2. Stability - nano Flashcards
Stability - micro/nano
Stabilisation > Electrostatic > DVLO > Steric > Salvation
Instability phenomena > Phase inversion > Creaming > Flocculation > Coalescence > Ostwald ripening
Emulsion stability
In a stable emulsion, droplets retain their initial character and remain uniformly distributed throughout the continuous phase
Emulsion instability can occur due to one or more of the following:
Phase inversion
Creaming
Flocculation
Coalescence
Ostwald ripening
Creaming is the opposite to;
interversion
Phase inversion
Oil-in-water emulsion stabilised by ionic surfactant/co-surfactant
If charge on emulsion droplet is reduced (with the addition of ions through buffer or drug)…
Emulsion droplets will come together
What happens when droplets are in contact?
Once droplets are in contact, interfacial surfactant film re-aligns forming water-in-oil droplets and phase inversion occurs
Phase inversion:
Conversion from oil-in-water emulsion to water-in-oil emulsion
What is creaming?
Due to density difference between oil and water, the oil droplets tend to concentrate at the top of the emulsion
To avoid this, increase the oil density or viscosity of the emulsion
What causes creaming?
Fat globules (coloured with dye) tend to accumulate as a cream layer on top of the milk
>Due to the differences in densities between
fat globules and the plasma phase of milk
Define Flocculation:
Flocculation is when two or more emulsion droplets aggregate without losing their individual identity
Larger droplets (> 2 µm) flocculate fastest and flocculation is promoted by creaming
What causes flocculation?
Addition of salt (Na3PO4) causes flocculation
What causes Coalescence?
Coalescence occurs when two or more droplets collide and form one larger droplet and is irreversible
What causes Coalescence?
It is caused by various factors, including surfactant type and concentration, pH, temperature etc
Arrested coalescence of adjoining inner cores
Ostwald ripening =
With poly-dispersed droplets, collision between two droplets may cause one bigger droplet and one smaller droplet
Upon repeated collisions…
the small droplets become very small and become solubilised in the continuous medium. They eventually diffuse and re-deposit on larger droplets making them even larger in size
Ostward ripening - for different periods of time
(a) 6 h
(b) 24 h
(c) 48 h
(d) 72 h
molecules binds together by d (close together)
DVLO
(Established byDerjaguin, Landau, Verwey, and Overbeek in the 1940)
Quantitative approach to the stability of lyophobic systems
Assumes the only interactions involved are
Van der Waals forces of attraction (VA)
Electrostatic repulsive forces (VR)
VT = VA + VR
VA = Van der Waals attraction
Energy of attraction varies with the distance (H) between the pairs of atoms or molecules or neighbouring particles with the inverse of the 6th power
VDW - equation
𝐴=1/𝐻^6
VR = Electrical repulsion
- Arises from the interaction of the electrical double layers surrounding pairs of particles
- Repulsive forces decay exponentially with distance
- Repulsive forces decay more rapidly than attractive forces therefore the attractive forces predominate over longer distances
VR = Electrical repulsion
Increasing charge on the double layer:
> Optimise the concentration of surfactant (don’t forget about the associated counterions)
> Optimise the pH
DVLO = VDW v VR
VDW = Explains why some colloidal particles aggregate
Emulsions > coalesce
Suspension > flocculation
VR = Explains why some colloidal particles stay separate
Combination of the two forces
Potential energy diagram, VT = VA + VR
The secondary minimum
> At large distances of separation, particles experience a minimal attraction
> Forces of attraction are weak, flocculation not coalescence, i.e. can re-disperse upon shaking
The primary maximum
As particles come closer together, they start to experience some repulsion which will peak at the primary maximum
Primary Max - the height of this repulsive force (Vmax) determines…
the stability of the system
Height value - effects
The height varies with different surfactants and electrolyte concentration
A high value will ensure coagulation is so slow that the system displays long term stability
The energy barrier that leads to irreversible particle aggregation
The primary maximum - addition of electrolyte (e,g. NaCl)
Neutralisation or reduction of charge on droplets
Decrease in Vmax
Destabilisation of the emulsion
The primary minimum
At close approach, van der Waals forces always dominate over repulsive electrostatic forces
A deep primary minimum is present
At this short inter-particle distance, particles/droplets coagulate irreversibly
DVLO theory applies to…
ionic surfactants VT = VA + VR
When a non-ionic surfactant stabilises an emulsion…
no electrostatic charge is present to stabilise the droplet (the charge on the droplet is neutral)
Hydrophilic polymer chains stabilise emulsions in one of two ways
1) Entropic (steric) effects
2) Osmotic (solvation) forces
How do non-ionic surfactants work?
Surfactant with polymeric head group e.g. Spans and Tweens
Examples of non-ionic surfactants:
Spans and Tweens
Entropic (steric) effects
When two particles come into close contact, the polymer chains start to overlap
This leads to a loss in the freedom of motion of the polymer chains, i.e. a loss of entropy
This situation is thermodynamically unfavourable and forces the droplets apart again
Osmotic (solvation) forces
When two particles come into close contact the polymer chains start to overlap, effectively leading to a concentrated polymer solution
Osmotic forces causes…
This induces an osmotic gradient in the solution: a concentrated polymer solution in the overlap region and a dilute solution in the bulk solution
What happens when water enters the concentrated region?
in attempt to dilute it and in doing so forces the polymer chains (and droplets) apart
Steric stabilisation - equation:
VT = VA + VS
A = attractive
S = steric & solvation forces
Forces - 4 main types
Van der Waals (attractive)
Electrostatic (repulsive)
Steric forces (repulsive)
Solvation forces (repulsive)
For charged colloids (emulsions stabilised by ionic surfactants)
> Van Der Waals
electrostatic forces
= the most important (both)
For uncharged colloids (emulsions stabilised by non-ionic surfactants)
> van der Waals and steric
solvation forces
= the most important (both)
Emulsions stabilised by ionic and non-ionic surfactants
Sometimes both electrostatic and steric & salvation forces can be present
DLVO equation becomes…
VT = VA + VS + VR
= This usually achieves best stability
Instability phenomena summary
6 steps
Creaming
Migration of the dispersed phase of an emulsion (liquid), under the influence of buoyancy. The particles float or sink, depending on their relative density. The substance particles remain essentially separated. The creaming of an emulsion is relatively easy process to reverse.
Sedimentation
The process of settling or being deposited as a sediment
Cracking
The permanent separation of an emulsion into its separate ingredients. Unlike creaming it can not be re-dispersed by shaking
Flocculation
Particles come together attracted by weak forces to form flocs or flakes
Coalescence
Two or more droplets merge to form one daughter particles
Ostwald ripening
Dissolution of small crystals or sol particles and the re-deposition of dissolved species on the surfaces of larger crystals or sol particles
