M7 Emulsions pt 2 Flashcards
What is the relationship between surfactant concentration and interfacial tension?
As we increase surfactant concentration, we decrease interfacial tension – up to a point, which is the CMC of the surfactant
what is interfacial tension
force that resist it from breaking up into smaller particules is interfacial tension
What is the relationship between the location of the surfactant monomers and interfacial tension?
Monomeric surfactant molecules accumulating at the surface and interface decrease surface/interfacial tension
* air considered hydrophobic -> will go to surface and orient which will dec interfacial tension
Explain the log concentration of surfactant vs. interfacial tension curve.
A: interfacial tension
B: log concentration of surfactant
C: critical micellar concentration
D: surfactant molecules without full coverage of surface
E: surface adsorption of monomeric surfactant molecules with full coverage of surface
F. air/oil – water interface G. micelle formation
What happens to surfaceant molecules in dilute dispersions vs more saturated
dilute: surfaceants orient themselves at oil.water interface
- as conc of surfactnat inc surface alyer will becomes saturated
- once saturated further increases will no longer affect interfacial tension or stability of system
- surfacnts mols will start forming mmicelles
What is the effect of micellization on interfacial tension?
Micelles formation after the CMC is reached does not influence interfacial tension
what is CNC
point at which michelle formation begins
- depends on structural characteristics to depends on surfactant
what happens to tendency to form michelles as the legnth of hydrocarbon chain inc
- increases the tendency to form micelles
micelle formation of non ionic surfactants vs ionic
ionic surfactants -> get repulsion between adjacent charged head groups that opposes michelle formation
*non ionic surfactants for micheles art conc << ionic ones
what does a higher CMC mean
- more monomers can go to interface ( more efficient)
*low CMC wll form micelles before oil ro water droplets are coated -> less efficient
What is meant by surface excess concentration of surfactant in the Gibbs equation?
- Γ = surface excess surfactant concentration
- portion of surfactant occupying the interface and contributes to lowering the interfacial tension
calc Q 32
when do you get mixed micelles
* some micelles ahve toils pointed out others with heads pointed out
If a blend of surfactants is used in the preparation of an emulsion → formation of mixed micelles
what is aggregation number
number of surfactant molecules that form one micelle
what are association colloids
micelles
what forms of surfactants are found below CMC and Above CMC
Below CMC: monomers
Above CMC: micelles
what does each varaible mean
- Γ: surface excess or surfactnat (surfactant mol at surface)
- moles/Cm2
- R: universal gas cosntant (8.314 x 107 erg/mole/K aka g cm2/s2
- T: abolute temp in K
- ∆γ: change in surface tension of mixture with change in surfactant conc
- dynes/cm OR g cm /s2
- C: concentration of surfactant in liq bulk
- unitless bc log
What is HLB
Hydrophile - Lipophile Balance
describing the hydrophilic-hydrophobic characteristics
ie. solubility of surfactants
the HLB scale ranges from1-20 (in some special cases up to 50)
HLB of something Lipophilic? Hydrophilic
HLB 0-10 = lipophilic
HLB: 10-20 = hydrophilic
What type of emulsion is formed when a surfactant of HLB=14 is used?
- o/w
*Hlb over 10 - hydrophilic
What type of emulsion is formed when a surfactant of HLB=5 is used?
w/o
Using the Tables below, select a pair of emulsifiers to prepare 100 g of an w/o emulsion containing 50g olive oil and 10g beeswax requiring a total of 7% w/w surfactant concentration
What emulsifying agents form monomolecular film around the disperse phase?
Ionic and non-ionic surfactants
*act by dec interfacial tension
What emulsifying agents form multimolecular film around the disperse phase?
Hydrocolloids such as polymers, acacia, gelatin
*sterioc hinderance (forming solid particle film)
What emulsifying agents form solid particle film around the disperse phase?
How does bentonite exert its emulsifying properties?
It forms a solid particle film on the surface of oil droplets
What are the desirable properties of surfactants?
non-ionic emulsifiers
*Depends on the emulsifying ability desired and the toxicity (determines route of administration) of each emulsifying agent.
less toxic and less iritant
What are the desirable properties of surfactants?
ionic emulsifiers
should not be given orally (irritant to GIT)
Cationic: toxic at low concentrations ⇒ limited to external use when antiseptic properties are required
Anionic: have high pH (alkaline) ⇒ unsuitable for broken skin
what is phase inversion? how can it be utilized?
- change in emulsion type w/o -> o/w
ex: during prep of o/w the water phase is poured into oil-> first add small amount of water to form w/o then subsequent addition of water will form an o/w - this order of addition results in better physical stability of final product
List a few frequently used antioxidant.
BHA, BHT, α-tocopherol, alkyl gallates (Propyl Gallate B.P.), ascorbic acid, ascorbyl palmitate
What equipment can be used for emulsification?
- Mechanical stirrers (rotating blades)
- used for low viscosity prep
- large quantities may be mixed
- Homogenizers
- small inlet orifice and presssure used to nicorporate oil
- not suitable for viscosity > 2000 cP
- ultrasonifiers
- sonic saves break down emulsified droplets
- produces uniform emulsion
- gentle -> powerful
- colloid mills (rotor/stator)
- common
What is creaming?
Emulsion instability caused by flocculation and upward sedimentation of oil droplets
reversible process
What is coalescence?
- complete fusion of droplets within an emulsion
leads to a ↓ number of droplets and ultimately separation of the two immiscible phases
– irreversible process
how do you determine rate of creaming or rate of sedimentation in an emulsion
Stokes law of sedimentation can be used to calculate sedimentation rate
in case of creaming sedimentation rate will be a negative number
what kind fo stress conditions are used for assessing emulsion shelf lfie
Aging and temperature increase
Centrifugation (to accelerate separation into two layers)
Agitation (to reflect conditions of shipping)
What is the reason for coalescence and what steps would you take to prevent coalescence in an emulsion?
- can be caused by low zeta potential, high surface free energy or high interfacial tension
to mitigate: add protective colloids, optimize carticle size, add more surfactant
*coalesence in w/o emulsions is prevented by presence of long hydrocarbon changs of surfactant molecules that project into oil phase
What is the reason for creaming and what steps would you take to prevent this in an emulsion?
- caused by flocculation and upward sedimentation
- add more rpotective colloids
add more surfactant
What is the reason for foaming and what steps would you take to prevent foaming in an emulsion?
too muhc surfactant
reduce the surfactant conc
What is the reason for microbial growth in an otherwise preserved emulsion and what steps would you take to prevent this in an emulsion?
preservative – surfactant interaction reduced antimicrobial function; add more or different preservative
What is the reason for unpleasant taste and what steps would you take to achieve palatability in an emulsion?
- inappropriate amount fo certain excipients: propylene glycol, glycerol, preservatives
- lack or inappropriate amount of sweetnener or flavouring agent
*reduce/optimize excipient conc, add more or diff sweetener or flavour
how to reduce rate of creaming/sedimentation
