Flashcards in Electrical properties of Dispersed systems Deck (42):
How do dispersed particles act to try to reduce surface area and surface energy?
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)
What will happen if the particels become permanently attached?
They will become larger particles/clumps that will likely sediment and may become impossible to re-disperse.
What decides if the particles bind or bounce off each other?
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
What are the three possible ways a surface can aqquire an electrical charge when in contact with water?
What is ion dissolution?
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.
What is ionisation?
If the molecules that are on the surface of the particle can be ionized the particle may acquire a surface charge.
What influences the Ionisation?
This is influenced by the pKa of the ionisable groups and the pH of the dispersant.
What is the charge at the isoelectric point?
pka = pH
There will not be an overall net charge.
What will be the overall charge if the pH is below the pKa?
Cations are gained, anions are lost, net POSITIVE CHARGE
What will be the overall charge if the pH is above the pKa?
cations are lost and anions are gained resulting in a NEGATIVE CHARGE
What is Ion Adsorption?
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
Describe the Ion distribution in disperse systems
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.
What is the Electrical Double Layer?
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:
Electrically neutral region.
What is the inner region of the Electrical Double Layer?
The surface charge of the particle itsel and the tightly adsorbed ions on the surface
What is the Diffuse region of the Electrical Double Layer?
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)
What is the Electrically neural region of the Electrical Double Layer?
Region outside the double layer. Equal amount of cations and anions present.
Describe what would happens in the Electrical Double Layer?
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)
What is the True surface in Ion distribution in disperse systems:
The edge of the particle itself
What is the stern plane of the Ion distribution in disperse systems:
The plane going through the middle of the adsorbed particles
What is the Shear plane Ion distribution in disperse systems:
The plane going around the adsorbed layer
What is the edge of the EDL (electrical double layer)?
The layer before the electrical neutral region.
What does the EDL theory tell us?
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.
Electrical Potential: What are we interested in?
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.
What happens to the potential as we increase the distance from the surface of the particle?
The potential decreases with distance from the surface i.e. the differences in the charges decreases.
What represents the highest potential?
The difference between the ENR and the particle surface
How would a graph representing the electrical double layer look?
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.
Where (and why) is the graph when at the ENR?
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.
What is the DeBye Huckel length?
This is the distance from the Stern plane to the ENR and it represents the thicknes of the double layer.
What does the DeBye Huckel length help us understand?
It helps us understand the affect the particle is going to have on the greater formulation.
What part of the graph represents the DeBye Huckel length?
The exponential region of the graph (after the stern plane)
What is K in the DeBye Huckel length formulation?
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
How can we reduce the DeBye Huckel length (1/k)?
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)
What contains lots of electrolytes that could affect formulations by affecting the DeBye Huckel lenth?
Flavourings and Preservatives.
The potential between the actual surface of the particle and the Electroneutral region of the solution is the :
Nernst Potential (E)
The potential between the Stern Plane and the Electroneutral region is the:
The potential between the Shear plane and the Electroneutral region is the:
Why is the Zeta potential so important?
It is the only value that we can influence as formulators
Describe the Zeta Potential:
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.
What does the Zeta potential indicate?
It indicates the degree of repulsion between adjenct (neighbouring), similarly charged, dispersed particles
The higher the Zeta potential...
The more stable the system is likely to be
What should the Zeta potential be for a dispersion to remain stable?
The zeta potential should be either greater than +30mV or less than -30mV