Flashcards in Particle-Particle interactions Deck (50):
Why do particles in a dispersed system tend to associate?
Because they have a large surface area, they associate in order to try to reduce their total surface energy.
When particels meet, they may: (3)
1) Rebound and remain seperate
2) Become temporarily attached
3) Become permanently attached
What dictates what happens when the particels do meet?
The balance of attractive and repulsive forces dictates what happens. Particle concentration also has a role.
Attractive interactions (VA): What do these result from?
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.
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?
H is multiplied by 12 so will have a big effect on the attractive interactions.
What happens to the attraction as the distance (H) increases?
The attraction decreases as the distance increases.
What charge are attractive forces all the time?
Repulsive interations (VR): What do these result from?
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.
Where do the repulsive interactions act over?
Approximately over the thinckness of the double layer
What would happen to the VR if the distance between the particels increases?
Bonus point: in relation to VA
VR also decreases with distance between particles (more sharply than va)
What do DVLO stand for?
Derjaguin and Landau, And Verwey and Overbeek theory
What is the DVLO theory?
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
What charge is Vr on the graph?
What charge is Va on the graph?
What force predominates at very low H (Interparticle distance)?
Va. Attraction predominates and the particles are very close together. This is the Primary Minimum
What force predominates at intermediate distance?
Vr. REpulsion predominates in the middle of the electrical double layer and particels are more spread out. This is the Primary Maximum
What is the force that predominates during large H (interparticulate distance)?
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.
What does the height of the primary Maximum on the graph depend on?
The height will depend on the Vr and therefore the Zeta potential (zeta potential = indicates degree of repulsion)
What is the force most favoured in pharmaceuticals?
Which axis on the graph tells us the total Forces?
What is the force when the graph line crosses the X-axis?
No repulsive or attractive force at this point. therefore Vt = 0
Why are particles at such high repulsion at the Primary Maximum?
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
WHat happens to the electrical double layer at the secondary minimum?
The electrical double layers are no longer overlapping therefore getting a low level of reversible attraction.
What does the depth of the Secondary Minimum depend on?
Depends on particle size
What is the binding of the two particels at Very low H?
Irreversible Van der Waal forces.
What is the aggregation of molecules in the Primary minimum phase?
Coagulation - that is irreversible
What is the aggregation of molecules in the Primary maximum phase?
What is the aggregation of molecules in the Secondary minimumphase?
flocculation that is reversible.
Describe a formualtion that would be fully coagulated:
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.
Describe a formualtion that would be Deflocculated:
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
Describe a formulation that would be Partially flocculated:
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.
WHat is the term used to describe the fully sedimentation of a partially flocculated system?
Clayed - the bottom is coloured, and the top transparrent.
WHat is the term used to describe the fully sedimentation of a deflocculated system?
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.
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?
The SLOW SEDIMENTATION characteristic of a deflocculated system. But there is a risk of caking and irreversible sedimentation so we settle for flocculated.
What is the best way to stabilise a suspension/colloid?
Altering the Zeta potential by wither altering the pH, or the electrolytes.
Altering the pH is the easiest and the most important.
What are the other ways of stabilising a suspension/colloid?
- Bridging agents: Surface active agents, Polyvalent electrolytes, or hydrophilic polymers.
- Steric stabilisation: not a good option, involves the physical building of a barrier.
What is the preferred Value of Zeta potential for a solution to make it most stable?
Over +30mV, or more negative than -30mV.
What happens if the Seta potential is not higher than +30, or lower than -30mV?
The suspension is unstable and is coagulated.
How does altering the pH change the Zeta potential?
More acidic, higher the Zeta potential, more basic, lower the Zeta potential.
What are the pH values required to change the zeta potential to a level that means the suspension is stable?
The pH must be less than 4.5 (more acidic) >+30mV
Or pH must be more than 8 (more basic)
What is the point where the graph crosses 0 on the Zeta potential graph called?
How do we alter the Zeta potential using electrolytes?
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.
What is the different amount of electrolytes that can be added to form the three diferent types of solutions?
Small amounts of electrolytes = flocculation (what we want)
Large amounts of electrolytes = Coagulation
Very large amounts of electrolytes = Charge reversal, deflocculation.
What affects the ability of an electrolyte to flocculate particels?
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.
Which type of electrolyte is most often used and why?
Although Trivalent has most effect, it is not often used due to toxicity, most often used is Divalent.
When would Bridging agents be used?
Only after trying changign the Zeta potetnial and if this was unsuccesful.
What is the method of adding a surface active agent as a bridging agent?
This isusing non-ionic SAAs to adsorb onto more than one particle, to form a bridge, thereofre forming loosely flocculated structures.
What is the method of Polyvalent Electrolytes as a bridging agent?
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.
What is the method of adding hydrophilic polymers of Bridging agents?
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.