral region, meaning that the repulsive forces are not large enough to stop attractive forces like Van der Waals from causing the particels to stick and clump together. As long as there is a difference in charges, it doesn't matter if +ive or -ive but must be more than 30.