Emulsions Flashcards
What are the different types of emulsions?
- Simple emulsions
- Multiple emulsions
- Microemulsions
- Micellar emulsions
How does one distinguish between o/w and w/o emulsion?
- Colour, feel on skin, filter paper, wetting, filter paper + CoCl2, fluorescence, conductivity, dye test, dilution
What are the basic components of emulsions?
- Oil phase
- Aqueous phase
- Emulsifying agent (a.k.a. emulgent)
What are the different types of surfactants?
- Anionic surfactants
- Cationic surfactants
- Amphoteric surfactants
- Nonionic surfactants
What are surfactants?
compounds with hydrophilic and lipophilic groups in molecular structure –> amphipathic nature causes molecules to be attached to interfaces
What are some examples of nonionic surfactants?
- Sorbitan esters and polyoxyethylene sorbitan esters
- Glycol and glycerol esters (glyceryl monostearate)
- Fatty acid polyglycol esters (POE fatty acid esters)
- Fatty alcohol polyglycol ethers
- Higher fatty alcohols
What are some examples of cationic surfactants?
Cetrimide, cetyl pyridium chloride, benzalkonium chloride
What are some commonly used hydrophilic colloids used in the formation of surfactants?
natural and synthetic clays, natural and synthetic gums, proteins
What are some ways that emulgents contribute to the stability of emulsions?
- Formation of rigid interfacial film
- Formation of electric double layer
- Increase viscosity of continuous phase
What happens when you use surfactants of a much lower HLB than the required HLB in the formation of a suspension?
phase inversion occurs if it is a o/w emulsion, otherwise stability will be very low
what are emulsions?
system consisting of 2 immisicible liquid phases, one dispersed as fine globules throughout the other
What are the phases in emulsions?
- Disperse/internal phase: liquid phase subdivided into globules
- Continuous/external phase: Liquid in which globules are dispersed in
What are microemulsions?
Emulsion has extremely small globules dispersed in the continuous phase, and appears transparent to the naked eye and cannot be seen under a light microscope
What are some desirable properties of an oil phase in an emulsion?
- Consistency
- “Feel” or tactile characteristic
- Stability
- Drug solubility
The two main types of oils used as oil phase in emulsions?
- Mineral oil
2. Vegetable oil (less stable than mineral oil)
What are the three main classes of emulgents?
- Surfactants
- Hydrophilic colloids
- Finely divided soilds
What are some factors affecting the selection of appropriate emulgents?
- Type of emulsion
- Compatibility with other components
- Toxicity of emulgent
- Cost
What type of emulsions do Anionic surfactants favour?
N.A: It depends on the specific anionic surfactant used.
E.g.
- Soaps of monovalent bases favour o/w
- Soaps of polyvalent bases favour w/o
Anionic surfactants: Amine soaps are not as strong as soaps of monovalent and polyvalent bases in terms of forming an emulsion. However, when is/are the scenario(s) in which amine soaps are more suitable?
For O/W emulsions where high pH cannot be tolerated (e.g. to reduce irritability of the skin)
An anionic surfactant that allows PO usage?
Quillaia saponins
Cationic surfactants favour the formulation of what type of emulsions?
o/w
On top of emulsifying property, what other property does cationic surfactants posses?
Antiseptic properties
If the drug compound to be formulated into an emulsion is negatively charged, is cationic surfactant suitable?
No, cationic surfactants are incompatible with anionic compounds
Advantage of non-ionic surfactants over ionic surfactants
Lower toxicity and irritancy
Hydrophilic colloids favour what kind of emulsions?
O/W emulsion
Two types of finely divided solids used in formation of emulsions, and their type of emulsions they favour
- Polar inorganic solids like metal hydroxides and non-swelling clays. Favour o/w
- Non-polar solids like carbon. Favour w/o
Without an emulgent, why are systems containing oil and water phase thermodynamically unstable?
A tension exists at the droplet interface because the two immiscible phases have different attractive forces for a molecule at the interface
Coupled with a droplet’s large surface area, the tension is very large, and each phase gradually coalesce in order to minimise tension in the system (hence unstable), forming a two phase system.
Both microemulsions and miscellar emulsions have one common property. They _________________
appear transparent to the naked eye
Electric double layer only applies to what kind of sufactants?
Ionic Surfactants
What are the three theories currently present that have been proposed to explain the formation of o/w and w/o?
- Bancroft’s theory:
- Oriented wedge theory
- Theory based on angle of contact
Describe Bancroft’s theory in the formation of o/w and w/o emulsions. What type of emulsion is this theory applicable to?
- Adsorption of surfactants at o/w interface give rise to interfacial film
- Film experience two interfacial tension between aqueous and oil phase respectively
- Film curves in direction of greater interfacial tension.
- Dispersed phase is on the side where film curves towards (higher interfacial tension)
Applicable when emulsions are formed using SURFACTANTS
Describe the oriented wedge theory in the formation of o/w and w/o emulsion, and what type of emulsion is this theory applicable to?
Only applicable for SOAPS
- Soaps have a polar head and a non-polar tail
- Soap molecules orientate at o/w interface in to allow more molecules to pack at the interface, forming a tightly packed film
- Relative location of polar heads determine type of emulsion
Describe the theory based on angle of contact in the formation of o/w and w/o emulsions. What type of emulgents does this theory apply to?
Applies to finely divided solids
- Solid particles attracted to the interface between the liquids
- The angle of contact formed by the solid particle at the interface determines type of emulsion
- The phase that wets the solid particles preferentially is the continuous phase
- Angle of contact is with relation to the aqueous phase
What are some properties must the finely divided solids have in order for the theory based on angle of contact to be applicable?
- Insoluble in both phases
- Preferentially wetted by one of the phases
- Can form thin interfacial film
- Colloidal size
What is HLB?
Hydrophile-lipophile balance is a concept for a surfactant consisting hydrophilic and lipophilic group.
The ratio of these two groups is termed HLB
Formula for HLB
HLB = 20 x Weight of hydrophilic group/MW
HLB 20 represents a molecule which is 100% hydrophilic
Is calculated HLB the optimal HLB for an emulsion?
No, it doe not take into consideration various factors affecting emulsifying efficiency of surfactants
After required HLB is calculated, outline the steps to determine the optimal HLB and surfactant blend
- From calculated HLB, use surfactant blends of different portions of chosen surfactants with HLB varying slightly from calculated HLB
- Plot a graph of emulsion stability against HLB to obtain optimal HLB
- Using optimal HLB, use different pairs of surfactants adjusted to give the optimal HLB
- Graphically, the surfactant pair that give rise to the highest emulsion stability is the most effective pair
List some additives used in the formulation of emulsion
- Sweetening agents
- Antibiotic preservatives: prevent growth of micro-organism that may be introduced into emulsion during use
- Antioxidants and chelating agents: Prevents oxidation of oils and other substances
- Others: Emulgents, oils, water, buffer, colours
List the techniques of emulsification
- Agent-in-water method
- Agent-in-oil method
- Nascent soap method
- Alternate addition method
Outline the agent-in-water method to form an emulsion
- Emulsifying agent added in water
- Oil added to the mixture with vigorous agitation
- This produces o/w emulsion directly
- For w/o emulsion, addition of oil continues until phase inversion occurs
Outline the agent-in-oil method to form an emulsion
- Emulsifying agent added in oil
- Water added to the mixture with vigorous agitation
- This produces w/o emulsion directly
- For o/w emulsion, addition of water continues until phase inversion occurs
Outline the Nascent soap method to form an emulsion. What emulsion is this method suitable for?
Suitable for emulsions with soap as emulgent
- Soaps are made of fatty acid + base
- Fatty acid dissolved in oil
- Base dissolved in water
- Soap is formed in situ when oil and aq phase brought to contact during mixing
- Whether o/w or w/o emulsion formed: depends on type of soap used
Outline the alternate addition method to form an emulsion. What is this method particularly suitable for?
Suitable for vegetable oils (from observations, no scientific explanation)
- Water and oil are added alternately in small amounts to the emulsifying agents
What are the types of emulsifying machines and their purpose?
- Simple stirring by mixers
- Colloid mill
- Vibrator and Ultrasonicator
2 and 3 are homogenisers
How do Colloid milling and Vibration and Ultrasonification contribute to the stability of emulsions?
They homogenise the emulsions by further reducing droplet sizes of dispersed phase
What is a stable emulsion characterised by?
No coalescence of disperse phase which can lead to separation of oil and aqueous phases
What are some phenomenon in which emulsion instability manifests?
- Creaming
- Flocculation
- Coalescence
- Cracking
Describe Creaming
- Globules rise or sediment depending on difference in specific gravities between phases
- This results in concentration of dispersed phase at top or bottom of system, readily redispersed on shaking
How is Stokes’ law applied in creaming phenomenon?
- Tell us the factors tha can be modified to reduce creaming rate and improve stability
- Velocity of droplets in emulsion cannot be calculated directly with stoke’s law due to hindrance of motion of particles
- The causes of hindrance of motion depends on concentration of droplets of disperse phase and this in turn depends on volume fraction of disperse phase
What factors in Stokes’ Law can be modified to reduce creaming rate?
- Radius of droplet
- Viscosity of continuous phase
- Difference in densities between dispersed and continuous phase
What factor NOT in Stokes’ Law can be modified to reduce creaming rate?
Volume fraction of dispersed phase (increase V fraction = reduce rate of creaming)
What is Flocculation?
The reversible aggregation of droplets of dispersed phase in the form of 3D clusters
Microscopically, when particles flocculate, why is the aggregation reversible?
- The individual flocculated particles do not lose their identity
- The mechanical or electrical barrier still surrounds each individual droplet, sufficient to prevent coalescence
How does flocculation increase the rate of creaming?
- Flocculated particles behave as single particles
- Stokes’ Law: the bigger the radius of particle, the higher rate of sedimentation or rise
- Hence rate of creaming increases
What is coalescence?
Droplets of dispersed phase IRREVERSIBLY join to form larger drops, eventually leading to decrease in number of droplets and finally cracking
Define cracking
A complete, irreversible breakdown of the emulsion, with coalescence of droplets and separation of two phases into 2 layers
List some chemical, physical and biological effects that can lead to cracking
- Addition of substances that are incompatible with emulgent
- pH changes
- Temperature changes
- Bacterial and fungal actions
Usually these affect emulgent functions which causes cracking of emulsion
List some incompatible pairs of substance - emulgents
- Strong acids + alkali-metal soaps
- Anionic compounds + cationic surfactants (and vice versa)
- High concentration of electrolytes + soaps
- Alcohol + gums (alcohol ppt gum out)
List the tests employed to determine the stability of emulsions
- Degree of separation
- Size analysis of globules
- Electrophoretic property
- Viscosity changes
Describe how degree of separation is used to determine the stability of emulsions
Ratio of volume of separated phase and total volume of emulsion is determined
- Note separated phase refer to the dispersed phase. Oil usually floats on top while aqueous phase usually settles at the bottom
Describe how size analysis of globules can be used to determine the stability of emulsions
- Mean size of globules determined by microscopic examination/electronic particle counting/laser diffraction
- Less stable emulsion show greater increase in mean globule size with time due to coalescence
Describe how electrophoretic property can be used to determine the stability of emulsions
- Emulsion conductivity is dependent on degree of dispersion
- For o/w emulsions, reduction in conductivity indicates oil droplet aggregation hence instability. Vice versa for water
- Measurements are made with pt electrodes on emulsions stored for short periods of time
Describe how determining viscosity changes can be used to determine the stability of emulsions
- Emulsion viscosity is affected by globule size and number
- Generally, viscosity increases over time. Emulsions with greater increase over time are less stable
- If cracking occurs, viscosity profile changes abruptly
