L2

Question 0

Why does attraction occur

Answer 0

Due to van der waals attraction between the molecules in particles.

Can’t be changed.

Question 1

Why does repulsion occur

Answer 1

Due to electrical repulsion between particles.

Can be altered.

Question 2

When particles approach each other both repulsion and attraction occurs, the force that predominates depends on…..

Answer 2

Size of the charge on the particles.

How close particles are to each other.

Question 3

When dispersed in an aqueous medium, solid particles can obtain a charge through…..

Answer 3

Ionisation of functional groups.

Adsorption of ions to the surface of the particles.

Question 4

When 2 particles approach each other, the electrical double layers will start to overlap, and this will cause…..

Answer 4

REPULSION between them, if the particles have the SAME surface charge

Question 5

DLVO theory: the total potential energy of interaction is….

Answer 5

Vtotal = Va + Vr

Question 6

Describe the primary minimum and what occurs in it. (5)

Answer 6

There is a high ATTRACTION between particles. 
Distance is small between particles. 
Particles will coalesce/aggregate. 
Particles cannot be redispersed. 
Low energy of interaction.

Question 7

Describe the primary maximum and what’s occurs in it. (3)

Answer 7

Region where REPULSION between particles occurs.

Prevents particles from interacting closely.

Most stable

Question 8

Describe the secondary minimum and what occurs in it. (5)

Answer 8

Region where attractive forces predominate. (Weak)
Magnitude of forces less than in PRIM MIN.
Prevents close approach to primary min, more physically stable.
Flocculation may occur but can be redispersed.
Allows accurate dose.

Question 9

When insoluble particles are dispersed, they may…..

Answer 9

Flocculate

Deflocculate

Question 10

What is flocculation

Answer 10

This is when particles come together, but are loosely bound.

Form ‘floccules’.

Question 11

What is deflocculation

Answer 11

Flocculated particles settle (or sediment) slowly, and aggregate, forming agglomerates –> caking.

Question 12

Explain how particles can form caking during storage.

Answer 12

Packing of particles, results in compression due to weight on top.
Enough energy –> primary maximum may be overcome.
Therefore, particles will be closer together at primary minimum.
Difficult to redisperse on shaking.

Question 13

What happens in flocculation?

Answer 13

Particles lie in SECONDARY minimum of each other.
WEAK attractive forces, EASILY redispersed.
Particles are LOOSELY bound –> floccules.
Each particle in a floccule is separated by thin film.

Question 14

What are the 3 ways sedimentation rate can be minimised?

Answer 14

REDUCE particle size

Increase DENSITY Of medium (add sugar)

Increase VISCOSITY of medium (add suspending agent)

Question 15

What is one problem with increasing thee density of medium (minimising sedimentation rate)?

Answer 15

Adding a sugar - May make viscosity too high.

Question 16

Name 5 facts about diffusible powders. DREEM

They are light.

Answer 16

Disperse easily
Remain suspended long enough for accurate dose
Easily wetted
Easy to formulate
May not require suspending agent

Question 17

Name examples of diffusible powders

Answer 17

Light magnesium carbonate

Magnesium trisilicate

Question 18

Are indiffusible powders heavy or light?

Answer 18

HEAVY.

Question 19

Indiffusible powders are ……… in vehicles.

Answer 19

INSOLUBLE.

They do not disperse evenly throughout upon shaking.

Question 20

Indiffusible powders settle…..

Answer 20

Too QUICKLY for an accurate dose to be poured

Question 21

Indiffusible powders require ………./………… to increase viscosity.

Answer 21

Thickening or suspending agents.