Disperse Systems (emulsions ) Flashcards
emulsion
a mixture of two immiscible (unmixable) liquids, typically consisting of one liquid dispersed in the form of small droplets within another liquid.
examples of emulsions in everyday life
creams
lotions
milk
mayonnaise
toothpaste
examples of pharmaceutical emulsions
Lotions, creams, ointments, vitamin drops
the two immiscible phases present in an emulsion
the dispersed phase(internal phase)
the continuous phase(external phase)
The dispersed phase, comprising small droplets, is dispersed within the continuous phase. In oil-in-water (O/W) emulsions, water is the continuous phase, and oil droplets are dispersed. In water-in-oil (W/O) emulsions, it is the opposite, with oil as the continuous phase and water droplets dispersed within it.
in an emulsion where one phase is dispersed in the other, is the emulsion stable?
no, because the dispersed droplets would eventually seperate from the continuous phase to reform the intial immiscible state
interface tension
a term used to describe the force acting at the interface between two immiscible phases, such as between two liquids or between a liquid and a gas.
It is the tendency of the interface to minimize its area, and it arises due to the difference in cohesive forces between molecules in the two phases.
state two functions of the interface tension
prevents the complete mixing of immiscible liquids. It creates a barrier that resists the penetration of one liquid into the other, maintaining the distinct separation of phases
influences the formation of droplets when one immiscible liquid is dispersed into another. It determines the size and shape of the droplets, with higher tension leading to smaller, more spherical droplets.
how are emulsions stabilized
via the addition of a surfactant or an emulsifying agent
function of an emulsifying agent or surfactant
To lower interfacial tension and stabilise emulsion
is an emulsifying agent the same as a surfactant
no
all emulsifying agents are surfactants, but not all surfactants are emulsifying agents. Emulsifying agents are a subset of surfactants that are specifically chosen for their ability to stabilize emulsions
surfactant
a broad category of compounds that have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts in their molecular structure.
This dual nature allows surfactants to reduce the surface tension between immiscible phases, such as oil and water. Surfactants can act as emulsifying agents when used to stabilize emulsions by preventing the coalescence of dispersed droplets.
if an emulsifying agent is not added to an emulsion what happens
the dispersion is unstable and the globules undergocoalescence to form two separate layers of water and oil
state some properties an emulsifying agent must possess
Be non-toxic.
Must not interfere with the stability and efficacy of the API.
Be compatible with other ingredients in the formulation.
Be stable to microorganisms.
Possess little or no odour, taste or colour.
Maintain stability of the emulsion for intended shelf-life.
Molecule has 2 parts: hydrophilic and hydrophobic
how does an emulsifying agent work
Forms a coherent film around the globules of the dispersed phaseand prevent the coalescence of the droplets.
when is it necessary for antioxidants to be added to emulsions
enhances stability by preventing autoxidation of the oil and rancidity/and or destruction of any sensitive components like vitamins, present in the emulsion
primary emulsion
The primary emulsion refers to the initial emulsion formed when two immiscible liquids are mixed.
In the context of O/W emulsions, it is the initial mixture where water is the continuous phase and oil is dispersed. For W/O emulsions, it is the initial mixture where oil is the continuous phase and water is dispersed. The primary emulsion is formed by combining the two phases along with an emulsifying agent.
secondary emulsion
Secondary emulsions can be formed by taking the primary emulsion and further emulsifying it with additional ingredients
This additional emulsification process can result in the creation of finer droplets within the primary emulsion. Secondary emulsions can have different properties, such as improved stability or altered texture, compared to the primary emulsion.
what could be the essence of secondary emulsions pharmaceutically(in terms of drug release)
By forming secondary emulsions with specific characteristics, pharmaceutical scientists can design controlled release systems. This allows for the gradual release of drugs over time, providing sustained therapeutic effects
which phase is the emulsifier typically more soluble in
the external/continuous phase
this is the bancroft rule
some differences btn o/w and w/o emulsions
Water soluble drugs are more quickly released from o/w emulsions while Oil soluble drugs are more quickly released from w/o emulsions
Preferred for internal use as bitter taste of oils can be masked while w/o preferred for external use
coarse emulsions
fine emulsions
microemulsions
mainly large droplets 10 μm to 100 μm
mainly small droplets 0.1 μm to 10 μm
very small droplets 10 to 100 nm
how does droplet size affect emulsion stability
the smaller the droplet size the higher the emulsion stability
HLB in the context of emulsion agents stands for?
Hydrophilic-Lipophilic Balance
the usual HLB range of emulsifying agents
0 to 20
Materials that are highly polar or hydrophilic have higher HLBs than materials that are less polar or lipophilic, true or false?
true
why might we be required to do if a surfactant with the desired HLB value is not available?
it may be necessary to mix two surfactants, then we calculate the HLB of the surfactant mixture using the formula
HLBmix = fA x HLBA + (1-fA) x HLBB
w/o emulsion can be diluted with water and vice versa, what is this based on
the solubility of the external phase of the emulsion
is oil a good conductor of electricity?
no, oil is a non conductor
name an example of a test used to determine whether emulsions are o/w or w/o
fluorescent test
Oils give fluorescence under UV light, while water does not.Therefore, o/w emulsion shows a spotty pattern while w/oemulsion fluoresces
creaming
sedimentation
the upward movent of dispersed droplets in an emulsion
the downwards movement of dispersed droplets in an emulsion
Aggregation or flocculation
dispersed droplets come together but do not fuse in an emulsion
colascence
the complete fusion of droplets in an emulsion
is a creamy emulsion likely to coalasce?
give a reason for your answer
yes, because of the close proximity of the droptlets at the top of the emulsion
how to decrease the chance of creaming
decrease droplet size
decrease density difference between the phases
increase viscosity
are colascence and breaking reversible?
no they are not
causes of breaking
Addition of a chemical that is incompatible with the emulsifying agent
Bacterial growth – may destroy the emulsifying agent. Temperature changes, such as heating or freezing.
is breaking the same as coalascence
no
breaking is a broader term that describes the overall instability of the emulsion, leading to phase separation. Coalescence is a specific process within this instability, involving the merging of individual droplets. Coalescence contributes to the overall breaking of the emulsion by reducing its stability and promoting the formation of larger droplets or distinct phases.
