L2 Flashcards
Why does attraction occur
Due to van der waals attraction between the molecules in particles.
Can’t be changed.
Why does repulsion occur
Due to electrical repulsion between particles.
Can be altered.
When particles approach each other both repulsion and attraction occurs, the force that predominates depends on…..
Size of the charge on the particles.
How close particles are to each other.
When dispersed in an aqueous medium, solid particles can obtain a charge through…..
Ionisation of functional groups.
Adsorption of ions to the surface of the particles.
When 2 particles approach each other, the electrical double layers will start to overlap, and this will cause…..
REPULSION between them, if the particles have the SAME surface charge
DLVO theory: the total potential energy of interaction is….
Vtotal = Va + Vr
Describe the primary minimum and what occurs in it. (5)
There is a high ATTRACTION between particles. Distance is small between particles. Particles will coalesce/aggregate. Particles cannot be redispersed. Low energy of interaction.
Describe the primary maximum and what’s occurs in it. (3)
Region where REPULSION between particles occurs.
Prevents particles from interacting closely.
Most stable
Describe the secondary minimum and what occurs in it. (5)
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.
When insoluble particles are dispersed, they may…..
Flocculate
Deflocculate
What is flocculation
This is when particles come together, but are loosely bound.
Form ‘floccules’.
What is deflocculation
Flocculated particles settle (or sediment) slowly, and aggregate, forming agglomerates –> caking.
Explain how particles can form caking during storage.
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.
What happens in flocculation?
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.
What are the 3 ways sedimentation rate can be minimised?
REDUCE particle size
Increase DENSITY Of medium (add sugar)
Increase VISCOSITY of medium (add suspending agent)
What is one problem with increasing thee density of medium (minimising sedimentation rate)?
Adding a sugar - May make viscosity too high.
Name 5 facts about diffusible powders. DREEM
They are light.
Disperse easily Remain suspended long enough for accurate dose Easily wetted Easy to formulate May not require suspending agent
Name examples of diffusible powders
Light magnesium carbonate
Magnesium trisilicate
Are indiffusible powders heavy or light?
HEAVY.
Indiffusible powders are ……… in vehicles.
INSOLUBLE.
They do not disperse evenly throughout upon shaking.
Indiffusible powders settle…..
Too QUICKLY for an accurate dose to be poured
Indiffusible powders require ………./………… to increase viscosity.
Thickening or suspending agents.
