Disperse Systems: Emulsions 8 Flashcards
1
Q
Emulsions
A
- Dispersion consisting of
-
Insoluble Liquid Globules
- (POLAR LIQUID phase)
-
Liquid/semisolid dispersing medium
- (NON-polar liquid phase)
- w/ aid of Emulsifying agent
- (surfactant), typically 2+ emulsifyers
-
Insoluble Liquid Globules
-
Primarily Coarse Dispersions
- 500-1000nm
-
Sometimes Colloidal Dispersions
-
10-200nm globule sizes
- nanoemulsions / microemulsions
-
10-200nm globule sizes
- NOT THERMODYNAMICALLY STABLE
2
Q
When is a colloid not a Colloid?
A
When it is an EMULSION
-
Milk is an COARSE dispersion
- we base it on size, the droplets need to be
- <500nm (<0.5um)
3
Q
Oil in Water
O/W
Emulsions
A
- Most Emulsions are Oil in water (THINK MAYONAISE)
- Oil droplets are dispersed as globules (nonpolar)
- throughout a Continious aqeuous phase (polar)
- Pharmacy practice uses mainly O/W emulsions
- Creams & Lotions
-
Oral to mask taste or convenient liquid dosage form
- protect from degradation
- Prenteral administration
- GOOD ELECTRICAL CONDUCTION
- Rinse out jar of mayo with WATER
- adding water will NOT cause PHASE SEPERATION
- Water soluble dyes work for OW
4
Q
Water in Oil
W/O
Emulsions
A
- Think BUTTER
- Water droplets are dispersed as globules
- throughout a Continious oil phase
-
Topical Drug formulations, Rare in pharmacy
- Ointments / sunscreen / sensitive skin
- Poor electrical conductance
- You can’t clean up butter with WATER
- need to add more OIL in order to not get phase seperation
-
Oil souluble dyes ( paprika / turmeric )
- will work for W/O emulsions
5
Q
Desired Properties
of Emulsions
A
- Emulsified material (=what comes first, O in O/W or W in W/o)
- SHOULD NOT seperate from the dispersing medium rapidly
- Globules should FLOCCULATE not coalesce
- Readily redispersed into UNIFORM mixture by MIXING
-
Globule size distribution & Specific surface area
- SHOULD NOT CHANGE
- Formulations should be prepared using readily available ingredients
- Should NOT BE too Viscous so it can pour freely
- but viscous enough to PREVENT settling
6
Q
Flocculation
for Emulsions
A
- Type of Agglomeration that is PREFERRED
- loose/fluffy conglomerate held together by VDW forces
- surface area is only slightly reduced
- Zeta Potential is close to zero
- Analagous to suspension except for:
- globules of the dispersed LIQUID phase
- vs particles of the dispersed SOLID phase
- are clustering together without caking in flocculation
- globules of the dispersed LIQUID phase
7
Q
Coalescence
in Emulsions
A
- Type of Agglomeration that is Not preferred
- compact liquid formed in which globule structure is lost
- interfacial surface area is minimized
- Zeta Potential is FAR from zero
- Analagous to suspension’s AGGREGATION except for:
- glubules of the dispersed LIQUID phase agglomerate to form a single mass as a seperate fused layer
- vs particles of the dispersed SOLID phase agglomerate to form a CAKE OF DENSELY PACKED PARTICLES
- –>TWO BULK LIQUID PHASES
-
vs –> One bulk liquid phase + Compacted solid particles
*
- glubules of the dispersed LIQUID phase agglomerate to form a single mass as a seperate fused layer
8
Q
Creaming
in emulsions
A
- Creaming = Sedementation in suspensions
-
Precurser to Coalescense
- vs sedementation as a precursor to aggregation
- When creaming occurs,
- the emulsion MAY or MAY NOT be easily reformed
-
In O/W emulsions, oil is lense dense than water so…
- Creaming –> FLOAT TO THE TOP
9
Q
Emulsifiers
A
- Needed to PREVENT coalescence
- because emulsions are thermodynamically UNSTABLE
-
Ways to create a barrier on the surface of dispersed globules:
-
SURFACTANT
- monomolecular layer of surfactant to alter interfacial tension
-
HydroPHILIC colloid particles, multimolecular layer
- Acacia, gelatin
- ONLY FOR O/W emulsions
-
Adsorb finely divided solids to the globules
- bentonite veegum for O/W
- carbon black for W/O
-
SURFACTANT
10
Q
Using Surfactants
A
- Typically TWO OR MORE surfactants to form stable O/W emulsion
-
HydroPHILIC surfactant –> in aquous phase
- Help form outerlayer around electric double layer
-
HydroPHOBIC / lipophilic surfactant
- adsorb to the surface of oil globule
-
HydroPHILIC surfactant –> in aquous phase
- Want film to be Densely packed
- cover the ENTIRE Surface
-
We DO NOT have a good predective theory
- Trial and error experimentation to determine whats good
11
Q
HLB Values
(hydrophile-lipophile balance)
for surfactants
A
- For O/W emulsions
- moderately HIGH HLB VALUES ( 9-12 )
-
For W/O emulsions
- moderately low HLB values ( 3-6 )
12
Q
Technique Matters!
A
- Emulsion like GRAVY is stabilized by hydrophilic colloid
- SHEER FORCE MATTERS
- TEMPERATURE MATERS
- Lumpy gravy is formed from failure to properly form the starch colloid.
- Caking starch particles
13
Q
Velosity of CREAMING
V
A
- Expressed by Stoke’s Law for dispersions containing less than 0.5% - 2% suspended material
- NEGATIVE V = particles FLOAT ON TOP
- Velocity is determined by:
- Diameter of globule
- Difference between densisties of globule/medium
- Gravity acceleration
- INVERSELY related to VISCOSITY of medium
14
Q
Breaking the Emulsion
=
Coalescense
A
- Coalescence to form a continuous phase from the formerly dispersed globules depends on multiple factors:
-
Globule Volume/Size
-
Reducing size does NOT always improve stability
- below 3-5um DOES, because brownian motion
- Uniform size retards creaming
-
Reducing size does NOT always improve stability
-
Dispersion medium viscosity
- Increasing Viscosity retards creaming
-
Phase Volume Ratio
- Phase Volume can NOT exceed 74% of total volume
- limiting to 50% increases stability
- Phase Volume can NOT exceed 74% of total volume
-
Zeta Potential
- Increasing globule zeta potential improves physical stability
- make globules more highly charged to increase electrostatic repulsion
-
Globule Volume/Size
15
Q
Preservatives
A
- Most pharmaceutical emulsions have ingredients that SUPPORT GROWTH of microorgamnisms
-
Preservatives are added for Antibacterial properties
- Prefer preservatives that prefer partion into the depersing medium
- Ex. Water in the O/W emulsion
- Prefer preservatives that prefer partion into the depersing medium
